Humans are making global warming worse, all right — but in more ways than you think.
The research, which was led by Hanqin Tian of Auburn University with a long list of 22 co-authors, points the finger at phenomena like 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.
And 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 much more. 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 vs. how much it is releasing. And 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 3.8 and 5.4 billion tons 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.
The picture would be a lot worse were it not for the fact that, at least for now, the research calculates that the biosphere is still pulling in more carbon dioxide than it is releasing. However, there are fears that, due to factors like worsening wildfires or carbon dioxide emissions from Arctic permafrost, this could change in the future, causing even more global warming gases to flow from the land to the atmosphere.
Surprisingly, the research finds that when it comes to these human-driven methane and nitrous oxide emissions, a key driver is Southern Asia, and rice cultivation in particular.
“Southern Asia has about 90% of the global rice fields and represents more than 60% of the world’s nitrogen fertilizer consumption, with 64%–81% of CH4 emissions and 36%–52% 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.”
Rice fields emit methane because microorganisms in these wetlands are simply very good producers of the gas, when the field is under water.
One caveat to this analysis is that it depends on converting three very different gases — carbon dioxide, methane and nitrous oxide — into the same units based on their “global warming potential” over 100 years. However, methane only lives in the atmosphere for 12 years. Nitrous oxide lives there for 114 years, and as for carbon dioxide, some of it lasts thousands of years.
So if we’re worried about warming in 2016 or 2017, or even 2050, all of these gases are a very big deal. But if we’re worried about warming over the very long term, of the sort that causes seas to rise and ice sheets to melt, carbon dioxide surges in importance.
Nonetheless, for this century, the results are a very big deal – and point to ways that we can weaken global warming without even targeting carbon dioxide.
“A lot of these additional emissions are ones that can be managed if we alter the ways in which we do things like agriculture,” Michalak said.
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