Researchers from the University of Michigan, led by Peter Reich, institute director of Institute for Global Change Biology there, just last month discovered that global warming can be forcing northern forest soils to release more carbon into the air than they have taken in. This is a bit of a bummer because scientists long assumed that forest soils are among the few remaining elements of forests that can slow climate change.
Northern forests play an important role in regulating the Earth’s climate by storing roughly 40% of the planet’s soil carbon. Soils essentially act as reservoirs, preventing carbon dioxide that trees absorb for photosynthesis from being spewed back into the atmosphere. According to new research led by Reich, however, as global temperatures continue to rise, more carbon is escaping from the soil than it is being added by plant growth. This may also turn the climate crisis to worst by increasing more carbon dioxide in the atmosphere.
“This is not good news because it suggests that, as the world warms, soils are going to give back some of their carbon to the atmosphere,” said Reich.
Unique Long-Term Experiment
In Nature Geoscience, scientists have published the first experiment that studied how soil and above-ground temperatures affect carbon emissions from forest soils. Most of the already published research in the area has been confined, in fact with most focusing on either the air or only soil temperatures and covered periods of only a few years. In contrast, the experiment of Reich was located in northern Minnesota where both soil and air temperature was controlled for over 12 years, so more results regarding the effects of warming over a longer period can be understood.
These experiments have been carried out over 72 plots at two sites in northern Minnesota. Two warming scenarios were created by the researchers: the first one raised the ambient by 1.7°C, or about 3°F, and the other raised the ambient by 3.3°C or about 6°F. The measurements have been compared with unaltered ambient temperatures so that changes can be assessed over time in carbon flux.
The emission of carbon dioxide from the soil was observed to increase by 7% in the modest warming case and 17% more in the more extreme case. Such a process for release of carbon dioxide from the soil is termed as soil respiration, spurred by the respiration of the roots of plants and of microorganisms living in the soil to break down organic food sources such as sugars, dead plant material, and other microorganisms, almost akin to the soil exhaling carbon dioxide after breaking down food as does a human being.
“The microbes are a lot like us. Some of what we eat is respired back to the atmosphere,” Reich said. “They use the same exact metabolic process we do to breathe CO2 back out into the air.”
Climate Change Implications
In cases of high soil respiration, increased temperatures would mean the release of more carbon to the atmosphere, hastening the rate of global warming. According to lead author Guopeng Liang, a postdoctoral researcher at Yale University, this carbon release could make it difficult for the world to achieve its goals on climate policy that have set out to restrict the rise in temperature.
The big picture story is that losing more carbon is always going to be a bad thing for the climate, Liang said.
However, the researchers found that the increase in carbon levels would not be as powerful as it could have been. Warming also led to increased dryness of soil as plants and soils lost moisture much faster. As soil microorganisms work better under wetter conditions for respiration, dry soil partly restrained the enhancement in carbon dioxide emissions. It allowed part of the possible elevation of carbon levels from the soil to be repressed.
As Reich summed it up, the take-home message here is that forests are going to lose more carbon than we would like. But maybe not as much as they would if this drying weren’t happening.
Long-term Challenges
The study points to the importance of understanding how climate change affects the balance between carbon storage and carbon release in forest ecosystems. Northern forests have long been considered an important carbon sink, but as the planet warms, their ability to serve as a carbon sink may soon diminish, propelling climate change acceleration.
The experiment conducted by Reich is not only comprehensive but also of long duration. Most experiments with comparable efforts usually do not last more than three or four years because of the financial burden. However, Reich’s study was supported by funds from the National Science Foundation, the U.S. Department of Energy, and the University of Minnesota. Therefore, Reich had enough funding for him to be able carry out long-term data gathering that provides more profound insights into climate change impacts on forest ecosystems.
Accounting for soil and air temperatures, along with soil moisture, the study gives a much better picture of how northern forests will react to climate warming. Much of the information from this study will help scientists when predicting and mitigating the impacts of climate change.
Conclusion
Far more significantly, however, potential impacts of this limitation are directly related to global warming: Northern forests are expected to have much reduced carbon sequestration capacities with increasing global temperatures, feeding directly into the challenge of controlling atmospheric carbon levels. As positive indicators for the results of Reich’s study, drier conditions may somewhat mitigate the increase of carbon emissions from the soils; however, the overall loss of carbon from these ecosystems is still a cause for concern. It is very important to understand the long-term implications of warming on carbon storage for effective strategies against climate change.
University of Michigan, Nature Geoscience