Climate Change Modelling

Introduction

Global warming is one of the main current societal problems. The observed global warming since the pre-industrial period (1850 – 1900) up to the end of 20th century (1986 – 2005) is estimated to be around 0.6°C (IPCC, 2013). The mean global surface temperature is expected to continue to rise in the 21st century due to human activity and an associated increase of greenhouse gas (GHG) concentrations. In its 5th assessment report (AR5) the Intergovernmental Panel on Climate Change (IPCC) examined four Representative Concentration Pathways (RCPs) of GHG concentration trajectories (IPCC, 2013). The projected warming (2081 – 2100 mean minus 1986 – 2005 mean) is 1±0.4°C for RCP2.6, 1.8±0.5°C for RCP4.5, 2.2±0.5°C for RCP6.0, and 3.7±0.7°C RCP8.5, given as multi-model mean ± standard deviation of the various IPCC models. In December 2015, many countries agreed to make an effort to reduce their emissions of GHG into the atmosphere in order to keep the global surface temperature rise <2°C above pre-industrial levels. This agreement was adopted under United Nations Framework Convention on Climate Change (UNFCCC), and it is now known as The Paris Climate Agreement. RCP2.6 is the only GHG concentration scenario that limits the global mean surface temperature increase at 2°C at the end of 21st century (van Vuuren et al., 2011).

We perform sensitivity experiments with CCM SOCOL to assess the influence of external forcings on the past and future climate change.

Results and Discussion

The role of different external forcings for the surface air temperature trends since 1600 is investigated using a number of sensitivity simulations with CCM SOCOL where one forcing or a combination of forcing is applied and the remaining forcings are held constant at preindustrial levels. The major part of the temperature increase simulated by all forcings is explained by the GHGs (72 %). The increasing solar activity (8 %) and the ozone trends (12 %) also contribute to the warming. The only negative signal (− 22 %) is related to stratospheric and tropospheric aerosols. All individual forcings (solar, ozone, GHG, aerosols) add up to only 70% of the full forcing experiment, but given the large uncertainties in the estimates this difference is not significant. The spatial structure of the global mean surface air temperature differences shows that the warming in the full forcing experiment is globally very uniform with some hints of polar amplifications in the northern high latitudes. This full forcing pattern is very similar to the changes of the GHG experiment, except for an overall larger trend. As in the global analysis, the temperature change associated with GHGs dominates the full forcing trend almost everywhere. The other three forcings display a much larger spatial heterogeneity and temperature changes are comparably small. For more information on experimental design and more results see Muthers et al. (2014)

Conclusions

The performance of SOCOL-MPIOM under changing external forcings is assessed for the period 1600–2100 using an ensemble of simulations. In the industrial period from 1850 onward SOCOL-MPIOM overestimates the global mean surface air temperature increase in comparison to observational data sets. Sensitivity simulations show that this overestimation can be attributed to a combination of factors: the solar forcing reconstruction, the simulated ozone changes, and incomplete aerosol effects and land use changes.

Future work will concentrate on the role of chemistry–climate feedbacks under changing external forcings.

References

For further information please contact: Dr. T. Sukhodolov