Response of high-latitude ecosystems to temperature overshoot scenarios

High-latitude soils contain almost twice as much carbon as the atmosphere, and the fate of this frozen organic matter under ongoing climate change is not well understood. Two recent studies led by scientists of the Max Planck Institute for Meteorology (MPI-M) are focused on the response of high-latitude ecosystems to temperature overshoots – climate pathways that are becoming more and more likely if we are to stabilize global temperatures at a desirable level. Philipp de Vrese, Victor Brovkin, Tobias Stacke, and Thomas Kleinen find that the temperature-dependencies of many terrestrial processes differ during the warming and the cooling phases of such an overshoot. Furthermore, they show that it will not only take Arctic soils several centuries to adjust to new climate conditions, but also that the consequences of a temperature overshoots may not be fully reversible.

Duvanni Yar, Siberia / Credit: Martin Heimann

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 CO­2 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.

 

 

 

Fig. 1. Simulated soil methane emissions in permafrost-affected regions as a function of global mean surface temperature. Grey dots show the emissions during the warming phase of different temperature overshoots, based on the SSP5-8.5 scenario. Colored dots indicate the fluxes during the cooling phases following forcing-peaks in 2025 (blue), 2050 (green), 2075 (yellow) and 2100 (red). Each dot represents the (20-member) ensemble mean, while the shaded areas indicate the spread between the ensemble minimum and maximum. The figure is representative of those areas that are affected by near-surface permafrost at the beginning of the 21st century. (from: de Vrese et al. (2021) Diverging responses of high-latitude CO2 and CH4 emissions in idealized climate change scenarios. doi:10.5194/tc-15-1097-2021. CC BY 4.0

 

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”.

 

 

 

Fig. 2 Long-term effects of temperature overshoots on the high-latitude water-, energy- and carbon cycle: differences between simulations initialized with soil carbon concentrations after and before a temperature overshoot that persist under non-transient atmospheric conditions. Relative difference in total soil water content (blue line; left y-axis), inputs of mineral nitrogen (yellow line; left y-axis) and net primary productivity (green line; left y-axis), as well as absolute differences in total terrestrial carbon (brown line; right y-axis) and Mai-October temperatures at a depth of 1 m (red line; right y-axis). With respect to soil water, nitrogen inputs, productivity and temperature the figure shows the average over the permafrost-affected regions, while the terrestrial carbon is an accumulated value. (from: de Vrese & Brovkin (2021) Timescales of the permafrost carbon cycle and legacy effects of temperature overshoot scenarios. doi.org/10.1038/s41467-021-23010-5. CC BY 4.0

 

Original publications:
de Vrese, P., Stacke, T., Kleinen, T., & Brovkin, V. (2021) Diverging responses of high-latitude CO2 and CH4 emissions in idealized climate change scenarios. The Cryosphere, 15, 1097-1130. doi: 10.5194/tc-15-1097-2021

de Vrese, P. & Brovkin, V. (2021) Timescales of the permafrost carbon cycle and legacy effects of temperature overshoot scenarios. Nature Communications. doi: 10.1038/s41467-021-23010-5
 

Contact:

Dr. Philipp de Vrese
Max Planck Institute for Meteorology
Email: philipp.de-vrese@we dont want spammpimet.mpg.de

Prof. Victor Brovkin
Max Planck Institute for Meteorology & Center for Earth System Research and Sustainability, Universität Hamburg
Email: victor.brovkin@we dont want spammpimet.mpg.de

Dr. Thomas Kleinen
Max Planck Institute for Meteorology
Email: thomas.kleinen@we dont want spammpimet.mpg.de

Dr. Tobias Stacke
Helmholtz-Zentrum Hereon
Email: tobias.stacke@we dont want spamhereon.de