Dr. Johns told the audience that the UNFCCC is framed around a dangerous climate change. There exists no hard and fast definition, but a canonical interpretation is that a global warming exceeding 2 degrees Celsius relative to the pre-industrial time would generate a critical situation. So keeping below the 2 degress Celsius target encapsulates in a simple way the stated climate policy objectives.
The study performed in the Ensembles project bridges between the previous (IPCC AR4) and the forthcoming (IPCC AR 5) climate experiments and the question was asked to what extent climate change is likely to be controlled through aggressive emissions reduction.
The old approach starts from socio-economic storylines and scenarios, explained Dr. Johns. The IPCC AR5 long term experimental design starts from representative concentration pathways. The researchers want to go in the other direction with modelling by going backwards.
A comparison has been performed between A1B unmitigated emissions which depart from "business as usual" and E1 mitigated emissions. This new mitigation scenario has been developed using the Image 2.4 integrated assessment model taking aggressive carbon emissions reduction measures relative to the A1B baseline.
E1 emissions reductions are achieved through global energy measurements. The equivalent CO2 concentration that would give the same radiative forcing as a combination of individual well-mixed greenhouse gases. The sulfate aerosol burden is not so easy to control, according to Dr. Johns.
There is a big spread among the different models when you compare them. The land use change is an imposed forcing in some models. E1 involves increases in agricultural productivity and reduced deforestation.
If we take a look at the global temperature response (11-yr means), the E1 response flattens at 2050, Dr. Johns showed. There is a similar spread and model ordering.
If we take into account the global precipitation response (11-yr means), there is a larger variation between models. The E1 stabilisation returns here. There is a larger spread in A1B than in E1 and there is
uncertainty in the sign of the response.
If we observe the precipitation versus the temperature factor, then we can see that for the mitigation phase the temperature is higher. We can also observe increasing stabilisation near the mitigation phase, according to Dr. Johns.
Most models show a quasi-linear behaviour. Global emissions reductions are needed to achieve the E1 pathway. The new design provides a good framework for comprehensive climate model intercomparison and potentially enables better prediction.
Better understanding is needed of proceses and feedbacks involved in the global and regional hydrological cycle including aerosols effects and rediative forcings, carbon cycle dynamics, and land use change in terrrestrial carbon system, concluded Dr. Johns.