The Climate Change scenarios come from simulations with Global Climate Models (GCMs).
Simulations come to each 250-600 km2, according to the climate model spatial resolution. The GCMs usually consider about 10-15 vertical layers.
Relevant centres in USA, UK, Germany, Russia, France, Canada, China, Australia and other countries developed climate models in the last 20 years. The last report (AR5, 2013) of IPCC Working Group 1 considered the results of CMIP5 Project, an international multi-model collaboration.
Besides the climate models, Climate Change scenarios rely on “Emission Scenarios”.
These emission scenarios estimate the amount of CO2 and other greenhouse gases launched to the atmosphere at each of the GCM cells.
In their former AR4 report, IPCC considered SRES scenarios, based on a study which they coordinated. Instead, the 2013 IPCC Report rely on the results of an international commitee, which developed new emission scenarios: RCP. These scenarios estimate greenhouse gas emissions during this century. The RCP scenarios consider current and future economical and social conditions, based on the foreseen population and economy growth.
Besides, the GCMs have shown man’s influence in global warming. Simulations and Observations do not match, unless man’s activity is considered besides natural forcing.
Usually, GCM simulation outputs are in higher scales than the needs of specific applications. Hence, GCM results must be downscaled.
There are several statistically and physically-based downscaling techniques which reduce GCMs outputs to scales of 50 km2 or even lower.
Most Climate-Change impact assessments rely on such downscaled data.