Past And Present Trends

While the budgets discussed so far represent current conditions, we need to know how these emissions have changed over the years before we can match the observed trends of concentrations shown in Figure 1 and represented in Eq. (1). Estimating past emissions, or a time series for each of these sources, is even more difficult than estimating current emission rates. The simplest approach is to assume that the current estimate of the total anthropogenic emissions is proportional to the human population, and the natural emissions have remained the same over the last 100 to 200 years. With these assumptions we can generate the emissions in Eq. (1) for the last century and calculate the expected concentrations. Although these assumptions are rough approximations, the results of this calculation explain the data quite well (Khalil and Rasmussen, 1994).

The assumption that anthropogenic emissions are proportional to human population breaks down in the recent decades. The atmospheric concentrations are not increasing as rapidly as would be expected if the anthropogenic emissions kept pace with the rising population. When we look at the data on the anthropogenic sources such as cattle populations or the area of rice fields, we find that these are not increasing any longer or are increasing very slowly. In the past these sources had been increasing at a rate proportional to human population. It seems then that there is a decoupling of the anthropogenic emissions from the human population. This circumstance makes the use of potential growth of human population an untenable surrogate for future emissions, even though it works well for the past.

An alternate and more detailed approach is to use the available agricultural and energy data to estimate how these emissions may have changed. Fortunately there are good records, going back a hundred years, on the number of cattle in the world and the hectares of rice harvested each year. Similar, but possibly less accurate estimates can also be made for the other anthropogenic sources based on archived records. We estimated the global emissions from the major sources over the last 100 years and calculated the expected concentrations using Eq. (1). The results are shown in Figure 6.

These results show that the available data for the calculation of global emissions over the last 100 years are in fact consistent with the observed concentrations shown in Figure 1. The long-term trends are driven by increases in rice agriculture and domestic cattle and collectively by the other anthropogenic sources. These same sources that led to the major increases of concentration over the last century are now stabilizing and causing the decreasing trends, at least in this model, and a more



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Figure 7 Comparison of trends calculated from measured methane concentrations vs. modeled methane concentrations.

the Ox molecules discussed earlier into (): and 0{1D), thus increasing the production of Ol I (Madronich and Granier. 1992). The increased OH would cause the trend of methane to slow down. Such a mechanism may contribute to the slowdown of the trend, but there is no experimental evidence that can pin down the magnitude of the OH trend. The increase of OH. if it is occurring, appears to be small and not sufficient to explain the entire observed slowdown (Krol et ul, 1998). It w ill take more w ork before we can say how much of the current trend is affected by possible changes of OH and how much is from the slowdown of emissions. For the present, however, it seems that the slowdown of emissions can be estimated, and these estimates of changing emissions are sufficient to explain the general patient of the observations as shown in Figure 1.

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