Long Term Response of the Earth System to Anthropogenic CO2 Emissions

U. Mikolajewicz, M. Gröger, E. Maier-Reimer, G. Schurgers, M. Vizcaíno & 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), terrestrial biosphere (LPJ), and ice sheets (SICOPOLIS).
Experiments: Starting from spinup with preindustrial conditions (CO2 prognostic). Prescribed CO2 concentrations increasing with 1% compound stabilized at 2x, 3x and 4x times the preindustrial level. Each experiment has been integrated for 1000 years. An additional experiment (4xrad280) has been integrated where the atmospheric radiation has been calculated with 280 ppm. This experiment has been used together with the 4x experiment to analyse the effect of climate changes on CO2 uptake.

2. Climate Changes


Fig.1: Time series, x-axis: time in years, all plots show 10-year running mean (high resolution)

The global mean warming (see fig. 1b) and the intensified freshwater at high latitudes (see fig. 1e) lead to a weakening of the deep water formation both in the North Atlantic (see fig 1c) as well as in the Southern Ocean (not shown). In the experiments with CO2 doubling and tripling this reduction is modest and the strength of the overturning reaches after a century almost the values of the control run, the overturning cells are shallower compared to the control run (compare figs. 1c and 1d). In the case of a quadrupling of CO2 concentrations deep water formation collapses completely. In the Ross Sea deep convection sets in after a few hundred years and intermediate water formation in the North Pacific strongly intensifies. In contrast the Atlantic shows no deeper convection, neither in the North Atlantic nor in the Weddell Sea. The result is a completely stagnant deep Atlantic without any significant overturning circulation.

2xCO2(high resolution):


3xCO2(high resolution):


4xCO2(high resolution):


Fig.2: Change in surface air temperature [K] averaged over the years 300 to 399 of the experiments relative to the climate of the control run.

The experiments with CO2 doubling (2x) and tripling (3x) show the typical warming patterns of other models. Stronger warming over land than over oceans, maximum warming in high northern latitudes. Over the north Atlantic an area with a weak cooling can be seen where the convection is reduced. The collapse of formation of North Atlantic Deep water in this experiment has substantially reduced the northward heat transport in the Atlantic. The result is a zone with vigorous cooling extending almost over the entire northern North Atlantic.

3. Changes in Ice Sheets


Fig.3: Time evolution of ice sheets [global sea level equivalent in m] (high resolution)



2xCO2 3xCO2 4xCO2

Fig.4: Change in ice thickness of Greenland at the end of the experiments in m

The doubling and tripling experiment show a rather similar pattern of changes in the Greenland ice sheet: Melting near the coast and an area of intensified accumulation due to stronger snowfall in North Greenland. In the experiment with fourdoubling the pattern is strongly changed. The cooling over the North Atlantic leads to a reduction in snowfall. The reduction in ice thickness in east Greenland is due to reduced snowfall rather than to melting. Consequently, the overall melting of Greenland is strongest in the CO2 tripling experiment. The overall contribution to global mean sea level rise is more than 1 m. However, this effect is almost completely balanced by an increase in the volume of the Antarctic ice sheet due to enhanced precipitation (see Fig. 3).

4.Carbon Uptake


Fig.5: Time series of uptake of anthropogenic carbon by terrestrial biosphere (solid) and ocean (dashed) displayed are 20 year running means. (high resolution)



During the phase of rising atmospheric CO2 concentrations the terrestrial biosphere and the ocean take up almost equal amounts of carbon. After the atmospheric composition is held constant the terrestrial uptake is reduced strongly and reaches almost equilibrium after approx. 100 years. The ocean uptake is continuing, but weakening through time. At the end of the experiments it is still almost 20% of the peak value. The reduced ventilation of the deep ocean, a weakening of the biological pump and the temperature dependence of carbon release from soil are responsible for the lower rate of carbon sequestration (see Fig. 5, experiments 4x and 4xrad280).

Conclusion:

This set of experiments shows the importance of using an earth system model to study the anticipated long term effects of anthropogenic CO2 emissions.

 
Updated:
Matthias Gröger (groeger@dkrz.de)
Maxim Sein
Last modified: December 07 2004.