The West Coast of the United States is still being bombarded with a continuous series of storms bringing extreme winds and rain, and causing a great deal of damage.
Given the destruction and the fact that there have been few comparable events in recent memory, it is natural to wonder about the influence of climate change. It turns out that for storms like these, that influence is a bit complicated, and it actually depends on the characteristic that is being considered.
In fact, for the two most impactful characteristics of extreme winds and extreme rainfall, we get opposite conclusions: Warming should weaken extreme winds in storms like these, but it should enhance extreme rainfall.
Climate change weakens extreme winds
Outside of the tropics, extreme winds are usually associated with “extratropical cyclones,” which is the technical term for the line of storms currently parading over the Pacific and into the West Coast. Extratropical cyclones are driven by pressure differences in the atmosphere, which in turn are largely driven by temperature contrasts at ground level.
In general, temperature contrasts at the surface decrease with warming because the colder polar regions are warming faster than the warmer tropical regions. In turn, as the climate warms—and temperature contrasts reduce—extratropical cyclones and their associated extreme winds should decrease in frequency and intensity.
This expectation is born out by more complex modeling. “The number of extratropical cyclones associated with intense surface wind speeds and undergoing explosive pressure deepening are projected to strongly decrease in the Northern Hemisphere winter,” as the IPCC puts it. And “the weakening of surface winds of extratropical cyclones in the Northern Hemisphere is attributed to the reduced low-level baroclinicity from sea surface temperature and sea ice changes (Harvey et al., 2014; Seiler and Zwiers, 2016; J. Wang et al., 2017a).”
The IPCC has also noted that, “...by the end of the century, climate models show that the number of extratropical cyclones associated with extreme winds will significantly decrease in the mid-and high latitudes of the Northern Hemisphere in winter…”
Thus, the high winds that have caused a lot of the damage from these storms are occurring despite climate change likely making them a bit weaker.
Extreme rainfall totals get larger as the planet warms
Apart from extreme wind, the other most impactful characteristic of these storms is their copious rainfall, which has resulted in widespread flooding. The moisture that supplies the rainfall in these storms is mostly transported from the tropics in narrow bands called “atmospheric rivers.” While there is some evidence that climate change directly impacts atmospheric river activity, its main influence is simply that a warmer atmosphere is capable of holding more water vapor and thus producing more rain.
As the IPCC puts it:
“There is high confidence that average and maximum extratropical precipitation rates will increase with warming, with the magnitude of the increases associated with increases in atmospheric water vapor.”
Changes in atmospheric water vapor are not the only contributing factor, so models for more precise calculations on changes in extreme rainfall must incorporate many more complex interactions.
A metric called RX5Day, which is the annual 5-day period with the most rainfall at a given location, is typically used to study changes in multi-day extreme rainfall totals in climate models. That metric is relevant to the parade of storms slamming the West Coast because the impressive rainfall totals are largely a result of rain being sustained for many consecutive days.
Both global climate models and high-resolution regional climate models show that most locations should see enhanced RX5Day totals as the planet continues to warm, but this enhancement is not particularly large over California. The reason for this is that the aforementioned reductions in the frequency and intensity of extratropical cyclones - so-called dynamical changes - could locally counteract some of the effects of increasing atmospheric water vapor.
So how much has climate change contributed to the large rainfall totals we've seen? Let’s use San Francisco as an example; the rain there over the last couple of weeks has been extreme, and it is representative of what's been seen over a large swath of the middle of California.
Since the streak of rain events began after Christmas, San Fransico’s 5-day period with the most rainfall was January 1-5, which totaled 5.53 inches.
Assuming for the moment that this 5-day period ends up being the 5-day period with the most rain this year, we can use the model calculations on RX5Day (charts below) to surmise how much climate change has contributed to the amount of rain on these days.
For San Francisco (black dot), we see that global warming to date (1.2°C) has enhanced RX5Day by about 4% relative to what it would have been in preindustrial times.
That means the 5.5 inches of rain that fell in San Francisco in the first week of January would have been more like 5.3 inches in a preindustrial climate. In a future world that experiences 3°C of global warming, this rainfall would total about 6 inches.
What the models say, then, is that for the extreme 5-day rainfall totals associated with these storms, the vast majority of the rain is a result of natural weather variability, and the contribution from climate change is in the range of several percentage points.
One could argue that the RX5Day metric is not extreme enough to capture the type of event we are seeing because what we are experiencing is much more severe than the typical annual five-day maximum rainfall total. However, looking at even more extreme events with return periods of ten and fifty years would not change the story much. One could also argue that five days is too short to capture the current event. After all, when all is said and done, the extremely wet period might last close to a month. But monthly timescale changes are calculated to be similar to RX5Day changes, so if we did this analysis on the monthly timescale, it would also paint a similar picture.
The above findings, however, do not reduce the importance of increasing societal resilience to events like this. We live in a climate that naturally produces all sorts of inclement weather, and it is not necessary for that weather to be radically changing for it to be wise to be prepared for it.