According to climate researchers, the increasing demand for evaporation is exacerbating the summer drought in California and the west.
The evaporation requirement is essentially the “thirst” of the atmosphere. It is calculated based on temperature, humidity, wind speed and solar radiation. It’s the sum total of evaporation and transpiration from plants and is powered by warmer global temperatures that are due to climate change.
The meteorological summer of 2021 in the adjacent United States, which runs from June to August, combined the extreme heat of the Dust Bowl summer of 1936.
Evaporation demand has driven nearly half of California into what the US Drought Monitor calls an “exceptional drought”. It causes soil and vegetation to dry faster, making fuels more dangerously flammable in summer and fall, the height of the California fire season. As a result, fuels burn faster and hotter.
California and the west have seen significant spikes in evaporation demand over the past half century, adding to the severity of the summer drought, said John Abatzoglou, associate professor and climate scientist at UC Merced.
The California drought is not just the result of scarce rainfall. It’s a combination of two things: a lack of rain and those thirsty atmospheric conditions that are drying up the landscape. For much of California, the summer and water year 2021 had the highest evaporation demand in 40 years, according to the National Integrated Drought Information System.
As Abatzoglou points out, Northern California experienced the third driest water year on record, along with the highest evaporation demand recorded – factors that “put this year in a league of its own”. These things are remarkable, he adds, “as the California drought hall is notorious of shame.”
Abatzoglou cites the Palmer Drought Severity Index, which he describes as a general catch-up drought index widely used in the USA that is part of the US Drought Monitor. By that standard, Northern California has its worst year on the instrumental record. The PDSI is a soil moisture index that takes into account both precipitation and evaporation requirements.
Abatzoglou sees the “demand” of the drought as something we are beginning to appreciate. He says there were dry droughts and then there were hot and dry droughts. The latter have promoted increased summer irrigation needs from agriculture in the Central Valley, an energy imbalance due to higher demand for heat, and reduced energy supplies due to the reduction in hydropower generation.
After some drought response, Northern California is having its worst year on record.
(Paul Duginski / Los Angeles Times)
The latest US Drought Monitor report, released on Thursday, paints a bleak picture of the situation in California and much of the West – as it has been for months. In the current data, about 46% of the state is classified as being in exceptional drought, while just over 42% are living in extreme drought. The remaining roughly 12% of the state is split equally between moderate and severe drought – that is, 100% of the state is affected by some drought.
The increased need for evaporation has exacerbated the dryness of the vegetation, which made more forest fires possible this year. Active forest fires like the Windy Fire and the KNP Complex fires were among those that continued to consume parched vegetation in California.
Evaporation demand was the highest on record in parts of northern and central California.
(Paul Duginski / Los Angeles Times)
The areas of exceptional and extreme drought on the drought monitor map pretty much match the pattern in which the state’s evaporation demand – the measure of a thirsty atmosphere – is considered the highest on record from June to August. This ox-blood-colored part of the map shows the greatest deviations from the normal California summer from 1980 to 2021.
In November, Science Daily quoted Abatzoglou as saying, “Increased evaporation needs with heating allows fuels to stay drier for longer periods of time. This is a recipe for more active fire seasons. “
This forecast was confirmed in summer 2021.
In addition, an April 2020 article in Science magazine that Abatzoglou was one of the authors estimated the massive, persistent drought afflicting the southwestern United States to be 46% more acute due to man-made climate change.
Another study by Abatzoglou and others several years ago estimated that the 2012-2014 drought in California was 8 to 27% more severe due to the warming climate.
There have always been cyclical droughts in California and the west, but scientific evidence suggests that man-made climate change is creating a warmer, more thirsty atmosphere that is drawing moisture away from the landscape.
“The increase in the need for evaporation is comparable to putting an extra straw into the drink,” said Abatzoglou. “It is now much easier to pour the cup off.”