Methane from the House of Tudor and the Ming Dynasty: Anthropogenic emissions in the sixteenth century

Chemosphere. VoL 29. No.5 . pp. 8-t3-854. 1994

Elsevter ScienceLtd Pnnted tn Great Britain 00..t5-6535/q4 $7 00-~.0.00

Pergamon 0045-6535(94)00222-3

M E T H A N E F R O M T H E H O U S E O F T U D O R AND T H E M I N G DYNASTY: A N T H R O P O G E N I C E M I S S I O N S IN T H E S I X T E E N T H C E N T U R Y

S. Subak

Centre for Social and Economic Research on the Global Environment University of East Anglia Norwich, Norfolk NR4 7TJ United Kingdom

ABSTRACT This paper presents an estimate of global anthropogenic methane (CH~) emissions during the early modern, pre-industrial, period based on the historical record and current estimates of CII4 emission factors. Methane emissions are estimated for biomass burning, enteric fermentation and irrigated agriculture. It is concluded that anthropogenic C1-14 emissions were likely to have been at least 55 Tg/CHJyr. in 1500 and preceding centuries, contributing to a total annual emissions flux of at least 210 Tg CH 4. At least half of the anthropogenic emissions were likely to have been due to biomass bunting, with the remaining from irrigated agriculture and animals. These conclusions suggest that the concentration of OH in the atmosphere may have been greater than today, given that recent deconvolution analysis indicates that a total source strength of only 170 Tg CH,~ a year during the 16th century is consistent with total estimated CH, sinks and natural sources at 20th century levels.

1. INTRODUCTION While the human population of five hundred years ago was burning very little fossil-fuel -- the main source of the greenhouse gas, carbon dioxide (COs) -- many of the people alive throughout the world during the 16th century were generating greater levels of greenhouse gases in the form of methane than do many of us today. During the reign of the English king, Henry VII (1485-1509). the average global per capita emissions of methane was likely to have been at least double the current level of 60 kg/CH,/yr.

While several CH,,

sources are chiefly 20th century phenomenon -- fossil fuel combustion, coal mines, natural gas systems and landfills -- the remaining anthropogenic sources were linked to activities vital to pie-industrial economies. The sources include livestock and irrigated agriculture, including rice production. Grassland burning for hunting and agriculture and to maintain diverse plant habitats, was likely to have been practised on a much greater scale than today (Pyne, 1994; Pyne, 1991; Anderson, 1994; Lewis, 1994).

Most of our understanding of global pre-industrial CIt,, emissions has been based on analyses of atmospheric CH, concentrations taken from ice core samples. While the resuhs vary somewhat among the 843

844

cores sampled, the records suggest fairly stable atmospheric CH, concentrations in the millennium 800-1800, usually varying only between 650 and 750 ppbv (Rasmussen and Khalil, 1984), compared with concentrations rising sharply during the last two hundred years to the current level of 18(]0 ppbv, as indicated in Figure 1 (Khalil and Rasmussen, 1990). Deconvoluting the source function based on observed CH+ concentrations and alternative assumptions of the known sinks, has provided estimates of pre-industrial emissions. This approach suggests that emissions in the 16th century were 170 Tg/yr, assuming OH concentrations -- the major removal process for methane -- were at current levels (Khalil and Rasmussen, 1994).

Alternatively, if OH

concentrations were 25 percent greater in the 16th century, annual emissions would have been about 200 Tg (Khalil and Rasmussen, 1994).

tO

--"-- Population CH4(I) --'-CH4(2)

5000 O

•~ 4000

~ 30oo

+

CH4 (3)

!,°

"---~--

CH4 (4)

0

/ /

/ / /

.

m ~ m _

t

t

!

1

t

I

I

t'

!

I

g

Fig.l. Methane concentrations and human population 800-2000 A.D. Concentrations from ice cores: (1) Camp Century 1963-66, (2) Byrd 1968, (3) Byrd 1971, as reported in Rasmussen and Khalil (1984), (4) Khalil and Rasmussen, 1990.

In contrast to the "top-down" deconvolution analysis, a range of emissions estimates is proposed based on historical and archaeological evidence, and current understanding ot" the CH+ emissions flux from different sources. An illustrative, continent and source-specific inventory of CH+ emissions during the early modern, but pre-industrial period is reconstructed. A theoretical emissions inventory for 1500 suggests that annual emissions were not likely to have been as low as 170 Tg/year. Global emissions were probably at least 200 Tg/yr., a level consistent with the view that OH concentrations were significantly greater in the pre-industrial atmosphere.

This "bottom-up" source-specific inventory can eventually be compazcd with future isotopic signature analysis of CH, concentrations in ice cores attempting to constrain the differe,t methane source strengths. In

845

addition, consideration of pre-industrial emissions can help outline the magnitude of current areas of carbon sink potential based on past environmental transformations and identify the difference in regions' opportunities for reducing CO: emissions through the land use sector.

The 16th century was selected because it predates the noted rise in CH4 concentrations of recent centuries, as shown in Figure 1, and this choice allows us to take advantage of the environmental histories that have been written of land use and agricultural practices upon the advent of the Europeans' arrival in the Americas.

Nonetheless, little distinguishes 1500 from 1400, 1300, and 12130 in the historical and

anthropological record used in this analysis, except for the difference in human population, which was likely to have been about 20 percent greater in 1500 than in the earlier period (McEvedy and Jones, 1978).

The climate of Europe was actually highly variable between the 13th and 16th centuries. Evidence from tree rings suggests that the 14th through 16th centuries encompassed a warm period in Scandinavia (Briffa et al., 1992). On the other hand, temperature in East China points towards cooler conditions than the long-run

average (Wang and Wang, 1991). The likely consequences for anthropogenic CH4 emissions were likely to have been slight in that even a 30 percent increase in fuel wood consumption in northern latitudes would not significantly affect the global emissions estimate proposed here.

2. METHANE INVENTORY 2.1. Enteric Fermentation Numerous environmental scholars and historians analyzing the scale of pre-industrial biomass burning and agricultural practices have concluded that specific quantification must be based largely on guesswork (e.g. Hall, et al., 1994; Pyne, 1994).

This paper does not seek to derive likely ranges in pre-industrial CH4

emissions, but to review aspects of the historical and archaeological record to estinaate a minimum level of anthropogenic emissions. Ruminants, particularly cattle, release significant amounts of methane through the digestive process known as enteric fermentation.

Other large mammals such as horses emit lesser amounts of methane.

Emissions increase with the amount of feed an animal eats; and animals eating lower quality feed emit proportionately more methane. The level of meat consumption in Europe during the early 16th century was greater than in the subsequent several centuries, and Europeans likely raised tens of millions of animals during this period (Braudel, 1973). For this inventory, estimates of domestic oxen and horse populations in Europe during the 17th century are scaled to estimates of the 16th century human population, and dairy cow populations are assumed to be comparable to oxen. Estimates for animal and human populations appear in Table 1. It is likely that 16th century Europeans fed their livestock somewhat I,. s than do today's commercial producers, and that the digestibility of the feed would also have been lower, iheSe conditions more closely

846

resemble feeding regimes in today's developing, rather than developed, countries. Therefore, current emission factors for developing countries are applied to estimates of past animal populations. Emissions factors of 35 kg,,'head/yr, for cattle and oxen and 10 kg/head/yr, for horses by Crutzen et al. (1986) were used. The result is a total of at least I Tg CH,/yr. in Europe.

Emissions in Asia were likely to have been greater given that the continent was at least four times more populous, and Asians commonly bred ungulates (Grzimek, 1989). Cattle and buffalo were prevalent in India during this period (Braudel, 1973). The Chinese kept very few cattle, but they raised buffalo and -- in contrast to the sub-continent-- horses for transport and farming (Braudel, 1973). Camels were widely used throughout Asia Minor, India, North Africa, and the Balkans (Braudel, 1973). For this inventory, camel populations in Asia were assumed to be the same as today's. Cattle population was scaled to India's estimated human population of the 16th century, buffalo population was scaled for the estimated population for all of Asia during that period. A horse population numbering 3 million in China was used based on Braudel's citation (1973) of Father de Magaillans in 1678.

In North America, the most numerous and the largest ruminants were the bison. They thrived in North America largely because they :ire migrating animals that can adapt to the frequent grassland burning set off by aboriginal human fire practices. One estimate puts the North American bison population at 60 million at the time of the first European exploration of the Great Plains (Grzimek, 1989). In comparison, current cattle and dairy cow population in North America is about 110 million (FAO, 1990). Assuming that emissions per bison were simil.'lr to today's emissions factors for water buffalo, emissions from bison alone were as much as 3 Tg/yr., nearly half as great as today's emissions from North American livestock.

In respect to migrating :mim:d populations, the continents of Africa and North America were the most comparable during this period, with large expanses of grasslands maintained by periodic burning, sustaining large populations of migrating animals. The density of large mammals in East and Southern Africa was likely to have been greater than the animal density of the Great Plains. A survey of ungulate populations in the Bison Range National Reserve in Wyoming revealed that in the early 1960s, the Reserve sustained 3.4 tonnes of ungulates per km2. compared with densities ranging from 4.4 T/kina to 23.6 T/kin2 in Zimbabwe, Tanzania, and Kenya (Bouliere, 1963). For this inventory, animal densities for significant CH,rproducing animals were selected from the low end of aerial surveys of animal densities in East African parks (Bouliere, 1963) applied to the current areas of Kenya, T:mzania, Uganda, Zambia, and Zimbabwe. Densities of one half of the above were assumed for East Zaire, imd one quarter densities for Angola, Botswana, Mozambique, Namibia, and South Africa. Camel and cattle population were based on per capita animal populations scaled to estimated human population of the 16th century. These estimates are summarized in Table 2. The total for Africa, 3 MT CHJyr., is about one third of estimated emissions from current African livestock populations (Subak et aL, 1993).

847

Table I: Human and Animal Population Estimates in 1500 North America 4'

Human

1

[ S & C. [ Am. 55'

I

Europe

Africa

81 *

46~,

Asia 280~ (105 India)b

Population (mil.) Animal Population (rail.)

7 horsesd

60 ¢

bison 12 oxend 12 cattle

$ elephants ¢, 52 wildebeestsf 38 buffalor, 2 giraffes r 1 camels, 18 cattle

25 cattle 13 buffalo 3 horses', 4 camels~

'Denevan, 1992; bMcEvedy and Jones, 1978; ~Grzimek, 1989; "Braudel, 1982; 'Braudel, 1973; fderived from Bouliere, 1963; "FAO, 1990.

Table 2: Methane Emissions in Africa in 1500 Animal Density' (T/kin2)

Animal Weightb ('13

Emission Factors"

Emissions

(kg CHJhead)

(Tg CH,/yr.)

Elephants

4.5

3.0

26

0.2

Wildebeest

2.0

0.2

13

0.7

Buffalo

3.6

0.5

34

1.3

Giraffes

0.6

1.2

50

0.1

Cattle

35

0.6

Camels

59

0.1

Total

3.0

'Various surveys inch,ded in Bouliere, 1973; bBouliere, 1973; 'Crutzen et al., 1986.

2.2. Irrigated Agriculture Amerindi:ms had experience both in cultivating the edges of natural wetlands and in transforming shallow lakes and seasonal wetlands (Whitmore and Turner, 1992; Denevan, 1970). The total area of wetland agriculture has not been quantificd but flooded fields in just one region, the San Jorge River basin of Colombia, may have covered 5(X),O00 ha, more than double the current area of wet rice cultivation in the entire country (Plazas and Falchetti, 1988; FAO, 1090). Wetlands were mix.lifted for cultivat;on in the Andes, the lowlands of South America, Mesoamerica and the Mississippi River basin (Plazas and Falch('tti, 1988). Givcn the extent of wetland conversion, historical anthropogenic emissions sites may currently be counted natural emissions sources. For this inventory, a level of 3 Tg/CHJyr. is comparable to today's CH4 emissions from rice paddies in Latin America is used (Subak et al., 1993). Arguably, this estimate may be conservative in light of the vast

848

area covered by wetl:md agricuhure, and because per area emissions of methane from raised field agriculture in some regions may have been higher than emissions from rice paddies (Barber et al., 1988; Schuetz et al., 1989).

Rice made up a greater proportion of the Asian diet in the pre-industrial period in China than it does today, comprising more than 90 percent of the caloric intake of a typical peasant (Braudel, 1982). The area required may have been greater, because pre-industrial cultivation may have been less productive on an area basis. Double cropping was practised throughout most of China as it is today and rice paddies, rather than dry cultivation, predominated (Braudel, 1973). An average growing season of 136 days, based on China's current crop calendar was used (Matthews et al., 1991), and an emission factor of 0.58 g/mS/day (Schuetz et al., 1989). Current per capita CH, emissions from rice paddies, scaled to estimated Asian population in 1500, would yield emissions of about 12 Tg/CHJyr.

2.3. Fuelwood Consumption Because pre-industrial household use of fuelwood was basically fulfilling the same energy and heat requirements as contemporary households that rely on biomass for energy, domestic consumption of biomass may have been stable over the past millennium at 1 tonne/person each year (Hall and Rosillo-Calle, 1991). Applying a fuelwood-specific emission factor for methane of 7.6 g CHJkg fuel (Hao and Ward, 1993), to this per capita consumption rate, results in a global total of 3 Tg CHatyr. Biomass for mercantile and industrial activities and for construction that place wood consumption are estimated to be about one quarter as great as domestic consumption (Kammen and Marino, 1993). Another possible source -- wood used for construction - would have released relatively less methane because much of the construction material would have been left to decay, rather than burned. Therefore, a total of 4 Tg/yr., comprised of 3 Tg from domestic and 1 Tg from industrial sources, is used.

2.4. Other Biomass Burning If grassland and savanna burning was as extensive as environmental historians suggest, these activities are likely to have been the single largest contributor to pre-industrial greenhouse gas emissions. Much of the large expanses of grassland in North America and Africa were maintained by i~criodic fires initiated by humans for hunting and the size of the broadcast fires were often uncontrolled. Archaeological evidence of high levels of pre-industrial burning has been found in deposits of charcoal in sediments and ice fields in Mesoamerica (Suman, 1991), and in the ecology of remnant prairie ecosystems of North America, which were abundant in pyrophilic plants that need periodic fires to germinate and control invading species.

For this inventory, an estimate of CH,~ emissions at a minimum of two thirds of current estimated emissions from biomass burning was used. This is based on the assumption that grassland burning exceeded current levels, wildfires were comparable, and burning for shifting cultivation was less extensive. Because the

849

emissions factors for grassland fires are Iov,er than fi~r fi)rests, due to higher combustion efficiency, overall emissions from grasslands will be less than would result from burning a comparable amount of fuel from forests.

An estimate of biomass burning in 1500 at five times current area has been proposed (Pyne, 1994).

If we assume that grassland burning covered five times the area of today's savanna fires (5 x 2.5 Pg fuel; Hao and Ward, 1993), and apply methane emission factors specific to tropical savanna fires (1.5 g/kg fuel carbon; Hao and Ward, 1993), CH4 emissions from grassland 'fires would have been about 19 Tg/yr. This figure compares with recent estimates of current CH4 emissions from grassland burning: Andreae (1991) calculates grassland emissions to be 17.3 Tg/CHJyr.; Hao and Ward's (1993) estimate of 4.0 Tg/CH,/yr. is based on lower emission factors. Current CH4 emissions from wildfires is estimated to be about 2 Tg/yr (Hao and Ward, 1993). Kammen and Mari,o (1993) have proposed that about 5 Tg CH2yr. arose from agriculitiral activities, chiefly slash-and-burn agriculture, in 1500.

3. INVENTORY RESULTS

Conservative assumptions about agricultural and biomass burning sources assessed here, suggest that non-natural emissions were at least 55 Tg annually during the early modem, pre-industrial period, as summarized in Table 3.

Because of the importance of grassland burning and large ruminants during this

period, the greatest emissions may have originated in the less densely settled regions of North America and Africa. Because of the absence of significant grassland burning and irrigated agriculture, per capita CH4 emissions, excluding wetlands and other natural sources, were likely to have been lower in Europe than in the other regions.

Anthropogenic emissions are estimated to have been at least 55 Tg, about 15 percent of

estimated current emissions (IPCC, 1992).

Table 3: Anthropogenic Methane Emissions by Source and Region in 1500 (Tg CHa'yr.)

Sources

Total

Animals

10

Irrigated and Wetland Agriculture

15

Biomass Burning

26

Woodfuel Consumption Total

North Amer.

I

JS &C J Amer.

4

1

3

10

2

1

9

1

14

5

J Asia 2

12

3

4 55

Europe [ Africa

3

A comparison of emissions estimates in 1500 and 1990 appears in Table 4.

4 3

12

21

Anthropogenic Ctt,,

emissions are estimated to have comprised about one quarter of total emissions, compared with a fraction of two thirds of total emissions today.

The fraction of total emissions originating from bacterial sources

850

(livestock, wetl;mds, irrigated agriculture, landfills) is estimated to have been 76 percent compared with 69 percent in 1990, as shown in Table 5. Table 4: Global Methane Emissions, 1500 and 199~ (Tg CH,,/yr.) Sources

1990 (IPCC, 1992)

(This Study)

Enteric Fermentation

I0

80

Irrigated and Wetland Agriculture

15

60

Biomass Burning

26

40

Woodfuel Consumption Other

180 155'

155

210

515 (332-857)

Natural Total

"Wetlands, 115; termites, 20; ocean, 10; freshwater, 5; CI-I,) hydrate 5 (IPCC, 1992).

Table 5: Emissions Sources by Type (%) r,.

,

Sources

1500

1990

Anthropogenic

26

65

Non-anthropogenic

74

35

Bacterial

76

69

Non-Bacterial

24

31

,,

.,

4. DISCUSSION

Biomass burning that took place during the 16th century, as well as during subsequent and previous centuries, helped to prevent grassland from reverting to woodlands and forest in many regions. Grasslands and savannas now comprise a significant sink potential on some continents. As some of those areas are now the site of new plantation establishment, late 20th century carbon sequestration projects may represent a return to the pre-anthropogenic or pre-fire biomass density of a region. For example in the United States, 19th and 20th century land use practices resulted in different changes in forest area. Both could be counted as contributions to the maintenance or establishment of carbon sinks. During much of the 20th century, forest growth likely led to a net increase in forest area and biomass density. During the 19th century, however, extensive land clearing resulted in a lower level of biomass density that was subsequently available for lower cost carbon sequestration projections (Rich:trds et al., 1983). Likewise, widespread grassland burning in the pre-industri.'d

851

period, including the 16th century, helped to create a portion of the carbon sink potential present today.

The anthropogenic portion of the CH, emissions estimate derived from this inventory could be variously described as 12 percent or 26 percent of total emissions, depending upon the definition of human activities. Irrigated agriculture, fuelwood consumption, and cattle are clearly anthropogenic sources and total 23 Tg/CH,/yr, in this approach, or 11 percent. If we include the remaining emissions from animals and biomass burning, anthropogenic emissions total 55 Tg/yr., or 26 percent of total emissions. Many of the large ruminants and mammals that dominated the grasslands of North America and Africa in pre-industrial times, the bison and wildebeest, are migrating animals that can survive frequent burning of their habitats. In this respect, humans helped to maintain the anir lal populations by keeping at bay less compatible biomes and comp/:ting species. Under this definition, migrating animals are anthropogenic sources although people used only a small percentage of the meat and skins of these animals. A similar rationale can be applied to the biomass burning source. A relatively small proportion of the area burned may have been used for agricultural purposes relative to today, but tot:d combustion surpassed the area affected by natural causes.

An inclusive definition of net anthropogenic emissions would consider the difference between calculated emissions and emissions as they might have been had humans never proliferated or mastered the use of fire. The question is then, would global CH4 emissions have been lower in a given pre-industrial periM if, for example, North America had remained populated by a different mix of animals, e.g. saber tooth cats, dire wolves and imperial mammoths (Wilson, 1993) rather than the large populations of migrating bison, elk, and moose that adapted well to the broadcast burning initiated by the native Americans. In other instances, the presence of humans may have reduced Ctl4 emissions from some sources. For example, the number of CH4-emitting beaver ponds declined during the last four centuries as a result of trapping in Canada and the United States (Nisbct, 1989).

Currently we are lacking historical data on CH4 isotopic composition that could help to fingerprint CH4 sources.

Both the mean carbon and hydrogen isotope ratios (~5~C-Ctl4 and 5 D-CH,) have been used to

distinguish among different contemporary CH4 sources. Such analysis may be especially useful for estimating the magnitude of biomass burning given that the isotope ratio for this source is so much lower than the estimated ratios for the other pre-industrial sources. Microbial, or bacterial, methane is rich in 12C and high in radiocarbon 14C. For example, the carbon isotope ratio for biomass burning is estimated to be -25 (%0, PDB), compared with -63, -60 and -58 (%0 , PDB) for the bacterial sources of rice paddies, enteric fermentation and natural wetlands, respectively (Whiticar, 1993). Mean values of less than -58 would signify a sizable non-bacterial source for meth:me. Applying the carbon isotope ratios to the pre-industrial budget estimated in this study, mean carbon isotope values would be -55.0, compared wi~h an estimate of -59.7, if we assume that emissions were entirely due to the natural sources listed in Table 5

Mean hydrogen isotopes

would be -327.0 (%o, SMOW) compared with -366.6 (%0, SMOW) from natur tl sources only (derived from

852

IPCC 1992 CH4 budgets from natural sources, and from isotope ratios in Whiticar, 1993).

A recent calculation of CI-t4 emissions during the 16th century suggests that a global total of 170 Tg./CHJyr. would be consistent with observed concentrations, assuming OH concentrations and the CH4 soil sink term were the same as current levels (Khalil and Rasmussen, 1994). Khalil and Rasmussen's (1994) estimate of emissions under alternative OH concentrations as cited here, was actually for the year 1595, but the results are also relevant for the somewhat earlier period considered in this analysis (Khalil, personal communication, 1994). Evidence exists that the CH4 soil sink was similar in pre-industrial times (Chappellaz et al., 1993). Given this proposed global emissions budget, and an anthropogenic contribution of at least 55

Tg/CH,/yr., we must cotaclude that either OH concentrations were significantly greater in the c:arly modern period, or that natural CH4 emissions were at the extreme low end of the current range of emissions estimates. A budget of 120 Tg/CHJyr. from natural sources compares with a current best-guess of 155 Tg/CHdyr, in a 116-300 Tg range (IPCC, 1992). Given that some regions experienced a notable cooling during the 16th century, globally-averaged CH4 emissions from wetlands may have been lower at that time. On the other hand, the total area of wetlands should have been greater in the pre-industrial period because wetlands have been drained in recent centuries to make room for agriculture and habitation. A number of analyses, however, point towards greater levels of OH concentrations in pre-industrial times.

Recent models suggest that OH

concentrations have decreased from pre-industrial levels by between 4 and 60 percent (Thompson, 1992).

5. CONCLUSIONS

If biomass burning, irrigated agriculture and animal husbandry were practised on the scale suggested by social and environmental historians of the early modem period, anthropogenic CH4 emissions were likely to have been at least 55 Tg/yr. in 1500 and preceding centuries, contributing to a total annual flux of at least 210 Tg/CH4. At least half of anthropogenic emissions were likely to have been due to biomass burning. Isotopic carbon ratio analysis indicating mean ratios o f - 5 5 (%0 , PDB) or less could help confirm a contribution of at least 15 percent of pre-industrial CH, concentrations as arising from biomass burning. Total emissions of at least 210 Tg/CH,/yr. suggests that the concentration of OH in the atmosphere would have been greater than today, given that recent deconvolution analysis concludes that a total source strength of only 170 Tg/CHdyr. in the 16th century is consistent with total estimated CH, sinks and natural sources at 20th century levels. If natural sources summed to 155 Tg during this period, the remainder from 170 leaves the improbably low figure of 15 Tg/CH, for anthropogenic emissions. In contrast, an anthropogcnic CH4 source strength of at least 55 Tg/yr during the 16th century is more supportable from available literature in the social sciences. This level of annual CH4 emissions would be consistent with a 25 percent increase in OH concentration above current levels, given that the other sinks are unchanged (Khalil and Rasmusser, 1994).

853

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