Over the past century, we've witnessed a rise in the greenhouse gas methane in our atmosphere. But for a few years at the turn of the millennium, methane levels tapered off and reached a stable state before starting to climb again. If we could just figure out what caused this slowdown, and replicate it, we might be able to alter the course of climate change.
Now two groups of researchers have gathered new data and analyzed the methane slowdown. And their results suggest it may be within our power to bring methane levels down again — and soon.
Published today in Nature, the two groups' research papers used climate data sets that were gathered independently, and they reach different (if complementary) conclusions.
UC Irvine Earth scientist Murat Aydin and colleagues believe that a decline in fossil fuel emissions led to the methane slowdown. They measured levels of greenhouse gas ethane, trapped in thick Antarctic ice, to determine whether fossil fuel emissions fluctuated as natural gas and green energy became more popular in the late twentieth century. Based on their tests, it seems that some of our energy policies may have paid off, at least temporarily. Aydin and colleagues write:
We speculate that the rising economic value of natural gas during the late twentieth century and the development of cleaner technologies led to sharp reductions in the release of light hydrocarbons into the atmosphere. Emissions linked specifically to the growing natural gas industry must have been more than offset by large reductions in the venting of light hydrocarbons, including methane and ethane, associated with production and processing of petroleum.
Fossil fuel emissions vary greatly over geographical regions, and the researchers offer this caveat:
Given that we do not have comparable observations from lower latitudes, it is possible that a shift in the location of fossil-fuel emissions towards lower latitudes within the Northern Hemisphere contributed to the observed ethane decline during 1980–2000. However, we estimate this effect to be relatively small . . .
So more data needs to be gathered, to determine whether these lowered emissions can be found throughout the globe.
Meanwhile, Aydin's colleague in the Earth sciences department at UC Irvine, Fuu Ming Kai, has determined that the methane slowdown can also be traced to changes in rice agriculture throughout Asia. Remember, greenhouse gases don't just come from driving trucks. They also come from factory farming. Though rice production has remained stable for several decades, farming techniques have changed dramatically. This could lead to reduced methane. Kai and colleagues write:
Accumulating evidence suggests, however, that application of chemical fertilizer and more efficient water use substantially reduces CH4 [methane] emissions per unit of area of rice production. CH4 fluxes in rice fields are strongly regulated by levels of organic fertilizer input. Chemical fertilizer application in China and India increased substantially from 1970 to 2005 . . . Other studies have shown that changing the water management of rice paddies can reduce CH4 emissions by about 10–80%. At the country level, increasing urban and industrial demands for water have increased since 1980 and have limited the water available for agriculture.
New fertilizers and reduced water use may actually have changed the composition of our atmosphere for the better for those years at the turn of the millennium.
The problem is that whatever trend these researchers tracked seems to be over. Methane levels are slowly rising again. Is it possible that we could reproduce the agricultural and fossil fuel use conditions that produced the slowdown? Now that would be a global scientific experiment I'd like to see.