- Email: [email protected]
Modelling and monitoring ‘Greenhouse’ warming
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that result from climatic change, to sort out the relative importance of biotic and abiotic influences on species abundances, and to predict the range of possible adaptations. If ecology is the study of factors that control the distribution and abundance of plants and animals, there is much ecology to be done before we can predict with certainty biotic responses to global change. The articles presented here represent an
important step toward understanding.
arriving
at that
References 1 Special Committee for the IGBP: McCarthy, J.J., chair (1988) The International Geosphere-Biosphere Program: A Study of Global Change (A Plan for Action: Global Change Report No. 4), IGBP Secretariat, Royal Swedish
Academy of Sciences 2 Barnola, J.M., Raynaud, D.,
_
A. Henderson-Sellers
The greenhouse effect is an unambiguous theory that is well understood by atmospheric and climatic scientists, and that successfully predicts temperatures on the Earth and on other planets. The greenhouse theory is based upon the fact (readily demonstrated by experiment) that whilst gases in the Earth’s atmosphere are transparent to incoming solar radiation, some of them absorb outgoing thermal (or heat) radiation emitted from the Earth’s surface. This radiative interaction between selected gases in the atmosphere (termed the greenhouse gases) and the outgoing heat radiation causes those gases to warm and, consequently, they themselves re-radiate heat in all A. Henderson-Sellers is at the School of EarthSciences, Macquarie University, North Ryde, New South Wales 2109, Australia.
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Korotkevich, Y.S. and Lorius, C. (1987) Nature 329,40&414 3 Davis, M.B., coordinator (1988) in Toward an Understanding of Global Change (Mooney, H.A., chair), pp. 69-106, National Academy Press
4 Peters, R.L. and Darling, J.D. (1985) Bioscience 35,707-717 5 Peters, R.L. and Lovejoy, T.E.,eds Consequences of the Greenhouse Effect for Biological Diversity: Proceedings of the World Wildlife Fund Conference, Yale
University Press (in press)
Modellingand Monitoring ‘Greenhouse’ Warming Greenhouse warming is now commonly accepted 6g politicians and the general public and is the su6iect of active research. This paper traces the greenhouse theory as far 6acf as 1827, highlighting new directions and significant advances over that time. Three main themes emerge: that certain radiative/y active gases are responsible for warming the planet; that humans are increasing the concentrations of these gases and hence inadvertently influencing this warming; that climate models are designed to permit prediction of the climatic changes resulting from changed loadings in these gases, 6ut that they have not yet achieved this goal of prediction.
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directions. Some of this re-radiated energy travels back down through the atmosphere to the surface and it is this additional heating of the surface, over and above the heating due to the absorption of solar radiation, that is termed the greenhouse effect. ‘Natural’ greenhouse heating turns the Earth from a planet unable to support life, with a global mean temperature of - 18°C into its present habitable state, with a global mean temperature of + 15°C. In the Earth’s atmosphere, unlike that of Mars and Venus, only trace gases (less than a few per cent of the total atmosphere) contribute to the greenhouse warming. These trace gases are water vapour, carbon dioxide (CO,), methane (CH,), nitrous oxide (N,O), tropospheric ozone (0,) and the chlorofluorocarbons (CFCsL although other gases such as carbon monoxide (CO) and sulphur dioxide (SO,) also make very small radiative contributions to the surface warming (see Centrepage Diagram, this issue). The most significant greenhouse agent in the Earth’s atmosphere is water vapour, which currently contributes about 100 of the 148 watts of additional, radiatively induced, heating of each square metre of the surface. Unfortunately, the amount of water vapour in the atmosphere is essentially uncontrollable, as the major source is evaporation from the oceans. Human activities do, however, modify the atmospheric water vapour loading: as temperatures rise in response to increases in $1 1990
CO, and other greenhouse gases, more water evaporates into the atmosphere, prompting greater temperature increases and thereby inducing further evaporation. This strong positive feedback between temperature and water vapour produces much larger temperature changes than would CO, and other gas increases alone. A brief history of greenhousetheory Since the history of greenhouse theory is not well known outside the atmospheric sciences, the following review provides a brief summary of key papers within the field. Emphasis is placed on the historical context of the subject, and on the significant papers that represented new directions and advances at their time. Recognition:heat can be absorbed by gases
The effects of radiative transfer of energy have been observed and described empirically for well over a hundred years. Indeed, radiant heat was recognized as similar (and later as identical) to light (i.e. a part, albeit invisible, of the electromagnetic spectrum) by Herschel in 1800’. Although theoretical understanding of the interaction between gases and radiation was impossible before the development of quantum theory in the early years of this century, the observation that certain gases absorbed heat radiation prompted the recognition over 150 years ago that there is considerable potential for mankind’s activities to modify the natural radiative budget of the planet. Carbon dioxide was the first, obvious ‘culprit’ and is, indeed, still used by most climate Elsev~er Science
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nodelling groups as a surrogate for
sented a paper to the Royal Swedish Academy of Sciences on ‘The influence of carbonic acid ICO,] in the air upon the temperature of the ground’. This paper was later communicateds to the Philosophical
Magazine and published in 1896. Arrhenius disagreed with Tyndall’s conclusion that the absorptive properties of water vapour were such that it made a larger contribution than CO,. He went on to calculate the variations in temperature under five different scenarios, in which CO, was 0.67, 1.5, 2.0, 2.5 and 3.0 times the observed atmospheric level (about 300 p.p.m.v.1. He also calculated latitudinal variations from 70”N to 60’s. The mean global warming for a doubling of CO, was 6°C (not too far removed from some current estimates). On the basis of his calculations, Arrhenius concluded that past glacial epochs may have occurred largely because of a reduction in atmospheric CO,. In 1903, he further suggested9 that most of the excess CO, from fossil fuel combustion may have been transferred to the oceans, thus displaying a remarkable awareness of cycling processes. Between 1897 and 1899, T.C. Chamberlin presented three papers detailing the geological implications of CO, theory. In 1897 he reviewedlO the current hypotheses of climatic change, and in 1898 and 1899 postulated*Jr the effects that limestone-forming periods (e.g. Carboniferous, Jurassic and Cretaceous) may have had in contributing to subsequent glacial epochs. A key paper in the history of terrestrial radiation studies is the work of CC. Simpson’*. By attempting to verify earlier calculations of outgoing radiation, Simpson identified the latitudinal variability of longwave radiation emissions (uniform between 50’S and 50°N, decreasing at the poles by around 20%). His conclusions questioned the assumption that water vapour is the only constituent of the atmosphere that absorbs and emits longwave radiation, leading to the suggestion that CO, could appreciably modify the estimates of outgoing radiation. This supposition was dealt with further in later worksr3,r4. By the late 1930s the role of atmospheric CO, had re-emerged. Callendarr5 estimated that between 1890 and 1938 around 150 million tons of CO, had been pumped into the atmosphere from the combustion of fossil fuels, of which 75% had remained in the atmosphere. Further, if all other factors remained
in equilibrium then anthropogenic activities would increase the mean global temperature by I. I“C per century, another remarkable example of foresight given that current General Circulation Models (GCMsl estimate that global temperatures should have risen by about l°C since about 1880. Subsequent workr6.17 reiterated the role of human climate forcing, suggesting that the increase in atmospheric CO, may account for the observed slight rise in average northern-latitude temperature during the first four decades of the 20th century. This was fundamentally a re-examination of the hypotheses of Arrheniu$ and Chamberlin2 in the context of human activities, and represents the transition from a theory of glacial climatic change through to the human influence on atmospheric CO, levels and its subsequent role as a climate forcing mechanism. During the 1950s the development of the greenhouse theory took on new dimensions. Research moved towards calculating the temperature increase given an atmospheric doubling of CO,. Gilbert Plassr8 calculated that the mean global surface temperatures would increase by 3.6”C if atmospheric CO, doubled, and would decrease by 3.8’C if it halved. These estimates were strongly criticized by Kaplan19, who rejected the hypothesis of a ‘clear atmosphere’ model that took no account of atmospheric water content. Kaplan was, however, concerned more with the effect of halving the CO, content, and concluded that if atmospheric water vapour and cloud cover were accounted for then the decrease would be closer to 1.8”C than to the 3.8’C that Plassrs suggested. There followed an exchange of views20,2’ that provided stronger evidence of the frailty of human egos than of scientific method. It was around the beginning of the 1960s that the misnomer ‘greenhouse effect’ was used to support an educational analogy of the Earth’s atmosphere (cf. Hare22, p. I 5). Unfortunately the term soon became popularized, and by 1963 there were already vain attempts*’ to clarify the confusion between a ‘greenhouse’ and an ‘atmospheric’ effect. 271
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Ground temperature,; water and energy flu held and computed,
Horizontal exchange between columns
SPHERIC Y 5, humidity,
Time step = 30 min Grid spacing = 3” X 3
LIvuuJ, LcilI$erature and Vertical exchange between levels
height held and computed
Fig. 1. Schematic representation of the processes that are parameterized in numerical climate models. Note the rather coarse spatial resolution (a few degrees of latitude and longitude and about ten layers in the atmosphere) as compared with the rather fine temporal resolution (about 30 minutes). The types of variable computed and retained to the next time step at the surface and in each ‘slice’ of each atmospheric column are indicated.
The first models Miiller24 provided the first attempt at a one-dimensional atmospheric model using fixed relative humidity and cloudiness. An increase in temperature of 1.5”C was estimated for a doubling of CO, from 300 p.p.m.v. to 600 p.p.m.v. He concluded that cloudiness diminished the radiation effects but not the temperature changes, and that the effect of a 10% increase in CO, could be compensated for completely by a change in atmospheric water content of 3% or in cloudiness of I % (of its value). In 1967, Manabe and Wetherald provided quantitative results for CO,-induced warming on the basis of a one-dimensional radiative-convective model with fixed relative humidity and cloudiness. They estimated a mean surface-temperature increase of 2.4’C. In 1971, Manabe*‘j was able to refine the Manabe and Wetherald model with a more elaborate treatment of infrared radiation transfer. The early 1970s also saw sufficient concern about climate modification to initiate the SMIC2’ report in 1971, In the same year, Rasool and Schneider28 observed that over the last few decades atmospheric CO, had increased by 7% and the aerosol content of the lower atmosphere by 272
as much as 100%. They also calculated the change in tropospheric temperature through a doubling of CO, as being 0.8”C using a onedimensional planetary radiation balance model with a fixed lapse rate, relative humidity, stratospheric temperature and cloudiness. Perhaps one of the most significant advances in the estimation of temperature change with a doubling of CO, was the development by 1975 of a three-dimensional global climatic model, reported by Manabe and Wetherald29. In comparison with modern GCMs (e.g. Fig. IL this limited-area model with idealized topography, fixed cloudiness and no heat transport by the oceans represented a very simplistic view. It was, however, able to provide some indication of how an increase in CO, concentration might affect the distribution of temperatures in the atmosphere, as opposed to simply Earth surface temperatures. It suggested that whilst the air temperatures in the troposphere would be likely to increase, the stratospheric temperatures would be likely to decrease. Furthermore, for the first time a model suggested that the hydrological cycle would be likely to increase in intensity. In 198 I Hansen et aL30 examined
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the main processes known to influence climate model sensitivity by inserting fixed absolute humidity, relative humidity, cloud formation (from fixed temperatures) and a moist adiabatic lapse rate individually into a one-dimensional radiative-convective model. By doing so, they were able to examine the effects of various climatic feedback mechanisms, particularly clouds and water vapour. They concluded that by the end of the century, the anthropogenic CO, warming factor will have risen above the ‘noise level’ of natural climatic variability (caused in part by variations in volcanic aerosols and the atmospheric dust veil index). The importance of the atmospheric water vapour feedback mechanism was compounded by the findings of Ramanathan et aL3’ using the US National Center for Atmospheric Research (NCAR) GCM. For the first time they were able to examine important mechanisms such as the way in which the vertical distribution of water vapour minimizes the lower stratospheric longwave cooling, and the dependence of the emissivity of cirrus clouds on liquid water content. The latter suggested that not only are clouds important in the modelling of atmospheric processes, but also the type of cloud and the radiative properties are crucial; high-level cirrus clouds appeared significantly to enhance the radiative cooling of the polar troposphere. TCrefirst model simulation of transient greenhouse warming By 1988 the modellers had increased the sensitivity of their GCMs by an order of magnitude, mainly through advances in computer technology and the development of supercomputers. Hansen et aLs2 provided three different scenarios using the Goddard Institute of Space Studies (GISS) GCM, assuming a continental exponential growth of trace gases for scenario A, a reduced linear growth of trace gases in scenario B, and a rapid curtailment of trace gases in scenario C. This experiment with transient CO, increase also recognized the importance of the other radiatively active gases in contributing to the greenhouse effect - not only atmospheric water vapour, methane
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and nitrous oxide, but also CFCs and azone. They concluded that scenario B appeared the most plausible, doubling CO, by 2060 AD (from 1958 levels). The GISS model was also able to distinguish some macroregional scale variations: the regions where an unambiguous warming is likely to appear earliest are ocean areas near Antarctica, China md the interior areas of Asia. Ramanathan4 provides a good, Jp-to-date view of the greenhouse :heory, emphasizing the importance of water vapour as a radiatively acLive gas as well as giving a summary of important climate feedback nechanisms, i.e. water, ice albedo, :loud and ocean-atmosphere inter.actions. At present, much of the ange in model predictions of the Narming due to doubling CO* is atributable to different treatments of ‘:loud parameterization33. Recently :he strength, and even the direction, of the temperature-water vapour l’eedback has been challenged34, vout the reality of this strong posi.:ive relationship has now been demonstrated using satellite obserllations35,36. Wicg making forthe green/rouseeffect By 1989, the greenhouse literaure had established a permanent ;liche in climate theory. Schneider3’, :+owever, provides a new dimension ‘vith his examination of science and policy making. This multi-disciplinary approach to the problem of the greenhouse theory argues the need tar greater involvement in future planning, given the ‘knock-on’ eflects of a global warming, i.e. sealevel rise, economic refugees, social and land-use planning changes, etc. The holistic approach represents a switch in emphasis: acknowledging !he trend in global warming, it opens the way for a greater involvement by other disciplines and, more importantly, it enables decision makers to incorporate possible future climatic change into policy decisions. The general circulation modellers are still investigating ways of improving the sensitivity of their models. Although there is a broad each model differs consensus :;omewhat ‘from its counterparts in physical construction and in the physical realism of the parameterizations employed, the number
and complexity of the feedbacks included, and the outcome of increased CO, levels. Inter-model variability still provides a high margin of uncertainty - for example, estimates of temperature change vary between 1.5”C and 4°C depending on the feedback mechanisms and responses. Predicting the consequencesof future greenhousewarming There are only two ways presently available to estimate how climate will change in the era of the greenhouse effect: the use of computerbased global climate models, and analogies with historical or palaeoclimatological records of periods when the Earth was warmer. The historical analogue approach has been used by a number of authors to construct warmer climate scenarios for Europe, America and Australia. There are two basic methods of using the instrumental record to create an analogy for the future: ( I ) comparison between climates of a preselected number of years (usually ten or twenty) that were the hottest on record and their coldest-year counterparts; and (2) comparison of a warm period with a cold period (e.g. for Australia, the cold period 1913-1945 has been compared with the more recent warm period of 1946-1978381. Comparison is made between rainfall, and sometimes even cloudiness, from the warm years as compared with the cold years. The assumption is that the same incremental change can be added to today’s conditions to give us a picture of how the climate will be in the next century. This historical analogue method of climate estimation has two major faults. Firstly, there is no reason to believe that the cold-to-warm changes seen in the historical record were caused by greenhouse gas increases, and therefore extrapolating these changes may indeed offer a picture of a warmer climate but this may be nothing like the climate that will be caused by greenhouse warming. The other major problem is that the temperature differences between cool and warm periods this century are about ten times smaller than the temperature differences that are expected by the middle of next century: differences of approximately 0.5”C as
compared with predicted temperature increases of 4°C or 5“C by 2030 AD. One of two assumptions must be made: (1) a linear scaling can be employed, or (2) the tenfold difference in temperatures can be ignored and the rainfall or cloudiness differences derived from the historical record employed directly as ‘predictions’ of the future climate. The palaeoclimatological approach has some advantages over historical analogues, the most important of which is that periods can be found in Earth’s history when temperatures were a few degrees, rather than a few tenths of a degree, warmer. Unfortunately, other information about these periods is much sparser than for the period of the historical record, so that the analogue is, at best, a very hazy one. The assumptions necessary and the qualitative nature of the palaeoreconstructions means that neither of these analogue methods is firmly founded. The only alternative mode of climatic prediction is the use of computer-based global climate models. Numerical global climate models (sometimes termed AGCMs to emphasize the fact that most such models represent predominantly the atmospheric component of the climate system) comprise an intricate set of computer programs that solve well-understood equations describing how pressure changes cause winds to blow, how energy is absorbed, temperatures change, moisture evaporates from the surface and precipitation falls (Fig. I). As is widely recognized, these global climate models offer a less than complete representation of the real climate system. In particular, the incorporation of ocean processes and sea ice - important components of the long-term climate - is still very simplistic, and clouds and surface features are modelled only very sketchily at present. Moreover, the spatial resolution is poor (grid elements are approximately 500 km x 500 km): details of relatively small atmospheric phenomena, such as thunderstorms, cannot be described explicitly, although modellers go to great pains to capture the ensemble effects of this ‘subgridscale weather’ over a few years and over broad regions. In one sense, this coarse resolution is 273
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warmest years: a7
I
I
1900
I
1920
I
c
I
1940
I
1960
I
I
1
1980
Year Fig. 2. Measured globally averaged (i.e. land and ocean) surface-air temperatures for this century, expressed as differences from the mean temperature across the years 1951-1980 (data derived from a variety of sources: see, for example, Ref. 391. Note that there are two periods of ‘warming’: 1915-1935 and 1975 to date. The latter period has been dominated by two large ENS0 (El Nitio-Southern Oscillation1 events. The Earth’s stratosphere has also been observed to be cooling fin agreement with climate model predictions). Although this stratospheric temperature record is much shorter (about 15 years in all), the lowest temperatures were observed in the 1980s I 1985, I986 and 1987).
unimportant:
since global climate models are designed to be useful for long periods, they do not have to forecast the exact timing and shape of each weather system precisely; rather, they only have to capture the statistics of a large number of these events. On the other hand, predictions made at the spatial resolution of current models are of little use to planners and policy makers. The poor spatial resolution of current AGCMs is the direct result of the very large number of calculations required for each model time-step. At present, the spatial resolution is as fine as is possible using the fastest supercomputers in the world. Computer power places other constraints on global model simulations too. The real-world experiment that humanity is currently conducting with the Earth involves a gradual increase in greenhouse gases. All but the most recent computer simulations have, in contrast, been ‘instantaneous doubling’ experiments; thus the model suddenly has twice as much radiative heating due to CO, as it had the day before. This sudden switch-on of extra heat causes immediate disequilibrium, so that the model has to be run on for a number of years until the atmosphere, surface and the upper ocean ‘catch up’. Although this time for re-equilibration is a few years, it is still very much shorter than a simulation that allows a gradual increase in CO, from the middle of the last century to the middle of the next. ‘Transient’ experiments3*, which follow the real-world gradual 274
increases, pensive the much doubling’
are very much more exin computer time than more common ‘switch-on experiments.
Conclusion Since the industrial Revolution, the atmospheric burden of CO, has increased from 260-280 p.p.m.v. to the 1988 value of 350 p.p.m.v. When a 25% increase in atmospheric CO, concentrations is imposed upon model simulations, predicted warmings are of about I’C. The temperature record, however (Fig. 21, shows a warming of only about 0.7”C. The constraint of computer power on simulation time and spatial resolution may be the primary reason for the failure, to date, to validate current climate models by hindcasting of temperature changes over the last century. Figure 2 is an average of all ‘acceptable’ surface-air temperatures worldwide; the number and geographical distribution of these observations varies considerably through the century. Moreover, recent critical evaluations of some of the temperature records included in this 88-year global average indicate that urban warming has enhanced some of the observed temperature increase. However, even if the most stringent corrections yet derived for the United States are applied, the global data still indicate a 0.5”~ warming this century. In addition, the 1980s appear to be the warmest decade on record and I98 I, 1987 and 1988 are the warmest years this century. Nonetheless, the discrepancy
between the climate model predictions and the observed facts persists. Possible explanations for this discrepancy include: (I) that the assumption of linearity in processes (this is the basis of the use of one quarter of doubled CO, predictions as representative of the effects of the 25% increase already observed) is invalid; (2) that climate models are too sensitive by a factor of two to increases in greenhouse gases; (3) that atmospheric climate models fail to account fully (or correctly) for the take-up of heat by the oceans; and (4) that the data record shown in Fig. 2, being incomplete and inhomogeneous, actually underestimates the global warming this century. Future research will pursue these and other topics vigorously, with the goal of matching improved climate model predictions to refined observational evidence. At present, the warming observed so far this century (Fig. 21 cannot be stated unambiguously to be the result of additional greenhouse warming caused by mankind’s activities. Moreover, it is unlikely that such an unambiguous statement will be possible in less than IO-15 years. On the other hand, any decision on the part of policy makers to delay all actions until certainty of cause and effect is assured is a value judgement that itself carries real costs if the evidence does ultimately ‘prove’ the models correct.
References I Scott Barr, E. I1961 I Infrared Phys. I, l-4 2 Chamberlin, T.C. I18991 1. Geol. 7, 454-584. 667-685. 751-787 3 Fourier, 1-B. (18271 Mem. Acad. R. Sci. Inst. Fr. 7, 569-604 4 Ramanathan, V. (1988) Science 240, 293-299 5 Tyndall, I. (18611 fhilos. Mag. 22, 169-194, 273-285 6 Langley, S.P. ( 18841 Researches on Solar /-/eat, Professional Papers of the Signal Service No. I5 7 Langley, S.P. (18861 Philos. Mag. 3 I, 394-409 8 Arrhenius, S. f 18961 fhilos. Msg. 41, 237-276 9 Arrhenius, S. ( 19031 Lehrboch der Kosmischen Physik (Vol. 21, Hirzel
IO Chamberlin, T.C. ll897l/. Geol. 5, 653-683 I I Chamberlin, T.C. t 18981I. Ceol. 6, 609-62 I I2 Simpson, G.C. (1928) Mem. R. Meteorol. Sot. 2 ( 16). 69-95 I3 Simpson, G.C. ( 1928) Mem. R. Meteorol. Sot. 3 (21), l-26 I4 Simpson, C.C. (1929) Mem. R. Meteorol.
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Sot. 3 (231, 53-78 15 Callendar, G.S. ( 1938) 0.1. R. Meteorol. Sot. 64, 223-237 16 Callendar, G.S. I19491Weather 4, 3 IO-3 I4 17 Callendar, G.S. (1958) Tel/us IO, 243-248 18 Plass, G.N. (1956) Te//us8, 140-153 I9 Kaplan, L.D. II9601 Tel/us 12, 204-208 20 Plass, G.N. II961 I Tel/us 13, 296-300 21 Kaplan, L.D. 11961) Tel/us 13, 301-302 22 Hare, F.K. ( 1961) The Restless Atmosphere, Hutchinson University Press 23 Fleagle, R.G. and Businger, I.A. ( 1963) An Introduction to Atmospheric Physics, Academic Press 24 Miiller, F. ( 196311. Geophys. Res. 68, 3877-3886
The detailed Neogene and Quaternary valeoclimatic reconstructions now available vrovide a means to test how species respond to environmental change. Paleontologic studies of marine organisms show that climatic change causes evolution (via cladogenesis and anagenesis), ecophenotypic variation, migration, morphologic stasis and extinction. Evolution during climatic change is a rare event relative to the number of climatic cycles that have occurred, but climate-related environmental barriers, usually temperature, may play an importgnt role in the isolation of populations during allopatric speciation.
The influence of environmental change on the distribution of organisms and on the barriers between populations and species has long been recognized as significant in geographical speciation’. Climate changes, including the associated sea-level and oceanographic changes, represent clear abiotic, extrinsic factors that influence organic evolution. Unfortunately, geologists’ understanding of Earth’s paleoclimatic history had, until recently, been extremely limited due to the fragmentary continental record of glacial and interglacial deposits, and the lack of reliable dating and correlation methods. Even for the most recent geological systems - the Neogene (23-l .6 million years ago) and the Quatemary ( I .6 million years ago to the present) the incomplete onshore geologic record did not provide the prerequisite climatic history adequately to test theories of speciation, in parThomasCroninand CynthiaSchneiderare at the US GeologicalSurvey,Reston,VA 22092,USA.
25 Manabe, S. and Wetherald, R.T. (19671 I. Atmos. Sci. 2 I, 361-385 26 Manabe, S. (1971) in Man’s Impact on Climate (Matthews, W.H. eta/., eds), pp. 249-264, Massachusetts Institute of Technology Press 27 Study of Man’s Impact on Climate (SMIC) II971 1 Inadvertent Climate Modification, Massachusetts Institute of Technology Press 28 Rasool, S.I. and Schneider, S.H. (1971) Science 173, 138-141 29 Manabe, S. and Wetherald, R.T. ( 1975) 1. Atmos. Sci. 32, 3-15 30 Hansen,l.eta/.1198l)Science213, 957-966 31 Ramanathan, V., Pitcher, EJ., Malone, R.C.
and Blackman, M.L. ( 1983) 1. A&OS. Sci. 40, 605-630 32 Hansen, 1. et al. ( 198811. Geophys. Res. 93,934 l-9364 33 Mitchell, I.F.B., Senior, CA. and Ingram, WI. 11989) Nature 341, 132-134 34 Lindzen, R.S. ( 19901 Bull. Am. Meteorol. Sot. 7 I, 288-299 35 Raval, A. and Ramanathan, V. (1989) Nature 342, 758-761 36 Levi, B.C. ( 1990) Phys. Today 43, 17-19 37 Schneider, S.H. (1989) Science 243, 771-781 38 Pittock. A.B. ( 1983) C/im. Change 5. 321-340 39 /ones, P.D. et a/. ( 1988) Nature 338, 790
ClimaticInfluenceson Species: Evidencefrom the FossilRecord Thomas M. Cronin and Cynthia E. Schneider titular, and biotic response to environmental change in general. During the last decade, a revolution in paleoclimate research has taken place. Paleobiologists now have the opportunity to examine directly how species respond to climatic change. The benthic and planktic foraminifers and ostracodes illustrated with scanning electron photomicrographs in Fig. 1 represent some of the microfossil groups used in paleoclimatology and evolutionary studies that will be referred to below. Organic evolution and environmental change can now be studied with greater spatial and temporal resolution due to improvements in four areas: (I 1 The continuous marine sedimentary record obtained from deep-sea drilling is generally superior to the emerged continental record for documenting climatic history. The development of the hydraulic piston core in the early 1980s was particularly important, because it allowed the recovery of undisturbed sequences of Cenozoic
ocean sediments having a temporal resolution of one sample per 103IO4 years, depending on sedimentation rate2. (2) High-resolution climatic proxy data obtained from deep-sea cores have led to the general acceptance of the Milankovitch theory of
Fig. I. (a) The benthic foraminifer Tubulogenerina butonensis from the late Miocene of Enewetak Atoll, Equatorial Pacific16;side view. (b) The marine ostracode Puriana mesacostalis from the earlyPleistocene Waccamaw Formation of South Carolina2’; left lateral view of female carapace. (c) Abapertural and (d) apertural views of the planktic foraminifer Cloborotalia truncatulinoides from the Pliocene of the southwest Pacific Ocean, DSDP core 590-A.
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that result from climatic change, to sort out the relative importance of biotic and abiotic influences on species abundances, and to predict the range of possible adaptations. If ecology is the study of factors that control the distribution and abundance of plants and animals, there is much ecology to be done before we can predict with certainty biotic responses to global change. The articles presented here represent an
important step toward understanding.
arriving
at that
References 1 Special Committee for the IGBP: McCarthy, J.J., chair (1988) The International Geosphere-Biosphere Program: A Study of Global Change (A Plan for Action: Global Change Report No. 4), IGBP Secretariat, Royal Swedish
Academy of Sciences 2 Barnola, J.M., Raynaud, D.,
_
A. Henderson-Sellers
The greenhouse effect is an unambiguous theory that is well understood by atmospheric and climatic scientists, and that successfully predicts temperatures on the Earth and on other planets. The greenhouse theory is based upon the fact (readily demonstrated by experiment) that whilst gases in the Earth’s atmosphere are transparent to incoming solar radiation, some of them absorb outgoing thermal (or heat) radiation emitted from the Earth’s surface. This radiative interaction between selected gases in the atmosphere (termed the greenhouse gases) and the outgoing heat radiation causes those gases to warm and, consequently, they themselves re-radiate heat in all A. Henderson-Sellers is at the School of EarthSciences, Macquarie University, North Ryde, New South Wales 2109, Australia.
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Korotkevich, Y.S. and Lorius, C. (1987) Nature 329,40&414 3 Davis, M.B., coordinator (1988) in Toward an Understanding of Global Change (Mooney, H.A., chair), pp. 69-106, National Academy Press
4 Peters, R.L. and Darling, J.D. (1985) Bioscience 35,707-717 5 Peters, R.L. and Lovejoy, T.E.,eds Consequences of the Greenhouse Effect for Biological Diversity: Proceedings of the World Wildlife Fund Conference, Yale
University Press (in press)
Modellingand Monitoring ‘Greenhouse’ Warming Greenhouse warming is now commonly accepted 6g politicians and the general public and is the su6iect of active research. This paper traces the greenhouse theory as far 6acf as 1827, highlighting new directions and significant advances over that time. Three main themes emerge: that certain radiative/y active gases are responsible for warming the planet; that humans are increasing the concentrations of these gases and hence inadvertently influencing this warming; that climate models are designed to permit prediction of the climatic changes resulting from changed loadings in these gases, 6ut that they have not yet achieved this goal of prediction.
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directions. Some of this re-radiated energy travels back down through the atmosphere to the surface and it is this additional heating of the surface, over and above the heating due to the absorption of solar radiation, that is termed the greenhouse effect. ‘Natural’ greenhouse heating turns the Earth from a planet unable to support life, with a global mean temperature of - 18°C into its present habitable state, with a global mean temperature of + 15°C. In the Earth’s atmosphere, unlike that of Mars and Venus, only trace gases (less than a few per cent of the total atmosphere) contribute to the greenhouse warming. These trace gases are water vapour, carbon dioxide (CO,), methane (CH,), nitrous oxide (N,O), tropospheric ozone (0,) and the chlorofluorocarbons (CFCsL although other gases such as carbon monoxide (CO) and sulphur dioxide (SO,) also make very small radiative contributions to the surface warming (see Centrepage Diagram, this issue). The most significant greenhouse agent in the Earth’s atmosphere is water vapour, which currently contributes about 100 of the 148 watts of additional, radiatively induced, heating of each square metre of the surface. Unfortunately, the amount of water vapour in the atmosphere is essentially uncontrollable, as the major source is evaporation from the oceans. Human activities do, however, modify the atmospheric water vapour loading: as temperatures rise in response to increases in $1 1990
CO, and other greenhouse gases, more water evaporates into the atmosphere, prompting greater temperature increases and thereby inducing further evaporation. This strong positive feedback between temperature and water vapour produces much larger temperature changes than would CO, and other gas increases alone. A brief history of greenhousetheory Since the history of greenhouse theory is not well known outside the atmospheric sciences, the following review provides a brief summary of key papers within the field. Emphasis is placed on the historical context of the subject, and on the significant papers that represented new directions and advances at their time. Recognition:heat can be absorbed by gases
The effects of radiative transfer of energy have been observed and described empirically for well over a hundred years. Indeed, radiant heat was recognized as similar (and later as identical) to light (i.e. a part, albeit invisible, of the electromagnetic spectrum) by Herschel in 1800’. Although theoretical understanding of the interaction between gases and radiation was impossible before the development of quantum theory in the early years of this century, the observation that certain gases absorbed heat radiation prompted the recognition over 150 years ago that there is considerable potential for mankind’s activities to modify the natural radiative budget of the planet. Carbon dioxide was the first, obvious ‘culprit’ and is, indeed, still used by most climate Elsev~er Science
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nodelling groups as a surrogate for
sented a paper to the Royal Swedish Academy of Sciences on ‘The influence of carbonic acid ICO,] in the air upon the temperature of the ground’. This paper was later communicateds to the Philosophical
Magazine and published in 1896. Arrhenius disagreed with Tyndall’s conclusion that the absorptive properties of water vapour were such that it made a larger contribution than CO,. He went on to calculate the variations in temperature under five different scenarios, in which CO, was 0.67, 1.5, 2.0, 2.5 and 3.0 times the observed atmospheric level (about 300 p.p.m.v.1. He also calculated latitudinal variations from 70”N to 60’s. The mean global warming for a doubling of CO, was 6°C (not too far removed from some current estimates). On the basis of his calculations, Arrhenius concluded that past glacial epochs may have occurred largely because of a reduction in atmospheric CO,. In 1903, he further suggested9 that most of the excess CO, from fossil fuel combustion may have been transferred to the oceans, thus displaying a remarkable awareness of cycling processes. Between 1897 and 1899, T.C. Chamberlin presented three papers detailing the geological implications of CO, theory. In 1897 he reviewedlO the current hypotheses of climatic change, and in 1898 and 1899 postulated*Jr the effects that limestone-forming periods (e.g. Carboniferous, Jurassic and Cretaceous) may have had in contributing to subsequent glacial epochs. A key paper in the history of terrestrial radiation studies is the work of CC. Simpson’*. By attempting to verify earlier calculations of outgoing radiation, Simpson identified the latitudinal variability of longwave radiation emissions (uniform between 50’S and 50°N, decreasing at the poles by around 20%). His conclusions questioned the assumption that water vapour is the only constituent of the atmosphere that absorbs and emits longwave radiation, leading to the suggestion that CO, could appreciably modify the estimates of outgoing radiation. This supposition was dealt with further in later worksr3,r4. By the late 1930s the role of atmospheric CO, had re-emerged. Callendarr5 estimated that between 1890 and 1938 around 150 million tons of CO, had been pumped into the atmosphere from the combustion of fossil fuels, of which 75% had remained in the atmosphere. Further, if all other factors remained
in equilibrium then anthropogenic activities would increase the mean global temperature by I. I“C per century, another remarkable example of foresight given that current General Circulation Models (GCMsl estimate that global temperatures should have risen by about l°C since about 1880. Subsequent workr6.17 reiterated the role of human climate forcing, suggesting that the increase in atmospheric CO, may account for the observed slight rise in average northern-latitude temperature during the first four decades of the 20th century. This was fundamentally a re-examination of the hypotheses of Arrheniu$ and Chamberlin2 in the context of human activities, and represents the transition from a theory of glacial climatic change through to the human influence on atmospheric CO, levels and its subsequent role as a climate forcing mechanism. During the 1950s the development of the greenhouse theory took on new dimensions. Research moved towards calculating the temperature increase given an atmospheric doubling of CO,. Gilbert Plassr8 calculated that the mean global surface temperatures would increase by 3.6”C if atmospheric CO, doubled, and would decrease by 3.8’C if it halved. These estimates were strongly criticized by Kaplan19, who rejected the hypothesis of a ‘clear atmosphere’ model that took no account of atmospheric water content. Kaplan was, however, concerned more with the effect of halving the CO, content, and concluded that if atmospheric water vapour and cloud cover were accounted for then the decrease would be closer to 1.8”C than to the 3.8’C that Plassrs suggested. There followed an exchange of views20,2’ that provided stronger evidence of the frailty of human egos than of scientific method. It was around the beginning of the 1960s that the misnomer ‘greenhouse effect’ was used to support an educational analogy of the Earth’s atmosphere (cf. Hare22, p. I 5). Unfortunately the term soon became popularized, and by 1963 there were already vain attempts*’ to clarify the confusion between a ‘greenhouse’ and an ‘atmospheric’ effect. 271
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Ground temperature,; water and energy flu held and computed,
Horizontal exchange between columns
SPHERIC Y 5, humidity,
Time step = 30 min Grid spacing = 3” X 3
LIvuuJ, LcilI$erature and Vertical exchange between levels
height held and computed
Fig. 1. Schematic representation of the processes that are parameterized in numerical climate models. Note the rather coarse spatial resolution (a few degrees of latitude and longitude and about ten layers in the atmosphere) as compared with the rather fine temporal resolution (about 30 minutes). The types of variable computed and retained to the next time step at the surface and in each ‘slice’ of each atmospheric column are indicated.
The first models Miiller24 provided the first attempt at a one-dimensional atmospheric model using fixed relative humidity and cloudiness. An increase in temperature of 1.5”C was estimated for a doubling of CO, from 300 p.p.m.v. to 600 p.p.m.v. He concluded that cloudiness diminished the radiation effects but not the temperature changes, and that the effect of a 10% increase in CO, could be compensated for completely by a change in atmospheric water content of 3% or in cloudiness of I % (of its value). In 1967, Manabe and Wetherald provided quantitative results for CO,-induced warming on the basis of a one-dimensional radiative-convective model with fixed relative humidity and cloudiness. They estimated a mean surface-temperature increase of 2.4’C. In 1971, Manabe*‘j was able to refine the Manabe and Wetherald model with a more elaborate treatment of infrared radiation transfer. The early 1970s also saw sufficient concern about climate modification to initiate the SMIC2’ report in 1971, In the same year, Rasool and Schneider28 observed that over the last few decades atmospheric CO, had increased by 7% and the aerosol content of the lower atmosphere by 272
as much as 100%. They also calculated the change in tropospheric temperature through a doubling of CO, as being 0.8”C using a onedimensional planetary radiation balance model with a fixed lapse rate, relative humidity, stratospheric temperature and cloudiness. Perhaps one of the most significant advances in the estimation of temperature change with a doubling of CO, was the development by 1975 of a three-dimensional global climatic model, reported by Manabe and Wetherald29. In comparison with modern GCMs (e.g. Fig. IL this limited-area model with idealized topography, fixed cloudiness and no heat transport by the oceans represented a very simplistic view. It was, however, able to provide some indication of how an increase in CO, concentration might affect the distribution of temperatures in the atmosphere, as opposed to simply Earth surface temperatures. It suggested that whilst the air temperatures in the troposphere would be likely to increase, the stratospheric temperatures would be likely to decrease. Furthermore, for the first time a model suggested that the hydrological cycle would be likely to increase in intensity. In 198 I Hansen et aL30 examined
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the main processes known to influence climate model sensitivity by inserting fixed absolute humidity, relative humidity, cloud formation (from fixed temperatures) and a moist adiabatic lapse rate individually into a one-dimensional radiative-convective model. By doing so, they were able to examine the effects of various climatic feedback mechanisms, particularly clouds and water vapour. They concluded that by the end of the century, the anthropogenic CO, warming factor will have risen above the ‘noise level’ of natural climatic variability (caused in part by variations in volcanic aerosols and the atmospheric dust veil index). The importance of the atmospheric water vapour feedback mechanism was compounded by the findings of Ramanathan et aL3’ using the US National Center for Atmospheric Research (NCAR) GCM. For the first time they were able to examine important mechanisms such as the way in which the vertical distribution of water vapour minimizes the lower stratospheric longwave cooling, and the dependence of the emissivity of cirrus clouds on liquid water content. The latter suggested that not only are clouds important in the modelling of atmospheric processes, but also the type of cloud and the radiative properties are crucial; high-level cirrus clouds appeared significantly to enhance the radiative cooling of the polar troposphere. TCrefirst model simulation of transient greenhouse warming By 1988 the modellers had increased the sensitivity of their GCMs by an order of magnitude, mainly through advances in computer technology and the development of supercomputers. Hansen et aLs2 provided three different scenarios using the Goddard Institute of Space Studies (GISS) GCM, assuming a continental exponential growth of trace gases for scenario A, a reduced linear growth of trace gases in scenario B, and a rapid curtailment of trace gases in scenario C. This experiment with transient CO, increase also recognized the importance of the other radiatively active gases in contributing to the greenhouse effect - not only atmospheric water vapour, methane
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and nitrous oxide, but also CFCs and azone. They concluded that scenario B appeared the most plausible, doubling CO, by 2060 AD (from 1958 levels). The GISS model was also able to distinguish some macroregional scale variations: the regions where an unambiguous warming is likely to appear earliest are ocean areas near Antarctica, China md the interior areas of Asia. Ramanathan4 provides a good, Jp-to-date view of the greenhouse :heory, emphasizing the importance of water vapour as a radiatively acLive gas as well as giving a summary of important climate feedback nechanisms, i.e. water, ice albedo, :loud and ocean-atmosphere inter.actions. At present, much of the ange in model predictions of the Narming due to doubling CO* is atributable to different treatments of ‘:loud parameterization33. Recently :he strength, and even the direction, of the temperature-water vapour l’eedback has been challenged34, vout the reality of this strong posi.:ive relationship has now been demonstrated using satellite obserllations35,36. Wicg making forthe green/rouseeffect By 1989, the greenhouse literaure had established a permanent ;liche in climate theory. Schneider3’, :+owever, provides a new dimension ‘vith his examination of science and policy making. This multi-disciplinary approach to the problem of the greenhouse theory argues the need tar greater involvement in future planning, given the ‘knock-on’ eflects of a global warming, i.e. sealevel rise, economic refugees, social and land-use planning changes, etc. The holistic approach represents a switch in emphasis: acknowledging !he trend in global warming, it opens the way for a greater involvement by other disciplines and, more importantly, it enables decision makers to incorporate possible future climatic change into policy decisions. The general circulation modellers are still investigating ways of improving the sensitivity of their models. Although there is a broad each model differs consensus :;omewhat ‘from its counterparts in physical construction and in the physical realism of the parameterizations employed, the number
and complexity of the feedbacks included, and the outcome of increased CO, levels. Inter-model variability still provides a high margin of uncertainty - for example, estimates of temperature change vary between 1.5”C and 4°C depending on the feedback mechanisms and responses. Predicting the consequencesof future greenhousewarming There are only two ways presently available to estimate how climate will change in the era of the greenhouse effect: the use of computerbased global climate models, and analogies with historical or palaeoclimatological records of periods when the Earth was warmer. The historical analogue approach has been used by a number of authors to construct warmer climate scenarios for Europe, America and Australia. There are two basic methods of using the instrumental record to create an analogy for the future: ( I ) comparison between climates of a preselected number of years (usually ten or twenty) that were the hottest on record and their coldest-year counterparts; and (2) comparison of a warm period with a cold period (e.g. for Australia, the cold period 1913-1945 has been compared with the more recent warm period of 1946-1978381. Comparison is made between rainfall, and sometimes even cloudiness, from the warm years as compared with the cold years. The assumption is that the same incremental change can be added to today’s conditions to give us a picture of how the climate will be in the next century. This historical analogue method of climate estimation has two major faults. Firstly, there is no reason to believe that the cold-to-warm changes seen in the historical record were caused by greenhouse gas increases, and therefore extrapolating these changes may indeed offer a picture of a warmer climate but this may be nothing like the climate that will be caused by greenhouse warming. The other major problem is that the temperature differences between cool and warm periods this century are about ten times smaller than the temperature differences that are expected by the middle of next century: differences of approximately 0.5”C as
compared with predicted temperature increases of 4°C or 5“C by 2030 AD. One of two assumptions must be made: (1) a linear scaling can be employed, or (2) the tenfold difference in temperatures can be ignored and the rainfall or cloudiness differences derived from the historical record employed directly as ‘predictions’ of the future climate. The palaeoclimatological approach has some advantages over historical analogues, the most important of which is that periods can be found in Earth’s history when temperatures were a few degrees, rather than a few tenths of a degree, warmer. Unfortunately, other information about these periods is much sparser than for the period of the historical record, so that the analogue is, at best, a very hazy one. The assumptions necessary and the qualitative nature of the palaeoreconstructions means that neither of these analogue methods is firmly founded. The only alternative mode of climatic prediction is the use of computer-based global climate models. Numerical global climate models (sometimes termed AGCMs to emphasize the fact that most such models represent predominantly the atmospheric component of the climate system) comprise an intricate set of computer programs that solve well-understood equations describing how pressure changes cause winds to blow, how energy is absorbed, temperatures change, moisture evaporates from the surface and precipitation falls (Fig. I). As is widely recognized, these global climate models offer a less than complete representation of the real climate system. In particular, the incorporation of ocean processes and sea ice - important components of the long-term climate - is still very simplistic, and clouds and surface features are modelled only very sketchily at present. Moreover, the spatial resolution is poor (grid elements are approximately 500 km x 500 km): details of relatively small atmospheric phenomena, such as thunderstorms, cannot be described explicitly, although modellers go to great pains to capture the ensemble effects of this ‘subgridscale weather’ over a few years and over broad regions. In one sense, this coarse resolution is 273
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warmest years: a7
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Year Fig. 2. Measured globally averaged (i.e. land and ocean) surface-air temperatures for this century, expressed as differences from the mean temperature across the years 1951-1980 (data derived from a variety of sources: see, for example, Ref. 391. Note that there are two periods of ‘warming’: 1915-1935 and 1975 to date. The latter period has been dominated by two large ENS0 (El Nitio-Southern Oscillation1 events. The Earth’s stratosphere has also been observed to be cooling fin agreement with climate model predictions). Although this stratospheric temperature record is much shorter (about 15 years in all), the lowest temperatures were observed in the 1980s I 1985, I986 and 1987).
unimportant:
since global climate models are designed to be useful for long periods, they do not have to forecast the exact timing and shape of each weather system precisely; rather, they only have to capture the statistics of a large number of these events. On the other hand, predictions made at the spatial resolution of current models are of little use to planners and policy makers. The poor spatial resolution of current AGCMs is the direct result of the very large number of calculations required for each model time-step. At present, the spatial resolution is as fine as is possible using the fastest supercomputers in the world. Computer power places other constraints on global model simulations too. The real-world experiment that humanity is currently conducting with the Earth involves a gradual increase in greenhouse gases. All but the most recent computer simulations have, in contrast, been ‘instantaneous doubling’ experiments; thus the model suddenly has twice as much radiative heating due to CO, as it had the day before. This sudden switch-on of extra heat causes immediate disequilibrium, so that the model has to be run on for a number of years until the atmosphere, surface and the upper ocean ‘catch up’. Although this time for re-equilibration is a few years, it is still very much shorter than a simulation that allows a gradual increase in CO, from the middle of the last century to the middle of the next. ‘Transient’ experiments3*, which follow the real-world gradual 274
increases, pensive the much doubling’
are very much more exin computer time than more common ‘switch-on experiments.
Conclusion Since the industrial Revolution, the atmospheric burden of CO, has increased from 260-280 p.p.m.v. to the 1988 value of 350 p.p.m.v. When a 25% increase in atmospheric CO, concentrations is imposed upon model simulations, predicted warmings are of about I’C. The temperature record, however (Fig. 21, shows a warming of only about 0.7”C. The constraint of computer power on simulation time and spatial resolution may be the primary reason for the failure, to date, to validate current climate models by hindcasting of temperature changes over the last century. Figure 2 is an average of all ‘acceptable’ surface-air temperatures worldwide; the number and geographical distribution of these observations varies considerably through the century. Moreover, recent critical evaluations of some of the temperature records included in this 88-year global average indicate that urban warming has enhanced some of the observed temperature increase. However, even if the most stringent corrections yet derived for the United States are applied, the global data still indicate a 0.5”~ warming this century. In addition, the 1980s appear to be the warmest decade on record and I98 I, 1987 and 1988 are the warmest years this century. Nonetheless, the discrepancy
between the climate model predictions and the observed facts persists. Possible explanations for this discrepancy include: (I) that the assumption of linearity in processes (this is the basis of the use of one quarter of doubled CO, predictions as representative of the effects of the 25% increase already observed) is invalid; (2) that climate models are too sensitive by a factor of two to increases in greenhouse gases; (3) that atmospheric climate models fail to account fully (or correctly) for the take-up of heat by the oceans; and (4) that the data record shown in Fig. 2, being incomplete and inhomogeneous, actually underestimates the global warming this century. Future research will pursue these and other topics vigorously, with the goal of matching improved climate model predictions to refined observational evidence. At present, the warming observed so far this century (Fig. 21 cannot be stated unambiguously to be the result of additional greenhouse warming caused by mankind’s activities. Moreover, it is unlikely that such an unambiguous statement will be possible in less than IO-15 years. On the other hand, any decision on the part of policy makers to delay all actions until certainty of cause and effect is assured is a value judgement that itself carries real costs if the evidence does ultimately ‘prove’ the models correct.
References I Scott Barr, E. I1961 I Infrared Phys. I, l-4 2 Chamberlin, T.C. I18991 1. Geol. 7, 454-584. 667-685. 751-787 3 Fourier, 1-B. (18271 Mem. Acad. R. Sci. Inst. Fr. 7, 569-604 4 Ramanathan, V. (1988) Science 240, 293-299 5 Tyndall, I. (18611 fhilos. Mag. 22, 169-194, 273-285 6 Langley, S.P. ( 18841 Researches on Solar /-/eat, Professional Papers of the Signal Service No. I5 7 Langley, S.P. (18861 Philos. Mag. 3 I, 394-409 8 Arrhenius, S. f 18961 fhilos. Msg. 41, 237-276 9 Arrhenius, S. ( 19031 Lehrboch der Kosmischen Physik (Vol. 21, Hirzel
IO Chamberlin, T.C. ll897l/. Geol. 5, 653-683 I I Chamberlin, T.C. t 18981I. Ceol. 6, 609-62 I I2 Simpson, G.C. (1928) Mem. R. Meteorol. Sot. 2 ( 16). 69-95 I3 Simpson, G.C. ( 1928) Mem. R. Meteorol. Sot. 3 (21), l-26 I4 Simpson, C.C. (1929) Mem. R. Meteorol.
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Sot. 3 (231, 53-78 15 Callendar, G.S. ( 1938) 0.1. R. Meteorol. Sot. 64, 223-237 16 Callendar, G.S. I19491Weather 4, 3 IO-3 I4 17 Callendar, G.S. (1958) Tel/us IO, 243-248 18 Plass, G.N. (1956) Te//us8, 140-153 I9 Kaplan, L.D. II9601 Tel/us 12, 204-208 20 Plass, G.N. II961 I Tel/us 13, 296-300 21 Kaplan, L.D. 11961) Tel/us 13, 301-302 22 Hare, F.K. ( 1961) The Restless Atmosphere, Hutchinson University Press 23 Fleagle, R.G. and Businger, I.A. ( 1963) An Introduction to Atmospheric Physics, Academic Press 24 Miiller, F. ( 196311. Geophys. Res. 68, 3877-3886
The detailed Neogene and Quaternary valeoclimatic reconstructions now available vrovide a means to test how species respond to environmental change. Paleontologic studies of marine organisms show that climatic change causes evolution (via cladogenesis and anagenesis), ecophenotypic variation, migration, morphologic stasis and extinction. Evolution during climatic change is a rare event relative to the number of climatic cycles that have occurred, but climate-related environmental barriers, usually temperature, may play an importgnt role in the isolation of populations during allopatric speciation.
The influence of environmental change on the distribution of organisms and on the barriers between populations and species has long been recognized as significant in geographical speciation’. Climate changes, including the associated sea-level and oceanographic changes, represent clear abiotic, extrinsic factors that influence organic evolution. Unfortunately, geologists’ understanding of Earth’s paleoclimatic history had, until recently, been extremely limited due to the fragmentary continental record of glacial and interglacial deposits, and the lack of reliable dating and correlation methods. Even for the most recent geological systems - the Neogene (23-l .6 million years ago) and the Quatemary ( I .6 million years ago to the present) the incomplete onshore geologic record did not provide the prerequisite climatic history adequately to test theories of speciation, in parThomasCroninand CynthiaSchneiderare at the US GeologicalSurvey,Reston,VA 22092,USA.
25 Manabe, S. and Wetherald, R.T. (19671 I. Atmos. Sci. 2 I, 361-385 26 Manabe, S. (1971) in Man’s Impact on Climate (Matthews, W.H. eta/., eds), pp. 249-264, Massachusetts Institute of Technology Press 27 Study of Man’s Impact on Climate (SMIC) II971 1 Inadvertent Climate Modification, Massachusetts Institute of Technology Press 28 Rasool, S.I. and Schneider, S.H. (1971) Science 173, 138-141 29 Manabe, S. and Wetherald, R.T. ( 1975) 1. Atmos. Sci. 32, 3-15 30 Hansen,l.eta/.1198l)Science213, 957-966 31 Ramanathan, V., Pitcher, EJ., Malone, R.C.
and Blackman, M.L. ( 1983) 1. A&OS. Sci. 40, 605-630 32 Hansen, 1. et al. ( 198811. Geophys. Res. 93,934 l-9364 33 Mitchell, I.F.B., Senior, CA. and Ingram, WI. 11989) Nature 341, 132-134 34 Lindzen, R.S. ( 19901 Bull. Am. Meteorol. Sot. 7 I, 288-299 35 Raval, A. and Ramanathan, V. (1989) Nature 342, 758-761 36 Levi, B.C. ( 1990) Phys. Today 43, 17-19 37 Schneider, S.H. (1989) Science 243, 771-781 38 Pittock. A.B. ( 1983) C/im. Change 5. 321-340 39 /ones, P.D. et a/. ( 1988) Nature 338, 790
ClimaticInfluenceson Species: Evidencefrom the FossilRecord Thomas M. Cronin and Cynthia E. Schneider titular, and biotic response to environmental change in general. During the last decade, a revolution in paleoclimate research has taken place. Paleobiologists now have the opportunity to examine directly how species respond to climatic change. The benthic and planktic foraminifers and ostracodes illustrated with scanning electron photomicrographs in Fig. 1 represent some of the microfossil groups used in paleoclimatology and evolutionary studies that will be referred to below. Organic evolution and environmental change can now be studied with greater spatial and temporal resolution due to improvements in four areas: (I 1 The continuous marine sedimentary record obtained from deep-sea drilling is generally superior to the emerged continental record for documenting climatic history. The development of the hydraulic piston core in the early 1980s was particularly important, because it allowed the recovery of undisturbed sequences of Cenozoic
ocean sediments having a temporal resolution of one sample per 103IO4 years, depending on sedimentation rate2. (2) High-resolution climatic proxy data obtained from deep-sea cores have led to the general acceptance of the Milankovitch theory of
Fig. I. (a) The benthic foraminifer Tubulogenerina butonensis from the late Miocene of Enewetak Atoll, Equatorial Pacific16;side view. (b) The marine ostracode Puriana mesacostalis from the earlyPleistocene Waccamaw Formation of South Carolina2’; left lateral view of female carapace. (c) Abapertural and (d) apertural views of the planktic foraminifer Cloborotalia truncatulinoides from the Pliocene of the southwest Pacific Ocean, DSDP core 590-A.
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