It is an ill-posed question whether the 2003 heatwave was caused, in a simple deterministic sense, by a modification of the external influences on climate—for example, increasing concentrations of greenhouse gases in the atmosphere—because almost any such weather event might have occurred by chance in an unmodified climate. However, it is possible to estimate by how much human activities may have increased the risk of the occurrence of such a heatwave.
This is from a 2004 article in Nature about the European heatwave of 2003. The same can probably be said about the Russian heatwave of 2010. Arguing about whether this was ‘caused’ by AGW doesn’t make sense, unless you clarify carefully what you really mean.
A single weather event can not be attributed one-to-one to an external forcing of the climate. But the chance of occurrence can. Therefore it is in principle possible to say something about the relative likelihood of a certain extreme event in an unperturbed climate versus that in a human-perturbed climate. Or, to put it differently, to assign a certain portion of the blame/causality to the external forcing.
The return period for European heatwaves: Human influence has increased the chance of occurrence 9 fold. Figure from a 2008 presentation by Myles Allen (slide nr 24).
Weather is not climate. But of course they are related. In fact, climate change is a consequence of the weather changing over time. Due to changes in the planetary energy balance the chance of certain weather conditions changes. Over time this becomes apparent as a change in the mean weather: The climate has then observably changed. You cannot have climate change without changes in the weather.
Nielsen-Gammon has a good article about how to deal with these attribution issues related to single weather events. (h/t Judith Curry over at Kloor’s)
In between these two extremes, where AGW in a sense causes everything and AGW changes the probabilities but doesn’t in a sense cause anything in particular, there are, I think, two possibilities for ways in which we can assign a portion of the blame to AGW if appropriate.
The first way is to consider the direct effect of AGW on the mean state of the atmosphere, and consider that change to the mean state as AGW’s contribution to individual weather events. I’ll call this the “additive effect” approach. (…)
So, for example, temperatures in large parts of Russia are averaging 6 C or more above normal. At this point, climate change (from all anthropogenic causes), according to best estimates, has produced about 1 C of warming in the area. So it would be fair to say that man has turned what would have been a 5 C heat wave into a 6 C heat wave. Climate change has added one degree to the heat wave.
This sounds like a small impact, but it’s not appropriate to attribute 1/6 of the impacts to anthropogenic climate effects. Deaths, fires, and the like increase nonlinearly with the severity of a heat wave, so anthropogenic effects have made this heat wave lots more than 20% more severe than it would have been otherwise.
(This is meant to be an illustration, not a rigorously quantitative attribution.)
The second way is to consider the estimated effect of AGW on the probability of extreme events. I’ll call this the “probabilistic approach”. (…)
For extreme events, we want to know how often a certain threshold is crossed. (…)
So, to correctly say that an individual event was likely caused by AGW, its probability of occurrence would need to have increased, because of AGW alone, by a factor of 3 or more over pre-AGW conditions.
(bold in original; “likely” in the IPCC sense refers to 67% to 90% chance, hence the factor of 3)
Michael Tobis puts the question as follows:
“Is the average time between persistent anomalies on this scale anywhere on earth in the undisturbed holocene climate much greater than a human lifetime?” In other words, is this so weird we would NEVER expect to see it at all?
There are two approaches to answering this question. One is statistical (mathematical), and the other is integrative (based on experience and expertise rather than number-crunching).
He also summarized his position nicely in a comment at Kloor’s:
Stopping at “attribution of causality of individual events is impossible” is not strong enough for the recent astonishing situation in Asia. “We are absolutely certain this couldn’t have happened in an undisrupted atmosphere” is too strong (…)
Why claim it is so unusual? Three reasons 1) this is an uncommonly stable and long-lived pattern (those are not good) 2) this is an uncommonly intense pattern, dipping all the way to the monsoon zone and fraternizing with the monsoon, to Pakistan’s great detriment and 3) the really big events tend not be in summer and not to be centered on hot spots but on cold spots. In other words, this blocking event has features dramatically different form other ones we have seen.
While there’s overwhelming observational evidence showing that humans are affecting climate, this evidence comes from long-term trends, rather than individual events.
Extreme events are related to climate change, however: the odds of them happening are much greater with climate change. (…)
We can estimate probabilities, but rarely can it be asserted with 100 percent confidence that there is a causal relationship between variables.
Tom Yulsman has an interesting article based on an interview with Peter Stott. What struck me though was Steve Bloom’s comment, comparing the manner in which doctors talk about the health risks of smoking versus how climate scientists talk about the climatological risks of greenhouse gas emissions. See also this pointy comment by SecularAnimist. Doctors tend to be much more upfront about the risks than climate scientists are (see also my previous post).