Given the trends of warmer temperatures and the 2012 drought conditions in Colorado, one might ask:
- How have changing conditions over the Southwest been symptomatic of human-induced climate change?
- Is recent Southwest dryness a symptom of human-induced climate change?
Observed vs. CMIP5The CMIP5 (Coupled Model Intercomparison Project Phase 5) dataset is a set of coordinated model experiments from 20 climate modelling groups around the world. This ensemble of different solutions can be used to examine the observed change in temperature with model simulations that include forcings due to increasing CO2. In the plot below, the observed temperature trend aligns with the rise in temperature due to human-induced forcings. However, the observed change in precipitation (drying) is not well simulated by the CMIP5 model runs.
Observed vs. AMIPThe AMIP (Atmospheric Model Intercomparison Project) is a standard experimental protocol for global atmospheric general circulation models (AGCMs). The AGCMs are constrained by realistic sea surface temperature and sea ice so the output is forced by ocean surface processes instead of greenhouse gasses. In the plot below, the precipitation much more closely matches the AMIP solutions, indicating that the primary driving force of the reduced precipitation is a natural variation of the ocean related to El Niño/La Niña cycles.