Climate change in the sub-Arctic - Assessing the importance of feedback mechanisms
This project focuses on feedbacks between climate and ecosystems in so called sub-Arctic environments. The sub-Arctic is a transition zone between the boreal and arctic climate. We study the interaction between climate and three different ecosystems surrounding Abisko in northern Sweden (see map below, figure 1). These ecosystems are mires, birch forests and heaths.
In the area surrounding Abisko, where the mean annual temperature is close to zero and precipitation is very low, extreme climate events could be exceptionally mild weather for a few days in winter or heavy rains in summer. Our aim is to understand how the ecosystem response to such changes directly or indirectly feed back to climate.
The reason why feedbacks are interesting to study is that we know little yet of how changes in the ecosystems may reinforce or offset the changes in climate that we observe today. One example is how an increased growth of shrubs and tall grass may lead to a thicker snow cover in winter. A thicker snow cover insulates the ground from the cold air, keeping it warmer during winter. In areas underlain by permafrost (ground that is not thawed for two or more consecutive years) such as so called palsa mires, this change may actually lead to thawing of the permafrost. Once the permafrost is thawed, the carbon stored in the mire can be released as carbon dioxide or methane. Both carbon dioxide and methane are greenhouse gases and would thus reinforce the ongoing warming, a so called positive feedback effect.
We study feedbacks both by experiments and modeling simulations. At one site called Storflaket (see map, figure 1) we manipulate the snow thickness during winter in order to study its effect on species composition, growth and greenhouse gas emissions. The greenhouse gases are measured using chambers (figure 2) connected by tubes to a gas analyzer.
One of the challenges is to quantify and compare different feedback effects with each other. For this purpose we need to use models. In this project we use a physically based process model called COUP, where each ecosystem is simulated separately. The model is driven by meteorological data such as temperature, precipitation and incoming radiation, and simulates the flow of energy and mass (water, carbon, nitrogen) trough a soil profile, defined for the specific ecosystem. The model can help us understand how processes are linked together and how different systems respond to different types of changes.