CMIP5 Experiments

The newest generation of climate models coordinated by the Coupled Model Inter-Comparison Project 5 (CMIP5) provides researchers and decision makers with the most up-to-date view of future climatic changes. Over 40 different modeling groups around the globe are participating in CMIP5. A suite of coordinated experiments was conducted to facilitate an intermodel comparison. This process involves each modeling group performing the same experiment on their model by using the same external forcings (i.e., increasing greenhouse gas and aerosol emissions).

Among the most widely used experiments in the CMIP5 are the long-term experiments predicated on projected changes in radiative forcing (additional energy trapped by the Earth-Atmosphere system, measured in W/m2, quantified relative to 1850s climate) by way of increasing atmospheric concentrations of greenhouse gases. Somewhat analogous to the previous generation of CMIP3 climate models forced by a variety of emission scenarios, the new CMIP5 experiments include several different future trajectories based on human actions called Representative Concentration Pathways (RCPs). Unlike previous emission scenarios that prescribed different rates of emissions as the basis for understanding atmospheric concentrations and future climate, the philosophy of the RCPs is to establish a range of changes in radiative forcing from which possible consequences (e.g., changes in emissions, land-use) can be derived.

For the MACA downscaling, we considered GCM outputs from the historical experiment for the years 1950-2005 and GCM outputs from the 2 future experiments RCP4.5 and RCP8.5 for 2006-2100. RCP4.5, refers to the experiment where an additional 4.5 W/m2 is trapped in the earth-atmosphere system by 2100 compared to preindustrial conditions, which is a future scenario of moderate climate action. RCP8.5, refers to the experiment where an additional 8.5 W/m2 is trapped by 2100, a future with no climate action and high emissions.