SAN FRANCISCO — In the midst of a two-decades-long megadrought that climate scientists say has been made worse because of rising global temperatures, California has been hit with an especially cold and wet winter, in which low temperature records have been set and the Sierra snowpack is poised to eclipse its all-time high.
But the whiplash from one extreme to another is consistent with climate change, experts say. Since the start of the Industrial Revolution, California’s average temperatures have risen between 2-4 degrees Fahrenheit, and the dramatic variability seen in recent years is a consequence of that fact.
“A lot of folks are saying, ‘Everything is getting more extreme, it’s wetter and drier and hotter and colder.’ One of those isn’t true,” UCLA climate scientist Daniel Swain told Yahoo News. “Three out of the four are in California. The one thing that isn’t happening is it’s not getting colder.”
Swain, whose blog, Weather West, documents California’s changing climate, noted that while many records have been set this year for the lowest high temperatures ever recorded on a given calendar day, that number pales in comparison to the number of daily heat records.
“We’ve broken dozens of daily cold records across the state in this event and this winter. Just in the last two years, we’ve broken hundreds, if not thousands, of the equivalent hot records in California,” Swain said. “You will continue to experience cold extremes even in a warming climate, but there’s so much evidence that we’re seeing far more of the record warm events than we are the record cold. We’ve quantified this on a U.S.-wide basis, it’s about a 3:1 ratio. In a stable climate, you’d expect it to be about 1:1.”
So, while some parts of Southern California saw rare blizzard conditions last week, and snow fell at unusually low elevations in places like the Bay Area, those events in no way disprove climate change, which is measured over a longer time horizon than a single week, month or season. Nor do the brutal cold snaps show that climate change is simultaneously causing the world to undergo cooling.
What is happening, Swain explained, is that rising temperatures have increased the amount of moisture that can be held by the Earth’s atmosphere, resulting in the kinds of extremes being felt in California. Under what is known as the Clausius-Clapeyron equation, for every degree Celsius of temperature rise — global average temperatures have risen roughly 1.1 C since mid-19th century — the atmosphere holds 7% more moisture. That dramatically increases the chances for extreme precipitation events, but it also means that, due to increased evaporation rates, the risk of droughts also rises.
“When it rains, it’s increasingly likely to pour, just because of basic thermodynamics, and when it’s not raining, when it’s sunny and hot — and, of course, increasingly hot due to climate change — it’s going to be easier to evaporate that water back into the atmosphere, leading to more arid conditions during that period, more rapidly intensifying droughts,” Swain said.
For California, climate scientists like Swain believe that the state's extreme weather of late is a manifestation of the Clausius-Clapeyron equation.
“We’ve seen two historically severe droughts in the past decade in California, both of which were probably comparable in magnitude to each other, but individually were the worst droughts in the modern instrumental record going back to the 1800s,” Swain said. "But this is also the same decade where we’ve had the wettest winter on record in much of Northern California, the wettest individual days on record in places like Sacramento and now Fresno and any number of cities and smaller places. A lot of Northern and central California, the single wettest day in the last 100 or 150 years has been within the last five or six years, in the same period when you had this historically severe, multiyear drought.”
In his acclaimed 2014 reboot of Carl Sagan’s “Cosmos,” astrophysicist Neil deGrasse Tyson laid out the prevailing scientific distinction between weather from climate, making the case that chaotic weather fluctuations were different from climate.
Since then, however, more data has emerged to link extreme weather events and climate change.
"We got a lot of blow-back five or 10 years ago when we first started talking about this because the evidence was not really there yet, but we were arguing, ‘Wait, we don’t have enough data yet, just wait a long enough period of time where this has been evolving for us to really see it in the real world,’” Swain said. “Now we have another decade of data and it has been a decade of exactly the kinds of extremes we’ve been talking about.”
To try to explain how the buildup of greenhouse gases have altered the climate, Swain used a metaphor of a giant sponge to describe the Earth’s atmosphere.
“A sponge works two ways: You can soak a bunch of water up that you spill on the counter and you can also wring out a lot of water out of a sponge,” he said. “For progressively larger sponges, you can soak up progressively more water and also wring out progressively more water.”
What that means for California in the years to come, he added, is “more ‘wet wets’ and more ‘dry drys.’”