From the Cliff High Volume Weather Blog
Cliff Mass
The media last week was buzzing with stories of a significantly increased threat of a major flood along the West Coast, known as Hurricane ARk. The immediate stimulation of this apocalyptic vision is arecent paper by Xingying Huang and Daniel Swain in the magazine Scientific advance.
This article shows that global warming has ready Essentially, the probability of catastrophic West Coast flooding has increased dramatically, and the likelihood of such flooding will increase dramatically by the end of the century.
In this blog, I will describe some of the serious problems with this research, which overstated the threat. And I’m going to show you the actual trend of heavy, prolonged rainfall.
What is ARk Storm?
The term denotes a record-breaking, prolonged, unimaginable heavy rain event along the West Coast (primarily California) that caused catastrophic flooding. It presents a double pun. First, the story of Noah’s ark in the bible, with its global flood after a long heavy rain (40 days and nights!). And second, atmospheric river (AR) events have a long return period (like 1000 years or “k”).
Atmospheric rivers are streams of moisture that are associated with virtually all heavy rainfall events along the West Coast (see satellite image of one below). Also called Pineapple Express events in our area. Think of them as a meteorological “whirlpool” that pours heavy rainfall as it hits the terrain of the western United States
The only ARk hurricane in history occurred in December 1861/January 1862 in California, over a period of about 30 days. Two to three feet of rain fell over much of central California, even more heavily in some mountain locations, due to about half a dozen extreme atmospheric river events. This downpour was accompanied by above-normal temperatures, melting snow and a period of relative humidity before that.
Larger areas of inland California were flooded (see estimated inundation levels below). There has been much loss of life and damage to buildings and farms.
This type of prolonged, extreme flooding in California has occurred many times before, with evidence provided by coastal deposits. Such events seem to happen every few hundred years.
Huang/Swain’s paper, based on global climate model simulations, scaled down to consider local variations using a high-resolution regional model, states that the probability of such extreme events has been greatly enhanced by global warming and will become further, more probable by the end of the century.
As you can imagine, such doomsday clickbait is honey to the media and there have been dozens of stories in this paper over the past few weeks. Unfortunately, there are serious problems with the paper and reality seems to disagree with its estimate.
Let me demonstrate this to you below.
As I noted above, Huang/Swain’s research is not based on observations but on a set (set of) climate model simulations. Regrettably, they made a serious mistake in using climate models forced by an overly strong global warming scenario (RCP8.5), which is often considered by the peer-reviewed literature to be impractical (warm up too much). Models driven by RCP8.5 also tend to poorly simulate the critical El Nino/La Nina (also known as ENSO) cycle.
RCP8.5, which makes the most draconian assumptions about fossil fuel use, produces more than twice as much warming as is reasonable. And because the water vapor content in the atmosphere increases fast with temperature, using the wrong scenario (with too much warming) is a very serious problem seeing the impact of precipitation. They should use RCP4.5.
Another problem with this article is their claim that the most extreme precipitation events in California occur during El Nino years. This is inconsistent with the observations: the neutral years (not El Nino and La Nina) are the years with the heaviest rainfall in that state. El Nino years have been associated with numerous mass rainfall events in southern California.
Inconsistency with observations
As a result of their modeling, the frequency and amplitude of multi-week heavy rainfall events should have increased (see their reprint Figure 5a below).Observations do not show that. There is no evidence of large increases or even changes in annual rainfall in California. This implies that something is wrong with their model simulation.
By Huang and Swain 2022
Let me prove this to you.
Below is the annual rainfall in San Francisco dating back to 1853. You can see the massive annual rainfall in 1862. Note that there is no uptrend for the next 150 years!! In fact, it is slightly bearish (there is a red linear trend line).
There is no upward trend of 30-day wettest events along the West Coast. Graphics by Professor John Christy
A more realistic look
The Huang/Swain paper makes big claims about global warming driving a significant increase in the potential frequency of massive ARkstorm rain events in California.
Unfortunately, there are major problems with their paper, including the assumption of an unrealistic increase in greenhouse gases. Their simulations compared poorly with observed trends showing no increase in ARk-like events or heavy rain.
So what do I think is the truth of the matter? (and I have published extensively in peer-reviewed literature on the effects of global warming on heavy rainfall along the West Coast)
Let’s base our projections on the assumption that greenhouse gases will follow the more realistic RCP4.5 scenarios. With advanced energy technology, the RCP2.6 scenario could be even better.
Assuming an RCP of 4.5, annual precipitation will not change appreciably for the West Coast for the rest of the century, possibly with a slight upward trend. Global warming will result in very heavy atmospheric river events increasing by 10-20%. So all else being the same, AT THE END OF THE CENTURY, the likelihood of an ARk storm will greatly increase.
But remember, it takes more moisture to make an ARk storm: you need not only record-breaking atmospheric rivers, but multiple rivers, all of which make landfall in the same area. In short, there must be many extremely unusual “tornadoes” occurring in a short time and hitting the same area.
And there’s more: to have an ARk record flood event, you need to have a large amount of snow and ice ready to melt in the mountains, which will lessen as the earth warms. And a prehistoric wet period.
No wonder it’s rare to get an ARk event! Lots of moving pieces. And for the most part, there’s no reason to expect an enhancement by global warming.
Furthermore, California has a huge storage capacity for water in its reservoir system, which 100 years ago did not exist. This could help create a buffer for the next ARk storm.
And one more thing to consider: reality does not follow the extreme precipitation increases predicted by climate models. So, models are likely overstating the effects of global warming. I am a modeler, have read articles and have attended endless seminars on climate modeling performance. Believe me, these models have big problems and many of the shortfalls lie in the cloud and precipitation areas.
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