It seems that even before industrialization humans had a significant impact on the Earth's atmosphere.
In a breakthrough study, US, Australian and New Zealand scientists have been able to `fingerprint' the sources of methane pollution with unprecedented accuracy, and been astonished at the results.
"Methane is the foul smelling gas you get from farm animals, rotting vegetation and coal," says Dominic Ferretti, at New Zealand's National Institute of Water and Atmospheric Research (NIWA).
Ferretti said they knew how it makes the world hotter by slowing down the radiation of heat back into space but they couldn't work out what proportions of the build-up came from natural, man-caused or industrial sources. Forest fires generate methane, as do bogs and fossil fuel burning and, of course, so do flatulent farm animals.
"Everyone has heard about how farting cows are endangering the planet and they do emit a significant part of the methane that goes into the air today, but our team was surprised by what the study told us about the history of methane emissions over the past 2,000 years," he said.
Using laboratories in each country, the project related the quantity and origin of concentrations of methane since the year 1 AD to the present day by analyzing tiny bubbles of air trapped in polar ice cores and samples collected from a range of atmospheric research stations in modern times.
Although concentrations of methane may have flat-lined before the industrial revolution the research revealed high levels of wildfire burning during hotter, drier climatic periods and higher inputs from the expansion of wetlands when it was cooler.
"They nearly cancel each other out. When one was up the other was down. And we think we see clear signs of human impact in this long before industrialization started the large scale burning of coal and gas," Ferretti says.
Burning of grasslands
Then, between 1500 and 1700, the proportion of methane found to have come from the burning of grasslands and forests declined sharply. Before this the farming methods of Native Americans included the burning of land. But between 1500-1600 the fires went out, a sure reflection of the gloomy statistic that tells of the huge number of people from among the native populations who died of diseases brought by European explorers and against which they had acquired no immunity.
"It is sobering to think that this terrible episode left its traces in the pattern of methane deposition in the polar ice, yet it also tells us that the large scale clearing of land by humans down the centuries since civilization emerged out of the last Ice Age may have had an impact on the atmosphere and the climate comparable in some ways to our industrial activities," Ferretti says.
Also, the levels of methane emitted because of large scale fires like those that ravage the rainforests of Indonesia each year peaked at the today's levels, 1,000 years ago.
Ferretti says other research is underway into possible evidence of widespread man-caused changes to the atmosphere far earlier than the 2,000-year time interval studied by this project.
"I think we need to look as closely as we can for links between climate change and pre-industrial human activity," he says.
Methane is considerably more effective than carbon dioxide in contributing to global warming, but fortunately, comprises only about 2 parts per million (ppm) of the lower atmosphere compared to more than 375ppm for CO2, where that concentration is rising so fast it is expected to exceed 500 ppm by as soon as 2040 and further lift average temperatures and worsen the impact of climate change in the process.
Ferretti says the research doesn't answer the question as to why methane levels in the atmosphere appeared to have stabilized from the late 1990s.
"We see signals of an upward spike in the early 90s that may be indicative of large leaks from natural gas pipelines, possibly related to the collapse of the Soviet Union. We don't know, although it is important to recognize that massive amounts of methane pollution can occur from badly managed fuel pipelines, coal mines, or other resource infrastructure, especially if it is in a remote location," he says.
"It may be related to better agricultural efficiency, or the biomass may be absorbing methane faster than it returns it to the atmosphere for reasons we haven't recognized," he says.
"This project tells us nothing is simple in atmospheric research, but it doesn't give comfort to anyone who thinks the issue of global warming is exaggerated," he says.
As to flatulent farm animals, Ferretti says he is familiar with current research in New Zealand into reducing the problem.
"It is hard to hazard a guess, but my gut feeling is that with genetic manipulation, we could readily reduce farming sources of methane by at least 10 percent," he says.
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