by Seeta A. Sistla, NOAA Climate and Global Change Postdoctoral Fellow, University of California Irvine


As winters' warm, storms and droughts intensify, and sea level rises, citizens across the world are increasingly concerned with the consequences of rising atmospheric greenhouse gas concentrations. Atmospheric carbon dioxide (CO2) concentration, a major greenhouse gas, is rising at a rate that is unprecedented in the recent history of the Earth. This rise in greenhouse gas concentrations increases the Earth's atmospheric insulation, changing our climate system. Climate change is of particular concern not only because of its direct effects on temperature, sea level, and precipitation, but also due to the indirect effects it has on ecosystems. The climate system is innately linked to the Earth's ecosystems, so as the climate changes, this will affect ecosystems, and these changes can in turn feedback to further affect the climate.


As a scientist, I study how a changing climate influences ecosystem processes, and how changes in ecosystem processes can in turn further affect the climate system. Identifying feedbacks is a critical component of the environmental sciences, as well as for understanding how other networks, including social, economic, and mechanical systems function. System feedbacks can be positive, negative, or a mixture of both positive and negative. Positive feedbacks magnify change and tend to destabilize systems, while negative feedbacks fight change and tend to stabilize systems. Most environmental systems are affected by a mixture of positive and negative feedbacks. A classic example of a positive feedback is the nuclear arms race between nations (represented by box A and B) that developed after World War II. A well known example of a negative ecological feedback is that of increasing abundance of prey (like deer, represented by circle A) causing predators who eat them (like wolves, represented by circle B) to increase, which then brings down the prey abundance.


Feedbacks between a changing climate and ecosystem processes are important to understand, because natural fluxes of carbon in and out of terrestrial systems are more than 10 times larger (annually) than the carbon released to the atmosphere by human activities alone. These fluxes include both the carbon taken in from the atmosphere by plants as CO2 that is fixed into organic material through the process known as photosynthesis, and carbon released back to the atmosphere through decomposition as CO2 and methane (CH4, another greenhouse gas).


Because these carbon fluxes are very large, small increases in the rates of decomposition relative to photosynthesis can have significant impacts on the atmospheric greenhouse gas concentrations. Increases in carbon emissions from soils—due to climate warming as well as land use (such as deforestation or agricultural disturbance of soils)—may outpace the rate of CO2 directly released by human activities (e.g. fossil fuel burning) in this century. Therefore, in addition to fossil fuel combustion, the future trajectory of the Earth's greenhouse gas concentrations will also be strongly regulated by the response of ecosystems themselves to climate change and human use of these systems.


My research is focused on understanding how arctic tundra carbon cycling is affected by warming. The Arctic is warming faster than any other biome on Earth. Up to 8 ͦ Celsius warming over the Arctic is projected to occur by the end of the century; it can be seen as the canary in the coal mine for understanding how biomes around the world may respond to climate change. Scientists are particularly interested in understanding how high latitudes are responding to warming because arctic systems store nearly half of all global soil carbon in permafrost, or permanently frozen soil.

This large store of permafrost soil carbon formed because the cold arctic climate limited soil decomposition. Therefore, even a slight rise in temperature could be enough to stimulate tundra decomposers and release substantial quantities of permafrost carbon into the atmosphere: a massive positive feedback to climate change. Projecting the extent to which warming will accelerate Arctic carbon emissions is complicated, however, because warming also accelerates arctic plant growth (which is a negative feedback to increasing atmospheric CO2 concentration).

To understand how the balance between faster decomposition and faster plant growth may play out in a warmer Arctic, I worked with a team of researchers to document the effects of 20 years of experimental warming on Arctic tundra plants and soil. This research took place at the longest-running tundra climate warming study in the world: the U.S. Arctic Long-Term Ecological Research site at Toolik Lake in northern Alaska.


This ecosystem-warming greenhouse experiment was started in 1989 (when I was 6 years old!) to observe the effects of sustained warming on the Arctic environment. In 2008, we found that the experimental warming increased shrub dominance and overall plant biomass. These findings were not unexpected: a similar 'greening' has been observed across the Arctic over the last several decades, which is attributed to climate warming.


The greenhouse study also provided some unexpected results, namely, that two decades of experimental warming had not changed the net amount of carbon stored in the seasonally-thawed layer of the permafrost soil. We found this soil carbon storage resilience to have occurred despite changes in vegetation and even the soil food web. This is because greater plant growth and changing soil conditions under the experimental warming treatment seems to have facilitated stabilizing feedbacks to soil carbon loss.


Greater plant productivity caused by the warmer temperatures — on average 2 degrees Celsius in the air and 1 degree in the soil to the permafrost —increased plant litter inputs to the soil. Unexpectedly, the soils in the greenhouse experiment developed higher winter temperatures, while the summer warming effect declined. This winter warming effect was caused because shrubs trap more snow than the lower-lying vegetation, which insulates the soil over the winter, creating warmer winter soil temperatures (and stimulating overwinter decomposer activity). Shrubs also increase summer soil shading, which appears to have reduced decomposer activity in the surface soil during the summer.


Greater plant growth and deeper thaw, meanwhile, also may have increased carbon availability in the deeper mineral layer that overlies the permafrost. We found the strongest biological effects of warming at depth, which we termed a "biotic awakening." Experimental warming stimulated mineral soil decomposers and the food web in this deeper soil expanded; we also observed a slight increase in the amount of carbon present in that horizon (likely due to increased litter inputs by the plants). These results could not have been foreseen when the experiments were started, because ecosystem feedbacks to climate change can take years to decades to develop.


Our study, along with others from the region, highlight the sensitivity of the deeper soil carbon to surface warming, and the importance that linked abiotic-biotic feedbacks can have on ecosystem carbon budgets. What remains to be seen is whether this resilience of tundra soil carbon stocks to long-term warming will be sustained, or whether we observed a transient phase that will eventually give way to accelerated carbon release as the indirect feedback of winter soil warming continues to develop. This phenomenon is critical to understand, because the mineral horizon contains the greatest proportion of the actively cycling tundra soil carbon; therefore, amplified decomposer activity at depth could have significant ramifications for future tundra CO2 release. Future studies will include more detailed investigation of the how experimental warming has altered the mineral soil, which may in yield clues into how the carbon cycle is changing at depth.


It remains challenging to forecast future climate conditions, in large part because of the complexity of ecosystem feedbacks to ongoing climate change. A major scientific goal is to improve our understanding of the carbon cycle in Arctic systems and elsewhere by identifying the magnitude of stabilizing and destabilizing ecosystem feedbacks to climate change. The hope is that this knowledge will help us to prepare for, adapt to, and potentially mitigate undesirable social and ecological consequences of rapid climate change across the world.


Explanation by Mundus maris: The author, Seeta A. Sistla, had recently completed her PhD at the time of writing the contribution. The full scientific article appeared in the prestigious weekly science journal "Nature". More information about Dr. Sistla’s work can be found at: https://sites.google.com/site/seetasistla