An Interview with Dr. Alexander Gershunov about the impacts of climate change on California, and the public health consequences that result.
BY ETHAN OLSON
Alexander Gershunov is a Research Meteorologist at the Scripps Institution of Oceanography.
Can you give me a general overview of what effects climate change is having and will continue to have on the West Coast of the United States, and can you describe the big climatological processes that power this change?
Okay, you know aside from general warming that's experienced all over the globe and has been happening consistently in California, there are extreme weather events that I'm particularly interested in. Because it's those extreme weather events that really have impact.
And some of those extremes are getting exacerbated by the steroids of climate change, and specifically in California — you know you tell me if you need me to talk more about the average warming impacts on snowpack and all that, but I'll just get into the extremes — so specifically in California heat waves have been on the rise for the last few decades, as they have been all over the globe. But in California and the Southwestern US there's a specific flavor of heat waves that is being bolstered by climate change — that is, humid heat waves, which are basically very hot during the day just like dry heat waves are, but the elevated humidity in the air prevents nighttime cooling. These humid heat waves remain very hot throughout the night, and so it's particularly in minimum temperatures where we see the largest increase in heat wave activity in California. It’s related to the warming of the oceans, and the impacts of humid heat waves are much more severe than those of dry heat waves.
The traditional dry heat waves in a semi-arid region of California basically elevate temperatures tremendously during the day, but then there is night time cooling that allows people and animals to have the respite from the heat. On the other hand, humid heat is actually much more difficult to weather, especially for this semi-arid region where we're not acclimated to humid heat. So, you know, when humidity is elevated, sweat doesn't cool you off; the process of sweating doesn't cool you off as efficiently as when the air is dry because it doesn't evaporate as effectively and it just sort of runs down your skin and makes you wet but doesn’t cool your body. So that's one thing, and the other one is the fact that the elevated nighttime temperatures and humidity also prevent us from getting the respite from the heat, and then the next day starts hotter. These humid heat waves tend to last longer and have a larger impact on human health.
Of course, you know in terms of intervention strategies, when it's the nighttime temperatures that are elevated you can’t tell people to go to cooling centers at night, right? There has to be some other way to ameliorate those impacts. And so that's the heatwave story.
But you know, they're also impacts on the hydro climate, basically the part of climate that has to do with water — precipitation — and we are expecting precipitation to change in a couple of important ways in California. One is that because California is a Mediterranean climate region — it basically sits between the sub tropics and mid-latitudes, so we get the dry subtropical summers and then the mid-latitude storms bring us most of the precipitation over the year — in response to a polar-amplified global warming, the subtropics expand and our subtropical summer is expected to get longer. This basically squeezes the wet season into a narrower season focused more and more on December, January, and February. So we are expecting to lose precipitation frequency: storms in the fall and spring specifically. This is the case for all of the Mediterranean climate regions around the world — there are about five of them, and these are all the regions where wine grapes also prefer to live.
The other signal that we are expecting is that the strongest winter storms that do make it into the region are expected to get stronger. And those are the atmospheric rivers that are responsible for most of our extreme precipitation events and floods, as well as contributing a lion's share to our water resources. Atmospheric Rivers have a relatively simple mechanism of producing precipitation, which is orographic. So basically there's a stream of moisture — water vapor — that hits the coastal mountain ranges of California, as well as the Sierra Nevada and the transverse ranges, and as that moist air mass is forced to rise over the mountains — they're not moving anywhere on these timescales — precipitation is squeezed out of them. And so in a warmer atmosphere, saturated conditions become wetter because the amount of water vapor that the atmosphere can carry — that's the saturation vapor pressure — increases nonlinearly with temperature. And so with a small increase in temperature, you get a disproportionately larger increase in its ability to carry moisture. And so these saturated conditions that atmospheric rivers are become wetter and more potent, and since the mountains are not moving anywhere on these timescales, they basically end up producing more precipitation.
Again, two climate change signals in California’s precipitation regime that are expected are a decrease in the frequency of precipitation, particularly in the fall and spring, and an increase in the intensity of the most extreme events. So that basically results in a smaller sample of storms that produce our total annual precipitation, and even if the total annual precipitation may not, on average, change, the swings between wet and dry years become larger. The volatility of the hydroclimate increases, and that's just basically a statistical fact called sampling variance. So the larger the sample of storms that produced our total annual precipitation, the more stable the precipitation regime and the less it changes from year to year. The smaller the sample of storms, the more of the total annual precipitation depends on that one, two, or three really big storms per year — whether they occur or not — and then the total annual gets even more volatile. And actually, California, especially Southern California, is the most volatile region in terms of precipitation in the United States because it's common for us to have, you know, drought years followed by flood years, and very rarely do we actually get our annual average normal amount of precipitation. So that volatility is going to get exacerbated by climate change.
That’s a pretty certain result, but in terms of total precipitation, it will probably decrease somewhat in Southern California and increase a little in Northern California. But all over California it’s going to get more volatile and more challenging in terms of management of water resources and flood risk and so on. So that’s that part, and then in how much detail do you want me to actually talk about it?
Can you go into a little bit more detail about the impact on water resources? Can you go into how climate change might affect water resource management — you know, the snowpack and how we supply water to people and farms and things like that.
Sure. Well, we have been losing snowpack for the last few decades, and specifically the ratio of rain to snow has been increasing. That's particularly true for lower elevations, which excludes the southern Sierra, the highest part of the Sierra Nevada range. This gradual loss of snow is also observed for most mountains around the world, as well as pretty much all of the mountainous Western US.
A lot of the precipitation that we get here, like I mentioned before, is from atmospheric rivers, and atmospheric rivers are typically warmer storms than the cold frontal cyclones that bring us most of the rest of our precipitation; so snow lines are higher with those storms. As climate change evolves, we expect an increasingly larger proportion of precipitation coming from atmospheric rivers, as these are specifically the events that are getting bolstered by climate change — and again, the warming directly impacts the conversion of snow to rain and the increasing or elevating snow lines.
So, the warmer storms and more of the precipitation coming from atmospheric rivers basically means that more of the precipitation runs off the ground once it falls in the winter, when it is much more challenging to capture in engineered reservoirs which were really designed to capture snowmelt in the spring when you know, it melts mostly in a very well-behaved way. But as climate change progresses, we get more of the precipitation running off in the winter, when reservoir managers are basically challenged with keeping enough space in the reservoir to capture runoff from from the storms that are coming. So during winter it is much more challenging to capture that water because you end up releasing water out to the ocean in order to make room for the next storms that are in the forecast. Whereas in the spring and summer when it doesn't really rain anymore, you can fill the reservoirs as much as the engineering allows. So you know this impact of climate change on the precipitation regime makes it much more difficult to retain the precipitation that actually falls, to capture and retain it in the reservoirs for the summer when it's really needed most.
Can you tell me more about how climate change is worsening the impacts of Californian wildfires, and touch on the role of Santa Ana winds?
There's so much to say about Santa Ana winds! On the one hand, they actually produce one of the types of heat waves that we get in California — that’s specifically coastal heat waves in the fall, winter, and spring. These heat waves are out of season and they are really just confined to the coastal zone. We do know that these out of season heat waves are associated with health impacts. There's a paper on that that came out last year.
Southern California is very interesting in the sense that you can actually get impactful heat waves all year long. But with climate change, we actually expect Santa Ana wind activity to decrease somewhat, in a very specific way. We expect it to decrease in the fall and spring, not really in the wintertime when it actually peaks, and it's sort of a similar signal to the precipitation regime change. And probably for the same reasons. But what that means is that with a later start of the wet season, which we expect with the changing precipitation that I described before, and a Santa Ana season that will always be much more active in the December than it ever was in October, which is our typical wildfire season, our wildfire season may be gradually shifting towards later in the year. And with dry fuels persisting into December, the peak of the Santa Ana winds season, you get typically much larger wildfires than you would get in October. In December you get back to back Santa Ana winds that have the potential of spreading wildfires to unprecedented proportions, which is what happened for example in December, 2017, when we had the Thomas Fire. The Thomas fire burned through most of December and into January, because there wasn't any precipitation and the fuels were totally dry until January 9th.
The Thomas Fire got really large, the biggest wildfire in California history at the time of its occurrence. It has been overshadowed by Northern California wildfires since then, but it's still the biggest in Southern California history, and it got so big because it was fanned by consecutive Santa Ana winds, and it was finally put out by an atmospheric river which caused the first precipitation of the season. Adding insult to injury, it resulted in debris flows that actually killed more people than the wildfire, because fire scars typically produce much larger runoff from them than healthy vegetated hill slopes; and that's because the vegetation has been burned out so the roots are not really holding the soil together. The soil itself for very high intensity burns becomes hydrophobic, which means that it repels water. So very little rain gets absorbed by the soil, and most of it just runs off taking everything with it — trees, boulders, houses, cars — and that’s an added risk.
And then smoke from wildfires is another major concern because wildfires in the coastal zone typically burn the sloping backcountry, where the Santa Ana winds, and the Diablos up north, blow the strongest. Even though the direct impacts of the wildfire are devastating, not many people live in regions where fires typically burn, but smoke gets transported to the very dense and diversely populated coastal zone, and the indirect impact on the wildfires via smoke exposure can actually be more important in terms of public health than the direct impacts.
We actually published a paper just last week that shows for the first time epidemiological evidence that on a population level smoke is up to 10 times as impactful to public health — respiratory health — as similar levels of pollution from other sources. In California, pollution from other major sources has been decreasing because of policy; air pollution from wildfires has been increasing, and will continue to increase with future warming.
I have a question about — maybe not a question — but can you go into a bit more detail about why we think climate change is correlated with more wildfires or bigger wildfires, other than the Santa Ana winds you were describing earlier?
Well, actually, we don't see any change in the Santa Ana winds yet, but we're expecting them to decrease somewhat, although there's some caveats attached to that. There are a couple of kinds of Santa Ana winds — two flavors: hot and cold, and it’s the hot ones that are.. So since we did a study about the projections of Santa Ana wind activity into the future, we learned that of the two flavors of Santa Ana winds — hot and cold — it’s the hot ones that are predominantly associated with wildfire because the cold ones are actually preceded by precipitation. These are pretty fresh results and we haven't done the projections on the flavors of Santa Ana winds yet, but I have a suspicion that it's probably the cold Santa Ana winds that are more sensitive to climate change. It’s probably the cold Santa Ana winds that are responsible for a decrease in Santa Ana wind activity that we saw in our projections. And if that's the case, then that is not good news for wildfires; but there hasn't really been any significant trend observed in Santa Ana wind behavior yet.
But we do see an increase in fire activity. And a part of that is due to fire management where we effectively put out the small fires; and that allows the vegetation to grow thicker than it was before, before we started putting out the small fires. And so the large fires that we can not control become a lot larger because there’s a lot more fuel there to burn.
That's one part of the story, but the other part of the story is that climate change through warming primarily impacts the dryness of the fuel because obviously, hotter summers, longer and hotter heat waves that we observe suck the moisture out of the plants, out of both the alive and dead fuels. And so that allows the fires to burn stronger and hotter when they do occur.
And also, we have sort of new emerging wildfire regimes that weren't really relevant before. For example, in August of 2020, we had wildfires that were actually exacerbated by heat. They occurred during a heat wave, and these wildfires were exacerbated by heat rather than wind because they occurred without Santa Ana winds. But the heat actually makes wildfires burn hotter and grow larger as well. There have also been several accounts of fire tornadoes associated with these wildfires that are driven by heat waves, which haven't really been observed historically in California.
So basically what we see is a wildfire season that's expanding to both earlier in the summer before the start of the Santa Ana winds, and later in the winter like the Thomas Fire that burned in December and into January because the start of the rainy season was delayed. And that's what we expect more of in the future. So we have an opportunity for longer, bigger wildfire seasons to occur with a warming climate. And, you know, the observed trends have certainly been consistent with that.
But there are actually a couple of different types of wildfires in California. One is the fuel-dominated wildfire that typically burns in mountains and forests. Those wildfires are typically ignited by lightning and they burn in the summertime, traditionally. Those are clearly getting larger, and that increase in fuel-dominated wildfire activity has been clearly tied to warming, which is also associated with less snow versus rain; less soil moisture in the summer, because whatever snow there is melts earlier — these are all observed trends — ; and then summertime temperatures, and particularly heat waves are getting hotter and stronger. And all that contributes to fuel-dominated wildfires to increase.
And then there are wind-dominated wildfires in the coastal zone. So the wind-dominated wildfires are the ones that I was describing specifically for the coastal zone, and they have a very different dynamic. Historically, they occur in the fall when the fuels are at their driest after a long dry Mediterranean summer, and when the first Santa Ana winds start before the first rains of winter. The fuels are extremely flammable and the conditions under those dry gusty hot winds are perfect for spreading humongous wildfires.
And so, at the coast, that's why we get our largest, most catastrophic wildfires in the fall, historically. As I said, that season is expanding because the fuels become even drier and more flammable with warming, and those dry fuels have more opportunity to persist longer into the winter, into the peak of the Santa Ana wind season in December as the rainy season is delayed by climate change. So there's two specific mechanisms, two different types of wildfires. Those two specific mechanisms, both of them, lead to an increase in wildfire for somewhat different reasons.
Great. I think we've got quite a bit of good information. Is there anything else that you want to go over though?
I guess what has been less studied is the compounding coincidence of extreme weather events that lead to impacts. So for example, heat waves and smoke from wildfires can compound to increase the impacts of just heat or just smoke alone, and we haven't quantified that compounding yet. That is a very important thing to do.
And I think part of the way that we're going to adapt to the changing climate, because we cannot completely mitigate our way out of it; even if we became carbon neutral tomorrow, we're still committed to some level of warming. The greenhouse gases that are already in the air are going to stay there for a while. It's a long-lived gas in the atmosphere, and you can't just stop global warming on a dime, even if there was political will to do that, or enough political will globally to do that. So we're going to have to adapt to some climate change, hopefully less rather than more, but part of that adaptation process is going to have to include advanced warning systems that address these types of impactful, extreme events, especially the ones that are being strongly exacerbated by climate change, where climate change acts like the steroids on which these extremes grow, like heat waves, wildfire, droughts, and floods.
So we have to learn how to predict them better, and at longer lead times. And then how to use that information in concert with the information about the impacts on health, energy agriculture, water resources; and make more intelligent decisions to reduce the risks associated with weather extremes on the steroids of climate change.