Ecologist advances understanding of the role of greenhouse gases

The paper, authored by a Montana State University ecologist and nearly two dozen other scientists around the world, takes a closer look at the role of greenhouse gases coming from land ecosystems by including methane and nitrous oxide emissions in addition to carbon dioxide simultaneously. Methane and nitrous oxide gases have the ability to trap more heat than carbon dioxide, but they make up a smaller portion of the atmosphere.

When observations and models only take into consideration how much carbon dioxide plants and other biological activity pull out of the atmosphere the results can look promising -- with the land actually absorbing the equivalent of about 25 percent of carbon dioxide from fossil fuel emissions and partially slowing down climate change.

"But we found that when you include the other two main greenhouse gases -- methane and nitrous oxide -- this completely changes the role of the land in that instead of having a cooling effect on the climate, it has a net warming effect," said Benjamin Poulter, an assistant professor with a dual appointment in MSU's Department of Ecology in the College of Letters and Science and the Montana Institute on Ecosystems.

Poulter is a co-author of the paper "The Terrestrial Biosphere as a Net Source of Greenhouse Gases to the Atmosphere," which was published March 10, in Nature, an international scientific journal that publishes peer-reviewed research in the life sciences.

Poulter said the study emphasizes that a multi-greenhouse gas perspective is critical to understanding the role of the land in regulating the earth's atmospheric chemistry and climate.

"This paper is important because it emphasizes that a full greenhouse gas perspective is needed when we talk about the role of the land in helping to offset greenhouse gases that are causing climate change," Poulter said. "It's thinking beyond carbon dioxide as the main greenhouse gas and opening up new options where methane and nitrous oxide reductions could be achieved."

The study identifies Asia as a hot spot for methane and nitrous oxide emissions -- methane being the result of rice agriculture and nitrous oxide resulting from over-applications of synthetic nitrogen fertilizers -- leading the researchers to point to improved land management practices as the quickest way to counteract the effect of the gases.

"A large part of the greenhouse gas emissions are a direct consequence of how the land is being managed," Poulter said. "This indicates to us that if we change our land management practices in agriculture, we can perhaps rapidly reduce methane and nitrous oxide concentrations and maintain the net cooling effect of the land from carbon dioxide uptake. Land management practices contributing to methane and nitrous oxide can possibly be changed more quickly than changing our energy production sector and are thus critical to address in the short term."

Poulter said that by using a "top down" atmospheric approach and a "bottom up" land approach, the team was able to reduce the uncertainties of the carbon budget on a global scale and come up with the best estimate to date on the greenhouse warming potential of the land, opening the door for further research.

"The papers present particularly novel and exciting findings at the global scale, and also identifies a lot of uncertainties and research gaps, which can hopefully be used to improve research in these particular areas and to emphasize what kinds of data and modeling approaches are needed in the future," Poulter said. "In terms of policy, the Paris Agreement coordinated by the United Nations Framework Convention on Climate Change highlighted the role of managing natural sources and sinks of greenhouse gases to address climate change, and this paper should help stakeholders appreciate a multi-gas approach when thinking about the role of the land in the earth system."

Another paper Poulter co-authored that focuses on Asia's contribution to the global carbon budget was recently published in Nature Communications, a scientific journal affiliated with Nature that covers all topics in physics, chemistry, earth sciences and biology.

Poulter said the paper, "Top-down Assessment of the Asian Carbon Budget since the Mid-1990s," provides the most recent perspective of carbon dioxide emissions and uptake from Asia, the location of four of the world's 10 largest national emitters of the gas.

"We find that over time there has been a lot of changes in land management, particularly in China where they've regrown a lot of forests," Poulter said. "The forests of China are acting as a land sink for carbon, which is good news because it's mitigating the rise of carbon dioxide concentrations in the atmosphere. But, over Southeast Asia, there's still a lot of deforestation taking place which is causing Southeast Asia to be a source of carbon dioxide to the atmosphere."

Poulter explained that unlike the study published in Nature, the research published in Nature Communications did not include a look at methane and nitrous oxide and the role they play in the climate.

"While this Nature Communications paper provides a reduced uncertainty estimate of the Asian carbon budget in a time of rapid economic change it does not address changes in nitrous oxide and methane emissions," he said. "And as we have shown, when you include those gases, it reverses the picture of what the land is doing from a climate perspective."