What happens when the entire Earth breathes more deeply?

Elizabeth Harrison

(UM News Service) A new study led by University of Montana researcher Zhihua Liu has found global warming is making the yearly rise and fall of carbon dioxide levels in the atmosphere more extreme, particularly in northern high latitudes.

Liu is part of UM’s Numerical Terradynamic Research Group, a leader in developing technology for satellite remote sensing and Earth system science. The new research, published recently in Nature Reviews Earth & Environment, highlights the critical impact of climate change on carbon dynamics and greenhouse gas emissions.

Scientists know the Earth breathes with the seasons. In spring and summer as temperatures warm, plants grow faster, leading northern ecosystems to pull – or breathe in – more carbon from the air. In fall and winter, plant growth slows, and northern ecosystems breathe back out. A warming climate means both the intake and loss of carbon accelerate.

Liu’s co-authors on the paper include UM researchers Ashley P. Ballantyne and John S. Kimball. They reveal the seasonal amplitude of atmospheric CO₂ in arctic and boreal terrestrial ecosystems (located between 50°N and 65°N latitude, bordered by grasslands or temperate forest to the south and arctic tundra to the north) has increased by 50% since the 1960s.

This rise is primarily driven by enhanced vegetation productivity, spurred by warmer temperatures and increased CO₂ levels. As vegetation flourishes in the spring and summer, the reduction of CO₂ deepens, followed by a rise in CO₂ emissions due to ecosystem respiration in the fall and winter.

“Warming temperatures are extending the growing season, allowing for greater photosynthesis and CO₂ drawdown,” Liu said. “However, this increased productivity is also accompanied by heightened respiration rates, which are offsetting some of the carbon sink potential in these regions.”

Scientists have worried for decades that a warming climate could trigger more respiration than plant growth, leading to even faster warming.

Ballantyne, an associate professor of bioclimatology, emphasized the collaborative nature of this research.

He said this study is the culmination of 15 years of planning and executing the Arctic Boreal Vulnerability Experiment (ABoVE). “It’s a Herculean lift by Dr. Liu and the team, synthesizing a vast amount of research into a comprehensive review,” he said.

Ballantyne also highlighted the importance of this synthesis.

“We’ve confirmed that more active plant growth is the main reason carbon is cycling faster in northern regions,” he said. “Scientists have noticed this trend for a while, but by combining data from multiple studies, we were able to better understand the specific processes driving this change.”

The research was conducted under the ABoVE mission, a NASA-sponsored international collaboration that is now transitioning into its synthesis phase.

The NTSG research center, located within the University’s W.A. Franke College of Forestry & Conservation, pioneers new approaches for landscape ecological and hydrological studies. NTSG is directed by Kimball, a UM professor of systems ecology.

Kimball said a major concern is respiration will begin to outpace productivity, leading to more greenhouse gas emissions from northern ecosystems that could further amplify global warming.

“The current generation of earth system models is highly sophisticated but fails to accurately represent the increasing CO₂ seasonal amplitude in northern high latitudes,” Kimball said. “This can lead to uncertainty regarding the role of northern ecosystems in global climate and future climate projections. Our study points to the need to better represent permafrost and other critical aspects of northern ecosystems in these models to better understand and predict future ecosystem and climate conditions.”