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The conundrum of forest expansion after the last ice age

How fast the Northern Hemisphere forest macro ecosystem tracks strongly warming climates such as projected for the near future is largely unknown. In a recent study published in Nature Communication, Anne Dallmeyer, Thomas Kleinen, Martin Claußen (Max-Planck-Institute for Meteorology and CEN, Uni. Hamburg), Nils Weitzel (now Uni. Tübingen), Xianyong Cao (now Chinese Academy of Sciences) and Ulrike Herzschuh (AWI and Uni. Potsdam) investigated the Northern Hemisphere forest expansion after the last glacial maximum. They compared a new synthesis of pollen-based reconstructions and a climate model simulation of the last 22,000 years. In the process, they discovered a difference of several thousand years in the expansion of forests. This conundrum challenges the paleo-climate community. Shortcomings in the model and the reconstructions could both contribute to this mismatch, but can technically not been disentangled so far, leaving the underlying causes unresolved.

Change in Northern Hemisphere forest coverage. Simulated (black) and reconstructed (red) mean forest cover on the Northern Hemisphere, north of 30oN, for the last 22,000 years and the respective uncertainty in forest cover (grey and red shadings). The late-glacial period (22-18 ka) and the Holocene are shaded.

The study was conducted as part of the BMBF-funded PalMod project (www.palmod.de) and the CLICCS cluster of excellence (www.cliccs.uni-hamburg.de).

 

Original paper:
Dallmeyer, A., Kleinen, T., Claussen, M., Weitzel, N., Cao, X. & Herzschuh, U. (2022). The deglacial forest conundrum. Nature Communications, 13: 6035. doi:10.1038/s41467-022-33646-6

Source: Max Planck Institute for Meteorology

 

 

What controls the millennial-scale climate variability in simulations of the last deglaciation? 

The transition between the last glacial maximum (LGM, about 21,000 years before present) and present, which is referred to as the last deglaciation, was characterized by a significant warming and a series of abrupt climate changes. By conducting a first systematic ensemble of hindcast simulations for the last deglaciation with the Max Planck Institute for Meteorology Earth System Model (MPI-ESM), Marie Kapsch, Uwe Mikolajewicz, Clemens Schannwell (scientists at Max Planck Institute for Meteorology) and Florian Ziemen (now at German Climate Computing Center) showed that MPI-ESM is capable of simulating abrupt climate changes. However, the exact sequence of abrupt events depends substantially on the glacial configuration prescribed from ice-sheet reconstructions and the method of distributing meltwater from retreating ice sheets. 

Figure: Transient model response to different ice-sheet boundary conditions and implementations of meltwater release for the simulations conducted for the study. (a) Global meltwater release, (b) Atlantic Meridional Overturning Circulation (AMOC) at 1,000 m depth and 26°N, (d) North Atlantic sea-surface temperature (SST) for simulations with GLAC-1D (black) and ICE-6G (red) ice-sheet boundary conditions as well as for ICE-6G ice sheets but a globally homogenous distribution of meltwater (blue) and no meltwater release (green). Vertical shadings mark approximate timings of the Bølling-Allerød warm period (left) and Younger Dryas cold period (right) according to proxy evidence (adapted from Kapsch et al., 2022).

Original publication:

Kapsch, M.-L., Mikolajewicz, U., Ziemen, F., and Schannwell, C. (2022) Ocean response in transient simulations of the last deglaciation dominated by underlying ice-sheet reconstruction and method of meltwater distribution. Geophysical Research Letters, 49. https://doi.org/10.1029/2021GL096767

Source: Max Planck Institute for Meteorology

 

 

 

Link between Abrupt Climate Changes and Deglaciation 

 

The large ice sheets over North America and Scandinavia disintegrated about 10,000-20,000 years ago during the most recent deglaciation. The causes of the transition between glacial and warm conditions are thought to be slow changes in the Earth's orbit and its orientation to the Sun, occurring over thousands of years, but they are also accompanied by abrupt changes in global ocean circulation that occurred in decades to centuries. These abrupt changes are thought to amplify the more gradual external influence by altering the exchange of heat and carbon dioxide between the ocean and the atmosphere, thereby enabling deglaciation to proceed. However, so far it has been a mystery why a similar link between gradual orbital changes and abrupt shifts in ocean circulation have not led to deglaciation earlier within a glacial cycle. The outstanding question has been ‘what is so special about deglacial shifts in ocean circulation as opposed to those earlier events?’.

A new study lead by Gregor Knorr from the Alfred Wegener Institute and an international team offers an explanation. With the help of climate simulations, the authors can show that the basic state, in particular the stratification in the ocean during the ice age was quite different from today. According to the new results, changes in ocean circulation at the end of the last ice age can lead to a doubling of the net warming rate across Antarctica. The authors argue that this led to increased global warming and greenhouse gas concentrations, accelerating the disintegration of the ice sheets. "Deglacial shifts in ocean circulation are special because they tap into deeper water masses that are on average less cold (compared to intermediate depths) and saltier than at any other time during a glacial cycle," points out Gregor Knorr, the study's lead author.

Stephen Barker, co-author from Cardiff University's School of Earth and Environmental Sciences, UK comments: “This provides an explanation for the failure of analogue events during sub-glacial conditions (e.g. MIS3) to produce a glacial termination.” Deciphering the critical ocean processes in a warming Earth is quite crucial for finding tipping points in the climate system, adds Gerrit Lohmann, co-author of the study.

 

Figure: Temperature difference between the glacial and interglacial anomalies in response to a weakening of the Atlantic meridional overturning circulation.  Shown are conditions between model years 100 and 200 as a 100 year mean, zonally averaged in the Atlantic sector (modified from Fig. 5 in Knorr et al., 2021).

 

Original Publication: Knorr, G., Barker, S., Zhang, X., Lohmann, G., Gong, G., Gierz, P., Stepanek, C., L. B. Stap: A salty deep ocean as a prerequisite for glacial termination. Nature Geoscience 14, 930–936 (2021). https://doi.org/10.1038/s41561-021-00857-3