Understanding how climate has changed, how it is likely to change in the future, and what this means for water, wildfire, ecosystems, species, and people requires complex interdisciplinary study. It also requires an understanding of uncertainty, as projections of the future, even if they are based on the best data and expert modeling available, should not be treated as "for certain" forecasts. Instead, scientists and policy makers rely on a suite of global circulation (GCM) models, each with its strengths and weaknesses, to develop plausible scenerios for our future.

Understanding climate change in the San Juan Mountain region has some special considerations. First, the mountains and valleys that make up our region are what scientists refer to as "complex topography" that is difficult to model on small geographic scales. Because of this complex terrain, more observing stations are needed to represent small scale patterns in climate. Second, in many cases, studies of climate effects (on ecosystems, species, etc.) for our region do not exist. Until they can be completed, our best knowledge comes from studies of similar regions.

The following is a compilation of the results of pertinant studies providing a brief summary for climate change observations and projections in Southwest Colorado. Understanding the existing and anticipated changes in climate is the first step in preparing for the impacts associated with changes.

Temperature, Evaporation/Evapotranspiration, Precipitation, Snowpack, and Streamflow

The climate summary, compiled by MSI, is being used by the Four Corners Office of Resource Efficency (4CORE) - for La Plata County, Colorado. The table at the end of this section summarizes the projected climate changes in Southwest Colorado and includes details on the source of these projections and level of certainty. The table demonstrates that changes in several factors are interrelated and that the impacts of these changes will be complex.

Temperature

Southwestern Colorado has warmed about 2° F in the last three decades (i.e., 1977 to 2007). This rate of warming is the same as for Western Colorado, but greater than the Western US, or any other region of the US except Alaska.
Temperatures are likely to increase by an additional 1.5 to 3.5 °F by 2025 and 2.5 to 5.5 °F by 2050.
Summers are projected to warm more than winters. By 2050, typical average monthly temperatures in the summer are projected to be as warm as or warmer than the hottest 10% of summers from 1950 to 1999.
The climate of the mountains is projected to migrate upward in elevation and the climate of the Desert Southwest to progress up into the valleys.
Examples of Potential Impacts:
- Changes in agricultural crop production and spread of agricultural pests.
- Earlier snowmelt and timing of peak river flows.
- Increased energy usage for heating and cooling.
- Increased heat-related illnesses and spread of disease.
- More amenity-led migration to the mountains from desert communities.

Evaporation and Evapotranspiration

Due to increased temperatures, the rate at which water is evaporated from water bodies, soil, and vegetation is very likely to increase.
This will make the environment drier even if precipitation stays the same.
Examples of Potential Impacts
- Decrease in streamflow and water stored in reservoirs, especially in hot, dry years.
- Decreased soil moisture for crops and natural vegetation.
- Increased irrigation needs or shifts to more drought-resistant crops.
- Shifts in elevation ranges of plants and animals.
- Increased wildfire occurrence and forest die-offs.

Precipitation

The San Juan Mountain region is difficult to model for precipitation due to complex topography and natural variability in precipitation patterns.
Projections of change in amounts of precipitation for the region are not in consensus. Some studies indicate that annual precipitation will decrease slightly while others project an increase in the winter.
Some models project more variable precipitation patterns with more frequent extreme events.
Examples of Potential Impacts:
- Increased storm events and severity.
- Change in aquatic habitat.

Snowpack and Streamflow

Warming temperatures are projected to have significant effects on snowpack, timing of snowmelt, and streamflow even without a decrease in precipitation.
It is likely that in the future more precipitation will fall as snow, snowpacks will decrease and melt earlier, and peak streamflow will occur earlier in the spring.
From 1978 to 2004, snowmelt already shifted about two weeks earlier in Western Colorado. Snowmelt has shifted even earlier in the Pacific Northwest and Northern California, however.
Decreases in the amount of water contained in snowpacks are projected to be greater at elevations below 8,200 feet (i.e., a 20 to 60% reduction of snowpack by the period 2040 to 2069). Above 8,200 feet, the snowpack is anticipated to decrease by 10 to 20%.
Examples of Potential Impacts
- More flooding in the spring.
- Reduced water availability in the summer.
- Shorter seasons for ski/snow and white water recreation industries.
- Increased summer recreation and tourism opportunities.

Literature Cited

Backlund, P., et al. 2008. The effects of climate change on agriculture, land resources, water resources, and biodiversity: Introduction. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Washington, DC., USA, pp. 362. http://www.sap43.ucar.edu/documents/SAP_4.3_6.18.pdf

Barnett, T.P.; Pierce, D.W.; Hidalgo, H.G.; Bonfils, C.; Santer, B.D.; Das, T.; Bala, G.; Wood, A.W.; Nozawa, T.; Mirin, A.A.; Cayan, D.R.; Dettinger, M.D. 2008. Human-Induced Changes in the Hydrology of the Western United States. Science. 19: 1080-1083.

Christensen, N and D.P. Lettenmaier. 2006. A multimodel ensemble approach to assessment of climate change impacts to the hydrology and water resources of the Colorado River Basin. Hydrology and Earth Systems Science Discussion, 3, 1-44.

Clow, D. 2008. Changes in the Timing of Snowmelt in Colorado, Presentation at the 50th Annual Convention of the Colorado Water Congress, January 23-25, Denver, CO.

Hamlet, A.F.; Mote, P.W.; Clark, M.P.; Lettenmaier, D.P. 2007. 20th century trends in runoff, evapotranspiration, and soil moisture in the Western U.S. Journal of Climate. 20: 1468-1486.

IPCC 2007: Climate Change 2007 Synthesis Report: Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Core Writing Team, Pachauri, R.K. and Reisinger, A. (Eds.), pp 104. http://www.ipcc.ch/ipccreports/ar4-syr.htm

IPCC 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. http://www.ipcc.ch/ipccreports/ar4-wg1.htm

IPCC 2008: IPCC Technical Paper VI, Climate and Water, Bates, B.C., Z.W. Kundzewicz, S. Wu and J.P. Palutikof (Eds.), 210 pp. http://www.ipcc.ch/ipccreports/technical-papers.htm

Miles, E.L.; Lettenmaier, D.P.; Mantua, N.J. [et al.]. 2007. HB1303 interim report: a comprehensive assessment of the impacts of climate change on the State of Washington. Seattle, WA: Climate Impacts Group, University of Washington.

Milly, PC, Dunne, KA, and Vecchia, AV. 2005. Global patterns of treds in streamflow ad water availability in a changing climte. Nature, 438, 347-350.

Mote, P.W.; Hamlet, A.F.; Clark, M.; Lettenmaier, D.P. 2005. Declining mountain snowpack in western North America. Bulletin of the American Meteorological Society. 86: 39-49.

National Academies of Science. 2007. Evaluating Progress of the U.S. Climate Change Science Program: Methods and Preliminary Results. Committee on Strategic Advice on the U.S. Climate Change Science Program, National Research Council. Pp. 178.

Salathé, E.P. 2006. Influences of a shift in North Pacific storm tracks on western North American precipitation under global warming. Geophysical Research Letters. 33: L19820.

Stewart, I.T.; Cayan, D.R.; Dettinger, M.D. 2005. Changes toward earlier streamflow timing across western North America. Journal of Climatology. 18: 1136–1155.
Timmermann, A.; Oberhuber, J.; Bacher, A.; Esch, M.; Latif, M.; Roeckner, E. 1999. Increased El Niño frequency in a climate model forced by future greenhouse warming. Nature. 398: 694-697.

Ranwala, I. 2008. Chapter 5 of Dissertation: 20th Century Climate Change In The San Juan Mountains In Southwest Colorado: Investigating long term trends in climate and hydrological variables and explaining the causes for a rapid climate change in the region between 1985-2005. Rutgers University. 35 pp.

Ray, A.J, Barsugli, J.J., Averyt, K.B., and 5 Others. 2008. Climate change in Colorado: A synthesis to support water resources management and adaptation. A report by the Western Water assessment for the Colorado Water Conservation Board. Pp. 53. http://cwcb.state.co.us/Home/ClimateChange/ClimateChangeInColoradoReport/