Projected future climate

Overview

Figure 1. Annually averaged temperature over the Colorado River Basin, 1895-2020, shown with the blue and red bars as anomalies from a late 20th- century (1971-2000) baseline. The gray line is a 10-year running average plotted on the 6th year, and the. The dashed yellow line is the linear trend from 1980-2020, showing 2.1°F of warming over that period. (Design: Jeff Lukas, updated from Lukas and Payton 2020, based on gridded climate data from NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/.)

As described in Recent climate change, the climate of the Colorado River Basin has become substantially warmer in the past 40 years, very likely due to human changes to the atmosphere and climate system. Meanwhile, average precipitation in the basin has not clearly changed, but 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. Neither the physics nor the GCMs present a clear picture of future precipitation change, however.

Relevance

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 in the absence of a large increase in average precipitation. More warming will also further stress the basin’s ecosystems and species, potentially beyond thresholds for viability in some cases.

Methods

Global climate models

Global climate models (GCMs) are extraordinarily complex math-based software programs that simulate the Earth’s climate system. GCMs partition the Earth into thousands of gridboxes or cells 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. They are the principal tools used by climate scientists to diagnose past and recent climate changes, and to generate physically plausible scenarios of the future climate, globally and regionally. GCMs share many characteristics with global weather prediction models used for short-term weather forecasts, but GCMs are run over much longer time frames (10-100 years or more), and represent additional processes that change slowly, such as ocean circulation and biogeochemical cycles (IPCC 2013, Lukas et al. 2014).

GCMs produce realistic simulations of the key physical phenomena, broad-scale patterns, and statistical characteristics of the historical and current global climate. CO-NM Regional Extreme Precipitation Study Volume VI November 2018 Page 11 of 57 Their simulations of future climate do differ among the several dozen different GCMs developed by research groups worldwide, which mainly reflects unresolved scientific uncertainty regarding some key climate processes, and the different ways that the respective modeling teams represent those processes in their models. For planning purposes, it is important to look across the range of model output for the possible direction and magnitude of future change, and not just focus on the average of the models. Consistency among the models in a particular outcome can be taken as a rough measure of scientific confidence in that outcome (IPCC 2013, Lukas et al. 2014). While the horizontal resolution (gridbox size) of the GCMs has progressively improved over the several model generations since the 1990s, the gridboxes, now typically 30- 80 miles on a side, are still too large to reflect the complex terrain of mountainous areas such as Colorado and New Mexico, or to directly represent processes like cloud formation or convective storms which occur at smaller, sub-grid scales (though clouds and storms are indirectly represented in GCMs through the use of model parameterizations). Thus, GCM output is often downscaled through statistical methods, or via regional climate models (RCMs), in an attempt to better capture more local changes to weather and climate. Simulations of future climate from GCMs are known as projections, as opposed to predictions or forecasts, because the projections are 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, four trajectories called representative concentration pathways (RCPs) have been developed; these are numerically labeled 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), and RCP8.5 (high emissions). Most published analyses of climate change focus on RCP 4.5 and/or RCP8.5; RCP2.6 is considered by many experts to be implausibly optimistic in its assumed reduction of emissions. Several dozen GCMs have been developed by over 20 modeling centers in 10 countries. Under the auspices of the Coupled Model Intercomparison Project (CMIP), the available models 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 last phase of CMIP and the most recent set of projections was CMIP5 (released in 2011-2012), which followed CMIP3 (2006); the next set of projections, CMIP6, will be released in the 2018-2020 timeframe. (There was no CMIP4.) A list of the modeling centers and the GCMs for which CMIP5 projections are available can be found at:

Future temperature

Figure 2. Fraction of the area of the Southwest U.S. (UT, CO, AZ, NM) experiencing extremely warm (upper 10%; red bars) and extremely cool (lower 10%; blue bars) daily high temperatures averaged over the summer (June-August), 1895-2020. Note the abrupt increase in the area experiencing extremely warm summer daily highs after 1995. The green line is a 9-point smoothing filter to emphasize decadal-scale variability. (Chart: NOAA NCEI https://www.ncdc.noaa.gov/extremes/cei/; annotation on y-axis by Jeff Lukas)

Future precipitation

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).

Figure 3. Upper Colorado River Basin water-year precipitation, 1900-2020 (green dots and line), with smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)

Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2020 (green dots and line), with smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)

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 (LINK), so it is advised that users of the tools familiarize themselves with these datasets as well.

NOAA NCEI Climate at a Glance - Regional Time Series

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 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.

Additional resources

State of the Science Report

Chapter 2 of the State of the Science report describes recent climate changes in greater detail, in section 2.10.

NCA4 Climate Science Special Report

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.

New and Notable Research (2020-present)

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Summary