An Earth system model, HadCM3C, is used to explore the consequences of the mitigation of greenhouse gas emissions compared to a ‘business as usual’ scenario.
The model is significantly more advanced than models typically used for providing mitigation advice. We use a collection of model runs, known as an ensemble, in order to include uncertainty in our understanding about the way Earth system processes operate, and how this uncertainty might affect future behaviour of the Earth system under different emissions scenarios.
The simulations represent a range of plausible climates, consistent with our current understanding of the behaviour of the Earth system. This ensemble cannot be viewed as ‘a distribution of likely climate change under different scenarios’ but instead adds new information which helps form that distribution. The main conclusions of this report are as follows:
- ‘Avoided’ global temperature change from the mitigation of greenhouse gas emissions, as compared with ‘business as usual’, is between 1°C and 3°C by 2100. As with previous studies, the temperature of the land surface, and in the Arctic region, increases more than the global mean temperature. The Earth system in our ensemble shows a wide range of behaviour, broadly consistent with previous advice, and findings from research with other models.
- Mitigation significantly reduces the uncertainty of future earth system behaviour changes.
- The Earth system is committed to change, even under a mitigation scenario.
- Atmospheric CO2 concentration reduces only slowly under quite severe reductions in greenhouse gas emissions. Once raised, changes in global temperature and precipitation are not reversed significantly by the end of the 21st century, even under aggressive mitigation.
- The surface mass balance of the Greenland ice sheet is strongly impacted by temperature change. With global temperatures limited to 2°C the ice sheet is extremely stable. The likelihood of mass loss increases to 50% at about 4.5°C.
- ‘Dieback’ of the Amazon rainforest is a low probability outcome, but is more likely and more severe under a business as usual scenario. ‘Dieback’ of the Amazon is a low probability, but high impact event and so continues to represent a significant risk.
- • Land carbon sinks change to sources in both business and usual and mitigation scenarios. This happens sooner with mitigation but the carbon loss is much more severe under the business as usual scenario.