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Climatic change and food production
Climatic change and food production
John Gribbin
Small
changes
in climate
disproportionate availability world
of food
reserves
reviews
and
on
when,
are low.
evidence
change
can have a
influence
as now, Dr Gribbin
for
concludes
climatic that
climate
is likely
to be much
variable
in future
than
the
middle
Longer
decades
periods
weather
in
agricultural Prudence stocks insurance probable
unfavourable of
areas
are
against
the
that
world
built an
major
expected. up
grain as
an
increasingly
series of poor harvests.
Dr Gribbin Science
it has been in
of
be
the more
of this century.
some
dictates should
the
is a visiting
fellow
Policy Research
sity of Sussex, Falmer,
at the
Unit, Univer-
Brighton,
UK.
This article draws extensively on the material in his book Forecasts, Famines and Freezes (Walker, New York and Wildwood, London; 1976).
’ Louis M. Thompson, ‘Weather variability, climatic change and grain production’, Science, Vol 188, 1975, pp, 535-541. See also, Committee Report to the National Oceanic and Atmospheric Administration, The Influence of Weather and Climate on United States Grain Yields; Bumper Crops or Droughts, (US
Department of Commerce, Washington, D.C., December 1973). * Nigel Calder, Ere Weather Machine, BBC Publications. London, 1974. 3 Stephen Schneider and Lynne Mesirow, The Genesis Strategy, Plenum, New York, 7976.
FOOD
POLICY
August
1976
Changes in climate which are not only small but local, in global terms, are sufficient to disturb the present world food supply system, and it has been partly through the variations in weather experienced in the key areas in recent years that the problem of climatic change and food production has begun to receive attention.’ Although evidence of a global shift in climatic patterns was certainly available at the beginning of this decade, such attention as this has received has focussed on the dramatic possibilities of the imminent arrival of a new Ice Age2, and even the more sober treatments have tended to stress the implications of a continuation of the cooling experienced by the Northern Hemisphere over the past 25 years or so (see Figure 1). For crop production, the immediate importance of such a cooling trend is the reduction it causes in the length of the growing season, with frosts occurring later in the spring and arriving earlier in the autumn. But these changes are small, and, as Thompson has stressed, cooler, wetter conditions are in fact better for the production of major crops in some of the key food producing areas of the globe - ‘wheat grown anywhere in the United States is adversely affected by higher than normal temperatures from flowering time until the crop is mature’ and ‘the summer of 1958 was cooler and wetter than normal in the middle latitudes of the Northern Hemisphere and resulted in new record yields in grain in both Russia and the United States’.’ Leaving aside for the moment the important question of how we define ‘normal’, the greatest hazard to production of these key crops is not the cooling trend as such, but the fact that this cooling trend is associated with a greater variability of weather, making it less predictable and producing greater extremes of all kinds - more common droughts and more common floods; more common hot summers and more common severe winters. Thompson suggests that if the weather was as variable from now until the year 2000 as it was in the corn and soybean belts of the USA from 1890 to 1955 then the average yield would be reduced by about 3%. This is not alarming in itself, perhaps, but that 3% reduction is not likely to be spread evenly over the 25 year period. Rather, we can expect that in some of those years the extremes of variation will produce very poor yields, which could give rise to a potentially disastrous situation unless steps are taken to provide a stockpile of food available to tide the world over the lean years, in an application of what Schneider has graphically termed the ‘Genesis Strategy’3.
301
Climatic change andfoodproduction
[[r’.i Figure
1.
mean
Top:
surface
past
century.
Murray
Five-year
Mitchell
Center
for
Variation
in global
temperatures
means
from
Atmospheric Variation over
temperatures
inferred
millenium,
data for England line
shows
Northern Murray
the from
by H.H.
20-y
Hemisphere,
mean past
-0 5 10.0 ? ;
Lamb. Solid for
the
9.5 -
calculated
by
Mitchell.
cj g
I
-
historical
means
1890
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10.5
Research of
1880
C
and the US National
(NCAR) Bottom:
1870
over the
9.0
__=
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I
1900
1910
1920
1930
1940
1950
1960
1970 “F
Mean Annual Temperature I I I I ;‘I..,‘\ I \ \ : 1’ “$entral England \ : \ \ .,’ \ \ I’\ \ \ ../’ \ \ \_-__,
I 1000
I
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The immediate problem of climate in terms of food production is not the cooling trend, but the greater uncertainty implied by the increased variability of the weather compared with the middle part of this century. What is now clear - and makes one very wary of using such descriptions as ‘normal’ in this context - is that by the standards of the past few hundred years the equitable years of the middle twentieth century were unusual, and the cooler, more variable conditions now being experienced are more typical of Northern Hemisphere weather during the past millenium.
The global picture
4 H.H. Lamb, personal communication. 5 Derek Winstanley, ‘Rainfall patterns and general atmospheric circulation’, Nature, Vol 245, 1973, pp 190-l 94.
302
From the 1890s to the mid-l 94Os, with only a slight dip around 1905, the global air temperature increased, by about 0.5”C; between the mid-1940s and 1970, however, global mean temperatures fell by about 0*3”C, and for the five year period 1968-72 the average temperature recorded by the nine ocean weather ships which are stationed between 35”N amd 66’N was more than 0.5”C below the peak of the 1940s. Every summer during the 1930s was warmer than the average for the century in the important corn belt of the USA, but the 1960s were as cool as the first decade of the century. Local effects - such as the recent run of mild winters in Britain - can mask the trend, but even locally the effects of the recent cooling are seen clearly in agricultural records. For example, harvests in England during 1960-73 were completed, on average, about nine days later than during the 1940~~. This change was largely the direct result of the cooling trend; the associated changes in rainfall only became clear when the recent prolonged droughts in the Sahel, in Ethiopia and elsewhere, and the repeated failure of the Indian monsoon, drew attention to the problem in the early 1970s. The basic problem across this belt of drought is a shift of rainfall belts to the south. Winstanley has linked these shifts to a changing pattern of atmospheric circulation which seems to provide the best available overall model, although it should be said that some meteorologists remain doubtful about its detailed accuracy although they have not been able to offer anything better as yet. The
FOOD
POLICY
August
1976
Climatic change andfoodproduction
Figure
2.
‘weak’ Weaker more
The
circulation
bringing
bringing
is associated in
the
tendency
to
further
push
high
and
the
the
tropical
rainfall coast
Middle
East.
rainfall
also squeezed
towards
expansion
of
Figure 3. Shifts since
have
the
Africa
Further
south,
(shaded)
is
the equator
by
the
As
to
circumpolar
footnote
5.
distribu-
well
as
the
squeezing towards the the recent changes in rainfall
led to more sea,
(H)
in rainfall
1960.
for south,
of North belt
flow. After Winstanley,
the
with
westerlies,
general
Mediterranean
general equator,
and
patterns.
low rainfall (L) to Britain, and a
depressions
tion
(top)
circulation
variability
with
the
‘strong’
(bottom)
key to this study is a comparison of the so-called ‘strong’ and ‘weak’ circulation patterns for the northern hemisphere. The strong type of circulation (Figure 2) corresponds to a situation in which the oscillation of the westerly winds that sweep around the North Pole is confined within a narrow range of latitudes. In this case, when the circumpolar vortex is contracted, the depressions which travel from west to east across the Atlantic and play a large part in determining the weather over the British Isles follow well defined paths. They bring changeable weather and high rainfall. Troughs do not reach far south from this belt so that rainfall over the Mediterranean and Middle East is, by and large, low. Further south still, with this pattern of circulation, the tropical wind patterns extend well to the north, and the monsoon rains reach relatively high latitudes. This kind of overall situation seems, in general, to be associated with warmer conditions in the Northern Hemisphere, such as those of the middle part of this century. But now the atmospheric circulation is moving, or has moved, towards the alternative mode - a weak pattern with an expanded circumpolar vortex, associated with a general cooling and increased ice cover at high latitudes. The effect of this change is to allow more erratic variation in the latitudinal distribution of the eastward moving depressions and of the circulation pattern in general, with the main flow of westerlies also shifted further to the south. The weather systems moving towards Britain and Europe travel more slowly and persistent anticyclones can become established, blocking the movement of depressions. For Britain, both the rainfall and the occurrence of westerly winds are decreased, while such rainfall as there is tends to be more concentrated in heavy bursts. With the troughs extending further south, rainfall over the Mediterranean and Middle East increases, but the tropical circulation belt is squeezed towards the equator so that monsoon rains do not extend so far to the north. (see Figure 3). The zonal circulation is decreasing in strength, rainfall over England is becoming less (see Figure 4 and Table I), and there are associated increases in rainfall over the Mediterranean and Middle East, with decreases in the rainfall over the Sahel, Ethiopia and northwest India. These changes have not proceeded uniformly over the past decades, and a significant jump occurred in 1972, a particularly disastrous year that tipped the balance of rainfall against the nomads of the Sahel. The importance of a global perspective in this situation was highlighted, however, by a study of snow and ice
so that
precipitation there
from
important the regions of the world.
FOOD POLICY
Auaust
is a net
over loss
nd since 1960
agricultural
1976
303
Climatic change andfoodproduction Table 1. Monthly Month
rainfall totals, 1973
England and Wales, since 1973 1974
1975
(mm) Standard
1976
average
1916-50 January February March April
Source: UK Met Office: see also Figure 4. The 12 months from May 1975 to April 1976 produced the driest one-year period since records began in 1727, with only 624mm of rain in the 12 month period, just 69% of the standard average.
in
Figure
4.
Wales,
October
Rainfall
1968
1973,
monthly
totals
nual figures monthly dicated.
(bottom)
totals The
to
1916-l replaced
950;
(top)
and
an-
the
month average’
inis
as that for the years
this
by a new
on 194 1 - 1970
and
given as twelve
‘standard
chosen arbitrarily
England
to September
is
soon
‘standard’
figures.
to
be
based
From the An-
Report of the Water Resources Board, 1974. See also Table 1. nual
May June July August September October November December Totals
44 40 24 67 84 63 91 63 86 57 52 67
117 98 47 14 40 66 77 95 144 99 125 69
117 31 81 71 47 21 66 52 107 36 73 50
738
991
752
61 40 50 21
92 66 57 60 63 55 79 81 76 92 95 88 904
cover in the Northern Hemisphere reported by Kukla and Kukla6 in 1974, which showed that there was a dramatic increase in the area of the Northern Hemisphere covered.by winter snow and ice only a year before the sudden shift towards the weak circulation pattern. It is not clear whether the change in snow and ice cover caused the sudden change in circulation, or whether the circulation change affected the buildup of snow and ice, or indeed whether both were caused by some other external influence. But it does seem clear that the two were related, and this implies that with a better understanding of the atmosphere it would have been possible to provide a year’s warning of the droughts which hit parts of Africa so severely from 1972 onwards. The political implications are profound, and it remains to be seen just what will happen if the continued monitoring of snow and ice cover shows another sudden increase of this kind sometime in the near future. Meanwhile, Perry’s prophetic comment in 1970 that ‘the decade of the 1960s may well be remembered as the first when climatologists became convinced that the climate over large parts of ,“S
mm 175 150
Monthly
Ramfall
Standard Monthly
n
(1916-1950) avera9e
/
304
FOOD
POLICY
August
1976
Climatic change andfoodproduction
the northern hemisphere was experiencing a reaction to the genial conditions of the earlier part of this century” seems more apposite than ever. The question of how long this ‘reaction’ will last, and how severe conditions are likely to become, is best answered by considering both the patterns of previous climatic changes and the physical mechanisms proposed to explain them.
Previous climatic changes
sG.J. Kukla and H.J. Kukla. ‘Increased in surface albedo the Northern Hemisphere’, Science, Vol 183, 1974, pp 709-7 14. ’ A.H. Perry, ‘The 1960’s - A Climatic Review’, Weather, Vol 25, 1970, pp 365370. B W. Dansgaard, S.J. Johnson, N. Reeh, N. Gundestrup, H.8. Clausen, and C.U. Hammer. ‘Climatic changes, Norsemen and modern man’, Nature, Vol 255, pp 24-28. 9 R.A. Bryson, ‘The Lessons of Climatic History’, Environmental Conservation, Vol 2, 1975, pp 163-l 70.
FOOD POLICY
August
1976
In order to understand climatic changes occurring over tens of years, we need information about past patterns of climatic change over hundreds of years, at least. The historical record is just sufficient for the task. Although climatic changes occurring over longer timescales - including full Ice Ages - are important, they are not directly relevant to the immediate problem of climate and food production to the year 2000, and will not be discussed in detail here. The first and most important lesson of the historical record is that there is no such thing as a climatic ‘normal’ in the sense that the word was used 50 or more years ago. Then it was felt that if a long enough series of observations could be amassed an average value would be produced for each element describing the climate, an average to which the climate at the observing site would always tend to return. Now it is recognised that climate is always changing on a variety of timescales. The problem is to determine the extent of these changes and to unravel the different timescales of change as far as possible from available records. The best long-term historical records come from only two parts of the world, China and Japan and northwest Europe, and here I shall concentrate on the latter evidence, together with archaeological evidence from North America which helps to fill in the outlines of climatic change around the North Atlantic over the past 1500 years. The best single continuous record of climate over that period comes from a 404m long ice core drilled from the glacier in central Greenland.8 Analysis of variations in the ratio of oxygen isotopes at different levels in the ice has enabled a temperature record to be produced going back for 1420 years. The new understanding of climatic changes on timescales of 60 to 200 years that this provides has given a new insight into the spread and decline of the Norse Atlantic colonies. Ninth century settlers in Iceland, it seems, flourished in a phase of rapid warming and were able to send their own colonists west to Greenland; but within a few hundred years a rapid deterioration of climate was a major contributory factor in the collapse of the Greenland colony. Even the Iceland colony survived by only the barest margin through the worst ravages of what has come to be known as the Little Ice Age, which was particularly marked from the 15th to the 19th centuries, and affected Britain most severely in the late 17th century. In summarising a variety of data about climatic change since the most recent full glaciation (about 10 000 years ago) Bryson9 has emphasised the integral behaviour of the atmosphere which brings the varying patterns of related change over the whole globe. It seems that the climatic changes which affected the Norse colonies after about AD 1200 were related to a cooling of the Arctic and expansion of the sea ice and climatic zones southward, in a manner similar to the recent pattern of change but more pronounced and with longer-term effects.
Climatic change andfoodproduction
lo R.A. Bryson and D.A. Barreis, ‘Climatic change and the Mill Creak people of Iowa’, J. Iowa Archaeol. Sot., Vol 15, 1968, pp l-34. l1 H.H. Lamb, The Changing Climate, Methuen, London, 1968.
306
In North America, this pattern of change shifted the summer westerlies southward from what is now southern Canada into what is now the northern USA, with reduced rainfall in the northern plains, the present corn belt, west Texas and much of the intermontane west, but with increased rainfall in the Pacific northwest, the southeast and much of the east coast.‘O An important reminder of the potential severity of such minor changes in climate in terms of their impact on human cultures is provided by the record of the Mill Creek people, who occupied a region near the corn belt of the present USA in the centuries just before AD 1200. At that time, the region was one of tall grass prairie uplands with wooded valley terraces and valley floors. Deer and corn were the Indians’ staple foods. The Arctic expansion and changing climate brought a long drought, probably because of westerly winds, more persistent than before, crossing the Rockies and putting the region into the rain shadow. Most of the forest disappeared; tall grass was replaced by short grass; farming villages in the west disappeared; and everywhere in the region the staple source of meat became the bison as the deer died out. The particular significance of these changes, as Bryson stresses9, is that the drought persisted for 200 years. In other words, the archaeological and other records show that minor climatic changes of the kind which have occurred in the past millenium are sufficient to bring 200 years of drought to the corn belt of North America, the region which today produces the surplus of grain on which many parts of the world have come to depend. The pattern of climatic changes in the Northern Hemisphere since the middle of this century has been in the same direction as those associated with the Arctic expansion of AD 1200, although the magnitude of the deterioration is not yet as great. Since, however, similar toolings in the past millenium have never lasted for less than forty years9, it seems that at the very least the present trend may return us to a situation more typical of the past few hundred years than of the past few decades - and this, of course, makes the interpretation of climatic patterns of the past few hundred years of great importance. Even with limited meteorological data it has proved possible to construct charts indicating the different patterns of mean sea level pressure over the past two or three hundred years.” Examination of the 40-year mean msl pressure distributions for January and July, for the two epochs 1790-1829 and 1900-39, shows that between the two epochs, the mean pressure gradients in January increased”. The earlier epoch marked a notable minimum in the strength of the zonal circulation and in Europe it was 1% or more cooler than in 1900-39. The cold winters of the years around 1800 can be explained by the weakness of the westerly flow towards Europe. The tendency since about 1950 towards a weaker circulation with more frequent blocking seems to mark a retreat from the 1900-39 situation, which itself marked something of a maximum in the strength of the zonal circulation. Clearly more information of this kind is needed, with pressure distributions determined for even earlier epochs wherever possible. Already, though, the clear lesson to be learnt from the pattern of historical climatic changes is that not only does climate vary but that climatic conditions in the middle decades of this century were around an optimum in terms of conditions for food production at middle
FOOD POLICY August
1976
Climatic
change
andfoodproduction
latitudes. It seems appropriate, especially in view of growing concern about the low level of food reserves today12, to reiterate Bryson’s comment that ‘combining the nature of recent climatic change with the present narrow margin of world food-grain reserves an urgent need to consider and react to the possibility of continued climatic variation is indicated’.
Mechanisms
‘* Lester Brown, ‘The world food prosoect’. Science, Vol 190, 1975, on 10531’059.. I3 Random noise in this context means any of the fluctuations which occur on a statistical basis simply because the climatic system (oceans, atmosphere, land and ice) contains so many particles interacting with one another. Even tossing a coin and noting the occurrence of heads and tails will produce a time series in which although the heads and tails may even out in the very long run, shorter runs dominated by a succession of heads or tails can give the appearance of regular even periodic - variations. The number of either/or interactions occuring such throughout the climatic system is immense, and it has been argued that all climatic variations may simply result from the accumulation of these random processes Although that view seems unnecessarily extreme, such noise must provide the ultimate limit to climatic predictability: a good discussion of the nature of the random noise problem in physical processes has been given recently by L.G. ‘Some thoughts about ranJacchia, domness’, Sky and Telescope, Vol 50, 1975, pp 371-374. I4 B.J. Mason, Symons Memorial Lecture to the Royal Meteorological Society, 1976 (to be published in Quarterly Journal of the Royal Meteorological Society). 15Stephen Schneider and Clifford Mass, ‘Volcanic dust, sunspots and temperature trends’, Science, Vol 190, 1975, pp 741746. I8 C.G. Abbott, Smithsonian Misc. Collections, Vol 448 No 7,1966, and K.Y. Kondratyev and G. Nikolsky, ‘Solar radiation and solar activity’, Quarter/y Journal of the Royal Meteorological Society, Vol 96, 1970, pp 509-522.
FOOD POLICY August 1976
If we understood the causes of climatic change (and assuming that such change is not simply the accumulation of random noise effects in the climatic system13) then prediction of future changes would be straightforward. Unfortunately, such an understanding is not yet at hand, but three mechanisms in particular seem to offer the greatest insight into patterns of climatic change over the past 10 000 years or so. First, there are the long-term cyclic effects produced by changes in the Earth’s orbit around the Sun and in the orientation of the spin axis of the Earth relative to the Sun. These ‘Milankovich’ effects have been discussed for decades, but have only become firmly established as a major contributory factor in the development and decay of Ice Ages in recent years, chiefly because a satisfactory chronology of past Ice Ages has only recently been available. There now seems little doubt, however, that these astronomical influences are important on timescales of thousands to hundreds of thousands of years, through the changes they produce in energy input at the latitudes of crucial importance for the spread of snow and ice cover14. This influence is now acting to produce a dip in Northern Hemisphere insolation over the next few thousand years. It seems that this will bottom out before reaching conditions as extreme as those which prevailed at the height of the most recent advance of the ice, nevertheless, the mode1 has formed the basis of some rather extravagant popularisations suggesting the imminent arrival of a new Ice Age and these, unfortunately, have diverted attention from the real climatic problems now facing us. An astronomical influence of more relevance to climatic change over a few decades seems to be the effects produced by the changing level of solar activity on the Earth’s atmosphere. Although the debate about the links between solar activity over the approximately 11 year ‘sunspot cycle’ and weather continues, Schneider and MasP have been able to reproduce many features of the observed record of temperature variations since 1600 with a model which incorporates an empirical dependence on solar activity. This dependence is no less controversial than other aspects of the debate about solar-terrestrial links, since it implies that the solar constant (now seen as a variable the solar parameter) is, for example, more than 2% less when there is no sunspot activity than when there is moderate activity. The empirical evidence for this comes from observations by balloon-borne instruments16. The most surprising feature of the controversy, perhaps, is that there is no evidence that the Sun is in fact constant to within 2% on a timescale of tens of years, and there is, in the light of the success of the Schneider-Mass model, an urgent need for a satellite to monitor the solar parameter from above the atmosphere to test the hypothesis. The other essentially empirical input to that model depends on the level of volcanic dust in the atmosphere, the third of the major
307
Climatic change andfoodproduction
mechanisms of climatic change on this sort of timescale. Dust produced by volcanoes can have a veiling effect which reduces the amount of insolation reaching the ground, possibly playing a part in the development of full Ice Age conditions and certainly affecting climate over shorter timescales and by smaller amounts”. The success of the combined solar activity/volcanic dust model in mimicking temperature changes of recent centuries, including the Little Ice Age and the cool decades around 1800, the warming in the first decades of this century and the subsequent cooling of the Northern Hemisphere, suggests that it could be a potentially powerful technique for forecasting. But this depends on a satisfactory method of predicting the variations linked with the solar cycle of activity, and although present theories suggest that this would imply that the minimum of temperature has not yet been reached in the present decline, any such extrapolation is to be treated with extreme caution, not least because of the likely disturbing influence of man’s activities. I7 H.H. Lamb, Volcanic dust in the atmosphere’, Philosophical Transactions of the Royal Society. Vol 266, 1970, pp 425-533. I8 S.H. Schneider, ‘On the carbon dioxideJournal of Atconfusion’, climate mospheric Sciences, Vol 32. 1975, pp 2060-2066. I9 By small, I mean particles which are aerosols, ie, they are held in suspension in the air for long periods of time. The sizes of these particles are in the range from fractions of a micron to a few tens of microns - much less than the sizes of dust particles. Unfortunately, the recent the possible concerning controversy harmful effects on the ozone layer of the atmosphere from excessive use of certain gases as propellants in so-called ‘aerosol’ spray cans has led to some confusion of terminology, which makes it necessary to stress that the aerosols discussed here are simply small particles, and have nothing to do with the spray can/ozone debate.
Anthropogenic
influences - real or imaginary?
It seems likely that man’s activities are indeed affecting the climate today and will continue to do so in the immediate future. But to how great an extent overall, and in which direction? Bryson has likened the production of atmospheric dust by man to a ‘human volcano’, with the implication that like the veiling effect of major volcanic activity this could hasten the decline in temperature, with associated atmospheric circulation changes, and perhaps even bring about an early arrival of the ‘next’ Ice Age. Alternatively, it has been argued with equal force that the production of carbon dioxide by the burning of fossil fuels is producing an increase of this in the atmosphere which will cause a pronounced global warming through the greenhouse effectis. The confusion is shown by Figure 5, where the two extreme possibilities are compared with actual temperature variations over the past hundred years or so. As for the effect of small particles in the atmosphere19 there is no clear indication which way they will influence the temperature 0.6 I
I
I
-
I
I
I
CO,
Warming
effect
I/
1
1980
2000
0.4 Figure how
5. Warming are
climate?
man’s At
say which
present
of two
more important significant Murray
or cooling activities
it is difficult
opposite
-or
to
effects
whether
is
either is
- as this plot produced Mitchell
e
-just
affecting
emphasises.
warming
‘greenhouse
increased
carbon dioxide and the sup-
posed cooling atmospheric out: other temperature
effect’
by The
due
to
effect due to increased dust
may
but
cancel
certainly
changes
each
308
influences
0.2
5
0
$J 2 $, g
-0.2
Q, f-
Tentative maximum particle cooling effec
-0.4
actual
during the past
100 year are too large to be explained by anthropogenic
& 2
alone.
-0.6 1860
1880
1900
1920
1940
1960
FOOD
POLICY
August
1976
Climatic change andfoodproduction
balance. ‘Gray’ particles will increase heat absorption if they are over a white surface, such as snow, but will produce a net cooling if they are over a dark surface, such as cleared land. And there are many other unanswered questions. The tiny particles may, for example, act as seeds for the growth of water droplets, encouraging the spread of new clouds, increasing planetary albedo and cooling the Earth. Or they may simply stick to droplets that would have formed anyway, making the existing clouds dirty so that on balance more heat is absorbed. Studies of this aspect of the climatic problem are only just beginning, and at present it seems most desirable to develop an understanding of the natural patterns of change in order that man’s influences can be calibrated against the natural processes. Since man’s influences are certainly increasing rapidly at present, there is considerable urgency about developing an understanding of the natural trends, before they become so distorted- by such external influences that comparison with the historical record becomes inappropriate.
The limitations
of conventional
forecasting
techniques
Studies of the history of climate, solar cycles, the Milankovich theory and volcanic dust effects all constitute one approach to climatic change. The other approach is to calculate how the atmosphere should respond to changes, and use the calculations in computer models to produce exact mathematical forecasts of how it will evolve - how the weather will change - starting from the present day situation. But even predicting the weather for a day or two ahead for a limited area of the globe is pushing the numerical simulation approach to its limits at present, and the task of developing a model to simulate in detail the entire atmosphere (or even one hemisphere) is well beyond current capabilities. Just as the best weather forecasts are obtained by combining computer modelling with conventional studies of meteorological parameters by experienced meteorologists, so even simple climate models can provide valuable insight into the processes of climatic change. Such models show, for example, the general circulation patterns of full Ice Ages, compared with those of the present,20 and confirm that increased ice cover is associated with increased aridity. However, even a hemispheric model with 10 000 grid points (used with great success in modelling general circulation by the UK Met Office) only has six points to cover the entire Indian sub-continent, so that while it is possible to include a general outline of the monsoon winds in the model there is no prospect of picking out the relatively small fluctuations responsible for recent droughts, floods and famines in the area. Numerical models, while invaluable as an aid in developing a broad understanding of the processes which are involved in changing both weather and climate, simply cannot cope with the complexities and detail which would be required for numerical modelling alone to provide us with accurate forecasts for the seasons, years and decades ahead. 2o Jill Williams, ‘The influence of snowcover on the atmospheric circulation’, Journal of Applied Metereorology, Vol 14, 1975, pp 137-l 52 (and references cited therein).
FOOD POLICY
August
1976
Prospects
to the year 2000
Table 2 provides a reminder of the susceptibility of present food reserves to any disturbance - what Brown calls the ‘index of world
309
Climatic change andfoodproduction Table 2. Index of world food security,
agased on carry-over stocks of grain at beginning of crop year in individual countries for year shown. The USDA has the coverage of recently expanded reserve stocks to include importing as well as exporting countries, thus the reserve levels are slightlv higher than those previously published. bpreliminaryestimates by USDA. Source: Lester Brown, ‘The world food prospect’, Science, Vol 190, 1975, pp 1053-1059.
*’ Gerald Leach in ‘Energy and food proFood Policy, Vol 1 No 1, duction’, November 1975, pp 62-73, discussed the relationship between energy and food production. Any climatic change, such as cooling, which affects demand for or the supply of energy must also have an indirect effect on food production by energy intensive agricultural technology.
Year
Graina
1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1 976b
163 176 149 153 147 151 115 144 159 188 168 130 148 108 111 100
1961 to 1976.
Grain equivalent of idle US cropland
Reserves (million metric tons)
Total
68 81 70 70 71 78 51 61 73 71 41 78 24 0 0 0
Reserves as days of world grain consumption
231 257 219 223 218 229 166 205 232 259 209 208 172 108 111 100
105 105 95 87 91 84 59 71 85 89 71 69 55 33 35 31
food security’.‘* Climatic change is just one of many factors which may bring extra pressure on these reserves, others include technology and the availability of energy*‘; but as Thompson notes’ ‘we must recognise that weather is becoming a limiting factor, as well as technology’. Although he stresses the difficulty of predicting long-term changes in climate, and the uncertainty of those predictions, Thompson concludes that it is important to monitor climatic changes and that agricultural scientists should be alert to changes that might be necessary to accommodate a cooling or a warming trend. He has calculated the effects of various combinations of changes in temperature and rainfall on the average wheat yields in six states of the USA. For such important crops as wheat, maize and soybeans, the likely problem is the increased variability in yields which can be expected if the weak circulation pattern continues to dominate. Using data gathered by Thompson, J M Ramirez and colleagues have evaluated the effects of changes in temperature and precipitation on wheat yields, given here as Table 3. Even allowing for the uncertainty of climatic prediction at present, it seems reasonable to pick out the two sections of this table corresponding to a decrease of 1°C and 0.5X as the most relevant for the immediate future in the light of the present natural trend, and to look most closely at the Table
3. Effects
of climatic
change
in wheat
yields
in six states of the USA.a
Change in wheat yields (bushel/acre) Climatic change
North Dakota
South Dakota
Kansas
Oklahoma
Illinois
Indiana
+3.08 +2.25 +1.22 -1.41 -3.02 494
+3.24 +2.18 +1.10 -1.13 -2.28 -3.45
No change in temperature Change in precipitation (%) a Average yields under ‘standard’climatic conditions are: North Dakota, 25.0; South Dakota, 2 1.1; Kansas, 25.9; Oklahoma, 25.2; illinois, 36.3; and Indiana, 36.3 bushels per acre. Figures taken from Thompson, footnote 1.
310
-30 -20 -10 +10 +20 +30
-3.70 -2.49 -1.07 +1.21 +2.39 +3.62
-1.85 -1.58 -0.67 +0.42 +0.60 +1.42
-2.84 -1.80 -0.85 -0.76 +1.41 +1.99
-2.81 -1.56 -0.62 i6.31 +0.30 -0.02
FOOD POLICY
August
1976
Climatic change andfoodproduction Table 3 continued.
No change in precipitation Changein:emperature ( C) -2O -lo -0.5O tO;5O +1 +2O
t1.18 M.68 -0.36 -0.41 -0.86 -1.90
t0.47 -1087 to.47 -0.55 -1.17 -1.64
+1.44
-2.00
t2.36
t1.69
to.74 +0.37 -0.38 -0.77 -1.57
-0.28 to.04 -0.40 -1.16 -3.76
t1.16 HI.62 -0.64 -1.24 -2.69
tO.88 HI.44 -0.46 -0.94 -1.93
t5.44 t4.61 t3.58 tO.94 -0.66 -2.48
+4.93 +3.88 t2.80 t0.57 -0.58 -1.76
t4.24 t3.41 t2.38 -0.25 -1.86 -3.68
i-4.12 +3.06 t1.98 -0.25 -1.40 -2.58
t3.70 t2.87 t1.84 -0.79 -2.39 4.22
t3.69 t2.63 +1.55 -0.68 -1.83 -3.01
t2.44 +1.61 i-O.58 -2.05 -3.66 -5.48
t2.78 t1.72 a.64 -1.59 -2.74 -3.91
t1.84 +1.01 -0.02 -2.65 4.25 4.08
t2.31 t1.25 HI.17 -2.06 -3.21 -4.39
xl.39 -0.44 -1.47 4.10 -5.71 -7.53
+1.32 UJ.26 -0.82 -3.05 -4.20 -5.38
A decrease of 2'C in temperature Change in precipitation(%) -30 -20 -10 +10 t20 +30
-2.52 -3.32 -0.06 +2.38 t4.41 +4.80
-2.29 -1,14 -0.03 +0.89 +1.07 +0.98
-1.40 -0.35 to.59 +2.30 +2.86 +3.43
-4.80 -3.56 -2.62 -1.69 -1.70 -2.01
A decrease of 1°C in temperature Change in precipitation(%I -30 -20 -10 +10 t20 +30
-2.62 -1.81 -0.56 t1.89 +3.07 +4.30
-1.88 -0.71 +0.29 +2.33 t1.47 +1.39
-2.10 -1.06 -0.11 +1.49 t2.16 t2.73
A decrease of 0.5”C Change in precipitation(%) -30 -20 -10 +10 +20 +30
-3.33 -2.13 -0.87 t1.58 t2.76 t3.99
-2.29 -1.11 -0.20 HI.80 +1.07 tO.98
in temperature
-2.47 -1.42 -0.48 +1.19 +1.79 +2.36
An increase of 0.5”C Change in precipitation(%) -30 -20 -10 +10 t20 t30
-4.11 -2.90 -1.48 HI.63 +1.98 +3.21
-3.31 -2.13 -1.22 -0.13 i-O.05 tO.03
-3.09 -1.83 -0.90 to.04 -003 -0.29
-2.77 -1.51 -0.58 to.35 tO.34 tO.38
in temperature
-3.22 -2.18 -1.25 to.36 t1.04 t1.61
-3.21 -1.96 -1.02 -0.09 -0.10 -0.42
An increase of l°C in temperature Change in precipitation(%I -30 -20 -10 t10 +20 t30
-4.56 -3.35 -1.93 tO.19 +1.53 +2.77
-3.92 -2.75 4.66 -0.66 -0.19 -0.65
-3.61 -2.75 -1.62 -0.01 t0.65 t1.22
An increase of 2’C Change in precipitation(%) -30 -20 -10 +10 +20 +30
FOOD POLICY
August
1976
-6.71 4.39 -3.14 -0.69 -0.49 t1.72
4.39 -3.22 -2.39 -1.13 -0.67 -1.12
-3.98 -2.72 -1.79 -0.85 -0.85 +1.18
in temperature
-4.41 -3.37 -2.42 -0.82 -0.16 i-O.42
-6.57 -5.31 4.38 -3.45 -3.45 -3.77
311
Climatic change andfoodproduction
figures for decreased rainfall within those sections. In the bad years to the end of this century, yields at least 10% below what we have come to think of as ‘normal’ are quite likely to occur, and the increased variability of climate is likely to produce bad years more often than in the past 25 years. Of course, in the good years yields could well be greater than ‘normal’, provided that the best use is made of the available land and other resources, and that there is no recurrence of a situation where farmers are paid not to grow crops, for fear of adverse effects on the market price. With increased climatic variability the hand to mouth situation implicit in the figures of Table 2 is simply inadequate to provide the necessary insurance against bad years. Prudent levels of grain stocks, particularly when so much of the world today depends on the surplus produced in North America, the top few percent of the total yield, should be pitched high enough to take account of the probability of greater climatic variation.
312
FOOD POLICY August 1976
John Gribbin
Small
changes
in climate
disproportionate availability world
of food
reserves
reviews
and
on
when,
are low.
evidence
change
can have a
influence
as now, Dr Gribbin
for
concludes
climatic that
climate
is likely
to be much
variable
in future
than
the
middle
Longer
decades
periods
weather
in
agricultural Prudence stocks insurance probable
unfavourable of
areas
are
against
the
that
world
built an
major
expected. up
grain as
an
increasingly
series of poor harvests.
Dr Gribbin Science
it has been in
of
be
the more
of this century.
some
dictates should
the
is a visiting
fellow
Policy Research
sity of Sussex, Falmer,
at the
Unit, Univer-
Brighton,
UK.
This article draws extensively on the material in his book Forecasts, Famines and Freezes (Walker, New York and Wildwood, London; 1976).
’ Louis M. Thompson, ‘Weather variability, climatic change and grain production’, Science, Vol 188, 1975, pp, 535-541. See also, Committee Report to the National Oceanic and Atmospheric Administration, The Influence of Weather and Climate on United States Grain Yields; Bumper Crops or Droughts, (US
Department of Commerce, Washington, D.C., December 1973). * Nigel Calder, Ere Weather Machine, BBC Publications. London, 1974. 3 Stephen Schneider and Lynne Mesirow, The Genesis Strategy, Plenum, New York, 7976.
FOOD
POLICY
August
1976
Changes in climate which are not only small but local, in global terms, are sufficient to disturb the present world food supply system, and it has been partly through the variations in weather experienced in the key areas in recent years that the problem of climatic change and food production has begun to receive attention.’ Although evidence of a global shift in climatic patterns was certainly available at the beginning of this decade, such attention as this has received has focussed on the dramatic possibilities of the imminent arrival of a new Ice Age2, and even the more sober treatments have tended to stress the implications of a continuation of the cooling experienced by the Northern Hemisphere over the past 25 years or so (see Figure 1). For crop production, the immediate importance of such a cooling trend is the reduction it causes in the length of the growing season, with frosts occurring later in the spring and arriving earlier in the autumn. But these changes are small, and, as Thompson has stressed, cooler, wetter conditions are in fact better for the production of major crops in some of the key food producing areas of the globe - ‘wheat grown anywhere in the United States is adversely affected by higher than normal temperatures from flowering time until the crop is mature’ and ‘the summer of 1958 was cooler and wetter than normal in the middle latitudes of the Northern Hemisphere and resulted in new record yields in grain in both Russia and the United States’.’ Leaving aside for the moment the important question of how we define ‘normal’, the greatest hazard to production of these key crops is not the cooling trend as such, but the fact that this cooling trend is associated with a greater variability of weather, making it less predictable and producing greater extremes of all kinds - more common droughts and more common floods; more common hot summers and more common severe winters. Thompson suggests that if the weather was as variable from now until the year 2000 as it was in the corn and soybean belts of the USA from 1890 to 1955 then the average yield would be reduced by about 3%. This is not alarming in itself, perhaps, but that 3% reduction is not likely to be spread evenly over the 25 year period. Rather, we can expect that in some of those years the extremes of variation will produce very poor yields, which could give rise to a potentially disastrous situation unless steps are taken to provide a stockpile of food available to tide the world over the lean years, in an application of what Schneider has graphically termed the ‘Genesis Strategy’3.
301
Climatic change andfoodproduction
[[r’.i Figure
1.
mean
Top:
surface
past
century.
Murray
Five-year
Mitchell
Center
for
Variation
in global
temperatures
means
from
Atmospheric Variation over
temperatures
inferred
millenium,
data for England line
shows
Northern Murray
the from
by H.H.
20-y
Hemisphere,
mean past
-0 5 10.0 ? ;
Lamb. Solid for
the
9.5 -
calculated
by
Mitchell.
cj g
I
-
historical
means
1890
I
10.5
Research of
1880
C
and the US National
(NCAR) Bottom:
1870
over the
9.0
__=
-
o-? AD
I
1900
1910
1920
1930
1940
1950
1960
1970 “F
Mean Annual Temperature I I I I ;‘I..,‘\ I \ \ : 1’ “$entral England \ : \ \ .,’ \ \ I’\ \ \ ../’ \ \ \_-__,
I 1000
I
I
I
I
I
I
I
I
1500
I
I 1960
The immediate problem of climate in terms of food production is not the cooling trend, but the greater uncertainty implied by the increased variability of the weather compared with the middle part of this century. What is now clear - and makes one very wary of using such descriptions as ‘normal’ in this context - is that by the standards of the past few hundred years the equitable years of the middle twentieth century were unusual, and the cooler, more variable conditions now being experienced are more typical of Northern Hemisphere weather during the past millenium.
The global picture
4 H.H. Lamb, personal communication. 5 Derek Winstanley, ‘Rainfall patterns and general atmospheric circulation’, Nature, Vol 245, 1973, pp 190-l 94.
302
From the 1890s to the mid-l 94Os, with only a slight dip around 1905, the global air temperature increased, by about 0.5”C; between the mid-1940s and 1970, however, global mean temperatures fell by about 0*3”C, and for the five year period 1968-72 the average temperature recorded by the nine ocean weather ships which are stationed between 35”N amd 66’N was more than 0.5”C below the peak of the 1940s. Every summer during the 1930s was warmer than the average for the century in the important corn belt of the USA, but the 1960s were as cool as the first decade of the century. Local effects - such as the recent run of mild winters in Britain - can mask the trend, but even locally the effects of the recent cooling are seen clearly in agricultural records. For example, harvests in England during 1960-73 were completed, on average, about nine days later than during the 1940~~. This change was largely the direct result of the cooling trend; the associated changes in rainfall only became clear when the recent prolonged droughts in the Sahel, in Ethiopia and elsewhere, and the repeated failure of the Indian monsoon, drew attention to the problem in the early 1970s. The basic problem across this belt of drought is a shift of rainfall belts to the south. Winstanley has linked these shifts to a changing pattern of atmospheric circulation which seems to provide the best available overall model, although it should be said that some meteorologists remain doubtful about its detailed accuracy although they have not been able to offer anything better as yet. The
FOOD
POLICY
August
1976
Climatic change andfoodproduction
Figure
2.
‘weak’ Weaker more
The
circulation
bringing
bringing
is associated in
the
tendency
to
further
push
high
and
the
the
tropical
rainfall coast
Middle
East.
rainfall
also squeezed
towards
expansion
of
Figure 3. Shifts since
have
the
Africa
Further
south,
(shaded)
is
the equator
by
the
As
to
circumpolar
footnote
5.
distribu-
well
as
the
squeezing towards the the recent changes in rainfall
led to more sea,
(H)
in rainfall
1960.
for south,
of North belt
flow. After Winstanley,
the
with
westerlies,
general
Mediterranean
general equator,
and
patterns.
low rainfall (L) to Britain, and a
depressions
tion
(top)
circulation
variability
with
the
‘strong’
(bottom)
key to this study is a comparison of the so-called ‘strong’ and ‘weak’ circulation patterns for the northern hemisphere. The strong type of circulation (Figure 2) corresponds to a situation in which the oscillation of the westerly winds that sweep around the North Pole is confined within a narrow range of latitudes. In this case, when the circumpolar vortex is contracted, the depressions which travel from west to east across the Atlantic and play a large part in determining the weather over the British Isles follow well defined paths. They bring changeable weather and high rainfall. Troughs do not reach far south from this belt so that rainfall over the Mediterranean and Middle East is, by and large, low. Further south still, with this pattern of circulation, the tropical wind patterns extend well to the north, and the monsoon rains reach relatively high latitudes. This kind of overall situation seems, in general, to be associated with warmer conditions in the Northern Hemisphere, such as those of the middle part of this century. But now the atmospheric circulation is moving, or has moved, towards the alternative mode - a weak pattern with an expanded circumpolar vortex, associated with a general cooling and increased ice cover at high latitudes. The effect of this change is to allow more erratic variation in the latitudinal distribution of the eastward moving depressions and of the circulation pattern in general, with the main flow of westerlies also shifted further to the south. The weather systems moving towards Britain and Europe travel more slowly and persistent anticyclones can become established, blocking the movement of depressions. For Britain, both the rainfall and the occurrence of westerly winds are decreased, while such rainfall as there is tends to be more concentrated in heavy bursts. With the troughs extending further south, rainfall over the Mediterranean and Middle East increases, but the tropical circulation belt is squeezed towards the equator so that monsoon rains do not extend so far to the north. (see Figure 3). The zonal circulation is decreasing in strength, rainfall over England is becoming less (see Figure 4 and Table I), and there are associated increases in rainfall over the Mediterranean and Middle East, with decreases in the rainfall over the Sahel, Ethiopia and northwest India. These changes have not proceeded uniformly over the past decades, and a significant jump occurred in 1972, a particularly disastrous year that tipped the balance of rainfall against the nomads of the Sahel. The importance of a global perspective in this situation was highlighted, however, by a study of snow and ice
so that
precipitation there
from
important the regions of the world.
FOOD POLICY
Auaust
is a net
over loss
nd since 1960
agricultural
1976
303
Climatic change andfoodproduction Table 1. Monthly Month
rainfall totals, 1973
England and Wales, since 1973 1974
1975
(mm) Standard
1976
average
1916-50 January February March April
Source: UK Met Office: see also Figure 4. The 12 months from May 1975 to April 1976 produced the driest one-year period since records began in 1727, with only 624mm of rain in the 12 month period, just 69% of the standard average.
in
Figure
4.
Wales,
October
Rainfall
1968
1973,
monthly
totals
nual figures monthly dicated.
(bottom)
totals The
to
1916-l replaced
950;
(top)
and
an-
the
month average’
inis
as that for the years
this
by a new
on 194 1 - 1970
and
given as twelve
‘standard
chosen arbitrarily
England
to September
is
soon
‘standard’
figures.
to
be
based
From the An-
Report of the Water Resources Board, 1974. See also Table 1. nual
May June July August September October November December Totals
44 40 24 67 84 63 91 63 86 57 52 67
117 98 47 14 40 66 77 95 144 99 125 69
117 31 81 71 47 21 66 52 107 36 73 50
738
991
752
61 40 50 21
92 66 57 60 63 55 79 81 76 92 95 88 904
cover in the Northern Hemisphere reported by Kukla and Kukla6 in 1974, which showed that there was a dramatic increase in the area of the Northern Hemisphere covered.by winter snow and ice only a year before the sudden shift towards the weak circulation pattern. It is not clear whether the change in snow and ice cover caused the sudden change in circulation, or whether the circulation change affected the buildup of snow and ice, or indeed whether both were caused by some other external influence. But it does seem clear that the two were related, and this implies that with a better understanding of the atmosphere it would have been possible to provide a year’s warning of the droughts which hit parts of Africa so severely from 1972 onwards. The political implications are profound, and it remains to be seen just what will happen if the continued monitoring of snow and ice cover shows another sudden increase of this kind sometime in the near future. Meanwhile, Perry’s prophetic comment in 1970 that ‘the decade of the 1960s may well be remembered as the first when climatologists became convinced that the climate over large parts of ,“S
mm 175 150
Monthly
Ramfall
Standard Monthly
n
(1916-1950) avera9e
/
304
FOOD
POLICY
August
1976
Climatic change andfoodproduction
the northern hemisphere was experiencing a reaction to the genial conditions of the earlier part of this century” seems more apposite than ever. The question of how long this ‘reaction’ will last, and how severe conditions are likely to become, is best answered by considering both the patterns of previous climatic changes and the physical mechanisms proposed to explain them.
Previous climatic changes
sG.J. Kukla and H.J. Kukla. ‘Increased in surface albedo the Northern Hemisphere’, Science, Vol 183, 1974, pp 709-7 14. ’ A.H. Perry, ‘The 1960’s - A Climatic Review’, Weather, Vol 25, 1970, pp 365370. B W. Dansgaard, S.J. Johnson, N. Reeh, N. Gundestrup, H.8. Clausen, and C.U. Hammer. ‘Climatic changes, Norsemen and modern man’, Nature, Vol 255, pp 24-28. 9 R.A. Bryson, ‘The Lessons of Climatic History’, Environmental Conservation, Vol 2, 1975, pp 163-l 70.
FOOD POLICY
August
1976
In order to understand climatic changes occurring over tens of years, we need information about past patterns of climatic change over hundreds of years, at least. The historical record is just sufficient for the task. Although climatic changes occurring over longer timescales - including full Ice Ages - are important, they are not directly relevant to the immediate problem of climate and food production to the year 2000, and will not be discussed in detail here. The first and most important lesson of the historical record is that there is no such thing as a climatic ‘normal’ in the sense that the word was used 50 or more years ago. Then it was felt that if a long enough series of observations could be amassed an average value would be produced for each element describing the climate, an average to which the climate at the observing site would always tend to return. Now it is recognised that climate is always changing on a variety of timescales. The problem is to determine the extent of these changes and to unravel the different timescales of change as far as possible from available records. The best long-term historical records come from only two parts of the world, China and Japan and northwest Europe, and here I shall concentrate on the latter evidence, together with archaeological evidence from North America which helps to fill in the outlines of climatic change around the North Atlantic over the past 1500 years. The best single continuous record of climate over that period comes from a 404m long ice core drilled from the glacier in central Greenland.8 Analysis of variations in the ratio of oxygen isotopes at different levels in the ice has enabled a temperature record to be produced going back for 1420 years. The new understanding of climatic changes on timescales of 60 to 200 years that this provides has given a new insight into the spread and decline of the Norse Atlantic colonies. Ninth century settlers in Iceland, it seems, flourished in a phase of rapid warming and were able to send their own colonists west to Greenland; but within a few hundred years a rapid deterioration of climate was a major contributory factor in the collapse of the Greenland colony. Even the Iceland colony survived by only the barest margin through the worst ravages of what has come to be known as the Little Ice Age, which was particularly marked from the 15th to the 19th centuries, and affected Britain most severely in the late 17th century. In summarising a variety of data about climatic change since the most recent full glaciation (about 10 000 years ago) Bryson9 has emphasised the integral behaviour of the atmosphere which brings the varying patterns of related change over the whole globe. It seems that the climatic changes which affected the Norse colonies after about AD 1200 were related to a cooling of the Arctic and expansion of the sea ice and climatic zones southward, in a manner similar to the recent pattern of change but more pronounced and with longer-term effects.
Climatic change andfoodproduction
lo R.A. Bryson and D.A. Barreis, ‘Climatic change and the Mill Creak people of Iowa’, J. Iowa Archaeol. Sot., Vol 15, 1968, pp l-34. l1 H.H. Lamb, The Changing Climate, Methuen, London, 1968.
306
In North America, this pattern of change shifted the summer westerlies southward from what is now southern Canada into what is now the northern USA, with reduced rainfall in the northern plains, the present corn belt, west Texas and much of the intermontane west, but with increased rainfall in the Pacific northwest, the southeast and much of the east coast.‘O An important reminder of the potential severity of such minor changes in climate in terms of their impact on human cultures is provided by the record of the Mill Creek people, who occupied a region near the corn belt of the present USA in the centuries just before AD 1200. At that time, the region was one of tall grass prairie uplands with wooded valley terraces and valley floors. Deer and corn were the Indians’ staple foods. The Arctic expansion and changing climate brought a long drought, probably because of westerly winds, more persistent than before, crossing the Rockies and putting the region into the rain shadow. Most of the forest disappeared; tall grass was replaced by short grass; farming villages in the west disappeared; and everywhere in the region the staple source of meat became the bison as the deer died out. The particular significance of these changes, as Bryson stresses9, is that the drought persisted for 200 years. In other words, the archaeological and other records show that minor climatic changes of the kind which have occurred in the past millenium are sufficient to bring 200 years of drought to the corn belt of North America, the region which today produces the surplus of grain on which many parts of the world have come to depend. The pattern of climatic changes in the Northern Hemisphere since the middle of this century has been in the same direction as those associated with the Arctic expansion of AD 1200, although the magnitude of the deterioration is not yet as great. Since, however, similar toolings in the past millenium have never lasted for less than forty years9, it seems that at the very least the present trend may return us to a situation more typical of the past few hundred years than of the past few decades - and this, of course, makes the interpretation of climatic patterns of the past few hundred years of great importance. Even with limited meteorological data it has proved possible to construct charts indicating the different patterns of mean sea level pressure over the past two or three hundred years.” Examination of the 40-year mean msl pressure distributions for January and July, for the two epochs 1790-1829 and 1900-39, shows that between the two epochs, the mean pressure gradients in January increased”. The earlier epoch marked a notable minimum in the strength of the zonal circulation and in Europe it was 1% or more cooler than in 1900-39. The cold winters of the years around 1800 can be explained by the weakness of the westerly flow towards Europe. The tendency since about 1950 towards a weaker circulation with more frequent blocking seems to mark a retreat from the 1900-39 situation, which itself marked something of a maximum in the strength of the zonal circulation. Clearly more information of this kind is needed, with pressure distributions determined for even earlier epochs wherever possible. Already, though, the clear lesson to be learnt from the pattern of historical climatic changes is that not only does climate vary but that climatic conditions in the middle decades of this century were around an optimum in terms of conditions for food production at middle
FOOD POLICY August
1976
Climatic
change
andfoodproduction
latitudes. It seems appropriate, especially in view of growing concern about the low level of food reserves today12, to reiterate Bryson’s comment that ‘combining the nature of recent climatic change with the present narrow margin of world food-grain reserves an urgent need to consider and react to the possibility of continued climatic variation is indicated’.
Mechanisms
‘* Lester Brown, ‘The world food prosoect’. Science, Vol 190, 1975, on 10531’059.. I3 Random noise in this context means any of the fluctuations which occur on a statistical basis simply because the climatic system (oceans, atmosphere, land and ice) contains so many particles interacting with one another. Even tossing a coin and noting the occurrence of heads and tails will produce a time series in which although the heads and tails may even out in the very long run, shorter runs dominated by a succession of heads or tails can give the appearance of regular even periodic - variations. The number of either/or interactions occuring such throughout the climatic system is immense, and it has been argued that all climatic variations may simply result from the accumulation of these random processes Although that view seems unnecessarily extreme, such noise must provide the ultimate limit to climatic predictability: a good discussion of the nature of the random noise problem in physical processes has been given recently by L.G. ‘Some thoughts about ranJacchia, domness’, Sky and Telescope, Vol 50, 1975, pp 371-374. I4 B.J. Mason, Symons Memorial Lecture to the Royal Meteorological Society, 1976 (to be published in Quarterly Journal of the Royal Meteorological Society). 15Stephen Schneider and Clifford Mass, ‘Volcanic dust, sunspots and temperature trends’, Science, Vol 190, 1975, pp 741746. I8 C.G. Abbott, Smithsonian Misc. Collections, Vol 448 No 7,1966, and K.Y. Kondratyev and G. Nikolsky, ‘Solar radiation and solar activity’, Quarter/y Journal of the Royal Meteorological Society, Vol 96, 1970, pp 509-522.
FOOD POLICY August 1976
If we understood the causes of climatic change (and assuming that such change is not simply the accumulation of random noise effects in the climatic system13) then prediction of future changes would be straightforward. Unfortunately, such an understanding is not yet at hand, but three mechanisms in particular seem to offer the greatest insight into patterns of climatic change over the past 10 000 years or so. First, there are the long-term cyclic effects produced by changes in the Earth’s orbit around the Sun and in the orientation of the spin axis of the Earth relative to the Sun. These ‘Milankovich’ effects have been discussed for decades, but have only become firmly established as a major contributory factor in the development and decay of Ice Ages in recent years, chiefly because a satisfactory chronology of past Ice Ages has only recently been available. There now seems little doubt, however, that these astronomical influences are important on timescales of thousands to hundreds of thousands of years, through the changes they produce in energy input at the latitudes of crucial importance for the spread of snow and ice cover14. This influence is now acting to produce a dip in Northern Hemisphere insolation over the next few thousand years. It seems that this will bottom out before reaching conditions as extreme as those which prevailed at the height of the most recent advance of the ice, nevertheless, the mode1 has formed the basis of some rather extravagant popularisations suggesting the imminent arrival of a new Ice Age and these, unfortunately, have diverted attention from the real climatic problems now facing us. An astronomical influence of more relevance to climatic change over a few decades seems to be the effects produced by the changing level of solar activity on the Earth’s atmosphere. Although the debate about the links between solar activity over the approximately 11 year ‘sunspot cycle’ and weather continues, Schneider and MasP have been able to reproduce many features of the observed record of temperature variations since 1600 with a model which incorporates an empirical dependence on solar activity. This dependence is no less controversial than other aspects of the debate about solar-terrestrial links, since it implies that the solar constant (now seen as a variable the solar parameter) is, for example, more than 2% less when there is no sunspot activity than when there is moderate activity. The empirical evidence for this comes from observations by balloon-borne instruments16. The most surprising feature of the controversy, perhaps, is that there is no evidence that the Sun is in fact constant to within 2% on a timescale of tens of years, and there is, in the light of the success of the Schneider-Mass model, an urgent need for a satellite to monitor the solar parameter from above the atmosphere to test the hypothesis. The other essentially empirical input to that model depends on the level of volcanic dust in the atmosphere, the third of the major
307
Climatic change andfoodproduction
mechanisms of climatic change on this sort of timescale. Dust produced by volcanoes can have a veiling effect which reduces the amount of insolation reaching the ground, possibly playing a part in the development of full Ice Age conditions and certainly affecting climate over shorter timescales and by smaller amounts”. The success of the combined solar activity/volcanic dust model in mimicking temperature changes of recent centuries, including the Little Ice Age and the cool decades around 1800, the warming in the first decades of this century and the subsequent cooling of the Northern Hemisphere, suggests that it could be a potentially powerful technique for forecasting. But this depends on a satisfactory method of predicting the variations linked with the solar cycle of activity, and although present theories suggest that this would imply that the minimum of temperature has not yet been reached in the present decline, any such extrapolation is to be treated with extreme caution, not least because of the likely disturbing influence of man’s activities. I7 H.H. Lamb, Volcanic dust in the atmosphere’, Philosophical Transactions of the Royal Society. Vol 266, 1970, pp 425-533. I8 S.H. Schneider, ‘On the carbon dioxideJournal of Atconfusion’, climate mospheric Sciences, Vol 32. 1975, pp 2060-2066. I9 By small, I mean particles which are aerosols, ie, they are held in suspension in the air for long periods of time. The sizes of these particles are in the range from fractions of a micron to a few tens of microns - much less than the sizes of dust particles. Unfortunately, the recent the possible concerning controversy harmful effects on the ozone layer of the atmosphere from excessive use of certain gases as propellants in so-called ‘aerosol’ spray cans has led to some confusion of terminology, which makes it necessary to stress that the aerosols discussed here are simply small particles, and have nothing to do with the spray can/ozone debate.
Anthropogenic
influences - real or imaginary?
It seems likely that man’s activities are indeed affecting the climate today and will continue to do so in the immediate future. But to how great an extent overall, and in which direction? Bryson has likened the production of atmospheric dust by man to a ‘human volcano’, with the implication that like the veiling effect of major volcanic activity this could hasten the decline in temperature, with associated atmospheric circulation changes, and perhaps even bring about an early arrival of the ‘next’ Ice Age. Alternatively, it has been argued with equal force that the production of carbon dioxide by the burning of fossil fuels is producing an increase of this in the atmosphere which will cause a pronounced global warming through the greenhouse effectis. The confusion is shown by Figure 5, where the two extreme possibilities are compared with actual temperature variations over the past hundred years or so. As for the effect of small particles in the atmosphere19 there is no clear indication which way they will influence the temperature 0.6 I
I
I
-
I
I
I
CO,
Warming
effect
I/
1
1980
2000
0.4 Figure how
5. Warming are
climate?
man’s At
say which
present
of two
more important significant Murray
or cooling activities
it is difficult
opposite
-or
to
effects
whether
is
either is
- as this plot produced Mitchell
e
-just
affecting
emphasises.
warming
‘greenhouse
increased
carbon dioxide and the sup-
posed cooling atmospheric out: other temperature
effect’
by The
due
to
effect due to increased dust
may
but
cancel
certainly
changes
each
308
influences
0.2
5
0
$J 2 $, g
-0.2
Q, f-
Tentative maximum particle cooling effec
-0.4
actual
during the past
100 year are too large to be explained by anthropogenic
& 2
alone.
-0.6 1860
1880
1900
1920
1940
1960
FOOD
POLICY
August
1976
Climatic change andfoodproduction
balance. ‘Gray’ particles will increase heat absorption if they are over a white surface, such as snow, but will produce a net cooling if they are over a dark surface, such as cleared land. And there are many other unanswered questions. The tiny particles may, for example, act as seeds for the growth of water droplets, encouraging the spread of new clouds, increasing planetary albedo and cooling the Earth. Or they may simply stick to droplets that would have formed anyway, making the existing clouds dirty so that on balance more heat is absorbed. Studies of this aspect of the climatic problem are only just beginning, and at present it seems most desirable to develop an understanding of the natural patterns of change in order that man’s influences can be calibrated against the natural processes. Since man’s influences are certainly increasing rapidly at present, there is considerable urgency about developing an understanding of the natural trends, before they become so distorted- by such external influences that comparison with the historical record becomes inappropriate.
The limitations
of conventional
forecasting
techniques
Studies of the history of climate, solar cycles, the Milankovich theory and volcanic dust effects all constitute one approach to climatic change. The other approach is to calculate how the atmosphere should respond to changes, and use the calculations in computer models to produce exact mathematical forecasts of how it will evolve - how the weather will change - starting from the present day situation. But even predicting the weather for a day or two ahead for a limited area of the globe is pushing the numerical simulation approach to its limits at present, and the task of developing a model to simulate in detail the entire atmosphere (or even one hemisphere) is well beyond current capabilities. Just as the best weather forecasts are obtained by combining computer modelling with conventional studies of meteorological parameters by experienced meteorologists, so even simple climate models can provide valuable insight into the processes of climatic change. Such models show, for example, the general circulation patterns of full Ice Ages, compared with those of the present,20 and confirm that increased ice cover is associated with increased aridity. However, even a hemispheric model with 10 000 grid points (used with great success in modelling general circulation by the UK Met Office) only has six points to cover the entire Indian sub-continent, so that while it is possible to include a general outline of the monsoon winds in the model there is no prospect of picking out the relatively small fluctuations responsible for recent droughts, floods and famines in the area. Numerical models, while invaluable as an aid in developing a broad understanding of the processes which are involved in changing both weather and climate, simply cannot cope with the complexities and detail which would be required for numerical modelling alone to provide us with accurate forecasts for the seasons, years and decades ahead. 2o Jill Williams, ‘The influence of snowcover on the atmospheric circulation’, Journal of Applied Metereorology, Vol 14, 1975, pp 137-l 52 (and references cited therein).
FOOD POLICY
August
1976
Prospects
to the year 2000
Table 2 provides a reminder of the susceptibility of present food reserves to any disturbance - what Brown calls the ‘index of world
309
Climatic change andfoodproduction Table 2. Index of world food security,
agased on carry-over stocks of grain at beginning of crop year in individual countries for year shown. The USDA has the coverage of recently expanded reserve stocks to include importing as well as exporting countries, thus the reserve levels are slightlv higher than those previously published. bpreliminaryestimates by USDA. Source: Lester Brown, ‘The world food prospect’, Science, Vol 190, 1975, pp 1053-1059.
*’ Gerald Leach in ‘Energy and food proFood Policy, Vol 1 No 1, duction’, November 1975, pp 62-73, discussed the relationship between energy and food production. Any climatic change, such as cooling, which affects demand for or the supply of energy must also have an indirect effect on food production by energy intensive agricultural technology.
Year
Graina
1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1 976b
163 176 149 153 147 151 115 144 159 188 168 130 148 108 111 100
1961 to 1976.
Grain equivalent of idle US cropland
Reserves (million metric tons)
Total
68 81 70 70 71 78 51 61 73 71 41 78 24 0 0 0
Reserves as days of world grain consumption
231 257 219 223 218 229 166 205 232 259 209 208 172 108 111 100
105 105 95 87 91 84 59 71 85 89 71 69 55 33 35 31
food security’.‘* Climatic change is just one of many factors which may bring extra pressure on these reserves, others include technology and the availability of energy*‘; but as Thompson notes’ ‘we must recognise that weather is becoming a limiting factor, as well as technology’. Although he stresses the difficulty of predicting long-term changes in climate, and the uncertainty of those predictions, Thompson concludes that it is important to monitor climatic changes and that agricultural scientists should be alert to changes that might be necessary to accommodate a cooling or a warming trend. He has calculated the effects of various combinations of changes in temperature and rainfall on the average wheat yields in six states of the USA. For such important crops as wheat, maize and soybeans, the likely problem is the increased variability in yields which can be expected if the weak circulation pattern continues to dominate. Using data gathered by Thompson, J M Ramirez and colleagues have evaluated the effects of changes in temperature and precipitation on wheat yields, given here as Table 3. Even allowing for the uncertainty of climatic prediction at present, it seems reasonable to pick out the two sections of this table corresponding to a decrease of 1°C and 0.5X as the most relevant for the immediate future in the light of the present natural trend, and to look most closely at the Table
3. Effects
of climatic
change
in wheat
yields
in six states of the USA.a
Change in wheat yields (bushel/acre) Climatic change
North Dakota
South Dakota
Kansas
Oklahoma
Illinois
Indiana
+3.08 +2.25 +1.22 -1.41 -3.02 494
+3.24 +2.18 +1.10 -1.13 -2.28 -3.45
No change in temperature Change in precipitation (%) a Average yields under ‘standard’climatic conditions are: North Dakota, 25.0; South Dakota, 2 1.1; Kansas, 25.9; Oklahoma, 25.2; illinois, 36.3; and Indiana, 36.3 bushels per acre. Figures taken from Thompson, footnote 1.
310
-30 -20 -10 +10 +20 +30
-3.70 -2.49 -1.07 +1.21 +2.39 +3.62
-1.85 -1.58 -0.67 +0.42 +0.60 +1.42
-2.84 -1.80 -0.85 -0.76 +1.41 +1.99
-2.81 -1.56 -0.62 i6.31 +0.30 -0.02
FOOD POLICY
August
1976
Climatic change andfoodproduction Table 3 continued.
No change in precipitation Changein:emperature ( C) -2O -lo -0.5O tO;5O +1 +2O
t1.18 M.68 -0.36 -0.41 -0.86 -1.90
t0.47 -1087 to.47 -0.55 -1.17 -1.64
+1.44
-2.00
t2.36
t1.69
to.74 +0.37 -0.38 -0.77 -1.57
-0.28 to.04 -0.40 -1.16 -3.76
t1.16 HI.62 -0.64 -1.24 -2.69
tO.88 HI.44 -0.46 -0.94 -1.93
t5.44 t4.61 t3.58 tO.94 -0.66 -2.48
+4.93 +3.88 t2.80 t0.57 -0.58 -1.76
t4.24 t3.41 t2.38 -0.25 -1.86 -3.68
i-4.12 +3.06 t1.98 -0.25 -1.40 -2.58
t3.70 t2.87 t1.84 -0.79 -2.39 4.22
t3.69 t2.63 +1.55 -0.68 -1.83 -3.01
t2.44 +1.61 i-O.58 -2.05 -3.66 -5.48
t2.78 t1.72 a.64 -1.59 -2.74 -3.91
t1.84 +1.01 -0.02 -2.65 4.25 4.08
t2.31 t1.25 HI.17 -2.06 -3.21 -4.39
xl.39 -0.44 -1.47 4.10 -5.71 -7.53
+1.32 UJ.26 -0.82 -3.05 -4.20 -5.38
A decrease of 2'C in temperature Change in precipitation(%) -30 -20 -10 +10 t20 +30
-2.52 -3.32 -0.06 +2.38 t4.41 +4.80
-2.29 -1,14 -0.03 +0.89 +1.07 +0.98
-1.40 -0.35 to.59 +2.30 +2.86 +3.43
-4.80 -3.56 -2.62 -1.69 -1.70 -2.01
A decrease of 1°C in temperature Change in precipitation(%I -30 -20 -10 +10 t20 +30
-2.62 -1.81 -0.56 t1.89 +3.07 +4.30
-1.88 -0.71 +0.29 +2.33 t1.47 +1.39
-2.10 -1.06 -0.11 +1.49 t2.16 t2.73
A decrease of 0.5”C Change in precipitation(%) -30 -20 -10 +10 +20 +30
-3.33 -2.13 -0.87 t1.58 t2.76 t3.99
-2.29 -1.11 -0.20 HI.80 +1.07 tO.98
in temperature
-2.47 -1.42 -0.48 +1.19 +1.79 +2.36
An increase of 0.5”C Change in precipitation(%) -30 -20 -10 +10 t20 t30
-4.11 -2.90 -1.48 HI.63 +1.98 +3.21
-3.31 -2.13 -1.22 -0.13 i-O.05 tO.03
-3.09 -1.83 -0.90 to.04 -003 -0.29
-2.77 -1.51 -0.58 to.35 tO.34 tO.38
in temperature
-3.22 -2.18 -1.25 to.36 t1.04 t1.61
-3.21 -1.96 -1.02 -0.09 -0.10 -0.42
An increase of l°C in temperature Change in precipitation(%I -30 -20 -10 t10 +20 t30
-4.56 -3.35 -1.93 tO.19 +1.53 +2.77
-3.92 -2.75 4.66 -0.66 -0.19 -0.65
-3.61 -2.75 -1.62 -0.01 t0.65 t1.22
An increase of 2’C Change in precipitation(%) -30 -20 -10 +10 +20 +30
FOOD POLICY
August
1976
-6.71 4.39 -3.14 -0.69 -0.49 t1.72
4.39 -3.22 -2.39 -1.13 -0.67 -1.12
-3.98 -2.72 -1.79 -0.85 -0.85 +1.18
in temperature
-4.41 -3.37 -2.42 -0.82 -0.16 i-O.42
-6.57 -5.31 4.38 -3.45 -3.45 -3.77
311
Climatic change andfoodproduction
figures for decreased rainfall within those sections. In the bad years to the end of this century, yields at least 10% below what we have come to think of as ‘normal’ are quite likely to occur, and the increased variability of climate is likely to produce bad years more often than in the past 25 years. Of course, in the good years yields could well be greater than ‘normal’, provided that the best use is made of the available land and other resources, and that there is no recurrence of a situation where farmers are paid not to grow crops, for fear of adverse effects on the market price. With increased climatic variability the hand to mouth situation implicit in the figures of Table 2 is simply inadequate to provide the necessary insurance against bad years. Prudent levels of grain stocks, particularly when so much of the world today depends on the surplus produced in North America, the top few percent of the total yield, should be pitched high enough to take account of the probability of greater climatic variation.
312
FOOD POLICY August 1976