subdivision and DenverClimate Change and Projections

Recent Climate Changes and their Causes

The climate of the Intermountain West is changing, with multiple independent measurements indicating an overall warming of about 2°F across the region in the past 30 years. While climate in our region has always been highly variable at annual, decadal, century, and longer time scales, the rate of recent change is unusually rapid. It is also consistent with the well-understood physical effects of the increasing accumulations of greenhouse gases, including carbon dioxide, in the atmosphere. While there has been no study specifically investigating whether the recent warming trends in Colorado, Utah, and Wyoming can be attributed to greenhouse gases, such attribution has been made at the continental (western North America) and global scales. It is reasonable to conclude that a substantial fraction of the recent warming in our region is due to anthropogenic climate change.

It is less clear whether the persistent drought conditions in our region since 2000 are related to anthropogenic climate change. The variations since 2000 in precipitation—the main driver of drought conditions—are consistent with the natural variability seen in long-term observed climate and paleoclimate records. However, the observed warming may have increased the severity of drought and exacerbated drought impacts, such as low streamflows.

Projections of Future Climate

Global Climate Models (GCMs) are highly sophisticated computer representations of the global climate system—the atmosphere, the oceans, ice sheets and sea ice, and the land surface—based on both physical laws and parameters derived from observation. GCMs are the principal tools used by climate scientists to diagnose the causes of climate variability and to make projections of future climate, given the potential trajectories (or scenarios) for greenhouse gas emissions and concentrations. (The word projection is used instead of forecast to indicate that the model output depends on these assumptions about future greenhouse gas concentrations.)

The consensus of projections from about 35 different GCMs compiled for the IPCC 5th Assessment Report (2013) is that the Intermountain West will warm by +2°F to +6.5°F by mid-century, relative to the 1971–2000 baseline. The range of projections reflects both the different greenhouse gas emission scenarios and differences among the models in how future climate will unfold under a given emissions scenarios. The projections show summers warming more than winters, and typical summer temperatures by 2050 will be as warm as or warmer than the hottest 10% of summers that occurred in the 20th century. Temperature regimes will effectively shift upslope and northwards as the climate warms.

The individual GCM projections have less agreement about whether average annual precipitation will increase or decrease in our region by 2050. The multi-model average shows little change in annual mean precipitation by 2050, though with a slight tendency towards drying in the southern part of the region, and wetter conditions in the northern part of the region. The models also suggest a seasonal shift in precipitation, with the combined effects of a northward-shifting storm track, potentially wetter storms and a drying of the sub-tropical regions globally resulting in more mid-winter precipitation, and in some areas, a decrease in late spring and summer precipitation.

Together, the uncertain changes in precipitation and the more certain impacts of warming lead to a broad range of plausible futures for water in the Intermountain West. Consistent themes across those futures include snowmelt and runoff occurring earlier in the spring, decreased late-summer streamflows, and increased water use by crops and other vegetation. For more information on the projected changes, see the assessment reports listed below.

Climate Change Assessments - from State-level to Global

WWA-CWCB Climate Change in Colorado Report (2014) - A synthesis of climate change science highlighting observed trends, modeling, and projections of temperature, precipitation, snowmelt, and runoff. While the report focuses on Colorado, much of the text and many of the figures are relevant to the broader Intermountain West region.

Assessment of Climate Change in the Southwest U.S. (2013) - A contribution to the National Climate Assessment Third Report, this report is a summary and synthesis of the past, present, and projected future of the region’s climate, emphasizing new information and understandings since 2009. (The 6-state 'Southwest' region includes Utah and Colorado, but not Wyoming.)

National Climate Assessment (NCA) Third Report (2014) - The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future. A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.

Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) Working Group I - The Physical Science Basis (2013) - The most comprehensive assessment of the physical science of climate change. The Working Group I report (~1000 pages) is summarized in the Summary for Policy Makers (~20 pages), and the Technical Summary (~70 pages).