Projected future climate
Overview
As described in Recent climate change, the climate of the Colorado River Basin has become substantially warmer in the past 40-50 years, mainly or entirely due to human changes to the atmosphere and climate system. Although average precipitation in the basin has not obviously changed over that period, as temperature has, the underlying atmospheric processes (e.g., circulation patterns, water vapor content) are already being influenced by the warming global climate.
Both basic physics and our most sophisticated tools (global climate models; GCMs) tell us that further warming will occur in the basin over at least the next several decades, at a similar or greater rate than what has been observed since 1980 (Figure 1). Neither the physics nor the GCMs present a clear picture of future precipitation change, however.
Continued warming of the basin’s climate will further impact the surface water balance and hydrology, leading to chronic drought conditions by today's standards, unless a large increase in average annual precipitation occurs--which is unlikely. Long-term declines in annual precipitation are possible, and more likely in the Lower Basin. More warming will also further stress the basin’s ecosystems and species.
Methods
Global climate models
Global climate models (GCMs) are highly complex math- and physics-based software programs that simulate the Earth’s climate system. GCMs partition the Earth into thousands of 3-D gridboxes or cells, typically 30 to 180 miles (50 to 300 km) on a side horizontally, and use equations based on both observations and fundamental physical laws to represent the movement of energy, air, water, and other constituents between the gridboxes. GCMs are the main tools used by climate scientists to diagnose past and recent climate changes, and to generate physically plausible scenarios of the future climate. At the global scale, GCMs produce realistic simulations of key physical phenomena, broad-scale patterns, and statistical characteristics of the historical and current climate, though this realism is weaker at finer scales. Since biogeochemical processes (e.g., carbon cycling) have been incorporated in many of the GCMs, these models are also known as Earth system models, or ESMs.
Several dozen GCMs have been developed by 30 modeling centers in 16 countries. The different GCMs often show somewhat different magnitudes and patterns of projected future climate change, which reflects unresolved scientific uncertainty regarding some key climate processes and the resultant different ways that the modeling teams represent those processes in their models. A major limitation across all GCMs remains their horizontal resolution; the gridboxes are too large to reflect the complex terrain of mountainous areas such as in the Colorado River Basin, or to directly simulate processes like cloud formation or convective storms which occur at smaller, sub-grid scales. Clouds and convective storms are indirectly represented in GCMs through parameterizations, which are generalized values based on observations or additional modeling.
Simulations of future climate from GCMs are known as projections, as opposed to predictions or forecasts, because any given projection is conditional on an assumed future trajectory for greenhouse gases and other human influences on climate. To represent the uncertainty in the future emissions of greenhouse gases, sets of multiple scenarios or storylines now called have been developed.
For the CMIP5 model runs (see below) the 4 scenarios were called representative concentration pathways (RCPs). These were numbered according to their impact on the Earth’s surface energy balance by 2100, in units of W/m2:
- RCP2.6 (low emissions)
- RCP4.5 (medium-low)
- RCP6.0 (medium)
- RCP8.5 (high emissions).
For the CMIP6 model runs, the 8 emissions scenarios were labeled first with a shared socioeconomic pathway (SSP), or one of 5 broad narratives of future socioeconomic drivers of emissions, and then one of several trajectories of emissions consistent with that narrative, resulting in a particular impact on the Earth’s surface energy balance by 2100, in units of W/m2 (like the RCPs):
- SSP1-1.9
- SSP1-2.6
- SSP4-3.4
- SSP5-3.4OS [OS = Overshoot]
- SSP2-4.5
- SSP4-6.0
- SSP3-7.0
- SSP5-8.5
Under the auspices of the Coupled Model Intercomparison Project (CMIP), the GCMs that have been developed around the world are run under standardized protocols, including emissions scenarios as described above, to produce future climate projections to support the periodic Intergovernmental Panel on Climate Change (IPCC) reports. The climate projections from CMIP's Phase 5 (CMIP5), were originally released around 2012. As of late 2024, most of the available GCM-based analyses of local impacts, e.g., for Colorado River Basin hydrology, are based on the CMIP5 projections. The most recent set of climate projections, from CMIP's Phase 5 (CMIP6), were released around 2020 and are increasingly being used in analyses for the Colorado River Basin and elsewhere.
The potential climate futures (temperature and precipitation) for the Colorado River Basin as portrayed by the CMIP6 models are generally similar to those seen in the CMIP5 models under comparable emissions scenarios with the key difference being that the range of CMIP6 is shifted towards greater future warming. However, a subset of the CMIP6 GCMs (the so-called hot models) may be modeling cloud feedbacks and anthropogenic aerosols in a way that leads to implausibly high rates of warming.
Downscaling
To resolve some of the issues with their coarse resolution, raw GCM output is often downscaled, or post-processed to represent more local scales--at most 50 km (31 mi.), and often 12 km (7 mi.) and smaller. Downscaling methods fall into two general categories: dynamical or statistical. In dynamical downscaling, a higher-resolution regional climate model (RCM) is run over the domain of interest, driven by boundary conditions taken from a GCM simulation. This produces localized projections that are better grounded in the physical processes at these scales, but with far higher computing costs than statistical downscaling, which is based purely on spatial statistical relationships among observed climate variables. But because statistical downscaling relies on historical relationships, it may not correctly represent localized future changes in those variables.
Many different downscaled CMIP5 datasets have been developed and publicly archived, serving as the basis for both interactive plotting and analysis tools (i.e., Climate change portals) and impact studies and assessments. Reflecting the disparity in the computational costs, the vast majority of these datasets were produced through statistical downscaling.
The general benefits of downscaling are that it facilitates the use of GCM data in local-impact modes (such as hydrologic models) that require higher-resolution inputs, and provides end users with projections that are at least superficially more relevant to their needs. But downscaling does necessarily not make the depiction of future change more accurate than in the underlying raw GCM projection. Different downscaling methods applied to the same GCM projection can result in quite different projected climate changes for a given region or location, especially for precipitation.
Future basin temperature
All of the GCM projections run for CMIP5 (Figure 2) and CMIP6 indicate that the Colorado River Basin will continue to warm in all seasons through the during the 21st century. By mid-century (2041-2070), the basin is projected by CMIP5 and CMIP6 models to warm by +3°F to +8°F over a 1971-2000 baseline, and by +2°F to +7°F over the most recent 30-year period (1991-2020). The range or uncertainty in the projected warming is due to both the emissions scenario (RCP) and the GCMs' different climate sensitivities to each increment of greenhouse gases in the atmosphere. After 2050, the range of projected warming under the higher-emissions scenarios (RCP8.5, SSP3-7.0, SSP5-8.5) increasingly diverges from the moderate-emissions scenarios (RCP 4.5, SSP2-4.5).
Even under a lower-end warming outcome (+4°F above 1971-2000), the “normal” years around 2055 would be warmer than the very warmest years of the last century. Slightly greater warming is consistently projected for the Upper Basin compared to Lower Basin, since the Upper Basin is further from the moderating influence of the Pacific Ocean. Warming is expected to be greater in summer and fall than the other seasons due to a land-surface feedback that is more prevalent and potent in those seasons: When soils dry out, the incoming solar energy that had been evaporating soil moisture can instead warm the land surface and the air above it.
Future basin precipitation
Compared to temperature, precipitation reflects a much more complex set of climate drivers and processes, and changes in precipitation are more difficult to predict than changes in temperature. In general, the GCMs agree that the prevailing westerly storm tracks across western North America will shift northward on average (potentially drying the Lower Basin and parts of the Upper Basin), but also that individual weather systems will carry more moisture, consistent with the effect of warming on the atmosphere's moisture-holding capacity. The models do not agree on future changes in ENSO (El Niño/La Niña) events, which influence storm tracks and precipitation, especially in the Lower Basin.
As a result of these mixed future tendencies, under higher and lower emissions scenarios the CMIP5 and CMIP6 GCMs show overall tendencies toward slightly higher annual precipitation in the Upper Basin (Figure 3) and toward lower annual precipitation in the Lower Basin. For the Upper Basin, the wetter-leaning of the models call for about 5–10% more annual precipitation by mid-century, while the drier-leaning models call for 5–10% less precipitation. For the Lower Basin, the wetter models call for up to 5% more annual precipitation, while the drier models call for 10–15% less precipitation.
The CMIP5 and CMIP6 GCMs show more pronounced tendencies for change in seasonal precipitation over the basin than annual precipitation. In winter (DJF), most models show increased precipitation over the Upper Basin. In spring (MAM), most models show decreased precipitation for the Lower Basin. In summer, most models show decreased precipitation for both the Upper and Lower Basins; however, the North American Monsoon is not simulated well in the GCMs, and confidence in the projected changes in summer precipitation is lower than for the other seasons.
Multi-decadal natural variability--as simulated by the GCMs--strongly influences the precipitation projections, as seen in the frequent excursions in the 30-year averages in Figure 3, up and down by 5% or more. These excursions, both in the models and in the real world, make it hard to discern trends that might be attributable to the "forced" (anthropogenic) climate changes, such as shifts in average storm tracks.
The ensemble median precipitation changes of RCP4.5 and RCP8.5 throughout the 21st century are very similar, though after 2050 the RCP8.5 ensemble does have greater spread (right side of Figure 3), suggesting that at least some of the models show larger responses in precipitation as a result of the higher greenhouse gases assumed in RCP8.5.
Most CMIP5 and CMIP6 GCMs also project that the variability in precipitation over the western U.S. will itself change, increasing at all time scales, including greater year-to-year variability. This would mean more frequent occurrence of both very dry and very wet years, and more frequent transitions from very dry to very wet conditions.
Future climate extremes and hazards
Hot extremes
Similar to what has been observed over the past 40 years in the basin and the Southwest U.S., the projected shift in the center of the distribution of temperatures (i.e., annually averaged temperature) is expected to manifest in large increases in extremely hot days and multi-day heat waves. Figure 4 shows that the number of days per year over 90°F is projected under RCP8.5 to increase by 20-60 days by mid-century over the middle and lower elevations of the basin; many high-elevation areas that do not currently experience temperatures above 90°F are expected to see them by mid-century.
Cold extremes
Cold extremes in all seasons are expected to decrease in frequency as the overall climate warms, although cold waves that do occur could be nearly as severe as historical cold waves.
Heavy and extreme precipitation
As noted in Recent climate change, warmer air can hold more moisture, also known as precipitable water (PW). Observations show that globally-averaged PW has increased in recent decades at a rate consistent with physical expectations, given the observed global warming. The CMIP5 and CMIP6 models consistently project there to be future increases in average PW globally as warming progresses. Changes in average PW for the Western U.S. are more variable across the CMIP5 and CMIP6 models because at the regional scale, PW is also a function of the frequency of wet weather patterns that transport and concentrate moisture--changes in which being less certain. The models do project that extreme PW values for this region (e.g., highest daily PW in a 10-year period) will increase, by about 10-20% by the 2050s per CMIP5.
With these projected increases in PW on the wettest days, extreme precipitation events over the region that includes the Colorado River Basin are also projected to become more frequent and intense. Over the the Southwest US (CA, NV, UT, CO, AZ, NM), what is currently a 1-in-5-year 2-day event is projected to become, by the 2050s, a 1-in-3-year event under both RCP4.5 and RCP8.5. [Jannsen et al 2014] The 1-in-20-year 1-day event is projected to increase in magnitude (rainfall depth) by mid-century by 10% under RCP4.5 and by 12% under RCP8.5 [USGCRP 2017]. These and other studies indicate that heavy and extreme precipitation days (>95th percentile of all days with precipitation) will contribute an increasingly greater fraction of the annual precipitation.
Drought
The projected higher temperatures for the basin will likely lead to greater frequency and severity of soil-moisture droughts in the basin as the increases in evapotranspiration that will result from warming either exacerbate long-term decreases in annual precipitation (if they occur) or cancel all but the largest future increases in precipitation. CMIP5 projections of near-surface soil moisture consistently indicate decreased summer (JJA) soil moisture over the Colorado River Basin, and most parts of the Western U.S. [USGCRP 2017]
For more on future projections of hydrologic drought and its key components (snowpack, runoff), see Projected future hydrology.
Wildfire
Wildfire occurrence and total areas burned per year are projected to increase substantially in the Rocky Mountain West by mid-century as warming leads to drying of the land surface and of fuels. [Lukas et al. 2014]. The length of the fire season is also projected to increase. While several other factors have led to increases in large fires and in area burned since 1980, and will continue to influence wildfire trends in the future (e.g., historical fire suppression, development of the wildland-urban interface, increase in human ignition sources), anthropogenic warming has been identified as a significant driver of increasing wildfire occurrence. [Williams and Abotzoglou 2018]
Data and tools
There are several web tools that can be used to explore downscaled CMIP5 model projections of future climate in graphical form, and also download the data being shown. A few caveats to these tools:
- The multi-model mean or median, which is usually highlighted in maps and plots, is not necessarily the most likely future--it's merely the middle of the distribution (range) of the model projections.
- Even within the same emissions scenario (RCP), the projected future climate change depicted by one tool can differ from others because:
- Different subsets of the >35 CMIP5 models are used in each downscaled dataset
- Each downscaling method may shift the change signal shown by the underlying models
- Different historical baseline periods and future periods may be used to analyze the data
- To repeat a point made above, the local resolution of downscaled data does not mean that it is more accurate; i.e., more likely to represent the actual future climate change
A User Guide to Climate Change Portals provides descriptions of many different web tools for visualizing and downloading CMIP5 model projections, including the three listed below.
The Climate Explorer
The Climate Explorer interactive tool (developed by NOAA and its partners) allows users to generate zoomable climate change maps for temperature and precipitation for the U.S. and northern Mexico, and time-series plots of projected future temperature and precipitation for U.S. counties. Other areas for analysis, such as river basin, cannot be selected. All projected climate data are from the LOCA-downscaled CMIP5 dataset (32 models).
Climate Toolbox
The Climate Toolbox is a diverse set of interactive tools developed by researchers at U. Cal-Merced and partners. All projected climate data are from the MACA-downscaled CMIP5 dataset (20 models).
Climate Toolbox - Climate Mapper
The Climate Mapper tool allows users to generate zoomable climate change maps for temperature as well as precipitation and a broad set of drought, agricultural, and fire-risk indicators. To access the projections, "Future: Projections" must be selected under Choose Data - Time Scale in the upper left. Projected climate data are from the MACA-downscaled CMIP5 dataset (20 models).
Climate Toolbox - Future Time Series
The Future Time Series tool allows users to generate time series for temperature as well as precipitation and a broad set of drought, agricultural, and fire-risk indicators. To access the projections, "Future: Projections" must be selected under Choose Data - Time Scale in the upper left. Projected climate data are from the MACA-downscaled CMIP5 dataset (20 models).
National Climate Change Viewer
The National Climate Change Viewer (NCCV) allows users to generate zoomable change maps, time series, monthly climographs, and scatterplots for multiple climate and hydrology variables, including area-averaged runoff (in inches or cm). Before the Viewer loads, users select one of two sets of analysis areas: (1) states/counties, or (2) river basins/watersheds (HUC2, HUC4, and HUC8), and one of two projected climate datasets: (1) MACA-downscaled CMIP5 (20 models under RCP4.5 and RCP8.5 emissions), or (2) LOCA-downscaled CMIP6 (23 models under SSP2-4.5, SSP3-7.0, and SSP5-8.5 emissions). The hydrology variables such as runoff are derived from the USGS Monthly Water-balance Model (MWBM) driven with the projected temperature and precipitation.
Additional resources
Colorado River Basin Climate and Hydrology: State of the Science report
Chapter 11 of the 2020 State of the Science report details the projected future climate for the basin, in section 11.6, as well the methods and data used in those projections: Global climate models (GCMs; 11.2); CMIP (11.3); Emissions scenarios (11.4); Downscaling (11.5).
Fifth National Climate Assessment (NCA5)
Chapter 2 (Climate Trends), Section 2.3, of the Fifth National Climate Assessment (NCA5; 2023) provides an overview of the projected impacts of climate change on temperature, precipitation, extreme heat, drought, and heavy and extreme precipitation, based on CMIP6 climate model projections.
Fourth National Climate Assessment (NCA4) CSSR
The 2018 Climate Science Special Report (CSSR), Volume 1 of the Fourth National Climate Assessment (NCA4), describes climate models, emissions scenarios, and projections in Chapter 4, and details future projections of climate variables and extreme events for the U.S., nased on CMIP5 climate models: Temperature changes (Chapter 6); Precipitation changes (Chapter 7); Droughts, floods, wildfire (Chapter 8); Extreme storms (Chapter 9).