The decades-long drying trend that increased fire risk in the US Southwest

The American Southwest is the driest it has been since the year 800 after a decade-long “megadrought”, creating tinderbox conditions that underlie the catastrophic fires in Los Angeles.

The heavy rain events of the past two winters in California were not enough to disrupt the long-term drying trend, and there has been barely a drop this winter.

Andrew J. Kruczkiewicz, a climate scientist at Columbia University, said the data showed the desert areas west of the Rocky Mountains and into Southern California had dried out over the past half-century.

The drier conditions increased the risk of bushfires, he said, though further analysis would be needed to definitively say the Los Angeles fires were caused by climate change.

“When we’re talking about wildfire risk – and in Australia, the ingredients are very similar – you need low relative humidity, you need drier than average conditions or dry conditions, and you need wind,” Kruczkiewicz said.

“These are the primary meteorological factors that lead to wildfire risk … not necessarily ignition of wildfires, but the spread. So the ingredients are there.”

Southern California, Nevada, Arizona, Utah, New Mexico, and parts of Colorado and Texas are in drought, figures from the National Oceanic and Atmospheric Administration (NOAA) show. Western Texas is in “exceptional drought”, the highest level.

The last time Los Angeles had rainfall of more than 2.54 millimetres was May last year, as this masthead reported last week. Only months before, in February, nearly 30 centimetres of rain hit the University of California, Los Angeles, in just two days. Scientists have described this oscillation between wet and dry as “hydroclimate whiplash”.

The drought is not a new phenomenon. An article in Nature Climate Change, led by researchers at the University of California, Los Angeles, in February 2022, labelled the period since 2000 a “megadrought” because it was so prolonged.

The researchers found megadroughts occurred repeatedly in the region west of the Rocky Mountains from southern Montana to northern Mexico from 800 to 1600, followed by a 400-year hiatus.

Human-caused climate change is responsible for about 42 per cent of the soil moisture deficit since 2000, the paper found, because warmer temperatures are increasing evaporation, which dries out soil and vegetation. From 2000 to 2021, temperatures in the region were 0.91 degrees higher than the average from 1950 to 1999.

The Palmer Drought Severity Index, based on NOAA figures published by the US Environment Protection Agency, suggests a long-term drying trend for six states in the Southwest over the 20th and 21st centuries. The index took average annual rainfall for Arizona, California, Colorado, Nevada, New Mexico and Utah.

The figures show several long dry spells followed by periods that are wetter than average since 1895 when the records began. But the period from the beginning of 1999 to the end of 2023 was the driest of the dry spells in the records – 20 times drier on average than 1924-48, and four times drier on average than 1949-1973.

A 2013 study in the Journal of Climate by researchers at Princeton University has linked deforestation in the Amazon Rainforest to reduced rainfall on the west coast of North America.

Kruczkiewicz said there were seasonal and regional variations within the drying trend. In California, the biggest drop in rainfall in California was in the September to November period, while the trend was weaker in December to February.

There were also other natural forces at play, Kruczkiewicz said, such as the El Nino-La Nina southern oscillation. La Nina causes wetter, rainier conditions in Australia but warmer, drier conditions on the US West Coast, and vice versa for El Nino.

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