Climate-Biosphere Interaction

Interactions between land surface processes and climate could lead to the phenomenon of multiple steady states. One famous example is the feedback between atmospheric circulation and the vegetation cover in North Africa that could explain the substantial greening of the Sahara in the mid-Holocene. Several studies suggest that, for present-day, both desert and green states are potentially stable in North Africa, and the transition between them could be rather abrupt (link to Martin’s page). For permafrost soils, interactions between soil temperature, water, and organic carbon could result in two different states: dry, organic-poor, warmer soil, and wet, organic-rich, colder soil. The difference between these two states could be regionally very substantial (Figure 3).

Another example of alternative steady states comes from the analysis of remote sensing data (Abis and Brovkin, Biogeosciences, 2017). When we studied the link between observed tree-cover fraction distribution in boreal regions with mean annual rainfall, minimum temperature, permafrost distribution, soil moisture, wildfire frequency, and soil texture, we found areas with potentially alternative tree cover states (forest, mixed, treeless) under the same environmental conditions. These areas, although encompassing a minor fraction of the boreal area (ca. 5 %), correspond to possible transition zones with a reduced resilience to disturbances. (Figure 4).

Processes on a fine scale are highly parameterized in large-scale models. If we consider an example of very small water bodies or ponds, they contribute disproportionally to the Arctic methane emissions. Methane concentrations in ponds vary strongly, calling for process-based understanding of variability. To do so, Rehder et al. (2021) categorized polygonal-tundra ponds in the Lena River Delta into three geomorphological types with distinct differences in drivers of methane concentrations: polygonal-center ponds, ice-wedge ponds and larger, merged polygonal ponds (Figure, left and center). They found that methane concentrations negatively correlate with the size of the pond: the smaller the pond, the higher the surface concentrations of methane (Figure, right). Also, no single driver (such as wind speed, water temperature, or water depth) could explain variability over all pond types suggesting that more complex upscaling methods such as process-based modeling are needed.

In short — very unlikely, but we cannot completely rule it out. Our group is involved in a joint project with the marine biogeochemistry groups focusing on the potential release of methane from sub-sea permafrost thawing.

• Group Ocean Biogeochemistry
• Group of Prof. Sandra Arndt (ULB)

During glacial cycles, the shallow Arctic shelf was land exposed to freezing temperatures that accumulated organic matter during short but productive summers. After the melting of the ice sheets, the shelf was flooded and frozen sediments are now slowly thawing from the top but also from the bottom due to a geothermal heat flux. Under warming scenarios, the ocean floor will warm and ice in the sediments will melt, allowing microbes to decompose previously frozen organic matter. This process leads to in-situ production of methane, one part of which could be eaten up by methane-consuming microbes in the sediments, and another part of which can escape into the atmosphere. The stronger the warming, the more ice will melt in the sediments (Figure 8). This process is very slow, but under certain scenarios, by the year 2300, the methane emissions from the shelf could be more than the methane emission from boreal wetlands (work in progress).