Has Global Warming Caused A Quantum Jump In Extreme Weather?

פורסם: 1 בנוב׳ 2012, 8:51 על ידי: Sustainability Org   [ עודכן 1 בנוב׳ 2012, 8:52 ]
By Joe Romm on Apr 17, 2012

“The climate has shifted to a new state capable of delivering rare & unprecedented weather events,” explained Weather Underground director of meteorology and former hurricane hunter, Dr. Jeff Masters.

Increasingly, scientists and meteorologists are asking whether global warming is driving a quantum jump — a non-linear shift — in our extreme weather.

We now have enough observations and analyses that a scientific literature on this subject has begun to emerge:

This is not your father’s climate, as Stu Ostro, senior director of weather communications at the Weather Channel has documented at great length (see this big PDF)

Peter Sinclair has put together an excellent video for the Yale Forum on why even the modest 1°C warming we’ve seen over the past century can cause a disproportionally large shift in our weather systems:


RealClimate ran an excellent, semi-technical explanation by Stefan Rahmstorf and Dim Coumou of Germany’s Potsdam Institute for Climate Impact Research. They explained how global warming sharply increases the likelihood of ‘outlandish’ heat waves (see charts below) and concluded:

So in summary: even in the most simple, linear case of a shift in the normal distribution, the probability for “outlandish” heat records increases greatly due to global warming. But the more outlandish a record is, the more would we suspect that non-linear feedbacks are at play – which could increase their likelihood even more.

Since this is an emerging field, it’s no surprise that not every climate scientist agrees. Martin Hoerling, who heads the “Climate Scene Investigators” at the National Oceanic and Atmospheric Administration, emailed blogger Andy Revkin a statement that included this truly remarkable sentence:

After all, the irony of extreme events is that the larger the magnitude the smaller the fractional contribution by human climate change.

This is the linear view of things: The modest amount of warming that we have had to date can have no more than a modest impact on any extreme event, large or small.

The emerging literature says otherwise. I asked for a comment by two leading climatologists, Michael Mann and Kevin Trenberth. Here is Mann, director of the Earth System Science Center at Penn State:

Hoerling’s statement hopelessly confuses two very different things, modest shifts in the average, and the dramatic impact those shifts can have on the incidence of extreme events. The anomalous recent U.S. heat is a manifestation of the latter. It is a demonstration of that we have loaded the random weather dice toward producing far more “sixes” than we would expect from chance alone.

UPDATE: Dr. Mann emailed me to clarify that he was using the word hopelessly “simply to indicate that I thought there was no possible justification for the connection that Hoerling was making.” If you read Hoerling’s email slamming Rahmstorf and Coumou, it’s clear that Mann’s statement is very mild in comparison.

And here is Trenberth, former head of the Climate Analysis Section at the US National Center for Atmospheric Research:

When climate changes, the percentage changes in frequency are always greatest for some extremes (what are referred to as the tails of the distribution). No matter how much climate changes, there is always weather and natural variability such as El Nino, and this can be beguiling. Extremes are inherently rare events but natural variability can break records all by itself. But when the climate has changed, and we know it has, the odds of a particular extreme can easily be magnified many fold: what would be in an unchanging climate a 1 in 1000 or even 1 in 1,000,000 or more event becomes a 1 in 100 year event, as has been documented in some cases. What were 500 year storms become 50 year storms: still not common but with much amplified odds. So while natural variability plays a key role, the effects are amplified by climate change: when natural variability is moving in the same direction as climate change, we not only break records, we smash them. And this is what we have seen especially in the past 2 years.

The above are largely statistical statements and climate research is also involved in understanding how these actually come about. While harder to prove, nonlinear effects often come into play to compound the issues as the accumulated effects of global warming set the stage for amplified responses. For example the extra heat from global warming dries the land and removes evaporative cooling as an option for the subsequent heat wave. Or extra moisture triggers instabilities in storms that make them much more severe than otherwise expected. Yet the phenomena that cause the extremes — thunderstorms, tornadoes, hurricanes, drought, blocking anticyclones — are familiar and it is easy to be led astray and say they are natural. Their effects and impacts are not.

Again, Rahmstorf and Coumou put this well in charts at the end of their post:

IPCC (2001) graph illustrating how a shift and/or widening of a probability distribution of temperatures affects the probability of extremes.

For illustration, let’s take the most simple case of a normal distribution that is shifted towards the warm end by a given amount – say one standard deviation. Then, a moderately extreme temperature that is 2 standard deviations above the mean becomes 4.5 times more likely (see graph below). But a seriously extreme temperature, that is 5 standard deviations above the mean, becomes 90 times more likely! Thus: the same amount of global warming boosts the probability of really extreme events, like the recent US heat wave, far more than it boosts more moderate events.

This is exactly the opposite of the claim that “the greater the extreme, the less global warming has to do with it.” The same is also true if the probability distribution is not shifted but widened by a constant factor. This is easy to show analytically for our math-minded readers.

Graph illustrating how the ratio of the probability of extremes (warmed climate divided by unchanged climate – this increased likelihood factor is shown as a dashed line, scale on right) depends on the value of the extreme.

As Coumou and Rahmstorf concluded in a Nature Climate Change piece,A decade of weather extremes” (subs. req’d):

It is very likely that several of the unprecedented extremes of the past decade would not have occurred without anthropogenic global warming…. The evidence is strong that anthropogenic, unprecedented heat and rainfall extremes are here — and are causing intense human suffering

The really worrisome part is we’ve warmed only about a degree and a half Fahrenheit in the past century. We are on track to warm more than five times times that or more this century (see M.I.T. doubles its 2095 warming projection to 10°F — with 866 ppm and Arctic warming of 20°F ).

In short, Mother Nature is just getting warmed up!

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