Response of high-latitude ecosystems to temperature overshoot scenarios

The delay in reducing anthropogenic carbon emissions increases the likelihood of temporarily overshooting the Paris Agreement’s long-term goal of limiting global warming to considerably less than 2°C above pre-industrial levels. Here, the response of high-latitude ecosystems plays an important role in the climate stabilization. The largest fraction of Arctic’s vast soil carbon pools is located in regions underlain by permafrost (perennially frozen ground), where they are becoming increasingly vulnerable to decomposition due to rising temperatures. At the same time, longer and warmer summers enhance plant productivity in the high latitudes, increasing the terrestrial carbon uptake. The net effect of these two opposing feedbacks is highly uncertain, but it is generally assumed that the Arctic will turn from a sink of atmospheric CO_­2 into a carbon source at some point during this century. Given our current greenhouse gas emissions and how close we already are to a 1.5°C-warming, the authors decided that it may not be enough to only look at the standard climate scenarios: ”We asked ourselves: What will happen if the warming is followed by a cooling phase that returns global temperatures to a more sustainable level? It seemed that no one knew how the Arctic will respond if we’ll need to reduce temperatures in order to meet the goals of the Paris Agreement.”

Using simulations with the MPI’s land surface model JSBACH, the researchers show that slow vegetation dynamics, long carbon turnover times and high soil water contents in permafrost regions lead to a large inertia, causing a hysteresis-like behavior in Artic land-surface processes. In particular, the methane emissions differ profoundly between the warming and the cooling phases of the temperature overshoot (Fig 1). This is because the CH₄ fluxes depend not only on the availability of decomposable material, but are also affected by the changes in the extent and location of wetlands that stem from a temporary warming of the Arctic. As a result, the high northern latitudes can constitute either a sink or a source of atmospheric CH₄ depending on whether temperatures are increasing or decreasing.

The authors then went on to investigate the states of the terrestrial Arctic that arise when the same steady climate is reached by different climate trajectories.  They show that the hysteresis-like behaviour, found in the first study, is not merely a transient phenomenon, as previously thought, but that temperature overshoots could even lead to irreversible changes in high-latitude ecosystems (Fig. 2). Multiple positive feedbacks among Arctic carbon, water, and energy cycles result in the steady state being partly determined by the soil organic matter content upon climate stabilization. The latter has a strong impact on the soil’s hydrological and thermal properties allowing overshoots to alter the boundary conditions under which physical and biophysical soil processes occur. The authors conclude: “We were aware that Arctic soils can exhibit a hysteresis-like behavior, but we had not expected the existence of multiple stable ecosystem states under the same climate conditions. This multi-stability is a quite a surprise and seems to be a unique feature of permafrost-affected regions. It will be very exciting to find out if and how this is connected to abrupt changes in case the leading state loses stability”.