In the last two decades of the 20th century up through 2006, scientists observed a reduced growth rate of atmospheric methane levels. While two papers released in the journal Nature are divided on the cause, both teams agree that human activities have lead to slower growth of atmospheric methane.
BBC reports one team of researchers from University of California, Irvine believe increased use of natural gas and more efficient use of fossil fuels have lead to a decrease in methane:
"Methane became economically valuable only during the second half of the 20th Century," said Dr Murat Aydin from the University of California, Irvine. "We think this had a role in it. We're not suggesting we used less fossil fuel, but because we were more careful about capturing the natural gas and using it as an energy resource, emissions of these gases into the atmosphere declined at the end of the 20th Century."
A different team of researchers from UC-Irvine came up with a different conclusion by linking a switch to artificial over organic fertilizers:
Traditionally rice farmers have used organic manure which contains high levels of methane. By using artificial fertilisers, the farmers have considerably reduced this amount.
"Approximately half of the decrease in methane can be explained by reduced emissions from rice agriculture in Asia associated with increases in fertiliser application and reductions in water use," says the lead author Dr Fuu Ming Kai.
Still, this second team did not find efficiency related to a decline in methane growth rate. Methane expert Paul Fraser from the Centre for Australian Weather and Climate Research -- who released a paper in 2008 stating that after eight years of near zero growth in methane emissions, levels were on the rise again -- was quoted by BBC:
"I think both studies are actually suggesting that human activities are playing a very important role in determining the methane levels in the atmosphere," he explained.
Methane, a greenhouse gas, traps more than 20 times more atmospheric heat than carbon dioxide and ABC Science reports that 60 percent of atmospheric methane is caused by human activities, which include farming and burning of fossil fuels. According to EPA, natural sources of methane include wetlands, oceans and forest fires.
We recently reported on skepticism among climate science as a leading scientist is being investigated for scientific misconduct and NASA data has revealed that the Earth is warming slower than what climate forecasts expected. With this division between causes of a decreased growth rate in atmospheric methane, BBC writes that according to Murat Aydin, both these papers suggest human activity plays a role in methane levels and it would be a mistake to add uncertainty of these papers to skepticism:
"As we use more and more fossil fuels, you can be sure it will start creeping up again slowly, I think it demonstrates pretty clearly that human activities have direct and pretty profound impacts on the levels of these gases in the atmosphere."