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 

Past abrupt changes provide evidence of cascading tipping points and ‘early warning signals’ in the Earth system

Can climate change result in a collapse in parts of the Earth system, what impacts would these events have on society, and can they be predicted? In the article published in Nature Geoscience, an international team of natural and social scientists have reviewed abrupt shifts in the Earth's past in order to sharpen their tools for predicting the future. They used well-documented abrupt changes of the past 30 thousand years of geological history to illustrate how abrupt changes propagate through the physical, ecological, and societal components of Earth System.

Prof. Victor Brovkin from the Max Planck Institute for Meteorology (MPI-M) and the leading author of the study, says “For humans, it is crucial to anticipate the future, we need to know what are surprises ahead. It sounds counterintuitive, but to foresee the future we may need to look into the past. A chance to detect abrupt changes and tipping points – when small changes lead to big impacts - increases with the length of observations. This is why analysis of abrupt changes and their cascades recorded in geological archives is of enormous importance.”

A possibility to decrypt an upcoming abrupt change in temporal or spatial patterns is a novel, powerful method called early warning signals. Dr. Sebastian Bathiany, an author from the Helmholtz-Zentrum Hereon, explains: “There are useful statistical indicators that can be interpreted as precursors of abrupt changes. Those include so-called slowing down before abrupt changes in oceanic circulation, or increased spatial variance of vegetation cover before the end of African humid period. At the same time, one needs to be cautious as some abrupt changes, such as the Black Sea flooding about 9.5 thousand years ago, cannot not be detected with such methods”.

For the study, it was important to decide on the conceptual framework for the analysis, including terminology. Prof. Martin Claussen, a co-author from MPI-M, comments: “How abrupt is abrupt? There are many definitions of abruptness, they are really context-dependent. Changes in most records evaluated in our study are about ten times faster than changes in the relevant forcing”.

 

A map of selected atmospheric, oceanographic, ecosystem, and societal records with abrupt changes or tipping points discussed in the article (Fig. 3 from Brovkin et al., 2021).

 

Abrupt changes in the Earth system are not limited to one particular domain but can cascade through space and time. Dr. Jonathan Donges, a co-author from PIK, comments: “ice-ocean interactions, for example, during onset of the Bølling-Allerød warming in the middle of transition from the last ice age to the current Holocene warm period, lead to cascading impacts in deep ocean anoxia, vegetation cover, and atmospheric CO₂ and CH₄ concentrations. These changes could also interact with and amplify each other, and propagate among different spatial scales, to eventually affect human hunter-gatherer societies at that time”.

Prof. Michael Barton, Arizona State University, USA, notes, "We are increasingly concerned about the potential for abrupt changes resulting from human impacts in coming decades. Equally important, however, are societal dynamics that can make seemingly resilient human systems vulnerable to abrupt economic or political change - or even collapse - from otherwise manageable environmental fluctuations. Study of past socio-environmental tipping points can give us important insights needed to plan for future ones."

“Earth’s recent past shows us how abrupt changes in the Earth system triggered cascading impacts on ecosystems and human societies, as they struggled to adapt," said Professor Tim Lenton, Director of the Global Systems Institute at the University of Exeter, UK. "We face the risk of cascading tipping points again now – but this time it is of our own making, and the impacts will be global. Faced with that risk we could do with some early warning signals: What examples from the past show is that different climate, ecological, or social systems all become slower at recovering from perturbations before they reach a tipping point – where they fail to recover at all."

This paper is an outcome of the workshop “Abrupt changes, thresholds, and tipping points in Earth history and future implications” held in Hamburg, Germany in November 2018. The workshop was officially endorsed by the Analysis, Integration and Modeling of the Earth System (AIMES) and Past Global Changes (PAGES) projects of Future Earth.

Original publication:

Brovkin, V., Brook, E., Williams J.W., Bathiany, S., Lenton, T.M., Barton, M., DeConto, R.M., Donges, J.F., et al. (2021) Past abrupt changes, tipping points and cascading impacts in the Earth system, Nature Geoscience, doi: 10.1038/s41561-021-00790-5

 

Source: Max Planck Institute for Meteorology

 

AMOC Recovery in CMIP Future Scenarios

The authors discuss the CMIP future scenarios RCP4.5 and RCP8.5 they redo with the AWI-ESM, a model that basis on AWI-CM (Rankow et al., 2018, Sidorenko et al., 2015) but includes interactive vegetation and an interactive Northern Hemispheric ice sheet model. The focus of the paper is on the effects of the melt-induced fresh water on the Atlantic meridional overturning circulation (AMOC). 

The results indicate, that AMOC is slowing down in both experiments, with and without included interactive ice sheet into the model system, for both future scenarios but starts to recover at the end of the 21st century (RCP4.5) and at the beginning of the 22nd century (RCP8.5), respectively.

Nevertheless, an interactive ice sheet model adds a strong decadal variability on the freshwater release, as a compensating effect, when the surface runoff is reduced by high accumulation rates.

The authors argue, that experiments that aim to parameterize the Greenland freshwater release by freshwater hosing have to be assessed critically, as this compensating effect is missing in climate models without interactive ice sheets. Moreover, they discuss, that the increasing net evaporation over the Atlantic and the resulting increase of the salinity may be the main driver of the AMOC recovery.

Fig.Time series of 11-year means and spatial changes of the Greenland Ice Sheet in the coupled simulations; shaded areas indicate 1 standard deviation. (a) The ice sheet’s total volume expressed as sea-level rise potential, (b) surface runoff from the icesheet model, (c) surface accumulation, (d) discharge, (e) the ice sheet’s total volume change, (f) surface runoff from the atmosphere model. For CTRL only the last 100 years are shown; (g and h) anomaly of ice sheet’s thickness for 2170 – 2199 for RCP4.5-ice and RCP8.5-ice respectively, (i and j) the ice sheet’s surface mass balance for 2170 – 2199 for RCP4.5-ice and RCP8.5-ice respectively. 

Ackermann, L. , Danek, C. , Gierz, P. and Lohmann, G. (2020) AMOC Recovery in a Multicentennial Scenario Using a Coupled Atmosphere‐Ocean‐Ice Sheet Model. Geophysical Research Letters, 47 (16). e2019GL086810. DOI 10.1029/2019GL086810.

Natural methane emissions – from the glacial to the present In a new study in Climate of the Past Kleinen, Mikolajewicz, and Brovkin (Max Planck Institute for Meteorology), were able to show that the changes in methane concentration between the Last Glacial Maximum (LGM, about 20000 years ago) and the preindustrial late Holocene (PI), 300 years ago, can be explained entirely by changes in the natural methane emissions caused by environmental changes.   Natural net emissions of methane in the present-day climate. Credit: Thomas Kleinen Kleinen, Thomas , Mikolajewicz, Uwe und Brovkin, Victor (2020) Terrestrial methane emissions from the Last Glacial Maximum to the preindustrial period. Climate of the Past, 16 (2). pp. 575-595. DOI 10.5194/cp-16-575-2020. Source: Max Planck Institute for Meteorology

PalMod I Highlights

 

Freshwater release and elevation loss affect climate during Heinrich events

 

A team of researchers around Dr. Florian Ziemen at the Max Planck Institute for Meteorology found that Heinrich events, climate changes during the last ice age, were caused by a succession of the effects of two mechanisms: iceberg calving, having effects on the ocean, and ice sheet elevation loss, having effects on the atmosphere. Using a novel model setup, they were able to study the relationship between the two individual effects. They were the first to observe the succession of both effects in one simulation.

Citation: Ziemen, F., Kapsch, M.-L., Klockmann, M., & Mikolajewicz, U. (2019). Heinrich events show two-stage climate response in transient glacial simulations. Climate of the Past, 15, 153-168. doi:10.5194/cp-15-153-2019

How cold was Antarctica during the last ice age?   [August 2018]  In a recent study by scientists from the Alfred Wegener Institute together with French colleagues temperature changes in Antarctica during the last ice age have been reconstructed. Ice core data and model results indicate a much stronger cooling of West Antarctica than East Antarctica during that time. Furthermore, the study enabled a new estimate of Antarctic ice sheet height changes during this past climate stage. The results of this study have been recently published in Nature Communications. Citation: Reconciling glacial Antarctic water stable isotopes with ice sheet topography and the isotopic paleothermometer; Martin Werner, Jean Jouzel, Valérie Masson-Delmotte & Gerrit Lohmann; Nature Communicationsvolume 9, Article number: 3537 (2018)

 

Coastal erosion in the Arctic intensifies global warming Sea level rise in the past led to the release of greenhouse gases from permafrost [September 2018]  The loss of arctic permafrost deposits by coastal erosion could amplify climate warming via the greenhouse effect. A study using sediment samples from the Sea of Okhotsk on the eastern coast of Russia led by AWI researchers revealed that the loss of Arctic permafrost at the end of the last glacial period led to repeated sudden increases in the carbon dioxide concentration in the atmosphere. press release (AWI) Citation: Winterfeld, M. et al., Deglacial mobilization of pre-aged terrestrial carbon from degrading permafrost. Nat. Commun. 9, 3666 doi: 10.1038/s41467-018-06080-w (2018).

 

Sea level fall during glaciation stabilized atmospheric CO2 by enhanced volcanic degassing   [Juli 2017] Throughout the last 800,000 years, Antarctic temperatures and atmospheric carbon dioxide concentrations showed a similar evolution. However, this was different during the transition to the last ice age: approximately 80,000 years ago, temperature declined, while the carbon dioxide content of the atmosphere remained relatively stable. An international research team led by the GEOMAR Helmholtz Centre for Ocean Research Kiel and the Alfred-Wegener-Institute Helmholtz Centre for Polar and Marine Research has now discovered that a falling sea level may have caused enhanced volcanic activity in the ocean, which can explain the anomaly. The results are published today in the journal Nature Communications. press release (AWI) Citation: Hasenclever, J. et al. Sea level fall during glaciation stabilized atmospheric CO2 by enhanced volcanic degassing. Nat. Commun. 8, 15867 doi: 10.1038/ncomms15867 (2017).

All PalMod publications are listed here.