How Global Warming Sharply Increases The Likelihood Of ‘Outlandish’ Heat Waves

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

The full 592-page (!) IPCC extreme weather report is out. Like most Intergovernmental Panel on Climate Change reports it has some value for people who don’t follow the science closely, which is to say the overwhelming majority of the media and policymakers.

Of course, the TV media ignored the summary report in November, so we will have to see if they pay any attention to this one now that the United States has just been through the most extreme winter heat wave in our recorded history.

But as Kevin Trenberth, one of the world’s leading experts on the link between climate change and extreme weather, put it to me in an email:

I have seen the chapter on the physical climate and I found it quite disappointing…. I don’t think it adds to AR4 [IPCC Fourth Assessment] much.

I agree with Trenberth that if, for instance, you want a more up to date and straightforward discussion of the impact of climate change on precipitation, you should just read his 2011 paper, “Changes in precipitation with climate change” (online here).

Indeed, the actual scientific literature from 2011 is generally more useful than this report — see “NOAA Study Finds Human-Caused Climate Change Already a Major Factor in More Frequent Mediterranean Droughts” and Hansen et al: “Extreme Heat Waves … in Texas and Oklahoma in 2011 and Moscow in 2010 Were ‘Caused’ by Global Warming.”

It is, as I wrote when the summary came out in November, the report is “Another Blown Chance to Explain the Catastrophes Coming If We Keep Doing Nothing.”  I also wrote that the summary has a good chart that hints at dust-bowlification, but is mostly silent on warming’s gravest threat to humanity.

The full report has more on drought, but fails to clearly describe what the literature now suggests is coming if we stay anywhere near our current emissions path. In 2010, the National Center for Atmospheric Research did a far more valuable literature review and analysis of what we face, which makes clear we risk multiple, devastating global droughts even on moderate emissions path.

In the case of extreme weather, my guess is that decades from now, people will look back on the staggering growth in off-the-charts “outlandish” extreme events in the past few years and conclude that a regime change had occurred in the climate. That change is probably a combination of the sharp loss in summer/fall Arctic sea ice and the sharp increase in ocean heat content.

We’re only in the past year or so seen analyses that demonstrate the human fingerprint in these uber-extreme events, including the studies above and these two:

So rather than citing this timely, but already out-of-date IPCC report, let me just repost below an excellent new piece from RealClimate by the authors of those two studies, who have been doing some of the best recent work in this area.

Extremely hot

By Stefan Rahmstorf and Dim Coumou

One claim frequently heard regarding extreme heat waves goes something like this: ”Since this heat wave broke the previous record by 5 °C, global warming can’t have much to do with it since that has been only 1 °C over the 20th century”. Here we explain why we find this logic doubly flawed.

One can ask two different questions about the influence of global warming on heat waves (Otto et al. 2012), and we take them in turn.

1. How much hotter did global warming make this heat wave?

We have some trouble with framing the question like this, because it tacitly assumes that the same weather situation would have also arisen without global warming, only at a (say) 1 °C lower temperature level. That need not be the case, of course, since weather is highly stochastic and global warming can also affect the circulation patterns of the atmosphere.

But even if we accept the basic premise (and it could be meant in a purely statistical sense, although that is not usually how it is expressed), would an average anthropogenic warming by 1 °C in the relevant location mean that 1 °C is also the amount added to an extreme event? Only in a linear climate system. Imagine a heat wave that pushes temperatures up to 30 °C in a world without global warming. In the same weather situation with global warming, you might expect that this weather then results in a 31 °C heat wave. But that could well be wrong. Possibly in the situation with warming, the soil has dried out over the previous months because of that extra 1 °C. So now you lost evaporative cooling, the incoming sunlight turns into sensible heat rather than a large fraction going into latent heat. That is a non-linear feedback, and not an imagined one. Detailed studies have shown that this may have played an important role during the European heat wave of 2003 (Schär et al. 2004).

The basic phenomenon is familiar to oceanographers: if the mean sea level in one location rises by 30 cm, this does not mean that the high-tide level also rises by 30 cm. In some cases it will be more, due to nonlinear feedback. I.e., a higher water level increases the flow cross-section (think of a tidal inlet) and reduces bottom friction so the tide rolls in faster, reaching a higher peak. The tidal range increases as well as the mean sea level.

Numerous other non-linear mechanisms are possible, which we are only beginning to understand – think of the recent studies that show how changes in snow cover or sea ice cover as a result of global warming affect weather systems. Or think of factors that could affect the stability of particularly strong blocking events. Thus, we’d be very cautious about making an essentially linear, deterministic argument about heat extremes to the public.

In the scientific literature, the influence of global warming on extreme events is therefore usually discussed in terms of probabilities, which is more fitted to stochastic events. The typical question asked is:

2. How much more likely did global warming make this heat wave?

For this question, it is easily shown that the logic “the greater the extreme, the less global warming has to do with it” is seriously flawed. The change in probability of certain temperature values being reached can be visualised with a probability density function (see Figure [above]). The probability distribution could be shifted unchanged towards warmer values, or it could be widened, or a combination of both (or some other deformation).

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 ofreally 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.

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.


Our Perspective article on the unprecedented extremes of the last decade was just published by Nature Climate Change: Coumou & Rahmstorf (2012) A decade of weather extremes (paywalled)


Otto et al., Reconciling two approaches to attribution of the 2010 Russian heat wave, Geophysical Research Letters 2012, VOL. 39, L04702, doi:10.1029/2011GL050422

Schär, C. et al. The role of increasing temperature variability in European summer heat waves. Nature 427, 332–336 (2004).

— Stefan Rahmstorf and Dim Coumou





25 Responses to How Global Warming Sharply Increases The Likelihood Of ‘Outlandish’ Heat Waves

  1. wili says:

    “In the case of extreme weather, my guess is that decades from now, people will look back on the staggering growth in off-the-charts “outlandish” extreme events in the past few years and conclude that a regime change had occurred in the climate.”

    That’s pretty much my take. By the time we can absolutely prove that we have suddenly shifted into a radically different climate regime in a very few years (as has happened in the past), it will have become absolutely obvious to anyone (but the Koch bro’s and their minions, of course).

  2. prokaryotes says:

    Given that most of the forecast of developments and extremes has been underestimated, we could assume that these IPCC projections very well present the lower impacts.

    Btw repost

  3. Rabid Doomsayer says:

    Has any study been done on the increase in variance of rainfall? This could produce a result far more noticeable than the increase in variance of teperature.

  4. B Waterhouse says:

    So in the summary paragraph at the end are they saying that the variance is increasing, i.e. that the tails of the bell curve are getting longer? Does the observed recent temperature data show that yet? If there is more water vapor and more energy in the atmosphere wouldn’t we expect the extremes to become even more extreme?

  5. prokaryotes says:

    Yes, because on Venus we assume, a runaway climate change took place and winds there are ~220 mph or 360 km/h. Based on this simple comparison it seems logical that somewhere between today calm wind speeds on earth and extreme winds after a runaway state has reached, take place. And this is exactly we we can observe with tornado uptake, and tornado uptake of extreme tornadoes EF-5 and such, hurricane wind speeds and more extreme winds during stormy weather.

    Another indicator for more violent weather (in particular wind speeds) are insurance claims, which also are highest on record. Partially to these developments, Munich Re is very vocal when it comes to action on climate change.

    All experts conclusions here are in agreement.

  6. Michael Stefan says:

    When referring to the recent U.S. heat wave, this post makes it sound like it is over, but it isn’t, at least not back down to normal; the monthly average where I live (St. Louis) has continued to rise (and will shatter the old monthly record, ending up around 15 degrees above normal). Today it was 82 for the high, 21 degrees above average (and a preliminary low of 66, 25 above average).

  7. Barry Saxifrage says:

    My view is that the Koch brothers almost certainly “know” what the climate is doing. They just don’t care.

    Nero, Marie Antoinette, Koch.

  8. Barry Saxifrage says:

    That explanation of distribution shifting is just plain excellent. Thanks!

    In particular I appreciate having the “increased likelihood factor” axis and curve for folks like me who understand the concepts but can’t do that math easily.

    It fits perfectly with Hansen’s paper on extreme heat (defined by standard deviations) going from much less than 1% to over 10% of earth’s surface now. He uses “loaded dice” as a metaphor.

    I’ve been trying to come up with a visual analogy of this radical shift in extremes that will work for everyday readers. This is super powerful info but hard to get to “ah hah” for most folks. If we can pull that off it could be the next “hockey stick” for general public.

    I’m experimenting with “climate roulette” wheel changing the amount of “red”…as well as a “wheel of misfortune” with the huge shift of extreme heat “dark red” going from less than 1% to 10%. This article provides more good ideas.

    The central message public needs to take in is that extreme heat events have gotten many, many times more likely to occur now. A small increase in overall warming creates a huge increase extreme events. The more extreme the event the more likely it was caused by the warming — 90 times more likely if it is a 5sd event!

  9. sidd says:

    Rainfall is interesting. The global distribution is much narrower than Gaussian. The mode of the distribution is moving lower, but the tails are increasing, more at the upper end. here is a look
    i have lumped the 5+ sigma deviations. the graf is derived from a monthly 2.5 degree lat/long grid of precip, plotted as the histogram of difference from local average divided by local std deviation


  10. Paul Magnus says:

    Selfishness and greed…

  11. Sasparilla says:

    An excellent article Joe, the explanation was very good and easily understood.

    After reflecting some (and spending some more time out in the summer warmth in IL today) I’ve come to a couple of realizations regarding this massive and sustained spring warming.

    This radical warming is a warning to us – which we won’t heed of course.

    The 2nd realization is that we are soooo lucky this did not happen in the middle of summer (add +30F or +20F temps to normal temps) – the crop destruction and heat deaths resulting would be horrifying, truly.

    Presumably the dice for a summer +30 (or +20) excursion from the norms are loaded and will get rolled at some point, its only a matter of time till they get loaded enough and hit. God help us when it happens.

  12. Spike says:

    If you have a normal distribution of people of various heights in a low room only one guy might bang his head on the roof when they jump. Raise the floor by a couple of inches and ten might hit the roof. We need to get this message across so the public can understand how small changes in global averages can greatly increase extremes. This is clear from the paleo record where a mile of ice was over where I now sit when the world was just 5-6C cooler.

  13. Peter says:

    Climate panel predicts weather disasters ahead

    WASHINGTON – Global warming is leading to such severe storms, droughts, and heat waves that nations should prepare for an unprecedented onslaught of deadly and costly weather disasters, an international panel of climate scientists says in a report issued Wednesday.

    The greatest danger from extreme weather is in highly populated, poor regions of the world, the report warns, but no corner of the globe – from Mumbai to Miami – is immune. The document by a Nobel Prize-winning panel of climate scientists forecasts stronger tropical cyclones and more frequent heat waves, deluges, and droughts.

    The 594-page report blames the scale of recent and future disasters on a combination of human-directed climate change, population shifts, and poverty.

    In the past, the Intergovernmental Panel on Climate Change, founded in 1988 by the United Nations, has focused on the slow, inexorable rise of temperatures and oceans as part of global warming. This report is the first to look at the less common but far more noticeable extreme weather changes, which recently have been costing on average $80 billion yearly in damage.

    “We mostly experience weather and climate through the extreme,’’ said Stanford University climate scientist Chris Field, who is one of the report’s top editors. “That’s where we have the losses. That’s where we have the insurance payments. That’s where things have the potential to fall apart.

    “There are lots of places that are already marginal for one reason or another,’’ Field said. But it’s not just poor areas: “There is disaster risk almost everywhere.’’

  14. a face in the clouds says:

    They know. Big Oil is already trying to build a defense case against environmental class actions and the World Court. BP’s “Beyond Petroleum” and the rare admissions by Exxon and Shell in recent years are by design.

    Keep picking at that scab, Dr. Gleick.

  15. Rabid Doomsayer says:

    I do not think summer temperatures twenty degrees above normal can happen just yet. This is more of a season shift, summer temperatures in spring. Perhaps someone more knowledgeable can help.

    You have some really scary thoughts. Here is another, one of the commentors at Tamino’s blog (Open Mind) did an exercise fitting an exponetial curve to winter PIOMAS data and got zero winter ice volume in 2030.

  16. Raul M. says:

    with higher moisture in the atmosphere, the atmosphere can also hold latent energy as well as kinetic energy?

  17. colinc says:

    I do not think summer temperatures twenty degrees above normal can happen just yet.

    Perhaps Europe 2003 is now just a distant glimmer of a mirage?

    Yes, I find the PIOMAS-data projections more than a little frightening. I think the “easiest” and perhaps most informative site to follow the “action” is Neven’s Arctic Sea Ice blog at …

  18. Joan Savage says:

    I think we need to know more about human thermo-regulation and particularly response to heat stress. There was a previous CP post on heat stress in football.
    Dr. Bindon of the Unniversity of Alabama has a nifty outline of facts on human thermoregulation, including diagrams on how the body regulates differently at different temperatures. e.g. At ambient 95F about 80% of cooling is from evaporation. Bindon also includes more about the number of days it takes to adapt to high temperature.

  19. Jody says:

    If this summer follows this winter, +30F or +20F temps to normal temps, watch for celebrity cooks using the side walk and their limo roofs as a cooking surface.

  20. tpinlb says:

    From Chapter 4 of the IPCC report on extreme events:

    “There is medium evidence and high agreement that long-term trends in normalized losses have not been attributed to natural or anthropogenic climate change… The statement about the absence of trends in impacts attributable to natural or anthropogenic climate change holds for tropical and extratropical storms and tornados… The absence of an attributable climate change signal in losses also holds for flood losses.”

    This would seem to contradict some of the statements posted here on Climate Progress. Is the IPCC wrong about this?

  21. Raul M. says:

    that floppy hat is about ready for a few more thin coats of cool white paint.
    Haven’t been paying much attention to the thermometer
    But, it sure seems that a hat that stays cool in the hot sun is good and my eyes don’t tan well so some new contact lenses that block the UV rays, yes.

  22. Raul M. says:

    South Florida and much of the Gulf seem to have made it to the extreme range of the UV index.
    I might be wrong but with UV rays if there is occlusion in the lense then UV rays would interact at the lens of the eye rather than making it to the back of the eyeball.
    So once cataracts start the UV rays would attract there to cause even more damage.
    Seems best to just wear the UV absorbing contact lenses and skip that difficulty.

  23. adelady says:

    Joan, speaking as one who’s lived through a record breaking heatwave, it’s not pretty. People die.

    5 days above 35C or 3 days over 40C are enough to qualify as a heatwave here. For those worried about large temperature increases, that really isn’t the main problem. It’s the relentless day after day – with little to no overnight relief – assault on your metabolism that is the real killer. You may or may not have a hottest ever record-breaking single day during such a sequence, but you don’t need it for there to be a devastating result.

    We now have hot weather coping strategies in place to try to prevent the excess deaths of that fortnight. But it’s not fun.