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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.

Last updated on 16 January 2013 by dana1981. View Archives

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Comments 51 to 79 out of 79:

  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. http://www.academicjournals.org/ajb/PDF/pdf2011/17Oct/Saad%20et%20al.pdf

    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 [ http://physicsworld.com/cws/article/news/2008/feb/26/earth-is-doomed-in-5-billion-years ]) 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: http://www.ciesin.columbia.edu/docs/002-193/fig5.gif
    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;
    http://en.wikipedia.org/wiki/List_of_weather_records

    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?

    This
    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 <http://www.metoffice.gov.uk/research/news/2014/uk-storms-and-floods>.  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.

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