Biogeochemical and biophysical interactions during the last interglacial

U. Mikolajewicz, M. Gröger, E. Maier-Reimer, G. Schurgers, M. Vizca&íno & A. Winguth*
Max-Planck-Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany, mikolajewicz@dkrz.de
*) Center for Climatic Research, Madison, USA



1. Introduction

Model: MPI Earth System Model for paleo studies including atmosphere (ECHAM), ocean (LSG), ocean biogeochemistry (HAMOCC), and terrestrial biosphere (LPJ).

Experiments:


Fig. 1: Spatial variations in the annual mean solar forcing (left) and temporal variations in the global mean solar forcing (right). (high resolution):

The earth system model was used in an experiment with a transient insolation forcing, corresponding to the insolation of 128 - 113 ky B.P., which represents the last interglacial and the start of glacial inception. In the beginning of the experiment, insolation is relatively high for the high latitudes of the northern hemisphere in summer (fig. 1). Towards the end of the experiment, the situation swaps, with relatively low insolation during summer, and higher insolation during winter on the northern hemisphere. A control run was performed with present-day insolation. The atmospheric CO2 concentration is calculated prognostically from the fluxes from land and ocean. To distinguish the role of the land surface, accelerated experiments were performed with a coupled land surface as well as with a prescribed land surface. Results:

2. Climate change and CO2
The increase in summer insolation over the mid- and high latitudes of the northern hemisphere causes a positive temperature anomaly for 128 - 123 ky B.P. (fig. 2). From 121 ky B.P. onwards, a negative anomaly occurs here. Winter temperatures north of 60oN follow the summer temperature patterns, between 122 and 113 ky B.P. a positive anomaly occurs in the tropics.


Fig. 2: Zonal mean temperature anomalies (K) for summer (JJA) and winter (DJF). Shown are 1000 year running means. (high resolution):

Atmospheric CO2 concentration The concentration of CO2 in the atmosphere increases during the experiments, from below 270 for 127 ky B.P. to 290 ppm for 116 ky B.P. (fig. 3). The increase is caused by a decrease of terrestrial carbon storage (see Biogeochemical interactions).


Fig. 3: Fig. 4. CO2 concentration for the insolation experiment and the control run. Shown are 1000 year running means. (high resolution):

The concentration of CO2 in the atmosphere increases during the experiments, from below 270 for 127 ky B.P. to 290 ppm for 116 ky B.P. (fig. 4). The increase is caused by a decrease of terrestrial carbon storage (see Biogeochemical interactions).

Biophysical interactions

Taiga-tundra feedback in the high latitudes
The feedback between temperature and forest presence enhances the temperature increase due to insolation changes. Whereas the accelerated experiment with a prescribed land surface reaches a temperature amplitude of 2.5 K (fig. 4c), the experiment with an interactive land surface shows a clear change in albedo (fig. 5b) and has a temperature amplitude of 5 K. The change in albedo is mainly caused by changes in snow albedo with forest presence, but the difference between forest and grass albedo plays a dominant role during summer.


Fig. 4: (a) forest fraction, (b) surface albedo, and (c) surface air temperature for the land surface 60o-90oN, excluding ice sheets. Shown are 0.8 ky running means. (high resolution):

Figure 6 shows that the accelerated experiment with a fixed land surface has basically the same circulation as the control run during summer, although the moisture content is increased slightly. For the accelerated experiment with a coupled land surface, the winds from the Atlantic increase and bring more moisture into North Africa.


Fig. 5: Summer (JJA) average winds at 850 hPa (vectors, m s-1) and integrated water content of the atmosphere (colours, kg m-2) for 126 ky B.P. (high resolution):



Biogeochemical interactions

Total terrstrial carbon storage
During the experiment, carbon storage in the terrestrial biosphere decreases, from 200 Pg C above the control run value for 125 ky B.P. to 150 Pg C below the control run value for 116 ky B.P. (fig. 6). The emitted carbon is taken up by the ocean and the atmosphere, thereby increasing the atmospheric CO2 concentration (fig. 4).


Fig. 6: Terrestrial carbon storage anomaly for the insolation experiment and the control run. Shown are 1000 year running means. (high resolution):

Zonal distribution of carbon storage
Experiments using only part of the climate data from the coupled experiment as forcing were used to explain the zonal distribution of carbon (fig. 7). For the warm phase in the beginning of the model, carbon storage increases in the latitudes north of 60oN, due to an expansion of the boreal forests caused by temperature increase. It decreases between 30oN and 60oN due to increased respiration, caused by temperature increase, despite a positive effect of increased radiation in these latitudes. An increase is seen between the equator and 30oN, caused by the enhanced monsoon. The experiment with only CO2 concentration as forcing did show only minor effects, which are counteracting the global trend in carbon storage (fig. 7).


Fig. 7: Zonal anomalies of terrestrial carbon storage (Pg C per degree latitude), for the fully coupled experiment and the experiments with only temperature, hydrology, radiation, or CO2 concentration as forcing. (high resolution):

Conclusions
Due to insolation changes, major shifts in vegetation occur in the high latitudes and the subtropical monsoon regions. - Positive feedbacks between land surface and atmosphere enhance this orbitally induced climate change, and thereby stimulate the vegetation shifts. Carbon storage in the terrestrial biosphere decreases during the Eemian, and causes thereby an increase of the atmospheric CO2 concentration. Carbon storage in the ocean increases as well. Largest effects on carbon storage are caused by changes in temperature: positive effects due to forest expansion and increased photosynthesis, negative effects due to increased. However, the net effect of temperature is varying, and changes in the hydrology and the radiation are important to explain the increase in atmospheric CO2 concentration.


Acknowledgements
This research was funded by the German Climate Research Program (DEKLIM) of the Federal Ministry of Ecucation and Research (BMBF).



UNDER CONSTRUCTION
Updated:
Matthias Gröger (matthias.groeger@zmaw.de)
Last modified: May 18 2006.