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?

The charts below compare the observed trends of March-April-May (MAM) temperature and precipitation to two different model systems. These box and whisker plots show the observed values (green dot) compared to the range of values from the model ensembles. The range is denoted by the line (whisker) between the maximum (red asterisk) and the minimum (blue asterisk). The bottom and top of the box are the 25th and 75th percentile (the lower and upper quartiles, respectively), and the band near the middle of the box is the 50th percentile (the median). In each of the plots, the top half shows the temperature comparison and the bottom half shows the precipitation comparison.

The 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.

The 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.