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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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

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Comments 26 to 50 out of 96:

  1. Tom, I would expect more tornadoes after adjusting for observation improvements. Those improvements cause most of the increase of your lower chart. The long term increase will be due to more convection and more CAPE (shown to be in a upward trend). I would not expect more strong tornadoes because of other needed ingredients: a strong jet stream for adequate storm motion and mid-level dry air for the strong downdraft (the updrafts are readily available of course). See for a simple explanation. The tornado season peaks well before the summer peak in heat because those other ingredients diminish in summer. The other ingredients with increase with global warming. Thus we would expect strong tornadoes earlier in the season when the jet stream is still in a more southerly position, primarily in the areas where dry air is available east of the Rockies. We would also expect strong tornadoes in northern locations where the jet stream stays stronger later in the season and dry air is still available from Canada. Weak tornadoes should increase under global warming especially earlier in the season.
  2. 25 Tom Curtis In your last tornado graph, I have read that the increase in total number is a result of the advanced tools meteorologists have for locating tornado activity and several storm chasers to find them and confirm touchdowns.

    [DB] "I have read"

    Norman, please be advised that when making a statement such as this intended to refute someone else's comment (which itself was based on sourced, linked data) you must be prepared to then back up your statement with a verifiable link to a reliable source.

  3. Thank you DB Here is one experts opinion on the matter of tornadoes and the apparent increase. More tornadoes or better detection?. Smaller tornadoes on increase, larger tornadoes may be decreasing in frequency.
  4. According to the NOAA data posted at #52, strong (EF3+) tornadoes have generally decreased over the past few decades. Whether smaller tornadoes have incrased of simply been detected better is still up for debate. Doppler radar has identified many tornadoes which would probably not have been spotted in years past.
  5. #25 Tom Curtis Here is an explanation of the increase in tornadoes in April. Check out what is needed to form a supercell thunderstorm. A strong jet stream is required. Here is the April anomaly for the Globe. You can see warmer than normal air in the Gulf of Mexico while the air in Canada was well below normal. Conditions are right for supercell thunderstorms. As Eric the Red pointed out in post #26, tornado season diminishes as the Canadian air warms rapidly in the summer months. There is not the strong differential and the jet stream weakens.
  6. "A strong jet stream is required." Upper or low-level jet or both? And not necessarily. You are clearly talking through your hat Norman. You might want to wander over to Desmogblog where I have been in a technical discussion with someone who claims to be in the know about these things. Regardless, the experts agree that trend now and in the future in tornadoes are hard to pin down, mostly on account of the poor nature of the data and changing building codes, and monitoring platforms etc. What we do have confidence in is that extreme precipitation events and severe thunderstorm events are likely to increase in certain areas as low-level moisture increases (see Desmogblog post for papers). In fact, extreme rainfall events are already on the increase.
  7. Albatross, I did wander over to Desmogblog and read your exchange with NicholasV. NicholasV does hold a similar position to mine concerning the Bill McKibben video. I did get that jet stream info from a Accuweather piece but rather than talk through my "hat" I am interested in updating my knowledge. I did take one course in college on meteorology but that was long ago. Here is a resource that I found that may help update my knowledge base. Online weather learning site. Reading your posts, you do seem a very intelligent person. Thanks for taking the time to share your views and resources.
  8. Albatross, Here is the Accuweather article about the jet stream. The claim is they predicted the severe weather in February. Accuweather article. Quote from article: " meteorologists knew by February of this year that the upcoming spring was going to be a wild one in terms of severe weather and flooding, and it was not because climate change was ongoing. The combination of a weakening La Nina and the anticipated sharp temperature anomaly gradient between the northern U.S. and the southern U.S. told us that the jet stream running across the U.S. would be abnormally strong this spring. A strong jet stream leads to more powerful storms and thunderstorms, which increases the chances of large tornadoes and widespread flooding."
  9. Text from the intermediate level seems broken.
  10. "I suppose what I am getting at at what stage can we start safely matching extreme weather events to AGW?" Doing this scientifically requires matching observations of weather to predictions from the climate models. As I understand it, only really disruptions to the hydrological cycle (drought and extreme precipitation events) are settled science within the predictions. Hurricanes are not. AR4 discussed extreme precipitation events but there was not really a long enough record to be making strong statements. Droughts will wax and wane in cycles but the pattern of drought is consistent with predictions. I dont believe you can link any single weather event to AGW - only trends in frequency and/or intensity.
  11. "I suppose what I am getting at at what stage can we start safely matching extreme weather events to AGW?" One thing you can do is look at the average increase in humidity due to raised temperatures. When we say that there's 4% more moisture aloft than before, you can get hold of the weather service's measurements of precipitation leading to floods and other damage. Then do some calculations with and without that 4%. What height would the floodwater have reached if there had been x% less water flowing into the system? The big issue here of course is that 4% is the global average increase. Choosing - let alone justifying - a more suitable number for a specific time and place is much harder. But this sort of calculation can be illustrative if not definitive. Without a 4% increase in humidity, floodwaters may only have reached 2 streets from the riverbank rather than the ... distance they did. Not terribly scientific, but may provoke thought. And much more useful for real life. AGW may not cause certain events directly, but it can worsen the impact of those that do occur.
  12. Adelady, I wholeheartedly agree. AGW does not directly cause these events, but can affect the impact due to warmer temperatures and higher humidity.
  13. A response to this page being cited; not a happy camper:
    I looked through your link, an UNSCIENTIFIC source. First there was NOTHING in there that showed any changes is because of our CO2, all it shows is there is changes in the climate system. DUH!!! OF COURSE. As they ADMIT they CANNOT attribute any of those events to AGW. Only that AGW might cause amplification of extreme weather events. Except, anyone who takes meteorology knows, the violence of a storm is based on the temperature differences in weather fronts. The warmer the cold air mass is, the LESS violent the storm. The evidence is clear, there is NO evidence of weather extremes changing.
    Response: Clarification: The italicized text is a quote from someone other than itscoldoutside.

    Why do they think AGW causes only cold air, not warm air, to get warmer?

    [Sph] blockquote added to clarify intent.

  14. itscoldoutside, "The evidence is clear, there is NO evidence of weather extremes changing." Really? Here is an interesting article that disagrees with your statement. Here we show that human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of data-covered parts of Northern Hemisphere land areas. And another article, this time focussed on England and Wales. The precise magnitude of the anthropogenic contribution remains uncertain, but in nine out of ten cases ourmodel results indicate that twentieth century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.
  15. @Hyperactive A response [quoted verbatim, hence the italics and caps, I encountered] to this page being cited; [as you can see they are] not a happy camper. The same author is citing CO2science as a "scientific" resource ....
  16. Ah, sorry I got a bit confused by your post.
  17. @Hyperactive, NP. BTW, They were asked to post their response here. They declined; instead posting that rant (which even I can even see is wrong at so many levels); thought you might all appreciate the reaction this site is getting :-) And, having a table of standard arguments makes things much easier; appreciated.
  18. itscoldoutside: Assuming you have cut & pasted this fine commentary from a response to one of your own comments, I was amused to see your interlocutor's comment "Except, anyone who takes meteorology knows, the violence of a storm is based on the temperature differences in weather fronts." I'm sure he or she was a professional or dedicated hobbyist. :)
  19. itscoldoutside: How about providing us with a link to the comment thread you are referencing.
  20. That @Hyperactive cites a UToronto paper is apt. (Unfortunately, it isn't possible to cite posts within the thread, and the interface is terrible - why I didn't bother. The exchange is fairly predictable).
  21. More unusual weather news, courtesy Jeff Masters: Rare February tropical disturbance drenching the Florida Keys Our calendars may say it's February, but Mother Nature's calendar says it's more like May in the waters of South Florida ... ... today's rare tropical disturbance over South Florida is symptomatic of how whacked-out our 2012 atmosphere has been. In isolation, the strange winter weather of 2011 - 2012 could be a natural rare occurrence, but there have been way too many strange atmospheric events in the past two years for them all to be simply an unusually long run of natural extremes. Something is definitely up with the weather, and it is clear to me that over the past two years, the climate has shifted to a new state capable of delivering rare and unprecedented weather events. But I'm sure some will say 'its happened before.' Right.
  22. The state and variety of a given ecological system plays a huge role in desertification. In most arid environments there are essentially 2 seasons- wet and dry. Wet seasons in an arid environment often exhibit torrential downfalls. Dry seasons are usually characterized by extreme drought. It makes sense that increasing global temperatures would increase the landmass of "arid" regions and increase the sporadic nature of rainfall (and overall precipitation levels). Certain biological systems have evolved to cope with extreme variations in rainfall(notably huge herds of ruminants and deep-rooted perennial grasses characterize arid environments). Bacteria within arid environments (which are usually grasslands) primarily survive the dry season's "bacterial holocaust" by living in the rumen of a grazer. Similarly, deep rooted perennial grasses improve hydrological cycles by reducing erosion, increasing water permeation into the soil and acting as a storehouse of water during the dry bacterial winter. Large herds roaming the land effectively manage the grass; they "trim the lawn" so to speak, and also apply bacteria ridden fertilizer to the land, and their hooves "till" up the hard packed soil surface (which is common in arid regions) "planting" the seeds of native perennials and increasing the soil's permeability. Increasing the number of perennial grasses/square foot also help the solar cycle through increased photosynthesis which helps bacteria cycling(more sugar available to the immediate environment). My point is that land management is the primary factor driving current trends in desertification- not atmospheric Co2 levels. A slightly warmer world with increased precipitation and more arid land, could actually have amazing benefits, if land managers took steps to encourage the biological systems that evolved to cope with such harsh climates, and thrive within them. From an ecological perspective, desertification is almost strictly a land management problem. Proper planning would heavily negate the potential problems associated with higher levels of more sporadic, "extreme" rainfall.
  23. @AHuntington1 #47: How about some references of peer-reviewed published science that support your currently unsubstantiated assertion that "desertification is almost strictly a land management problem." Also what exactly do you mean when you say "from an ecological perspective"? Finally, do you believe that the southward expansion of the Sahara desert is a "land management problem"?
  24. Oh, when I say "arid environments" i am referring to environments with extremely sporadic, sometimes torrential rainfall contrasted with extreme drought. Not completely lacking in water but with extreme temporal variations in water level. This as opposed to a rainforest, which has (relatively) consistent temporal moisture content.
  25. AHuntington, are you aware of the relationship between the Hadley cells and desertification in the horse latitudes? Try Johanson & Fu (2009) for starters. From the abstract:
    Observations show that the Hadley cell has widened by about 2°–5° since 1979. This widening and the concomitant poleward displacement of the subtropical dry zones may be accompanied by large-scale drying near 30°N and 30°S. Such drying poses a risk to inhabitants of these regions who are accustomed to established rainfall patterns.

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