Thomas Raddatz

Climate-carbon cycle feedback analysis: Results from the C4MIP model intercomparison

Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5°C. All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.

Sensitivity of a coupled climate‐carbon cycle model to large volcanic eruptions during the last millennium

The sensitivity of the climate–biogeochemistry system to volcanic eruptions is investigated using the comprehensive Earth System Model developed at the Max Planck Institute for Meteorology. The model includes an interactive carbon cycle with modules for terrestrial biosphere as well as ocean biogeochemistry. The volcanic forcing is based on a recent reconstruction for the last 1200 yr. An ensemble of five simulations is performed and the averaged response of the system is analysed in particular for the largest eruption of the last millennium in the year 1258. After this eruption, the global annual mean temperature drops by 1 K and recovers slowly during 10 yr. Atmospheric CO2 concentration declines during 4 yr after the eruption by ca. 2 ppmv to its minimum value and then starts to increase towards the pre-eruption level. This CO2 decrease is explained mainly by reduced heterotrophic respiration on land in response to the surface cooling, which leads to increased carbon storage in soils, mostly in tropical and subtropical regions. The ocean acts as a weak carbon sink, which is primarily due to temperature-induced solubility. This sink saturates 2 yr after the eruption, earlier than the land uptake.

Contribution of anthropogenic land cover change emissions to pre-industrial atmospheric CO2.

Based on a recent reconstruction of anthropogenic land cover change (ALCC), we derive the associated CO2 emissions since 800ADby two independent methods: a bookkeeping approach and a process model. The results are comparedwith the pre-industrial development of atmospheric CO2 known from antarctic ice cores. Our results show that pre-industrial CO2 emissions from ALCC have been relevant for the pre-industrial carbon cycle, although before 1750 AD their trace in atmospheric CO2 is obscured by other processes of similar magnitude. After 1750 AD, the situation is different: the steep increase in atmospheric CO2 until 1850 AD—this is before fossil fuel emissions rose to significant values—is to a substantial part explained by growing emissions from ALCC.

Anthropogenically induced changes in twentieth century mineral dust burden and the associated impact on radiative forcing

We investigate the relative importance of climate change (CC) and anthropogenic land cover change (ALCC) for the dust emissions and burden changes between the late nineteenth century and today. For this purpose, the climate-aerosol model ECHAM6-HAM2 is complemented by a new scheme to derive potential dust sources at runtime using the vegetation cover provided by the land component JSBACH of ECHAM6. Dust emissions are computed online using information from the ECHAM6 atmospheric component. This allows us to account for changes in land cover and climate interactively and to distinguish between emissions from natural and agricultural dust sources. For todays climate we find that nearly 10% of dust particles are emitted from agricultural areas. According to our simulations, global annual dust emissions have increased by 25% between the late nineteenth century and today (e.g., from 729 Tg/a to 912 Tg/a). Globally, CC and ALCC (e.g., agricultural expansion) have both contributed to this change (56% and 40%, respectively). There are however large regional differences. For example, change in dust emissions in Africa are clearly dominated by CC. Global dust burden have increased by 24.5% since the late nineteenth century, which results in a clear-sky radiative forcing at top of the atmosphere of −0.14 W/m2. Based on these findings, we recommend that both climate changes and anthropogenic land cover changes should be considered when investigating long-term changes in dust emissions.

Climate variability-induced uncertainty in mid-Holocene atmosphere-ocean-vegetation feedbacks

[1] Previous modelling studies have shown that the response of the ocean and the vegetation to mid-Holocene insolation feeds back on the climate. There is less consensus, however, on the relative magnitude of the two feedbacks and the strength of the synergy between them. This discrepancy may arise partly from the statistical uncertainty caused by internal climate variability as the common analysis period is only about a century. Therefore, we have performed an ensemble of centennial-scale simulations using the general circulation model ECHAM5/ JSBACH-MPIOM. The direct atmospheric response and the weak atmosphere-vegetation feedback are statistically robust. The synergy is always weak and it changes sign between the ensemble members. The simulations, including a dynamic ocean, show a large variability at sea-ice margins. This variability leads to a sampling error which affects the magnitude of the diagnosed feedbacks.

Different response of surface temperature and air temperature todeforestation in climate models

When quantifying temperature changes induced by deforestation (e.g., cooling in high latitudes, warming in low latitudes), satellite data, in situ observations, and climate models differ concerning the height at which the temperature is typically measured/simulated. In this study the effects of deforestation on surface temperature, near-surface air temperature, and lower atmospheric temperature are compared by analyzing the biogeophysical temperature effects of large-scale deforestation in the Max Planck Institute Earth System Model (MPI-ESM) separately for local effects (which are only apparent at the location of deforestation) and nonlocal effects (which are also apparent elsewhere). While the nonlocal effects (cooling in most regions) influence the temperature of the surface and lowest atmospheric layer equally, the local effects (warming in the tropics but a cooling in the higher latitudes) mainly affect the temperature of the surface. In agreement with observation-based studies, the local effects on surface and near-surface air temperature respond differently in the MPI-ESM, both concerning the magnitude of local temperature changes and the latitude at which the local deforestation effects turn from a cooling to a warming (at 45–55 N for surface temperature and around 35 N for near-surface air temperature). Subsequently, our single-model results are compared to model data from multiple climate models from the Climate Model Intercomparison Project (CMIP5). This inter-model comparison shows that in the northern midlatitudes, both concerning the summer warming and winter cooling, near-surface air temperature is affected by the local effects only about half as strongly as surface temperature. This study shows that the choice of temperature variable has a considerable effect on the observed and simulated temperature change. Studies about the biogeophysical effects of deforestation must carefully choose which temperature to consider. Published by Copernicus Publications on behalf of the European Geosciences Union. 474 J. Winckler et al.: Tsurf vs. T2m response to deforestation

Interactions between climate and vegetation at high northern latitudes during the mid-Holocene

The mid-Holocene climate, about 6000 years before present, is investigated with the comprehensive general circulation model ECHAM5/JSBACH-MPIOM at high northern latitudes. Applying a factor-separation technique, we isolate the contributions of the atmosphere, the atmosphere-vegetation feedback, the atmosphere-ocean feedback and their synergy to the midHolocene climate change signal. The mid-Holocene climate signal shows a modification of the seasonal cycle at the high northern latitudes compared to pre-industrial climate. This is characterised by increased temperatures in summer, autumn and winter, and a cooler climate in spring. The summer warming is primarily caused by the direct response of the atmosphere to the change in insolation. The autumn temperature rise, however, results not only from the direct atmospheric signal but is also amplified by the atmosphere-ocean feedback. The winter warming is primarily induced by the atmosphere-ocean feedback, counteracting the cooling caused by the the direct atmospheric signal. In spring, the temperature decrease is a combined effect of the direct atmospheric signal and the atmosphere-ocean feedback. The atmosphere-vegetation feedback compensates this cooling only marginally. The synergy between the atmosphere-ocean and atmosphere-vegetation feedback results in slight warming for all seasons. In summary, the direct mid-Holocene climate response to the change in insolation is mainly modified by the atmosphere-ocean feedback. In contrast, the atmosphere-vegetation feedback influences the mid-Holocene climate signal only marginally. We test the statistical robustness of the results. The atmosphere response and the atmospherevegetation feedback are statistically robust. By contrast, the factors derived from simulations with an interactive ocean are sensitive to long-term anomalies in sea-ice cover. Nevertheless, the statistical testing confirms that the most important modification of the direct climate response to the orbital forcing can be related to the atmosphere-ocean feedback. A detailed analysis of the atmosphere-vegetation feedback shows that the expansion of forest during the mid-Holocene causes two opposing effects in spring. On the one hand, the increase in forest results in a reduction in surface albedo and, thus, enhances the absorption of solar radiation which leads to a near-surface air-temperature rise. On the other hand, the expansion of forest favours the increase in transpiration and, thus, an increase in cloud fraction, which, in turn, dampens the warming signal. Furthermore, it is investigated to what extent the strength of the atmosphere-vegetation feedback depends on the parametrisation of the albedo of snow. A parametrisation with a strong reduction in the albedo of snow by deciduous trees increases the temperature signal regionally. Simulations with the albedo of snow depending on the age of snow show a regional increase in temperature as well. However, the large-scale temperature signal of the atmosphere-vegetation feedback simulated with ECHAM5/JSBACH remains weak compared to previous studies.