Recent climate change
The climate of the Colorado River Basin has clearly changed in the past several decades, becoming substantially warmer, by over 2°F since 1980. This observed warming is synchronous with warming over that period on continental and global scales, and that has been firmly attributed to human activity such as the emission of greenhouse gases. This pervasive increase in the basin’s temperature has significant implications for the surface water balance and hydrology.
Over recent decades, average precipitation in the basin has not clearly changed relative to the high year-to-year and decadal variability. However, there is evidence that circulation patterns are shifting due to anthropogenic climate change, and precipitation may be responding to those shifts in ways that cannot yet be discerned from the background variability.
The water infrastructure and policies of today, and many other land uses, are predicated on and attuned to the climate of 20th century. Similarly, the basin’s ecosystems and species have evolved with the relatively stable late-Holocene climate of the past 4,000 years. Increasingly novel conditions of temperature and other climate factors are putting increasing stresses on these systems.
After seeing little overall warming from the beginning of observations in the late 1800s to the 1970s, annually averaged basin temperatures have risen sharply, with a persistent warming trend of about 0.5°F per decade since 1980. The period from 2009-2018 was the warmest 10-year period on record (since 1895), and of the 13 warmest years on record in the basin, 11 have occurred from 2000 onwards. Each of the four meteorological seasons has warmed at rates similar to the trend in annual temperatures. The warming trends for the Lower Basin and Upper Basin are comparable.
This warming trend in annual temperatures observed since 1980 in the basin is 20-50% greater than the warming trends during the same period observed across the Western U.S, over the lower 48 states, and globally. The global warming since the mid-20th century has been attributed to anthropogenic effects--mainly the emissions and rising atmospheric concentrations of greenhouse gases--with natural influences playing a minor role at best. The evidence indicates that these anthropogenic influences are also mainly responsible for the recent regional warming as seen in the basin.
As the seasonal and annual averages of basin temperatures have shifted, the tails of the temperature distribution have also shifted, with extremely high seasonal, monthly, and daily temperatures, including heat waves, becoming much more frequent (Figure 2).
As discussed in Climate patterns and variability, annual precipitation in the basin is highly variable, and there are no recent trends in annual or seasonal precipitation that clearly emerge from the background “noise” of historical year-to-year and decadal variability, as with temperature. But the most recent two decades does stand out for overall dryness, which is accentuated by the preceding two wet decades in the 1980s and 1990s (Figures 3 & 4). By slight margins, 2000-2020 has been the driest 21-year period on record in both the Upper Basin (with 94% of the 20th-century average) and Lower Basin (88% of the 20th-century average). The period since 2000 also includes the driest single water years on record in the Upper Basin (2018) and Lower Basin (2002).
Several studies have indicated that the overall lower precipitation since 2000 is likely part of natural variability in the basin’s climate, rather than due to longer-lasting changes in atmospheric circulation related to anthropogenic warming. Some more recent studies, however, suggest that anthropogenic climate change is in fact exerting a long-term tendency towards reduced precipitation. However, the tendency that was identified is small relative to the natural fluctuations. This ambiguity about causation of precipitation change is expected to continue; even if a systematic anthropogenic change in precipitation is occurring, it may not be obvious for several decades. This is very different than the situation with temperature change.
One well-known consequence of warming has implications for precipitation at shorter timescales (hourly, daily, weekly): warmer air can hold more moisture. So in principle, under a warmer climate, any weather pattern that brings a moist air mass into the region can generate a larger precipitation event, all else equal. Recent and substantial increases in heavy and extreme precipitation events (daily and multi-day) have been clearly observed in most parts of the world, and much of the U.S., consistent with that physical expectation.
However, over the southwestern U.S., including the Colorado River Basin, consistent trends in these events have not been observed over the last several decades. Increases in extreme precipitation are seen for some timescales and frequencies of event, but most analyses have not shown increases. Similar to the situation with annual precipitation in the basin, it may be the case that natural variability--in this case, variability in the frequency of weather patterns that produce extreme precipitation--may be masking anthropogenic influences that are also occurring.
A broad suite of drought indicators are used by interests in the basin and beyond to anticipate and monitor drying of soils, crops and other vegetation, and wildfire fuels. (Trends in indicators of hydrologic drought such as snowpack and streamflow are discussed in Recent hydrologic changes.) With a few exceptions , all of these drought indices include temperature in their formulation. Accordingly, given the observed warming trend described earlier, one would expect these indices to show increases in the frequency and/or severity of drought conditions in recent decades, and in fact they do show increasing drought in the basin.
The Palmer Drought Index is a simple emulation of the balance of moisture inputs (via monthly precipitation) and evaporative losses (temperature) from the soil column, accounting for the memory of water storage in the soil. It is widely used in agriculture as well as general drought monitoring. Figure 5 shows the historical record of August PDSI, reflecting drought conditions leading up to and through the growing season, for the Upper Basin.
It shows that fluctuations in PDSI are mainly driven by precipitation variability, but accentuated by the warmer temperatures that typically accompany drier-than-average years. It also shows show the impact of systematically warming temperatures over the last 40 years in causing a steeper decline in PDSI (i.e., increasing drought) than in precipitation over that period (compare with figures X and X). The average August PDSI in the Upper Basin (-2.59) as well as in the Lower Basin (-2.79) since 2000 would be classified as moderate drought, with over a third of those years falling under extreme drought.
Note that PDSI has been criticized for being unrealistically sensitive to temperature, especially systematic increases in temperature that are beyond the historical calibration range of PDSI, as is the case with the last 20 years or so. So while the recent downward PDSI trend expresses a robust physical concept (warming makes soil moisture drought more severe), the actual values may be questionable.
Data and tools
There are several climate tools that are useful for plotting and examining time-series and recent trends in temperature, precipitation, and other climate variables over specific areas (states, counties, river basins, etc.). Each tool depicts one or more gridded climate datasets, so users of the tools may want to familiarize themselves with these datasets as well; see Weather and climate monitoring and Chapter 4 of the State of the Science report.
The “CAG” tool is a versatile tool that can be used to generate many types of charts, maps, and analyses from NOAA’s official nClimGrid monthly gridded (5 km) climate dataset. The link above opens the Regional and Time Series options, allowing users to plot temperature, precipitation, and other variables for the Upper Basin, Lower Basin, or many other U.S. basins and regions. All variables can be plotted from 1895 to present.
This tool, developed by researchers at the U. of California-Merced and partners, generates time series plots for any point, county, HUC8 watershed, or user-selected area from the gridMET gridded (4 km) climate dataset. Many more variables are available from gridMET than from NOAA's nClimGrid; however, gridMET only extends back to 1979. Some drought-related variables calculated from other climate datasets are available through this tool.
State of the Science Report
Chapter 2 of the State of the Science report describes the basin's recent climate changes in greater detail, in section 2.10.
The 2018 Climate Science Special Report (CSSR), Volume 1 of the Fourth National Climate Assessment (NCA4), describes the historical record and likely causes of recent temperature change (Ch 6.1, 6.2) and recent precipitation change (Ch. 7.1) in the U.S.