Climate change’s hidden catalyst

New study points finger at a major, yet often ignored sector: global agriculture

Thestar.com
March 10, 2016
By Chris Mooney

Humans are making global warming worse, all right - but in more ways than you think.

That’s the result of a study published Thursday by the journal Nature, which finds that the Earth’s land “biosphere,” defined as all the plants, animals and micro-organisms living on the surface of the Earth (excluding the oceans), is now a “net source” of greenhouse gases to the atmosphere. Thus, the biosphere is now adding to global warming in much the same way that our fossil fuels are.

The research, led by Hanqin Tian of Auburn University with 22 co-authors, points the finger at phenomena such as animal agriculture, rice cultivation and waste disposal as key features of climate change that don’t get much attention, but that overall are causing the Earth to heat up even more than it would otherwise.

It’s yet another finding that underscores the importance of a sector that has, somewhat surprisingly, largely escaped attention in the climate debate: global agriculture. And the world will need to produce even more food to keep up with growing populations, says Tian.

To understand the new study, you have to consider that greenhouse gases that warm the planet are far more numerous than carbon dioxide, which gets the lion’s share of the attention. There’s also methane, which causes much more warming over a 10-year time horizon (but dissipates far more quickly), and nitrous oxide, to name a few.

These gases pack quite a punch. “The methane global warming potential is 28 times larger than carbon dioxide,” says Tian. “And nitrous oxide is 265 times greater than carbon dioxide, in terms of global warming potential” over 100 years, he added.

“These two gases are really important non-CO2 greenhouse gases,” Tian said.

Methane doesn’t just emerge from leaks from oil and gas operations, it is also belched by ruminants (i.e., cows) and emerges from wetlands, landfills and other sources. Nitrous oxide, meanwhile, is emitted from nitrogen fertilizers used in agriculture, among other sources.

In the new study, the researchers calculated a comprehensive, worldwide inventory of how much methane, nitrous oxide and carbon dioxide the land biosphere is pulling in each year compared to how much it is releasing. They found that while land-based living things are still pulling in more carbon dioxide each year than they themselves are giving off — rendering the biosphere a carbon dioxide “sink” — they are a net source of both methane and nitrous oxide.

Moreover, when you then convert the three gases to a comparable unit based on their potential to warm the planet over a 100-year time frame, the planet’s biosphere works out to be a net source of greenhouse gases, causing a warming comparable to the effect of between 3.4 and 4.9 billion tonnes of carbon dioxide emissions per year. That’s close to what the U.S. itself emits annually.

“Human actions not only are emitting greenhouse gases based on our own activities, but also are causing plants and animals and microbes to be net emitters of greenhouse gases as well,” said Anna Michalak, a co-author of the study with the Carnegie Institution for Science in Stanford, Calif.

This overturns prior assumptions that the Earth’s land is a carbon “sink,” which did not take into account these other greenhouse gases.
Surprisingly, the research finds that when it comes to these human-driven methane and nitrous oxide emissions, a key driver is Southern Asia - the area’s rice cultivation in particular.

“Southern Asia has about 90 per cent of the global rice fields and represents more than 60 per cent of the world’s nitrogen fertilizer consumption, with 64 per cent to 81 per cent of CH4 emissions and 36 per cent to 52 per cent of N2O emissions derived from the agriculture and waste sectors,” the study says.

“Given the large footprint of agriculture in Southern Asia, improved fertilizer use efficiency, rice management and animal diets could substantially reduce global agricultural N2O and CH4 emissions,” the study adds.

Rice fields emit methane because micro-organisms in these wetlands are very good producers of the gas when the fields are under water.