Climate Change in Colorado
Climate Change in Colorado
Chapter 2 - Changes in Colorado's Climate
Climate variable/event Recent trend Projected future change Confidence in change
Average Temperature Warmer Warmer Very high
Annual Precipitation Lower Uncertain Low


2.1 Overview

Colorado’s Average Climate

Colorado’s climate reflects its mid-continental location, high elevations, and the complex topography of the mountains, plains, and plateaus. Topographic influences on weather and climate processes result in large variations in climate over short distances. Wind, humidity, temperature, and precipitation patterns are all modulated by sharp changes in elevation and the orientation of mountain ranges and valleys (Doesken et al. 2003).

The state’s interior location results in frequent sunshine, low humidity, and large variations in daily temperature ranges and annual temperature variability. The distance from large sources of moisture (i.e., Pacific Ocean and Gulf of Mexico) results in lighter precipitation for the lower elevations. High mountain ranges benefit from Pacific moisture moving eastward during the winter months.

Average Temperature

For most parts of the state, on average, January tends to be the coldest month of the year, and July is the warmest (Figure 2.1). Topography plays a role in temperatures – in general, temperatures decrease with elevation. Average high elevation temperatures (over 10,000 feet above sea level [asl]) range from single digits in the winter months to 60s and 70s (°F) in the summer. For lower elevation areas and the plains (elevations around 5,000 ft asl or less), average temperatures dip to the teens in the winter and frequently top the 90s (°F) in the summer. Middle elevations offer warm temperatures in the summer, but rarely into the 90s, with frequent single digit temperatures in the winter. Extremes across the state range from negative temperatures (with winter temperatures observed below -40°F in the high mountain valleys) to triple digits (over 110°F occurring in the lower river valleys of the eastern plains of Colorado).

Average Precipitation

Topography also plays an important role in influencing precipitation processes and patterns. Precipitation typically increases with elevation in all seasons, but especially in winter when nearly all moisture falls as snow. The seasonal cycle of precipitation is highly dependent on location (Figure 2.1). The Eastern Plains are generally wetter during the spring and summer months, with a May peak in northeast Colorado and a July peak in southeast Colorado. The higher mountain areas tend to be wetter during the winter and early spring months, and southwest Colorado’s wettest months coincide with the occurrence of the North American Monsoon in August and September. Annual precipitation totals are less than 10 inches in the San Luis Valley, while the high mountain ranges typically receive over 40 inches of liquid precipitation in one year (with amounts observed between 60 and 80 inches in wet years).

Figure 2.1

1991-2020 normal monthly precipitation (green bars), daily average maximum (red line) and daily average minimum (blue line) temperatures for eight National Weather Service Cooperative Observer Program (COOP) stations around the state. Precipitation in inches and temperature in degrees Fahrenheit. Locations of the eight stations are labeled on the top left map.

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For recent trends and variability in temperature and precipitation, we have relied on NOAA nClimGrid, a gridded dataset based on weather observations from hundreds of sites across Colorado, and corrected for biases from changes in instrumentation, changes in the daily time of observation, moves in station location and other inhomogeneities. An earlier version of nClimGrid was used in the 2014 report. See Appendix A for more information on this dataset and a comparison with a similar dataset.

For likely future changes in temperature and precipitation, as with the previous two reports, we relied on the simulations (projections) from global climate models (GCMs). The 2014 report featured results from the then-latest global archive of GCM projections, known as CMIP5 (Coupled Model Intercomparison Project, Phase 5; see Appendix A for more information about the CMIPs). The 2014 report also compared those results from the previous archive (CMIP3).

In this report, we show results from CMIP5 as well as the most recent archive of GCM projections (CMIP6) that was released in 2020-21. The projections from CMIP6 have not yet been used to generate basin-scale projections of hydrology and water resources (such as in Chapter 3); thus, we have chosen to emphasize CMIP5 projections throughout Chapters 2, 3, and 4 to maintain consistency among the analyses. We also examine the differences between the CMIP5 and CMIP6 projections for Colorado.

2.2 Temperature

The most fundamental and pervasive effect of anthropogenic (human-caused) climate change is an overall warming of the climate system. This global warming has manifested in nearly all regions of the world in the past several decades.

Observed temperature changes

Colorado statewide temperatures have warmed since systematic instrumental observation records began in the late 19th century (Fig. 2.3). When compared to the 1971-2000 average, only one year in the 21st century had below-average annual temperature. Seven of the top 10 hottest years on record have occurred since 2010. Recent mean temperatures (2001-2022) have averaged 1.4°F warmer than the 1971-2000 average (45.1°F).

Figure 2.3

Colorado statewide temperature anomaly (°F) with respect to the 1971-2000 average of 45.1°F. The 1895- 2022 trend (yellow dashed), and 1980-2022 (red dashed) lines are included.

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We analyzed temperature changes by season, both long-term (from 1895-2022) and more recent trends (1980-2022). From 1895-2022, the winter season (Dec-Jan-Feb) shows the greatest warming (Table 2.2). However, since 1980, winter warming has diminished, largely due to recent cooling observed in February. Fall season (Sep-Oct-Nov) temperatures have warmed more than any other season for 1980-2022 (Table 2.2). While all seasons have exhibited increasing trends in both short- and long-term periods, with seasonal changes ranging between +1°F to +3°F for the 1980-2022 period.

Statewide 1895-2022 change 1980-2022 change
Winter +3.3°F +1.0°F
Spring +2.6°F +1.7°F
Summer +2.7°F +2.5°F
Fall +2.1°F +3.1°F
Annual +2.9°F +2.3°F

Table 2.2: Changes in statewide average annual and seasonal temperature as calculated by the linear trend, 1895- 2022 (middle column) and 1980-2022 (right column).

We also analyzed seasonal and annual temperature changes for each of the 11 alternate climate divisions (see sidebar for description of climate divisions). Figure 2.4 shows the seasonal changes in temperature for each division for the recent period of 1980 to 2022. Most notably, the greatest warming has occurred in the fall (Fig. 2.4d) for each climate division. Summer warming has also been significant (Fig. 2.4c), with larger changes in the western climate divisions. The south and the west have observed more warming in the spring (Fig. 2.4b). The Northern Front Range (including the majority of the state’s population) has experienced little to no warming in spring, while the Central Mountains and South Park area experienced little to no warming during the winter (Fig. 2.4a). Annually, the greatest warming has been observed over the Southwest and San Luis Valley climate regions.

Figure 2.4

Changes in observed climate division temperatures, 1980-2022, for (a) winter, December-January-February, (b) spring, March-April-May, (c) summer, June-July-August, and (d) fall, September-October-November.

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Attribution of the observed trends

The pervasive observed warming trends across Colorado are comparable, in terms of timing and magnitude, to warming trends that have been observed regionally, nationally, and globally. At the global scale, human influence has been the main driver of observed warming in the past several decades (USGCRP 2017, IPCC 2021). The warming trend in the southwest U.S., including Colorado, has likewise been primarily attributed to human influence (Lehner et al. 2018). Figure 2.5 shows that the trajectory of observed annual average temperature for Colorado (gray) since 1950 is comparable to the trajectories of median modeled temperatures from the CMIP3 (yellow) and CMIP5 (orange) climate model ensembles. These model runs assume greenhouse gas emissions and atmospheric concentrations similar to what has actually occurred through 2022. The similarity between the observed and modeled statewide warming trends is consistent with the evidence at broader spatial scales that indicates human influence has played a substantial role in Colorado’s recent warming trend.

Figure 2.5

Observed statewide annual average temperatures 1950-2022 (same data as in Figure 2.3), compared with the median historical simulation plus the median future projection from the CMIP3 and CMIP5 climate model ensembles, respectively. (Data: Observations: NOAA NCEI nClimGrid,; CMIP3 and CMIP5 projections: GDO-DCP,

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Future temperature projections

There is very high confidence that the climate of Colorado will continue to warm in all seasons through the mid-21st century, given our understanding of the physical mechanisms for warming, the observed warming trend, and climate model projections. While the magnitude of warming is uncertain, by 2050, Colorado’s average annual temperatures will likely match or exceed the very warmest years of the past, bringing large changes in the frequency and severity of heat waves, as we will discuss in Section 4.1. Note that in the analyses below, we focus on the medium-low emissions scenario RCP4.5, used for the CMIP5 climate model runs, and its counterpart SSP2-4.5, used for the CMIP6 model runs. The section “Emissions Scenarios” in Appendix A explains why we focused on these scenarios and provides more information about these and other emissions scenarios.

Under RCP4.5, Colorado statewide annual temperatures are projected by the CMIP5 climate models to warm by +2.5°F to +5°F compared to the late 20th century (1971-2000) average (Figure 2.6). We continue to use this 1971-2000 baseline to maintain consistency with the analysis of climate projections in the 2008 and 2014 reports, and in other state reports such as the Colorado Water Plan. Colorado has already warmed by about 1.5°F beyond this baseline, as detailed below.

Figure 2.6

Projected future temperature change for Colorado statewide for a 2050-centered period (2035-2064) relative to 1971-2000, from the CMIP5 and CMIP6 climate models under medium-low emissions scenarios. The solid orange bars show the middle 80% of the model projections (10th-90th percentiles); the two orange dashes show the minimum and maximum projections; the open squares show the median projections.

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Under a comparable emissions scenario (SSP2-4.5), the CMIP6 models show a range of warming that is shifted upward, especially at the low end, compared to CMIP5, showing +3.5°F to +5°F warming for Colorado, as taken from the 10th to 90th percentiles of the projected values. The two red bars on the right side of Figure 2.6 show that the climate change scenarios for 2050 used in the Colorado Water Plan (CWCB 2015, CWCB 2023) are within the range of both CMIP5 and CMIP6 under 4.5 emissions scenarios. It is not surprising that the CMIP6 models show overall warmer futures for Colorado than CMIP5, since the global temperature response of the CMIP6 models given additional increments of greenhouse gases (i.e., climate sensitivity) is overall higher than for the CMIP5 models (see Appendix A for more detail on the CMIP6 “hot” model issue).

It is important to remember that Colorado has already observed, through 2022, a substantial fraction of the projected warming relative to the 1971-2000 baseline: about +1.5°F, depending on the calculation method (Fig. 2.6). Thus, the projected statewide warming for 2050 shown by the CMIP5 models is +1.0°F to +3.5°F relative to “today”, and in the CMIP6 models, +2.0°F to +3.5°F relative to today. The fact that Colorado has already experienced +1.5°F of warming relative to 1971-2000 suggests that the lowest-warming projections in the CMIP5 ensemble, below the 10th percentile, are now very unlikely outcomes.

Most of the projections under a medium-low (4.5) emissions scenario, whether from CMIP5 or CMIP6, show a mid-century climate that is, on average, at least 3°F warmer than the 1971–2000 baseline. If this does occur, an “average” year in 2050 will be warmer than the very warmest individual years observed through 2022 (Figure 2.7).

For a later future period centered on 2070 (2055-2084), the CMIP5 models under medium-low (RCP4.5) emissions scenario projects Colorado statewide temperatures to have warmed +3.0°F to +6.5°F of warming relative to 1971-2000, and +1.5°F to +5.0°F of warming relative to today. For the same 2070-centered period, the CMIP6 models under a comparable emissions scenario (SSP2-4.5) show warming of +4.0°F to +7.0°F relative to 1971-2000, and so +2.5°F to +5.5°F of warming relative to today. As seen in Figure 2.7, the difference between the CMIP5 and CMIP6 median warming under the 4.5 scenarios increases to about 1.0°F by 2070.

Figure 2.7

Projected change in Colorado statewide average annual temperatures to 2100, relative to a 1971-2000 baseline, from CMIP5 models (median and range) and CMIP6 models (median only) under medium-low emissions scenarios (RCP4.5, SSP2-4.5), compared to observed temperatures through 2022.

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With continued warming over the next few decades, the future temperatures at every location in Colorado will become more like those currently experienced in places that are to the south, or lower in elevation. With 2°F of further warming, the seasonal temperature regime for Denver would become more like the current temperatures in Pueblo. With 4°F of further warming, Denver’s temperature regime would be similar to Lamar today. With 6°F of further warming, Denver’s temperatures would be slightly warmer than the current temperatures in the warmest parts of the lower Arkansas Valley (Las Animas and La Junta), and similar to Albuquerque, New Mexico. Note that this comparison only speaks to temperatures, not precipitation; Denver is very unlikely to experience a large decline in precipitation that would make the overall climate like Albuquerque's, even with 6°F of warming.

Under a given emissions scenario (e.g., RCP4.5), the differences in warming across the various projections have two sources. The primary one is that the various climate models have different inherent sensitivity to each increment of greenhouse gases, because of how physical feedbacks are represented in each model. The second and lesser source is the “noise” of model-simulated natural (internal) variability. The 30-year averaging period (e.g., 2035-2064) used here is designed to reduce this noise, but some projections will happen to simulate a relatively warmer, or cooler, few decades in the middle of the longer-term warming trend, and we cannot easily distinguish the noise from the background signal (the warming trend). The effect of this noise is more problematic for the precipitation projections than for temperature projections, as will be discussed in section 2.3.

Figure 2.8 shows the statewide seasonal temperature changes projected by CMIP5 models under RCP4.5, using the same data as shown in Figures 2.6 and 2.7. Overall, summer and fall show slightly greater future warming than winter and spring, though the differences between the seasons are relatively small compared to the magnitude of the overall projected warming. The CMIP6 models show the same pattern: summer and fall are expected to warm slightly more than winter and spring.

Figure 2.8

Projected future change in seasonal temperatures for Colorado statewide for a 2050-centered period (2035-2064) relative to 1971-2000, from CMIP5 (36 models/projections) under a medium-low emissions scenario (RCP4.5). The solid orange bars show the middle 80% of the model projections (10th-90th percentiles); the two orange dashes show the minimum and maximum projections; the open squares show the median projections.

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Downscaled (regional) projections of temperature

The “raw” output from climate models provides useful estimates of future climate changes at the global scale down to statewide scales. But the spatial resolution of the data, generally 100-km (60-mi) to 300-km (180-mi) grid boxes in the midlatitudes, is too coarse to adequately represent the complex terrain of Colorado and its effects on climate, or for the data to be used as inputs for watershed hydrology modeling or other impact modeling. Thus, global climate model output is typically downscaled through statistical methods, or via higher-resolution regional climate models (RCMs), in order to better represent localized changes to weather and climate, and to facilitate further modeling. The process of downscaling also includes a bias-correction step which adjusts for systematic biases or offsets between the model-projected climate at regional scales and the observed historical climate, over the period of overlap between the two (e.g., 1950-2005).

For a closer look at how the projected future climate change may vary in different areas in Colorado, we analyzed the CMIP5-LOCA (LOcalized Constructed Analogs) statistically downscaled climate projection dataset developed by Pierce et al. (2014). These projections were not available at the time of the 2014 Report, but they have since been used in many climate assessments and studies, including USGCRP (2017, 2018), Lukas et al. (2020), and Reclamation (2021). Taking the 11 alternative climate divisions described earlier in this chapter, we obtained CMIP5-LOCA data for a 0.75° x 0.75° (40 mi./64 km x 52 mi./83 km) quadrangle within each division.

Figure 2.9 shows the projected change in annual average temperature under RCP4.5 between the historical baseline (1971-2000) and the 2050-centered future period (2035-2064) for the 11 alternative climate divisions. All of them are expected to see substantial future warming into this mid-century period. Slightly greater future warming is generally seen in the divisions in Western Colorado and the northern Front Range, with slightly less warming seen in South Park and the San Luis Valley divisions. Keep in mind that these differences in the projected changes in average temperature between the divisions, which are at most 0.7°F, are much smaller than the overall warming across all divisions (median: 4.1°F), or the uncertainty in the warming across the ensemble of 32 projections (generally ± 2°F). The key point is that all parts of the state are expected to warm at rates that are similar to the statewide average.

Figure 2.9

Projected future change in annual average temperature in 11 alternative Colorado climate divisions for a 2050-centered period (2035-2064) relative to 1971-2000, from an ensemble of 32 CMIP5-LOCA climate projections under a medium-low emissions scenario (RCP4.5). The solid orange bars show the middle 80% of the model projections (10th to 90th percentiles); the two orange dashes show the minimum and maximum projections; the open squares show the median projections.

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2.3 Precipitation

In Colorado, statewide precipitation exhibits high variability at both year-to-year and longer-term decadal timescales (Figure 2.10). With respect to the 1971-2000 average, annual precipitation has varied from 6 inches below average to 6 inches above average. The smoothed time series (Fig. 2.10, gray line) shows frequent extended dry periods with wet periods in between.

Figure 2.10

Colorado statewide water year precipitation anomaly (inches) with respect to the 1971-2000 average of 18.51 inches. Smoothed 10-year running mean (gray line) included.

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Observed precipitation changes

Since the relatively wetter periods of the 1980s and 1990s, Colorado has experienced more persistent dry conditions since 2000. The differences in precipitation and temperature variability necessitate different approaches in analyzing their changes. Rather than calculating a linear trend, we calculate the difference in precipitation between the period 2001-2022 and the period 1951-2000. Statewide, precipitation was 4% lower in 2001-2022 compared to the 1951-2000 average (Table 2.3). These decreases have largely been concentrated in spring, summer, and autumn.

Statewide Change from 1950-2000 to 2001-2022
Winter +3%
Spring -7%
Summer -6%
Fall -5%
Annual -4%

Table 2.3: Recent changes in statewide annual and seasonal precipitation, as calculated by the difference between the 1950-2000 average and the 2001-2022 average.

Dry conditions since 2000 have been particularly notable in western Colorado, with the Southwest division having precipitation decreases of 22%, 11%, and 12% in spring, summer, and fall, respectively (Fig. 2.11b, 2.11c, 2.11d). In contrast, winter precipitation increased over this period, but the increase was largely observed in lower-elevation regions of Colorado, where winter is typically the driest part of the year, thus the seasonal change had less impact on annual precipitation (Fig. 2.11a). The higher elevations saw relatively small changes in winter precipitation over this period.

Figure 2.11

Percent change in precipitation between the periods 1951-2000 and 2001-2022, for (a) winter, December- January-February, (b) spring, March-April-May, (c) summer, June-July- August, and (d) fall, September-October-November (d).

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Colorado’s precipitation variability is partially modulated by the El Niño-Southern Oscillation (ENSO). ENSO is an episodic interaction that occurs between the tropical Pacific Ocean and the atmosphere, which results in the occurrence of three different phases (recurring every 2 to 7 years): El Niño (warmer ocean temperatures in the tropical Pacific), La Niña (cooler ocean temperatures), and neutral (when there is neither an El Niño or La Niña). Variability in ENSO strongly influences global weather patterns. Coastal areas of the U.S. tend to have the strongest correlations with ENSO variability. The general pattern in the western U.S. is that wetter conditions are favored in the Southwest during an El Niño and wetter conditions are favored in the Northwest during a La Niña. With Colorado on the eastern edge of these areas (far from the ocean), and bisecting the two regions latitudinally, the state’s relationship with ENSO is more complex.

La Niña winters tend to be wetter for our northern and central mountains (Fig. 2.12, DJF panel). Aside from that signal, La Niña is generally associated with drier conditions around the state. El Niño favors wetter conditions along the Front Range and west slope in the spring, in northeast Colorado in the summer, and over large portions of the state in the fall (Fig. 2.12). While the relationship between Colorado precipitation and ENSO does exist, ENSO only accounts for a small percentage of precipitation variability. While ENSO forecasts can be used as a guide for more or less favorable precipitation patterns around the state, its year-to-year predictive potential is limited.

Figure 2.12

General relationship between El Niño Southern Oscillation and Colorado seasonal precipitation. Areas of correlation are shaded red when El Niño tends to be wetter and blue if La Niña tends to be wetter.

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El Niño conditions occurred more frequently in the 1980s and 1990s, while La Niña conditions have been much more common since the turn of the century. Colorado’s climate connection with ENSO, and its relative frequencies over the last 40-50 years, may have partially contributed to the persistent dry conditions observed over most of the state since 1980.

Future precipitation projections

The future direction of precipitation change in Colorado is much less certain than for temperature change. The climate models lack consensus about whether Colorado will on average see less, more, or about the same annual precipitation in the future, reflecting potentially offsetting physical mechanisms, as well as the greater complexity of the physical processes controlling precipitation compared to temperature. The climate models—CMIP3, CMIP5, and CMIP6—consistently project is the northernmost U.S. states and Canada will see overall higher annual precipitation in the future, and that the far Southwest and Mexico will see lower annual precipitation in the future. Colorado is in a transition zone between these regions of greater model consensus; this has opposing implications for the northern (more likely wetter) and southern (more likely drier) portions of Colorado, as will be explored in the next section, on downscaled projections of future precipitation.

Figure 2.13 illustrates the projected changes in statewide annual precipitation for Colorado from CMIP5 and CMIP6 models straddle the no-change line under a medium-low emissions scenario, with some projections showing wetter conditions for 2050 (2035-2064) and some showing drier conditions for 2050. The two blue bars on the right side of Figure 2.13 show that the climate change scenarios for precipitation in 2050 used in the Colorado Water Plan (CWCB 2015, CWCB 2023) are within the range of both CMIP5 and CMIP6 under 4.5 emissions scenarios, although the “Hot & Dry” scenario is not as dry as many of the projected precipitation outcomes. Note that even the 90th percentile (+6%) and 10th percentile (-5%) changes shown by the models are much smaller than the observed year-to-year variability in statewide precipitation (+30% to – 40%), although these changes are similar to the largest observed deviations in running 30-year averages in precipitation.

Figure 2.13

Projected future change in average annual precipitation for Colorado statewide for a 2050-centered period (2035-2064) relative to 1971-2000, from the CMIP5 and CMIP6 climate models under medium-low emissions scenarios. The solid blue and brown bars show the middle 80% of the model projections (10th-90th percentiles); the two dashes show the minimum and maximum projections; the open squares show the median projections.

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Most CMIP5 projections also show increases in year-to-year and decadal variability in annual precipitation for Colorado and the interior West over the next several decades (Lukas et al. 2014; Pendergrass et al. 2017). This suggests more frequent occurrences of both very dry and very wet years, and multi-year periods, than seen in the historical record. It also suggests more frequent oscillations from one extreme to the other, such as from 2018 to 2019.

With each model generation since CMIP3, there has been a slight shift towards wetter outcomes. However, the range of projected changes (i.e., model uncertainty) has not shrunk from CMIP5 to CMIP6. For a 2070-centered period, the CMIP5 models show the range of precipitation outcomes shifted slightly wetter than for 2050.

Figure 2.14 shows the seasonal precipitation changes projected by CMIP5 models under RCP4.5 for a 2050-centered period, using the same dataset shown in Figures 2.13. The slight overall model signal towards increased annual precipitation (far left) is strongly accentuated for winter (Dec-Feb) precipitation and to a lesser degree for spring (Mar-May) precipitation. Summer (Jun-Aug) precipitation shows the largest range and uncertainty across the models, with the greatest tendency towards large decreases among the seasons. The projections for fall (Sep-Nov) precipitation are very similar to annual, with a slight tendency towards increased precipitation. The CMIP6 models show outcomes for seasonal precipitation that are very similar to CMIP5.

Figure 2.14

Projected future change in seasonal precipitation for Colorado statewide for a 2050-centered period (2035-2064) relative to 1971-2000, from CMIP5 (36 models/projections) under a medium-low emissions scenario (RCP4.5). The solid blue and brown bars show the middle 80% of the model projections (10th-90th percentiles); the two dashes show the minimum and maximum projections; the open squares show the median projections.

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The climate models disagree about the direction of change in future precipitation for Colorado in part because they disagree about how much the average storm track in the fall, winter, and spring over the western U.S. will shift northward. This northward shift has been observed already and is expected to continue, as a consequence of warming-induced expansion of the dry subtropical high-pressure zone that dominates the climate in the region south of Colorado (Harvey et al. 2020; McAfee et al. 2011). At the same time, individual storms that affect Colorado will tend to be wetter, as a warmer atmosphere holds more moisture (Seager et al. 2010); the implications of this relationship for extreme precipitation will be explored in Chapter 4.

A second key factor leading to model disagreement regarding precipitation change is how ENSO will change in a much warmer climate. Some CMIP5 and CMIP6 models show more frequent and intense El Niño events (on average associated with wetter conditions for Colorado), while others show more frequent and intense La Niña events (associated with drier conditions). None of the CMIP5 and CMIP6 model simulations capture the recent observed sea-surface temperature (SST) trends in the tropical Pacific, which show a systematic shift towards a more La Niña-like SST gradient from east to west. It is not clear if this shift is associated with anthropogenic influences on the climate system (Seager et al. 2019; Heede et al. 2020; Lee et al. 2022) or natural (internal) variability (Zhang et al. 2021) . If this observed trend towards a more La Niña-like tropical Pacific is in fact anthropogenically forced, then drier precipitation outcomes for Colorado would be more likely to occur over the next several decades.

As described earlier in Chapter 2, observed annual precipitation for Colorado from 2000 through 2022 was about 4% lower than the second half of the 20th century (1951-2000). While several studies suggest that this recent period of reduced precipitation across the southwest U.S. is likely due to natural variability (Barnett et al. 2008; Hoerling et al. 2010; Lehner et al. 2018), other analyses suggest that there is a long-term anthropogenic trend towards lower precipitation in the southwest U.S., including Colorado—though this effect is small enough to be overwhelmed by natural variability on decadal timescales (Gao et al. 2011; Hoerling et al. 2019).

If any anthropogenic decrease in Colorado’s average annual precipitation does occur over the rest of the 21st century, as a large minority of the projections indicate, that would substantially worsen the impacts of warming temperatures on future hydrology. Conversely, only a relatively large increase in statewide annual precipitation (>5%) would ameliorate the impacts of future warming. That outcome, while not off the table, cannot be counted on.

Again, note that the climate models simulate the natural (internal) variability in precipitation as well as the anthropogenic (forced) change signal. Each projection from one climate model simulates a unique sequence of variability (e.g., ENSO events), not synchronized with other models. Even when using a 30-year averaging period (e.g., 2035-2064) for calculation of future change, some long-term variability is picked up in the future “change” for a given model projection. This is consistent with how the real future climate will evolve: there will still be variability in precipitation (whose characteristics may change), which will potentially be superimposed on a forced trend in precipitation.

Downscaled (regional) projections of precipitation

For a closer look at how the projected future precipitation changes may vary in different regions of Colorado, we analyzed the CMIP5-LOCA downscaled climate projection dataset, as described under Temperature (section 2.2, above).

Figure 2.15 shows the projected change in annual precipitation, under RCP4.5, between the historical baseline (1971-2000) and the 2050-centered future period (2035-2064) for the 11 alternative Colorado climate divisions. In each division, the downscaled model projections do not agree on the direction of future precipitation change, with the range of projections extending from large increases to large decreases, as with the statewide projections (Figure 2.15). But in general, the ranges of projections for the northern divisions (Northwest, N. Mtns, N. Front Range, Northeast) are shifted towards wetter outcomes than for the southern divisions (Southwest, San Luis Valley, Southeast). Whatever the overall future change in annual precipitation for Colorado as a whole--more, less, or about the same--the southern divisions are likely to have a drier outcome than the rest of the state, especially the northern divisions.

Figure 2.15

Projected future change in annual precipitation in 11 alternative Colorado climate divisions for a 2050- centered period (2035-2064) relative to 1971-2000, from an ensemble of 32 CMIP5-LOCA climate projections under a medium-low emissions scenario (RCP4.5). The solid blue and brown bars show the middle 80% of the model projections (10th-90th percentiles); the two dashes show the minimum and maximum projections; the open squares show the median projections.

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