During the last year there have been many extreme weather events, including the Moscow heatwave, and flooding in the US, Pakistan, Australia, Brazil, and Sri Lanka. That 2010 was a very hot year and that the planet is warming and that human activities are a major cause of the warming are undeniable. The question is often asked whether the cause of the the extreme events listed above is global warming.
Before considering the views of Climate Scientists the following video from the US ABC is a useful introduction:
Summary of Expert Opinion
This is a complex issue, so what do climate experts have to say about it?
Here is my summary of the points made by the experts.
1. The probability of the occurrance of extreme events will change most as they are in the tail of the probability distribution.
The diagram below helps to make this important concept clear. The diagram is referring to temperature events, but could just as easily involve hydrological events, such as droughts or floods. The probabilities of many events can be described in terms of the bell shaped normal curve. The diagram shows two normal curves, which have different means (averages). Note with the left (cooler) curve the very hot temperatures (marked in dark red) are very unlikely but if the mean temperature is increased (the right curve) the very hot temperatures become much more likely.
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2. the extra warming increases the amount of water vapour in the atmosphere which invigorates storms, and generally makes the hydrological (water) cycle more active.
3. the extra warming also leads to hotter weather.
4. although it is often stated that it is impossible to attribute individual weather events to global warming, scientists are developing methods of determining the degree to which global warming caused specific events (“fractional attributable risk”). One such study Stott et al that can be read at this link, concluded:
Using a threshold for mean summer temperature that was exceeded in 2003, but in no other year since the start of the instrumental record in 1851, we estimate it is very likely (confidence level >90%)that human influence has at least doubled the risk of a heatwave exceeding this threshold magnitude.
The complexity of these issues, and the degree to which they can be easily misrepresented is illustrated in this post on Tropical Cyclomes.
If the scientists are right and our warming of the planet is partly to blame for increased extreme weather, of particular concern is that we have only had 3/4 0C of warming so far. What will the world be like when we have had 20C of warming?
The Experts Speak for Themselves
The UK MET Office and the Royal Society released a joint statement in 2009, which can be found at this link. The whole report is worth reading, but here is the section that refers to extreme weather events:
We expect some of the most significant impacts of climate change to occur when natural variability is exacerbated by long-term global warming, so that even small changes in global temperatures can produce damaging local and regional effects. Year on year the evidence is growing that damaging climate and weather events - potentially intensified by global warming - are already happening and beginning to affect society and ecosystems. This includes:
* In the UK, heavier daily rainfall leading to local flooding such as in the summer of 2007;
* Increased risk of summer heat waves such as the summers of 2003 across the UK and Europe;
* Around the world, increasing incidence of extreme weather events with unprecedented levels of damage to society and infrastructure. This year's unusually destructive typhoon season in South East Asia, while not easy to attribute directly to climate change, illustrates the vulnerabilities to such events;
* Sea level rises leading to dangerous exposure of populations in, for example, Bangladesh, the Maldives and other island states;
* Persistent droughts, leading to pressures on water and food resources, and the increasing incidence of forest fires in regions where future projections indicate long term reductions in rainfall, such as South West Australia and the Mediterranean.
These emerging signals are consistent with what we expect from our projections, giving us confidence in the science and models that underpin them. In the absence of action to mitigate climate change, we can expect much larger changes in the coming decades than have been seen so
Here are the views of three prominent climate scientists in their own words.
According to James Hanson at this link, the language used by experts is often confusing to the general public:
Was global warming the cause of the 2010 heat wave in Moscow, the 2003 heat wave in Europe, the all-time record high temperatures reached in many Asian nations in 2010, the incredible Pakistan flood in 2010? The standard scientist answer is “you cannot blame a specific weather/climate event on global warming.” That answer, to the public, translates as “no”.
However, if the question were posed as “would these events have occurred if atmospheric carbon dioxide had remained at its pre-industrial level of 280 ppm?”, an appropriate answer in that case is “almost certainly not.” That answer, to the public, translates as “yes”, i.e., humans probably bear a responsibility for the extreme event.
In either case, the scientist usually goes on to say something about probabilities and how those are changing because of global warming. But the extended discussion, to much of the public, is chatter. The initial answer is all important.
Although either answer can be defended as “correct”, we suggest that leading with the standard caveat “you cannot blame…” is misleading and allows a misinterpretation about the danger of increasing extreme events. Extreme events, by definition, are on the tail of the probability distribution. Events in the tail of the distribution are the ones that change most in frequency of occurrence as the distribution shifts due to global warming.
For example, the “hundred year flood” was once something that you had better be aware of, but it was not very likely soon and you could get reasonably priced insurance. But the probability distribution function does not need to shift very far for the 100-year event to be occurring several times a century, along with a good chance of at least one 500-year event.
Kevin Trenberth (KT) in an inteview with Joe Rohmm (JR) at this link, gave the following opinion:
JR: It seems to me the media hasn’t figured out a way to talk about this so they often just don’t talk about it at all.
KT: That’s correct.
JR: And as a result the public never learns the connection to climate change. I’m just wondering if you have any comments about that and what you would suggest is the right way to talk about it and the like.
KT: I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.
Also in the interview Trenberth discusses hard evidence that extreme events were increasing:
... there was a study a few years ago now, that only went through 2002 but it was looking at the 20th century and at that point the average rainfall over the 48 contiguous states had gone up 7% but the heavy rainfall events had gone up 20%. And so the heavy rainfall events have been documented as increasing across the United States.
... There was a more recent study that looked at somewhat different statistics and showed that the really heavy rainfall events — the top 1% and the top 0.3% — had gone up at even more alarming levels something like 27% as I recall over the last 30 or 40 years [actually 1967 to 2006]. And indeed most of those changes have occurred since about 1970.
There was a more recent study that looked at somewhat different statistics and showed that the really heavy rainfall events — the top 1% and the top 0.3% — had gone up at even more alarming levels something like 27% as I recall over the last 30 or 40 years [actually 1967 to 2006]. And indeed most of those changes have occurred since about 1970.
In the video below, Ben Santer makes some of the points already introduced by Hansen and Trenberth, but also discusses some new work on determining the liklehood of extreme events using the fractional attributable risk method found in other disciplines. His final point is also important: it is not surprising that we are getting extreme weather events as we warm up the planet.
In written testomony presented to the US Congress, Santer described the attribution of extreme climate events to global warming in the following terms:
We are now capable of making informed scientific statements regarding the influence of human activities on the likelihood of extreme events (75, 76, 77).
As noted previously, computer models can be used to perform the control experiment (no human effects on climate) that we cannot perform in the real world. Using the “unforced” climate variability from a multi-century control run, it is possible to determine how many times an extreme event of a given magnitude should have been observed in the absence of human interference. The probability of obtaining the same extreme event is then calculated in a perturbed climate – for example, in a model experiment with historical or future increases in greenhouse gases, or under some specified change in mean climate (78). Comparison of the frequencies of extremes in the control and perturbed experiments allows climate scientists to make probabilistic statements about how human-induced climate change may have altered the likelihood of the extreme event (53, 78, 79). This is sometimes referred to as an assessment of “fractional attributable risk” (78).
Recently, a “fractional attributable risk” study of the 2003 European summer heat wave concluded that “there is a greater than 90% chance that over half the risk of European summer temperatures exceeding a threshold of 1.6 K is attributable to human influence on climate” (78).
This study (and related work) illustrates that the “D&A” [detection and attribution] community has moved beyond analysis of changes in the mean state of the climate. We now apply rigorous statistical methods to the problem of estimating how human activities may alter the probability of occurrence extreme events. The demonstration of human culpability in changing these risks is likely to have significant implications for the debate on policy responses to climate change.
(Note that the numbers in brackets refer to peer reviewed publications. The details of the publications can be found in the document at the link above.)
Will Steffen is one of Australia's premier climate scientists. That he agrees that global warming will increase the probability of extreme events can be found at this link.
Here is one final link that contains plenty of valuable information: A wave that could drown the world
Recent studies linking warming and extreme weather:
Greenhouse gasses and British flooding
Two new studies , here
UPDATE
Here is Tamino's take on this issue in November 2012.
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