This is the third edition of Climate Change in Colorado. In 2008, the Western Water Assessment program at the University of Colorado Boulder, in collaboration with the Colorado Water Conservation Board (CWCB), produced the first edition, Climate Change in Colorado: A Synthesis to Support Water Resources Management and Adaptation (Ray et al. 2008). The 2008 report synthesized the current science on the physical aspects of climate change relevant to evaluating future impacts on Colorado’s water resources. It presented scientific analyses to support future studies and state efforts to develop a water adaptation plan. The 2008 Report was notable for being one of the first state-level climate change assessments; today, similar assessments have been conducted in at least 25 states.
Several years later, the CWCB partnered with the Western Water Assessment to undertake a thorough update and revision of the 2008 Report, the result of which was also called Climate Change in Colorado: A Synthesis to Support Water Resources Management and Adaptation (Lukas et al. 2014). This update again covered the observed trends and future projections of hydroclimate variables — including temperature, precipitation, snowmelt, and runoff — that determine both water supply and demand for the state. The 2014 report also had a broader scope and more detail in many areas than the 2008 report, doubling its length compared to its predecessor.
The main findings about recent climate trends and projected future climate change for Colorado were consistent between the two reports: Colorado’s climate has become much warmer and further warming is expected; precipitation has been highly variable, and its future direction was uncertain; significant future changes to the water cycle are likely due to the effects of warming alone, including decreases in snowpack and runoff.
The information in the 2008 and 2014 reports has been used as guidance for many statewide and local water planning and climate planning documents and processes, including the Colorado River Water Availability Study (CRWAS; CWCB 2012 and CWCB 2019a), Colorado Water Plan (CWCB 2023), Colorado Climate Plan (State of Colorado 2018), and the Colorado Drought Mitigation and Response Plan (CWCB 2018). The 2008 and 2014 reports have each been cited in about 100 peer-reviewed studies, indicating that they have been highly regarded and relied upon in the scientific community as well.
For this third edition of Climate Change in Colorado, we cover a similar scope, though with overall less detail compared to the 2014 report. The core function of the report is still to describe recent trends in Colorado’s climate and hydrology and interpret the model-based projections of future climate and hydrology. Compared to its predecessors, this report has greater coverage of climate extremes and hazards, including heat waves, droughts, wildfires, and extreme precipitation and floods. The overall societal impacts of climate change will be driven by both changes in these extreme events and by pervasive changes in the average climate.
While this report provides a scientific basis to prompt further studies of water resources impacts and support planning and adaptation efforts, the assessment of specific local sensitivities and vulnerabilities is beyond the scope of this report. Several other resources, including the Colorado Water Plan (CWCB 2023), the Analysis and Technical Update to the Colorado Water Plan (CWCB 2019b) and the Colorado River Water Availability Study (CRWAS), Phase II (CWCB 2019a) provide more detailed assessments of climate change impacts on water resources at the basin scale and smaller. The Colorado Climate Change Vulnerability Study (Gordon and Ojima 2015) focused on vulnerabilities due to climate change across multiple sectors, including water. A forthcoming compendium study to this report will further describe climate impacts and vulnerabilities based on hazards identified here.
The key findings of this report are summarized at the beginning of each chapter and in the Executive Summary that precedes Chapter 1.
Chapter 2 provides analysis of the observed and projected changes in temperature and precipitation. Chapter 3 provides analysis of the observed and projected changes in Colorado’s water, including snowpack, streamflow volume and timing, soil moisture, and evapotranspiration. Chapter 4 assesses the observed and projected changes in different climate hazards and extremes. Appendix A provides supplemental information on the observational climate dataset and the climate model projections used in the report.
While Colorado’s climate has characteristics specific to our state's particular geography, it plays out within a much broader arena: the global climate system. Since the 2014 report, the evidence that human influences have impacted the global climate system has only strengthened. The latest report from the Intergovernmental Panel on Climate Change (IPCC 2023) states that
It is unequivocal that human influence has warmed the atmosphere, ocean, and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere [snow and ice] and biosphere have occurred. (Summary for Policymakers, p. 4)
The observed globally averaged warming of the earth’s surface (land and ocean) as of early 2023 has reached 1.4°F (0.8°C) since 1980, and a total of 2.0°F (1.1°C) relative to the 1850-1900 period. It is estimated that the total warming influence of human drivers is responsible for all the globally averaged warming relative to 1850-1900, while solar and volcanic drivers and natural climate variability have had little net effect over this period (IPCC 2023). The most important human drivers of warming are the increases in greenhouse gases, principally carbon dioxide (CO2) and methane (CH4), which have acted to trap additional heat in the lower atmosphere and at the earth’s surface compared to pre-industrial (<1750) conditions. As of 2022, the annually averaged level of CO2 in the atmosphere was at 420 parts per million (ppm), higher than at any time in at least 2 million years (IPCC 2023). The anthropogenic (human-caused) increases in CO2 (+50%) and methane (+150%) since 1750 greatly exceed the natural changes in those two gases that occurred over thousands of years between past glacial and interglacial periods (IPCC 2023).
The rapid global warming of the past several decades, a rate unprecedented in at least 2000 years, is associated with pervasive changes to the earth system (IPCC 2023), including:
These global climate trends and changes have shaped the recent evolution of Colorado's climate and will continue to do so. Like nearly every other part of the globe, Colorado has warmed substantially over the past century, particularly since the 1980s, as described in Chapter 2. Figure 1.1 shows that the overall trajectory of Colorado’s observed temperatures since 1900 has closely followed those of U.S. and global temperatures. Like other land areas, Colorado has warmed more than the global average - which mainly reflects the slower-warming oceans that cover 71% of the Earth's surface. This relative difference between Colorado's warming and the globally averaged warming is expected in future warming as well.
Observed 5-year running mean surface temperature departure from a 20th century baseline for Colorado (black), the U.S. (red), and the globe (orange). Data from NOAA National Centers for Environmental Information.