Note: I’ll be using California to represent Southwest drought for this blog entry.
Introduction: The current state of the California drought
Let’s start with the graphic I’ve chosen for the banner above, courtesy of the Drought Monitor maintained by U of NV Las Vegas. The shading shows the degree of soil moisture dryness, as indicated by the Intensity legend at the lower right of the diagram. Drought Impact Types are indicated by areas bounded in black lines and are labeled short-term (labeled S, up to and including 6 months. affecting agriculture and grasslands) and/or long-term (labeled L, greater than 6 months, affecting the above plus general hydrology and ecology). As of February 8, 2022, more than half of the lower 48 (CONUS) is drier than normal, with various short- and long-term impacts based on the persistence of dry conditions.
The drought in California has been ongoing, with a couple of breaks, since about 2000, with serious impacts on agriculture (S), general hydrology/water supply, and ecology, and a ramping up of forest fire size and damage (L). None of the breaks in the drought have restored long term water supplies to normal over the last 22 years.
If we use reservoir levels as a measure of long term surface and subsurface water storage overall, reservoir levels from last week tell a troubling story. After December’s record precipitation over CA, it has not rained since January 7 in northern CA, and not since January 16 in the southern portion of the state. The graphic below shows that last week’s levels were, except for three reservoirs, below normal for this time of year. Current levels are indicated by the blue shading, normal levels by green horizontal lines, and total capacity by the full rectangle (yellow and blue shading).
Most reservoirs are below their levels in 2021 on the same date as well (not shown, see story link in preceding text and figure description above).
How Did CA Get Here?
It’s not only lack of precipitation
The role of anomalous heat in development and intensification of drought cannot be overstated. As dryness increases and evaporation from soils and evapotranspiration from vegetation decreases, a larger share of incoming solar energy increases the ambient temperature. We can see both processes in the graphics of 20-month periods (Jan. through August of the following year) and 20-year centered averages for precipitation (left) and temperature (right) for the Southwest since 1895.
The striking 20-year centered average temperature increase since the late 1970s reflects both the long-term drying of the region and the impact of greenhouse gases. Also note the extreme dry departure of precipitation in the most recent 20-month period (Jan. 2020 — Aug. 2021), which was the driest over the last 126 year record. There’ll be more on the contribution of anthropogenic global warming (AGW) to the Southwestern US/California drought later, after we look at the drought forecast from now through spring of 2023.
Near-term Expectations for Precipitation and Drought
February is the climatologically wettest month in much of CA, because the storm track is furthest south. However, for years with cold equatorial eastern Pacific sea surface temperatures (the well-known La Nina) like this one, drier than normal weather is typical in winter, especially in CA.
The medium range forecast (14 days) shows 0-0.5” of precipitation over the southern half of CA, with from 0.05” to about 3” of precipitation over the northern half. In all areas this represents only from near 0% to about 50% of what should be received during the period. March is expected to have near normal precipitation over the southern 80% of the state, with above normal amounts restricted to the north.
Starting in April, California and the rest of the US west coast begins its dry season. In its most recent projection, the NOAA Climate Prediction Center (CPC) indicated a continuation of drought over CA and the rest of the Southwest through at least April 30, 2022. Also of concern is the large area of drought persistence (brown) over the Rocky Mountains, and western and southern Great Plains. This is also common when the central and eastern Pacific is in its La Nina phase. Note the expectation that drought will expand eastward and southward through April 30. None of this bodes well for winter wheat in the Southern Plains or warm season wheat further north.
I took a quick peek at next year’s projected CA wet season from the CPC (predictions for three month periods from OND 2020 through JFM 2023) and it holds to drier than normal weather from CA into NV, AZ, and parts of OR, UT, and ID. Needless to say, however, predictive ability does decrease over increased projection time.
Why This May Be the New Normal for CA
Research on historical droughts
A press release was issued early this week on a study published in the journal Nature: Climate Change on the 2000-2021 southwestern US drought. A study from 2020, using soil moisture proxies such as tree rings from carbon-14 dated wood, concluded that 2000-2018 was the driest 19-year period since the megadrought (i.e. a drought lasting two decades or more) of the late 1500s (1571-1593). In the earlier study’s conclusions, the authors expected that the plentiful precipitation of the 2019 water year would terminate the 2000-2018 extended drought.
However, rather than ending, the drought was further exacerbated in 2020 and the extremely dry and hot 2021 (from reconstructed data, driest since 1729). The additional two years dominated by drought resulted in the current designation of 2000-2021 as a megadrought in the most recently published 2022 study. This megadrought is the first to occur in 400 years, and represents the driest period as measured by soil moisture and their proxies over the last 1200 years.
Previous research has shown that the period from 1600 back to 800 CE was punctuated by seven megadroughts in the southwest US, one of which lasted for 30 years (1271-1300). These were caused just by natural climate variability. In contrast, the subsequent 400 years were unusual in terms of relatively stable precipitation and temperature. Based on the reconstructed record alone, the current megadrought may mark a return to an old normal. It’s also an ominous reminder that development of the Southwest was based on the continuation of what may have been an anomalous stable regime. But how much of the current situation is made worse by AGW?
Comparison of recent observed climate changes to climate models
The most recent study of Southwestern drought used models currently used for climate projections based on anthropogenic global scenarios rerun for the 1901-2021 observational period (not requiring reconstruction with proxies), but without increases to greenhouse gas concentrations. The researchers then used the difference between the two runs to estimate the AGW contribution to the 2000-2021 megadrought.
results
The first graphic used the full reconstructed soil moisture time series to compare to the current 2000-2021 megadrought, and to the 2021 extreme. See the caption for a full description.
Right: rank of current drought severity (per the legends to the right) with and without AGW. Top shows the 22 year drought rank, and the bottom the rank for 2021 alone. Ranking is based on the full reconstructed record
The researchers estimate that about 42% of the current megadrought and 19% of last year’s even more extreme dryness appears to be attributable to AGW.
Next we have a closer look at the megadroughts of 800-1600 compared to the current one. I won’t go into an extended description of the statistical methods used to verify the significance of the results, beyond that the soil moisture deficit graphic below include plus and minus 95-percentile bounds based on drought reconstructions (shaded, with a 2.5% chance of a correct value beyond the upper or lower bound).
Right: Same as left but for each drought from beginning to end, with the blue line again representing the current drought without the effect of anthropogenic greenhouse gas emissions. Without AGW, the current drought would not have begun until 2007, using the definition in the article text.
Soil moisture deficits in the current drought evolved at least as quickly as in any of the droughts over the past 1200 years, and the contribution from AGW is clear.
Conclusions/Speculations
- Drought has not occurred over the last four centuries to the degree or intensity in the US Southwest as in the previous 800 years.
- The development of large cities and agricultural regions with high water demand in the last 100 years occurred during a long period of stable climate (both precipitation and temperature), which was able to support such development.
- About 55,000,000 people live in CA, AZ, UT, and NV. The current reversion to more frequent drought, while new to the region since its development, is likely to continue, at least in part because of AGW.
- While not directly discussed above, water storage in much of the region is at record low levels.
- The Colorado River watershed supplies water for drinking and power generation for 40 million people in this region. The snow pack that provides water to the river has been running below normal during the megadrought. Reservoirs fed by the Colorado (Lake Powell and Lake Mead have been at record low levels.
- The Sierra Nevada provides snow melt for large reservoirs and the Central Valley of CA (Sacramento and San Joaquin Rivers). The snow pack here too has been adversely affected by the megadrought. Reservoirs dependent on snowmelt are also at seriously low levels.
- The current hydrologic infrastructure may not be sufficient for the current population, let alone any future growth.
- There will have to be significant changes to the hydrologic infrastructure (adding desalinization of seawater? Diverting water from wetter regions?) and to lifestyles relative to water use (Recycling wastewater at the household level?).
- Agriculture will need to convert to growing crops with less water demand than those grown presently, or to use water more efficiently.
- Another more extreme alternative would be to reduce the population of the Southwest to a sustainable level based on expected water availability, which may be difficult to determine.