Thomas Kleinen

Comparison of modeled and reconstructed changes in forest cover through the past 8000 years: Eurasian perspective

Reproducing the tree cover changes throughout the Holocene is a challenge for land surface–atmosphere models. Here, results of a transient Holocene simulation of the coupled climate–carbon cycle model, CLIMBER2-LPJ, driven by changes in orbital forcing, are compared with pollen data and pollen-based reconstructions for several regions of Eurasia in terms of changes in tree fraction. The decline in tree fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the Holocene. The cooler and drier trend at the eastern side of the Eurasian continent, in Mongolia and China, also led to a decrease in tree cover in both model and data. In contrast, the Holocene trend towards a cooler climate in the continental interior (Kazakhstan) is accompanied by an increase in woody cover. There a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon-affected regions. In general the model-data comparison demonstrates that climate-driven changes during the Holocene result in a non-homogeneous pattern of tree cover change across the Eurasian continent. For the Eifel region in Germany, the model suggests a relatively moist and cool climate and dense tree cover. The Holzmaar pollen record agrees with the model for the intervals 8–3 ka and 1.7–1.3 ka BP, but suggests great reduction of the tree cover 3–2 ka and after 1.3 ka BP, when highly developed settlements and agriculture spread in the region.

The influence of climate on peatland extent in Western Siberia since the Last Glacial Maximum.

Boreal and subarctic peatlands are an important dynamical component of the earth system. They are sensitive to climate change, and could either continue to serve as a carbon sink or become a carbon source. Climatic thresholds for switching peatlands from sink to source are not well defined, and therefore, incorporating peatlands into Earth system models is a challenging task. Here we introduce a climatic index, warm precipitation excess, to delineate the potential geographic distribution of boreal peatlands for a given climate and landscape morphology. This allows us to explain the present-day distribution of peatlands in Western Siberia, their absence during the Last Glacial Maximum, their expansion during the mid-Holocene, and to form a working hypothesis about the trend to peatland degradation in the southern taiga belt of Western Siberia under an RCP 8.5 scenario for the projected climate in year 2100.

Causes of Regional Change—Land Cover

Anthropogenic land-cover change (ALCC) is one of the few climate forcings for which the net direction of the climate response over the last two centuries is still not known. The uncertainty is due to the often counteracting temperature responses to the many biogeophysical effects and to the biogeochemical versus biogeophysical effects. Palaeoecological studies show that the major transformation of the landscape by anthropogenic activities in the southern zone of the Baltic Sea basin occurred between 6000 and 3000/2500 cal year BP. The only modelling study of the biogeophysical effects of past ALCCs on regional climate in north-western Europe suggests that deforestation between 6000 and 200 cal year BP may have caused significant change in winter and summer temperature. There is no indication that deforestation in the Baltic Sea area since AD 1850 would have been a major cause of the recent climate warming in the region through a positive biogeochemical feedback. Several model studies suggest that boreal reforestation might not be an effective climate warming mitigation tool as it might lead to increased warming through biogeophysical processes.

Path-dependent reductions in CO 2 emission budgets caused by permafrost carbon release

Emission budgets are defined as the cumulative amount of anthropogenic CO2 emission compatible with a global temperature-change target. The simplicity of the concept has made it attractive to policy-makers, yet it relies on a linear approximation of the global carbon–climate system’s response to anthropogenic CO2 emissions. Here we investigate how emission budgets are impacted by the inclusion of CO2 and CH4 emissions caused by permafrost thaw, a non-linear and tipping process of the Earth system. We use the compact Earth system model OSCAR v2.2.1, in which parameterizations of permafrost thaw, soil organic matter decomposition and CO2 and CH4 emission were introduced based on four complex land surface models that specifically represent high-latitude processes. We found that permafrost carbon release makes emission budgets path dependent (that is, budgets also depend on the pathway followed to reach the target). The median remaining budget for the 2 °C target reduces by 8% (1–25%) if the target is avoided and net negative emissions prove feasible, by 13% (2–34%) if they do not prove feasible, by 16% (3–44%) if the target is overshot by 0.5 °C and by 25% (5–63%) if it is overshot by 1 °C. (Uncertainties are the minimum-to-maximum range across the permafrost models and scenarios.) For the 1.5 °C target, reductions in the median remaining budget range from ~10% to more than 100%. We conclude that the world is closer to exceeding the budget for the long-term target of the Paris Climate Agreement than previously thought.Carbon release from permafrost thaw would substantially decrease the amount of carbon emissions required to meet climate targets, according to climate simulations.

Widespread global peatland establishment and persistence for the last 130,000 years

Claire C. Treat, Thomas Kleinen, Nils Broothaerts, April S. Dalton, René Dommain, Thomas A. Douglas, Judith Drexler, Sarah A. Finkelstein, Guido Grosse, Geoff Hope, Jack Hutchings, Miriam C. Jones, Peter Kuhry, Terri Lacourse, Outi Lähteenoja, Julie Loisel, Bastiaan Notebaert, Richard Payne, Dorothy Peteet, A. Britta K. Sannel, Jonathan M. Stelling, Jens Strauss, Graeme T. Swindles, Julie Talbot, Charles Tarnocai, Gert Verstraeten, Christopher J. Williams, Zhengyu Xia, Zicheng Yu, Minna Väliranta, Martina Hättestrand, Helena Alexanderson, Victor Brovkin

Comparison of Climate and Carbon Cycle Dynamics During Late Quaternary Interglacials

Within the project COIN we investigated climate and carbon cycle changes during late Quaternary interglacials using ice core and terrestrial archives, as well as earth system models. The Holocene carbon cycle dynamics can be explained both in models and data by natural forcings, where the increase in CO2 is due to oceanic carbon release, while the land is a carbon sink. Climate changes during MIS 11.3 were mainly driven by insolation changes, showing substantial differences within the interglacial. Terrestrial reconstructions and model results agree, though data coverage leaves room for improvement. The carbon cycle dynamics during MIS 11.3 can generally be explained by the same forcing mechanisms as for the Holocene, while model and data disagree during MIS 5.5, showing an increasing CO2 trend in the model though reconstructions are constant.