Think of it as the new normal.
That’s the conclusion that has emerged from the latest update of the 30-year climate “normal” in the Southwest, according to the National Oceanic and Atmospheric Administration.
Over the past century, the average temperature in Arizona had risen by more than 2 degrees and the average rainfall has declined by more than 10%, according to the running tally of average temperature and rainfall — calculated in 30-year blocks.
The change has been especially notable in the past 30 years — which includes one of the worst droughts in the past 1,000 years.
The entire country’s heating up — but in the East and South the higher temperatures have brought more rainfall. In the West, high temperatures seem to go hand-in-hand with less rain and snow. Both the winter snowpack and the summer monsoon have become more unreliable.
Climate experts say their computer models show that the heat-trapping pollutants in the atmosphere have worsened natural cycles of heat and drought. Droughts as well as hot, dry stretches occur naturally in the Southwest, based on irregular, largely unpredictable variations in the climate.
However, the extra heating caused by greenhouse gases has made those hot, dry periods more frequent and more intense. One recent study based on tree-ring data suggested that in the past 20 years the heat-trapping gases turned what would have been a moderate drought into what may ultimately rank as the worst drought in 1,200 years.
The latest estimate of the new normal comes from data gathered by 15,000 U.S. weather stations, many with data gathered continuously since 1890. The latest 30-year average measures the period from 1991 to 2020. NOAA also maintains a database that compiles 15-year averages and updates the averages every 10 years.
In the past decade, temperatures in the Southwest have risen by an average of between half a degree and 1 degree F, while rainfall has dropped by 10%. Turns out, the 1980s were a relatively wet decade — while the region has been setting records for heat since the turn of the century.
In Phoenix, the average annual temperature increased by 0.6 of a degree and rainfall decreased from 8 inches to 7.2 inches.
Climate experts say the climate for centuries has gone through these cycles of drought and abundance. The recent meta-analysis of tree-ring data from 1,586 sites documented four other intense droughts, in the late 800s, the mid-1100s, the 1200s and the 1500s. The driest 19-year period in that record struck in the 1500s, although the resumption of exceptional drought across most of Arizona may soon top that record.
Researchers from the University of Arizona, Columbia University and elsewhere used 31 climate models to estimate the impact of the heat-trapping gases building up in the atmosphere on those natural climate cycles. They concluded that greenhouse gases added 2.2 degrees to the temperature in the current drought, making it 47% more severe, according to the research published in the journal Science.
The rise in average temperatures has a host of effects by changing rainfall patterns, decreasing snowpack, drying out the soil, increasing the water demand by plants, lengthening droughts and increasing weather instability, according to an analysis of an array of studies published in the Proceedings of the National Academy of Sciences.
In the Southwest, “This translates into progressively lower river flows, drier landscapes, higher forest mortality and more severe and widespread wildfires — not year on year, but instead a clear, longer-term trend towards greater aridification, a trend that only climate action can stop.”
Each 2 degrees increase in temperature increases the amount of water vapor the atmosphere can hold by 7%. However, the increased heat also increases the amount of water plants draw from the soil — as well as the moisture that evaporates from the soil, lakes, rivers and oceans.
So the warming trend has resulted in more rain in the East and South, but a 50% decline in the flow of the Colorado River.
“The impact of warming on the West’s river flows, soils, and forests is now unequivocal. Will higher precipitation save the day?” asked the authors of the PNAS study.
Don’t count on it.
“The net result of these more frequent and severe hot-dry events translates into a climate that can manifest increasing aridity and extreme event impacts, particularly in summer, even if the mean annual climate paradoxically becomes wetter.”
The increase in wildfires is an example of the impact of even a few degrees of average temperature increase.
The increased dryness driven by the higher temperatures has also stressed water use and storage — as evidenced by the projection that Arizona will this year lose most of its Colorado River allotment for the first time since completion of the Central Arizona Project.
“Across North America, greater aridity is being offset with increased groundwater use, but this strategy has limits in many places, such as the Southwest and the High Plains, where groundwater use exceeds recharge and is thus unsustainable ... Unfortunately, climate change and this aridification are likely irreversible on human timescales, so the sooner emissions of greenhouse gases to the atmosphere are eliminated, the sooner the aridification of North America will stop getting worse,” concluded Jonathan Overpeck and Bradley Udall, the lead authors of the PNAS meta-study.