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Is extreme weather caused by global warming?

What the science says...

Select a level... Basic Intermediate

Global warming amplifies the risk factors for extreme weather events - and that is all that Climate Science claims.

Climate Myth...

Extreme weather isn't caused by global warming

"The 30 major droughts of the 20th century were likely natural in all respects; and, hence, they are "indicative of what could also happen in the future," as Narisma et al. state in their concluding paragraph. And happen they will. Consequently, the next time a serious drought takes hold of some part of the world and the likes of Al Gore blame it on the "carbon footprints" of you and your family, ask them why just the opposite of what their hypothesis suggests actually occurred over the course of the 20th century, i.e., why, when the earth warmed - and at a rate and to a degree that they claim was unprecedented overthousands of years - the rate-of-occurrence of severe regional droughts actually declined." (source: CO2 Science)

Whenever there is an extreme weather event, such as a flood or drought, people ask whether that event was caused by global warming. Unfortunately, there is no straightforward answer to this question. Weather is highly variable and extreme weather events have always happened. Detecting trends takes time, particularly when observational records are rare or even missing in certain regions. An increase in extreme weather is expected with global warming because rising temperatures affect weather parameters in several ways. Changes in the frequency of extreme events coinciding with global warming have already been observed, and there is increasing evidence that some of these changes are caused by the impacts of human activities on the climate.

How global warming affects weather parameters

Rising temperatures can have several effects on the factors involved in weather. For example:

  • They increase the rate of evapotranspiration, which is the total evaporation of water from soil, plants and water bodies. This can have a direct effect on the fequency and intensity of droughts.
  • A warmer atmosphere can hold more water vapour. The atmosphere now holds 4% more water vapour than it did 40 years ago as a result of increasing temperatures. This increases the risk of extreme rainfall events.
  • Changes in sea-surface temperatures (SSTs) also have an effect by bringing about associated changes in atmospheric circulation and precipitation. This has been implicated in some droughts, particularly in the tropics.

These changes don't automatically generate extreme weather events but they change the odds that such events will take place. It is equivalent to the loading of dice, leading to one side being heavier, so that a certain outcome becomes more likely. In the context of global warming, this means that rising temperatures increase the odds of extreme events occurring.

Changes in extreme weather events are already being observed

In the US, the Global Changes Research Program published a report in 2009 entitled Global Climate Change Impacts in the US. The National Climate Change chapter reports the following findings for recent decades:

  • Heavy rainfall events have increased both in frequency and in intensity by 20%, and are the main cause behind the increase in overall precipitation in the US. The Northeast and Midwest have seen the greatest increase in such events.
  • The frequency of drought has increased in areas such as the Southeast and the West, and decreased in other areas. Rising temperatures make droughts more severe and/or widespread, and also lead to the earlier melting of snowpacks, which can exacerbate problems in vulnerable areas.
  • Atlantic hurricanes have increased both in power and frequency, coinciding with warming oceans that provide energy to these storms. In the Eastern Pacific, there have been fewer but stronger hurricanes recently. More research is needed to better understand the extent to which other factors, such as atmospheric stability and circulation, affect hurricane development.

Similarly, Australia has seen the odds of both heavy rainfalls and droughts increase, and similar patterns are being observed worldwide, coinciding with rising temperatures over the past 50 years.  Heat waves are also occurring more frequently as temperatures shift upwards:

Source: NASA/Goddard Space Flight Center GISS and Scientific Visualization Studio

In conclusion, although it isn't possible to state that global warming is causing a particular extreme event, it is wrong to say that global warming has no effect on the weather. Rising air and sea temperatures have a number of effects on the water cycle, and this increases the odds for more extreme weather events.

Basic rebuttal written by dana1981

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 7 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 51 to 96 out of 96:

  1. John Hartz, essentially, yes I do believe that the spread of the Sahara has been largely the result of mismanagement of land. Increasing rates of desertification across the globe are the result of the same mismanagement. When I say ecological perspective, I mean a perspective that takes into account the ability of bio-diverse ecosystems (or lack thereof) to influence local climate and desertification in general (water cycles, decomposition cycles, solar cycles, mineral cycles, carbon cycles). Ecological systems have evolved for millions of years in order to withstand and thrive in conditions with extreme variations in local rainfall over time. As humans learn to work with and exploit these natural systems, the "negative" aspects of more sporadic rain will be reduced and the positive aspects enhanced.
  2. DSL, that is fascinating! This completely makes sense, as wetter regions can handle a high level of mismanagement (such as overgrazing) in comparison to drier regions with more sporadic temporal fluctuations in rainfall. As the planet warms regional rainfall patterns would change (horse latitudes begin drying), this makes the local environment have higher fluctuations in rainfall (and drier) and more susceptible to mismanagement (such as overgrazing) and subsequent desertification. For this reason anthropogenic desertification will always be faster in environments with higher seasonal fluctuation in rainfall over places with a less seasonal fluctuation in rainfall.
  3. whoops...please excuse the grammar. i forgot the old adage, "post once, edit twice"..
  4. Bacteria within arid environments (which are usually grasslands) primarily survive the dry season's "bacterial holocaust" by living in the rumen of a grazer.
    No, rumen-adapted bacteria primarily survive by living in the rumen of a grazer. Bacteria that live in arid environments have other adaptations to survive, including sporulation and simple dispersal by wind, water, and/or on fomites. Internal bacterial communities are remarkably resistant things, and don't much allow externally-adapted species to board the ark whenever the rains don't come. Apart from this, there is a gaping logical fallacy in your overall argument about climate change and land use. The form is:
    If A, then B. B, therefore A.
    I wonder if you realise what it is?
  5. @AHuntington1 #51: You have conveniently ignored my request to provide documentation for your assertions. Personal opinion and sweeping generalizations have virtually no value in a discussion of scientific evidence and findings.
  6. John Hartz, well here is one piece of documentation implicating overgrazing (a complete land management issue) as a causative factor in desertification. There are many more- just google overgrazing and desertification if you wish to access them (or I will post more). Bernard J., Of course rumen-adapted bacteria primarily survive by living in the rumen of a grazer- I didn't mean to imply that they survive outside the animal for long.- lol I am talking about the bacterial carcasses, water, fiber, nitrogen etc. that the fecal matter and urine of massive herds of ruminants provide to decomposers on the drier soil surface- extending their lifespan a little further through the hot bacterial winter. Ruminants fertilize, chop grass, and till the earth's surface simultaneously. Lay out my non sequitur for me please, I am having trouble seeing it.
  7. AHuntingdon1 A: "Overgrazing can cause desertification" (not detested) BTW, this does not jibe with your comment @51, last paragraph B: "Desertification is observed, therefore overgrazing must have been happening" (incorrect conclusion, aka logical fallacy) Correct: Desertification can be caused by other factors aside from overgrazing, or any grazing. For instance, a change in climate, particularly increasing T alongside reduced rainfall, can cause desertification. When posting at SkS, try making cohesize statements as you would in a written text for students, not blurbs that can be misinterpreted. You maybe perceived as a troll.
  8. AHuntington, thank you for the clarification from "desertification" to "anthropogenic desertification." For a minute there, I thought you were trying to blame the general process of desertification on humans. Humans can obviously cause desertification, but circulation-based desertification is the primary mechanism and has been for the duration.
  9. @AHuntington1#56: Thanks for the providing the link to the peer-reviewed paper, "Nature and causes of land degradation and desertification in Libya: Need for sustainable land management." In the paper's Abstract, the authors state: "Among others, overexploitation of natural resources, inappropriate land use planning, insufficient water resources etc. are the main factors escalating the process of desertification and deteriorating environmental quality." Some of the factors cited are caused by human activity and others are not even identified. My position is that the natural environment existing at any location on Earth has been and will continue to be affected by changes in the planet's global climate system. From my perspective, your sweeping assertion that desertification is primarily caused by human activity at the local and regional scale may hold true in the short-run, but will not hold true in the long-run. In the long-run, desertification has been and will continue to be driven by changes in the Earth's global climate system caused primarily by the burning of fossil fuels and deforestation. Note: When I refer to the "global climate system" I mean the standard definition used by climate scientists. The entire system includes the atmosphere, the aquasphere, the cyrosphere, the biosphere, and the lithosphere.
  10. @AHuntington1 #56: As they say, "What's good for the goose is good for the gander" Please Google "Climate change and desertification" and read some of materials listed. You just might learn something new.
  11. John Hartz, I am not disputing the fact that climate change is a driving factor in desertification. The ecosystem's ability to cope with a change in local climate is a bigger causal factor in desertification. When the horse latitudes heat up (whether from anthropogenic emissions sooner, or the sun slowly enveloping the earth later [ ]) rainfall will become more sparse and sporadic. There are certain ecological mechanisms that have evolved to cope with such sporadic moisture. If humans properly exploited these mechanisms- changed our behavior to fit the changing climate- desertification as we know it (climate change characterized by increasing erosion, destruction of biodiversity, and breakdown of ecosystems) could be avoided to a high extent. You mention burning fossil fuels, and deforestation as potential causal candidates for desertification. I don't disagree, although overgrazing, deforestation, and local ecosystem destruction are also major players. Co2 driven climate change would cause rainfall patterns to change. Whether human management of bio-diverse ecosystems (or lack thereof) can cope with these changes determines desertification. DSL, if you believe that human emissions of fossil fuels, and human errors such as overgrazing, deforestation, etc are causing desertification, you believe that all desertification is anthropogenic. Isn't this correct? What aspects of desertification are not anthropogenic? So the only issue on which we seem to disagree (so far as I can tell) is my contention that higher atmospheric Co2 is not as big a driving force of desertification as human promotion or demotion of bio-diversity. Ecosystem management is really the issue here. If humans stopped burning all fossil fuels, the rate of deforestation would probably skyrocket (as deforestation is already primarily a fuel issue in developing countries). Mismanaged cattle would still be roaming around. Biodiversity would likely decrease. Desertification will likely continue, regardless of human Co2 emissions, unless land management is addressed. Ecosystem destruction is at the root of desertification. gws, my argument is more like this: 1. Land management errors cause desertification (undisputed). 2. Therefore, without changing these specific land management issues (eg. overgrazing, deforestation, species extinction, burning the grasslands too much, etc.), desertification will continue to occur, regardless of human emissions of Co2. Human emissions might be a factor in causing the initial climate change, but human land management practices are the primary reason for subsequent desertification.
  12. AHuntington: "DSL, if you believe that human emissions of fossil fuels, and human errors such as overgrazing, deforestation, etc are causing desertification, you believe that all desertification is anthropogenic. Isn't this correct? What aspects of desertification are not anthropogenic?" Hadley-type circulation existed before significant human modification of the atmosphere. Hadley-type circulation can be modified by other major forcings (solar, volcanic aerosols, etc.). Desertification has occurred as large-scale circulation patterns have shifted in response to major forcings over the course of Earth's history.
  13. AHuntington, The main cause of deserts is the Hadley Cells (and in a few special cases altitude). This is readily apparent by noting that the latitudes at which deserts occur are the same (above and below the equator) and is readily explained by the mechanics of the Hadley Cells. You may argue all you want to otherwise, and certainly some of those arguments will be valid in some cases, but this does not change the fact that one major, unavoidable and already observed effect of global warming will be the expansion of the Hadley Cells, which will in turn necessarily and proportionally expand the existing deserts poleward. Texas, the American Southwest, Mediterranean Europe and others are in for a very rough ride.
  14. DSL, Hadley-type circulation did exist prior to anthropogenic CO2 emissions increasing, this results in deserts with sporadic fluctuations in rainfall over time. As to why Hadley cells are widening, I was under the impression that you thought anthropogenic CO2 emissions were responsible. Thus the spreading of deserts or desertification would be anthropogenic. you said, " Desertification has occurred as large-scale circulation patterns have shifted in response to major forcings over the course of Earth's history. " I agree completely, and add that since humans have existed on earth, we have become a factor that can either rapidly intensify natural desertification and ecosystem breakdown, or promote biodiversity, and ecosystem growth. Sphaerica, you said, "The main cause of deserts is the Hadley Cells (and in a few special cases altitude). This is readily apparent by noting that the latitudes at which deserts occur are the same (above and below the equator) and is readily explained by the mechanics of the Hadley Cells." 1. I never said that the Hadley cells weren't an important factor. They are not the only factor however, this is apparent when looking at this graph: Notice the northern arid regions that are experiencing desertification. 2. I feel like you are missing my argument, but because I have tried to explain myself (and you seemingly ignore my points) and you haven't posed any specific questions re the information I have presented, I will not elaborate further (unless you have specific questions or criticisms of the points I am making, of course). you said, "some of those arguments will be valid in some cases, but this does not change the fact that one major, unavoidable and already observed effect of global warming will be the expansion of the Hadley Cells, which will in turn necessarily and proportionally expand the existing deserts poleward." Well good, we are in complete agreement on this point.
  15. Pardon my confusion, AHuntington. I was responding to this claim of yours @ 47: "From an ecological perspective, desertification is almost strictly a land management problem." and then especially this: "DSL, if you believe that human emissions of fossil fuels, and human errors such as overgrazing, deforestation, etc are causing desertification, you believe that all desertification is anthropogenic. Isn't this correct? What aspects of desertification are not anthropogenic?" and now you give me this: "I agree completely, and add that since humans have existed on earth, we have become a factor that can either rapidly intensify natural desertification and ecosystem breakdown, or promote biodiversity, and ecosystem growth." which I agree with. The second quote suggests that you were not aware of any natural mechanisms for desertification. I'm suggesting that you be a little more precise in your presentation. Your first quote could be excused because you did say "anthropogenic desertification" in the preceding paragraph. The second quote is an inexcusable non sequitur: it does not follow that if I believe the enhanced greenhouse effect can cause desertification that I then must believe that all desertification is human caused.
  16. DSL, the second quote doesn't suggest that I am unaware of such mechanisms (which I already acknowledged, and explained how Hadley cell widening fits into my understanding of desertification). My point is that desertification as we know it is primarily dependent on the biodiversity of a given region, and the extent that humans choose to degrade or enhance the local ecology. Widening Hadley cells are responsible for some changing rainfall patterns, but any environment on earth that already experiences extreme temporal variations in moisture are at risk of desertification via human mismanagement of land (thus desertification spreads far beyond the Horse latitudes). Land management is at the root of desertification.
  17. No, AH. It's best to be precise. Small-scale desertification can occur through land mismanagement. Large-scale desertification can occur through climate-scale changes in atmospheric and oceanic circulation. Land mismanagement is at the root of some types of desertification, but not desertification in general.
  18. DSL, you said, " Land mismanagement is at the root of some types of desertification, but not desertification in general. " But it is (especially if you believe that human CO2 emissions cause Hadley cell widening)- changing rainfall patters or widening Hadley cells do not always cause desertification, ecosystems can and do adapt to changing rainfall patterns (a grassland, which can be much more tolerant of drought, can replace a rainforest and vice versa). The human element which directly reduces the efficiency of the water, mineral, solar, and decomposition cycles create desertification. So to be more precise, the vast majority of desertification is the result of mismanagement of land, as opposed to humans burning fossil fuels (burning trees and grass are land management issues).
  19. AH, are you suggesting that humans are at the root of most of the desertification in the 4.5 billion years of Earth's existence? Perhaps you're thinking of desertification that has occurred only within the last couple of hundred years. Humans are not responsible for those big dry belts at the horse latitudes. Those dwarf any desertification caused by humans. Why am I counting standing deserts as desertification? Because those big circulation cells aren't permanent. Their development and shifting created deserts. Isn't this all a little obvious?
  20. Response to Clyde: Why do you think we need to be able to say AGW caused a particular extreme weather event? That's like saying we shouldn't warn people of the dangers of smoking until science can prove that a particular lung cancer sufferer developed lung cancer because they smoked and not because of any other reason. As your own link states, "the emerging ability, arising from improvements in climate models, to calculate how anthropogenic global warming will change, or has changed" is being watched with interest by "lawyers, insurers and climate negotiators" because "nations, communities and individual citizens may begin to seek compensation for losses and damage arising from global warming", but:
    It is more difficult to make the case for ‘usefulness’. None of the industry and government experts at the workshop could think of any concrete example in which an attribution might inform business or political decision-making.
    Response: [DB] Added hyperlink to referenced link.
  21. I hope I have put this in the right place.


    At the risk of being accused of 'cherry-picking' single events, I ask everyone to take note of these dates;

    Typoon Tip -1979
    Marble Bar - 1923-24
    Furnace Creek Ranch (formerly Greenland Ranch) - 1913
    Oodnadatta, South Australia - 1960

    ...actually, instead of listing them, please go here;

    Now open the SkS trend calculator, HADCRUT4, 1850-2013, moving average - 1 month.

    I'm not interested in temps or trends as such, this is not a 'it's getting hotter or colder' question. I'd like you to take note of the 'extreme' weather events from the link above and see which period(s) the bulk of them occur in.

    Have weather events become more 'extreme'? Yes...............and it's not because of Global Warming.... ;) .

    Response: [JH] Please note that posting comments here at SkS is a privilege, not a right. This privilege can and will be rescinded if the posting individual continues to treat adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum. Moderating this site is a tiresome chore, particularly when commentators repeatedly submit offensive or off-topic posts. We really appreciate people's cooperation in abiding by the Comments Policy, which is largely responsible for the quality of this site. Finally, please understand that moderation policies are not open for discussion. If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is in the offing. Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.
  22. snafu...   You know that you're not making a lot of sense here.  How do you come to the conclusion that weather events are getting more extreme but not because of global warming?

    Response: [JH] Snafu is also skating on the thin ice of sloganeering.]
  23. snafu:

    At the risk of being accused of 'cherry-picking' single events [...]

    Got it in one.

    A handful of individual weather events is not the equivalent of a global aggregate of extreme weather events over long time frames.

  24. Snafu - lets get this clear. Science works like this - you build hypotheses into models from which you make predictions. You test these predictions against the real data. Now you think you doing this. "Look AGW says xyz, and he is abc which shows its wrong". However, what you are really doing is constructing a straw man agument because climate models predict no such thing and all the handwaving in the world doesnt change that. So instead of constructing your own wild ideas on what the science predicts you could instead see what is actually predicted and compare that with systematic analysis of global records.

    So far you are sloganeering. Before we waste further time, perhaps we should ask whether data would change your mind or are you only looking for something to sure up an opinion that wasnt based on data in the first place?

  25. Has anyone seen Bjorn Lomborg's latest:

    Don’t blame climate change for extreme weather

    He makes a lot of claims allegedly based in the IPCC's 600-page report, 

    Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation

    ...basically saying that the benefits of AGW will outweigh the costs until pretty far down the road.  I don't have time to verify his claims myself; has anyone seen a rebuttal to his argument?

    No, climate change will not be good for the world

    and this
    NY Times Says Earth Has Unlimited Carrying Capacity, So Forget Climate Change and Party On, Homo Sapiens!

    are relevant, but I'm specifically concerned with his claim that it is the IPCC itself which is saying these things.  I'm getting attacked by a guy in a debate who says, "You've been talking all along about how a skeptic must accept the scientific consensus... now you're disagreeing with it when it doesn't suit your ideology!"

  26. dvaytw, in my experience the fact that Lomborg wrote something is in and of itself a solid indicator that it is unlikely to be true.

    More specifically on the IPCC, see this article on how the leaked draft of the impact report actually shows dire consequences indeed, rather than the low impact being falsely claimed by Lomborg and other climate fiction writers.

    Unfortunately, I haven't seen a point by point rebuttal of that particular article. However, all his claims are all fairly generic denier talking points which have been debunked on this site and thus rebuttals can be found in the 'most used myths' or via the search box.

    My suggestion would be to ask your 'skeptic' to quote the IPCC report itself saying that there is a scientific consensus that AGW is not increasing extreme events.... rather than Lomborg falsely making that claim. There is no such conclusion in the IPCC reports. It's fiction.

  27. Please excuse a very basic question, prompted by the flooding here in the UK and a report on it at <>.  I understand (a) CO2 etc will raise the temperature and moisture content of warm air; (b) precipitation generally occurs when warm moist air meets colder air.  Is the strength of precipitation proportional to the difference in temperature between the two air masses (that causes water vapour to condense)?  If so, will not the colder air mass on average also be warmer, so the differential remains roughly constant with rising SST, and precipitation is not necessarily heavier?  (If the answer requires maths, please include.) Thanks.

  28. Cedders - the simple answer is that there isn't a linear relationship between the two - the moisture-holding capacity of the air increases faster for any given increase in atmospheric temperature. See the Clausius-Clapeyron relation.

  29. Of course!  There's a roughly exponential factor besides the difference in temperatures.  "water holding capacity of air increases by about 7% per 1°C warming" Trenberth KE (2011) Changes in precipitation with climate change. Clim Res 47:123-138

    Thanks for sorting that out for me, Rob.

  30. Hey SkS'ers.  I've been engaging in my guilty hobby of debating denialist trolls again.  Have been having a pretty lengthy one on Quora.   Basically the only point he continues to give me trouble on is on the question of definitions.  He claims that in "real science", terms are defined precisely.  He then goes on to say that this doesn't happen in climatology; he uses the example of "extreme weather".  If one goes to SREX, 3.1.2., one gets the following definition:

    This report defines an ‘extreme climate or weather event’ or ‘climate extreme’ as “the occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper (or lower) ends of the range of observed values of the variable” 

    It goes on to explain a lot of issues, advantages, disadvantages etc with the different methods of determining threshold values.  His argument is that with such a broad definition (he refuses to even call it a definition), they can basically call anything they want "extreme weather".  

    I have responded that the IPCC isn't dictating definitions to scientists; further that the broadness of the definition isn't a problem with the science but a feature of reality that scientists must deal with.  He claims that it should be easy to come up with a clear and precise definition (he suggests the standard deviation of the index).  He asks me how, for example, the IPCC can give a probability that ACC is affecting extreme precipitation when they can't even precisely define what extreme precipitation is.

    Can anyone offer help on this?   An example in another science where a definition is range migh be very helpful, or anything else y'all can offer.

  31. Dvaytw, it is interesting that your "friend" demands that climate change should have an "all or zero" effect on extreme weather events, and that extreme weather events themselves be "all or zero" extreme/non-extreme, with no fuzziness of inbetween status.

    Your "friend" is playing word-games to deceive you and/or himself.  Some things can be defined precisely, but some (just as valid in concept) can only be defined with fuzzy logic.

    When is sunset exactly - when the sun first touches the horizon / when it is halfway below the horizon / or when the final bright limb of the sun is occluded by the horizon?  And mean sea-level horizon? : viewed from what altitude?  Five-foot or fifty-foot eye-point?

    When does full night start? At sunset, or after the "civil" twilight finishes - or after the "astronomical" twilight?  It is all rather arbitrary and imprecise, to a greater or lesser degree.  Fuzzy.  But it is crazy to deny the real usefulness of concepts such as sunset and twilight.

    Does your "friend" think that men and women are separate concepts?  Can he tell them apart?  Always?  How precisely does he define the difference?  Appearance is sometimes confusing or deceptive.  Sometimes  in rare cases for a newborn, the expert paediatricians can be perplexed in making the male/female decision - even with the closest examination of the genitals.  Blood hormonal levels can be indecisive.  Even when the genome is clearly XX or XY, the outcome can sometimes be apparently "wrong".

    Or does your "friend" maintain there is no difference between man and woman?

  32. Well said, Eclectic.  And this person isn't my friend.  

  33. PS - I'm wondering, though: can anyone show me where to find the definition the IPCC uses for actual calculation of its probabilities regarding the contribution of ACC to extreme precipitation?  Or if not that, at least, how to find this definition on one particular study of extreme precipitation?

    I took Eclectic's point back to the argument, but he maintains that nevertheless a definition must be chosen if calculations are going to be made from it, and this makes sense to me with my child's understanding of the topic, at least.  

    And in case anyone is wondering, no, I am not a denialist troll trying to covertly advance these arguments myself.  Personally I think the argument is ridiculous; I am just trying to solve the puzzle of how to defeat it without simply dismissing it out of hand.

  34. dvaytw @83, you might find that individual studies use their own statistical cutoff points for what counts as "extreme weather".  As the IPCC itself does not do research, they must rely on those studies, and therefore cannot use a specific cut off point, but must rely on those that exist in the literature.  If you look at Box 3.1 in the SREX, the IPCC says:

    "A large amount of the available scientific literature on climate extremes is based on the use of so-called ‘extreme indices’, which can either be based on the probability of occurrence of given quantities or on threshold exceedances (Section 3.1.2). Typical indices that are seen in the scientific literature include the number, percentage, or fraction of days with maximum temperature (Tmax) or minimum temperature (Tmin), below the 1st, 5th, or 10th percentile, or above the 90th, 95th, or 99th percentile, generally defined for given time frames (days, month, season, annual) with respect to the 1961-1990 reference time period. Commonly, indices for 10th and 90th percentiles of Tmax/Tmin computed on daily time frames are referred to as ‘cold/warm days/nights’ (e.g., Figures 3-3 and 3-4; Tables 3-1 to 3-3, and Section 3.3.1; see also Glossary). Other definitions relate to, for example, the number of days above specific absolute temperature or precipitation thresholds, or more complex definitions related to the length or persistence of climate extremes. Some advantages of using predefined extreme indices are that they allow some comparability across modelling and observational studies and across regions (although with limitations noted below). Moreover, in the case of observations, derived indices may be easier to obtain than is the case with daily temperature and precipitation data, which are not always distributed by meteorological services. Peterson and Manton (2008) discuss collaborative international efforts to monitor extremes by employing extreme indices. Typically, although not exclusively, extreme indices used in the scientific literature reflect ‘moderate extremes,’ for example, events occurring as often as 5 or 10% of the time. More extreme ‘extremes’ are often investigated using Extreme Value Theory (EVT) due to sampling issues (see below)."

    This does not mean that "extreme values" are not precisely defined in original research.  Nor does it mean the IPCC treats the issue imprecisely.  However, to not overstate the case, they cannot use a precise common definition where one does not exist in the literature.  What they do do is take the relative rigour of the statistical cut offs used in individual studies in weighing the evidence from the literature.

    As noted on the bottom of the quoted section above, there are often very good reasons for the different cutoffs in different studies.  Specifically, if the phenomenon in question has a limited data set, using a 2.5% cut off may result in insufficient data for normal statistical methods.  EVT can be used in these cases sometimes, but is not as robust.  Droping the cut off may allow the scientists to note, and discuss changes in frequencies that actually exist even when the change is not yet robustly detectable in "extreme extremes".

    On top of that, some events are extreme events, and known to be extreme events even when no precise statistical criteria is defined.  An example is any hurricane/cyclone/typhoon.  Any such is an extreme event with regard to wind speed, and probably precipitation and storm surge as well.  To expect the IPCC's discussion of extreme events to exclude all cyclones except the 5% of strongest cyclones would be absurd, but the statistical cut off based just on wind speed for cyclones will not be precisely known (even though clearly it will meet any reasonable such cutoff).  The IPCC definition, by not specifying precise values allows discussions of the frequencies and strengths of all cyclones (and tornadoes, and floods) without arbitrary cutoffs being used to exclude relevant data.

  35. Thanks, Tom.  I appreciate the immense patience you folks have with my endless queeries.  Your answer is very informative, but can you help me with a specific point?  The guy has this inane idea that extremes are defined nowhere in the literature.  He asks, 

    "So, what is the threshold variable for extreme rain events in Sydney? How would that be calculated?"

    Can you answer this, or at least point me to a definition of "extreme rain" in an individual study?  It sounds ridiculous, but he seems to think this is all just smoke and mirrors.  I want to point him to something very concrete and graspable.

  36. dvaytw @85:

    In my opinion, the World Meterological Organization (WMO) sets the "gold standard" with respect to meterological terms. You can access the definition of "extreme weather" and other terms by going to the WMO's Metroterm* webpage. 

    *METEOTERM is WMO terminology database. It contains specialized terminology in six languages: Arabic, Chinese, English, French, Russian and Spanish. It includes the International Meteorological Vocabulary, the International Glossary of Hydrology and terms from related sciences that appear in WMO documents.

  37. John Hartz @86, I find the IPCC glossaries sufficiently exact and easy to access.  In the case of "extreme weather event", so also did meteoterm:

    "extreme weather event
    IPCC 4th Assessment Report, WG 1 Glossary
    An extreme weather event is an event that is rare at a particular place and time of year. Definitions of rare vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of the observed probability density function. By definition, the characteristics of what is called extreme weather may vary from place to place in an absolute sense. Single extreme events cannot be simply and directly attributed to anthropogenic climate change, as there is always a finite chance the event in question might have occurred naturally. When a pattern of extreme weather persists for some time, such as a season, it may be classed as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., drought or heavy rainfall over a season).
    IPCC 4th Assessment Report, WG 1 Glossary"

  38. davytw @85, for precipitation, extreme events are typically defined in terms of the return interval for a certain amount of precipitation over a given time period.  Consider the following examples of return intervals from four meteorological stations in South West Western Australia:

    The charts show the one day rainfalls for various return intervals for data from 1930-1965 (black) and 1966-2001 (red).  A return interval of 0.1 means, approximately, that you would average 10 such events a year.  A return interval of 50 means that, on average you would experience only 1 such event every 50 years.  You may not think of an event with a return interval of 0.1 years as an extreme event, but it means that the chance of experiencing such a rainfall event on any given day is 2.74%, ie, well within a 5% threshold, and almost within a 2.5% threshold.  Of course, such events are not likely to cause anything more than local flooding if that - but they have the great virtue of being sufficiently frequent as to lend themselves to statistical analysis of changes in the return interval.

    You can see that above.  For return interval below 0.5-1 (depending on the station), there are statistically significant changes in the return interval for a given quantity of rainfall.  Above that, even though the rainfall events become more extreme, because they become rarer the change ceases to be statistically significant, at least for individual values.

    These charts show a decline in return interval, ie, a lower frequency of extreme rainfall events and hence likely a lower frequency of local or more extreme flooding.  As this is for South West Western Australia, climate models predict such a reduction as a result of global warming (with a consequent increased frequency of drought); although the paper from which this analysis comes attributes it to other causes.

    A similar calculation could be done for Sydney, and would be done by the same methods.  Problems do arise.  The return interval for very high rainfall events (>100 years) are predicted from best fit curves such as shown on those graphs.  They are, however, unreliable.  

    The Brisbane region, for example, has experienced at least 4 extreme rainfall events with a purported return interval >500 years in the last 150 years, most recently in the 2011 flood.  Two of those events were in the 1890s, and it is possible that two such events occurred within weeks of each other in the 1890s.  Clearly the best fit curve is not a reliable predictor at the top of the range.  The 2011 flood was unique among these events on a number of grounds, including that it occurred at the time of the greatest flooding of Queensland by arial extent on record, it occurred in the run down from an El Nino (with the second greatest flooding of Queensland by extent occurring earlier the previous year in El Nino conditions).  All, or nearly all other major floods in Brisbane have been associated with La Ninas.  Further, it occurred without the involvement of any cyclone, unlike the others among the 4 or 5 rainfall events with >500 year notional return intervals, which all occurred when a cyclone, or the remnants of a cyclone tracked along the South East Queensland coast (either just off shore as in 1974) or in the Brisbane valley as in other occasions.  Naturally, these unique features suggest global warming was a significant factor in the size of the flood; but because of the low return intervals (notional or actual), that cannot be shown statistically.  That is why studies of extremes in precipitation concentrate on the much lower impact, higher return rate events.

  39. dvaytw @85:

    You may also want to recommend that your debating partner check out the recently released report, Attribution of Extreme Weather Events in the Context of Climate Change, National Acadamies Press. 

    The report was authored by the Committee on Extreme Weather Events and Climate Change Attribution; Board on Atmospheric Sciences and Climate; Division on Earth and Life Studies; National Academies of Sciences, Engineering, and Medicine.


    As climate has warmed over recent years, a new pattern of more frequent and more intense weather events has unfolded across the globe. Climate models simulate such changes in extreme events, and some of the reasons for the changes are well understood. Warming increases the likelihood of extremely hot days and nights, favors increased atmospheric moisture that may result in more frequent heavy rainfall and snowfall, and leads to evaporation that can exacerbate droughts.

    Even with evidence of these broad trends, scientists cautioned in the past that individual weather events couldn't be attributed to climate change. Now, with advances in understanding the climate science behind extreme events and the science of extreme event attribution, such blanket statements may not be accurate. The relatively young science of extreme event attribution seeks to tease out the influence of human-cause climate change from other factors, such as natural sources of variability like El Niño, as contributors to individual extreme events.

    Event attribution can answer questions about how much climate change influenced the probability or intensity of a specific type of weather event. As event attribution capabilities improve, they could help inform choices about assessing and managing risk, and in guiding climate adaptation strategies. This report examines the current state of science of extreme weather attribution, and identifies ways to move the science forward to improve attribution capabilities.

  40. BOOM!!! Direct hit!!!  THAT'S what I'm talking about, guys!!!  Thanks!!!

  41. Hallo, this is my first post. I am Giancarlo Pace, ex- astronomer. I am spending some time checking some deniers' arguments. Most of them are easily debunked and do not deserve scientifical attention. 

    However, I did find something that still sounds reasonable to my non-expert ears. Bjorn Lomborg (who is not exactly a deniers but seems to be not too worried about climate change) shows in a video of a terrible youtube channel, a plot that seems to indicate that droughts are decreasing. Misteriously, he does not indicate the name of the authors of the paper, he says that it is a Nature paper of 2014. However, the paper exists:  Hao et al. 2014

    The paper does not state any decline in droughts, but actually their Figure 5 shows what definitely seems to be a decline in number of droughts since the 80s.

    If you want to watch the video, here it is:


    I read here that collectively, the number of extreme weather events is declining, but I only get an insurance company as source. Do you have some scientific research on it?


    Thanks a lot

  42. I searched for some info on USS storm trends and how they relate to temperature change. But I couldn't find anything pertinent, so I decided to put together something myself that compares storm frequency to temperatures.

    The NOAA Storm Database has records of "Thunderstorm Wind" events going back to the 1950s, so I downloaded the years 1975-2015 (380,026 events in total).

    For temperature, I chose the NASA GISTEMP annual means for the Northern Hemisphere.

    Here's the graph of storm counts and temperatures for the past 35 years, showing a strong correlation between the two (97.35%):

    Annual US Storm Frequency and Temperature


    [RH] Adjusted image size. Please limit your images to 500px in width.


    Some non-expert comments to Giancarlo and Terry, in lieu of a more informed response.

    Giancarlo - yes, it'd surprise me a litle if increased drought hasn't been unambiguously detected, although it has been unambiguously projected.  Would be interested to take that figure 5 in Hao up to present and see if the trend is positive yet.  I find examples of projections in the USA in the National Climate Assessment, and in Cook, B. I., Ault, T. R. & Smerdon, J. E. Sci. Adv. 1, e1400082 (2015) covered here and hereCook also detects extreme drought in the Levant.  It does seem there has been differences of emphasis in the literature recently.  Sheffield et al 'Little change in global drought over the past 60 years' seems like a downward estimate compared to the usual measure (PDSI) of past drought, and is put in context by John Holdren in an article reproduced on this site.  He also refers to a summary in research by Schwalm et al that 'severity and incidence of climatic extremes, including drought, have increased'.  Spinoni et al (2013) finds a small increase in each of global drought frequency, duration and severity from 1951-2010, but big regional variations that are disruptive.

    Responding to Terry's graph, this is nice work, but I'm sure someone must looking at this professionally, who you may be able to contact.  The increase in thunderstorms you show is so dramatic it must surely be an artefact, maybe of increased reporting?  Secondly, a correlation between two time series each of which are increasing may well come to >90% (even more so if smoothed), but be misleading since both are dependent on time. 

    It strikes me that new measures may be useful for examining trends in 'extreme' weather rather than total weather, for example, maximum hourly rainfall, variance of rainfall, or collating data on water tables and aquifers.  Not sure how realistic or expensive that would be.


  44. How does Green-house-gas driven warming effect the diurnal temperature cycle and what impact would that be expected to have on weather? 

    My understanding is that one would expect night-time lows to rise more than day-time highs.  I believe this has been observed and stated as evidence for AGW.  Thus, the difference between daily highs and lows should actually be reduced by AGW, which in turn presumably would tend to moderate extreme weather events (winds and precipitation). 

    Of course, I'm not a weather expert, so I may be missing some important effect.  For example, the changes in surface temperatures may  be different from those at higher altitudes, driving cloud formation.

  45. SemiChemE,

    It is more complicated that just the change in diurnal temperature.  The US Climate Change Assessment (from 2014) is the best source of scientific information about changes in the USA.  If you are not from the USA,  the information there will generally be similar to other parts of the world.  Effects vary a lot from region to region.

    The most consistent predictions are that heat waves, drought and flooding will increase.  During the winter that means heavy snows (like they currently are having the the USA).  Significant increases in heavy precipitation have been measured across the entire USA but are most pronounced in the North East where there has been a 70% increase in the heavist precipitation events.  (Heavy precipitation = more floods).  Hurricanes like Sandy are more common already.  Indeed, Sandy was the latest hurricane in recorded history in the North East by several weeks.  Record heat waves are already common.

    It is unclear if the total number of Hurricanes will increase or decrease, but the strongest hurricanes are expected to be more common.  Already hurricanes average about 15mph stronger than 50 years ago.  Since the power of the wind goes up with the cube of the speed, 15 mph is a big effect.  Some models suggest that the melting Greenland Ice sheet will cool off the North Atlantic so much that super storms start to occur.  Hopefully those models will turn out to be incorrect.

    We may get lucky and less diural difference means less extreme weather.  Or we may not be lucky and more energy in the system means  more and stronger storms.  In either case we expect more floods and more drought.  Meters of sea level rise are also a given.  The question is how fast the sea level will rise.  Don't buy land in Miami.

    The report I cited is much less biased than I am.  See what they say.

    Watts Up With That is a site well known for misleading and outright false reports.  If you want to understand the science be careful about who you trust on the internet.  Scientists recommend Skeptical Science.

  46. Weather extremes are often not a direct thermodynamically-driven consequence of increasing temperatures and humidity, but often indirectly driven by global warming via associated changes in the dynamics of weather systems. A prominent example is the projected poleward shift of the storm tracks under global warming scenarios (Ref. 1). The storm tracks are the accumulated footprint of the pathways of numerous extratropical cyclones, which then can cause, among others, extreme precipitation events. If the storm tracks shift poleward, storm frequencies change from one location to another. Storm intensities must be treated distinct from storm frequencies and their clustering.

    One related example are weather fronts, along which most precipitation in an extratropical cyclone is formed. Forecasters may use slightly different definitions of what exactly a weather front is, but it is clear that extreme precipitation events (or hail) are often triggered ahead of cold fronts, due to the related forced vertical motion, for example during a hot summer day with high convective potential. It is known that the intensity of weather fronts scales with the precipitation during the following hours.

    Consequently, changes in extreme events are often linked to changes in weather systems and related changes in their local frequencies and their intensities. This acts as an additional dynamical driver of changes in extreme weather, in tandem with the more direct thermodynamic change, i.e., the increase in the atmosphere's water holding capacity, due to global warming. Trends in the frequency of weather fronts and their intensity changes are presented for example in Ref. 2.

    Selected references:

    (1) Chang, E. K. M., Y. Guo, and X. Xia (2012), CMIP5 multimodel ensemble projection of storm track change under global warming, J. Geophys. Res., 117, D23118, doi:10.1029/2012JD018578.

    (2) Schemm, S., M. Sprenger, O. Martius, H. Wernli, and M. Zimmer (2017), Increase in the number of extremely strong fronts over Europe? A study based on ERA‐Interim reanalysis (1979–2014), Geophys. Res. Lett., 44, 553–561, doi:10.1002/2016GL071451.

    [Nature Climate Change, V7, page 96 (2017):]

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