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California's rapid descent into extreme drought is a climate detective story ? and the outcome could be disastrous

Tuesday June 22, 2021 - 01:37 AEST

In the space of little more than a month California has plunged into a water crisis.


At the end of March, the US state's drought situation was severe and widespread, but only a small area was rated as being in exceptional drought.


Then in April something very strange happened that caught water experts by surprise.


The winter snowfalls that provide more than a third of California's water supplies weren't great, but they weren't disastrous.


"This year on April 1 for California we were anywhere from 60 to 80 per cent of normal (snow levels)" US Desert Research Institute climatologist Daniel McEvoy said.


"We were concerned, but it didn't look as bad as 2015, when there was no snow on April 1.


"But then some interesting things happened."


Here today ?


April was unusually warm and dry in California and Dr McEvoy watched the snow melt faster than he had ever seen before.


"Within a month we went from around 70 per cent of normal down to around 30 or 40 per cent of normal," he said.


"What was interesting was just how fast it happened after April 1 ? and how quickly we got into trouble."


By mid-June, most of the state was declared to be in extreme drought, up from less than a third in late March.


A third of California was declared to be in the worst category, exceptional, up from five per cent in late March.


Somehow much of the water from melted snow that California was relying on to break the drought never made it into reservoirs.


It was as if it had disappeared into thin air.


Where did the water go?


Some climate scientists could not ignore California's extraordinary previous summer of heat and fire in their search for clues. 


"I really think this is largely to do with the role of unprecedented, record-breaking heat," prominent University of California drought and bushfire scientist Daniel Swain said.


"Extreme heat essentially means there is much more evaporation than there would be in the absence of extreme heat.


"It's really that simple."


But a number of top Australian climate scientists suggested it might be the other way around.


"It is often the lack of evaporation driving the temperatures up, rather than vice versa," Anna Ukkola from the University of NSW said.


"In Australia, evaporation normally declines during drought and this in turn increases temperatures."


Dr Ukkola said scientists did not have strong evidence for increasing evaporation with global warming in Australia.


She said the observations suggested evaporation had remained stable over most regions during the last three to four decades.


"This is because temperature is just one factor among many that influence evaporation rates" Dr Ukkola said.


"I think the problem is that 'higher temperatures increase evaporation' feels intuitive and is nice and simple.


"But in practice the physics is more complicated and this often gets ignored."


Summer of heatwaves


In April last year poor snowfalls were followed by California's first heatwave of the season.


In August 2020, California suffered its worst heatwave in history, with Furnace Creek in Death Valley recording what may be the hottest temperature ever recorded on Earth ? 54.4 degrees Celsius.


The heatwave intensified the wildfires that went on to burn four per cent of the state, in what was the worst fire season in California's modern history.


"What happened last August and through October, which coincided with the extraordinary sequence of wildfire events here, are just record-shattering and searingly high temperatures," Dr Swain said.


"These were all-time high temperatures, hottest months, hottest days on record, in places with quite long records.


"It was just unrelenting."


California's big dry


By winter California's soils and plants were at near-record levels of dryness.


"When we had the early and rapid snowmelt, the soil was so dry that most of it likely went into saturating the soil instead of running off into the streams and reservoirs," Dr McEvoy said.


Snow and snowmelt also evaporated into the atmosphere or were sucked up by thirsty plants.


"The extreme temperatures have played a big role in just really sucking the moisture out of the soil ? but it's not the only thing that contributes to snowmelt," Dr McEvoy said.


"Solar radiation is a big one, and where you have bare soils, it's going to dry those soils out faster and it's going to stress the vegetation more."


Flash drought


The sudden escalation of drought in California has parallels with so-called "flash droughts", according to Ailie Gallant from Monash University.


"A flash drought is where evaporation plays much more of a prominent role than it usually does in a drought, and because it plays a more prominent role, you descend into drought quite quickly," she said.


"So areas can go from being a bit dry to really severe drought in a matter of weeks or months."


But like Dr Ukkola, Dr Gallant cautioned against seeing heat as the primary driver of increased evaporation.


"Saying that increased temperature means increased evaporation is vastly simplistic, because the role of temperature in evaporation is actually more like a second or third order effect," Dr Gallant said.


"It's more things like [solar] radiation, wind and humidity levels in the atmosphere."


Hot, dry summer looms


The reservoirs that supply California's vast irrigation networks are now at record low levels and farmers could see their water allocations slashed to as low as five per cent.


The same processes that have dried out soils have dried out fuel for bushfires.


Former California fire chief Ken Pimlott said fire conditions were looking worse than last year.


"California burned four million acres last year, which was unprecedented ? and nothing's changed," he said.


"In reality it's actually gotten worse in terms of fuel moisture.


"Fuel moistures are trending down significantly and on top of that, dry wind events that are really becoming more and more frequent and are accelerating in intensity.


"By mid to late summer, early fall, the potential again for catastrophic fire is there."







- ABC

© ABC 2021

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