By Judith Curry
Does global warming make extreme weather events worse?
The IPCC SREX found limited evidence of global warming worsening extreme events. Nevertheless, there are a lot of climate scientists that think global warming is worsening extreme events.
A post at RealClimate argues:
Here is the #1 flawed reasoning you will have seen about this question: it is the classic confusion between absence of evidence and evidence for absence of an effect of global warming on extreme weather events. Sounds complicated? It isn’t.
The two most fundamental properties of extreme events are that they are rare (by definition) and highly random. These two aspects (together with limitations in the data we have) make it very hard to demonstrate any significant changes. And they make it very easy to find all sorts of statistics that do not show an effect of global warming – even if it exists and is quite large.
The fundamental issue here is not even one of attribution – rather it is detecting a meaningful change. For record high temperatures, it is fairly straightforward to expect more records as average temperatures increase. But this is much more difficult for drought, hurricanes and other extreme weather events. The RC post argues that physical reasoning is sufficient, e.g. warmer sea surface temperatures drive more intense hurricanes. The RC post concludes:
While statistical studies on extremes are plagued by signal-to-noise issues and only give unequivocal results in a few cases with good data (like for temperature extremes), we have another, more useful source of information: physics. For example, basic physics means that rising temperatures will drive sea levels up, as is in fact observed. Higher sea level to start from will clearly make a storm surge (like that of the storms Sandy and Haiyan) run up higher. By adding 1+1 we therefore know that sea-level rise is increasing the damage from storm surges – probably decades before this can be statistically proven with observational data. –
With good physical reasons to expect the dice are loaded, we should not fool ourselves with reassuring-looking but uninformative statistics. Some statistics show significant changes – but many are simply too noisy to show anything. It would be foolish to just play on until the loading of the dice finally becomes evident even in highly noisy statistics. By then we will have paid a high price for our complacency.
The RC post seems in part motivated by Roger Pielke Jr’s 538 piece:
If an increase in extreme weather events due to global warming is hard to prove by statistics amongst all the noise, how much harder is it to demonstrate an increase in damage cost due to global warming? Very much harder! A number of confounding socio-economic factors clouds this issue which are very hard to quantify and disentangle. Some factors act to increase the damage, like larger property values in harm’s way. Some act to decrease it, like more solid buildings (whether from better building codes or simply as a result of increased wealth) and better early warnings. Thus it is not surprising that the literature on this subject overall gives inconclusive results. Some studies find significant damage trends after adjusting for GDP, some don’t, tempting some pundits to play cite-what-I-like. The fact that the increase in damage cost is about as large as the increase in GDP (as recently argued at FiveThirtyEight) is certainly no strong evidence against an effect of global warming on damage cost. Like the stranger’s dozen rolls of dice in the pub, one simply cannot tell from these data.
I’m not comfortable with Pielke’s assertion that climate change has played no role in the observed increase in damages from natural hazards; I don’t see how the data he cites support such a confident assertion.
There is an even more significant problem with Pielke’s analysis. In a nutshell, he addresses trend detection when what we need is event risk assessment. The two would be equivalent if the actuarial data was the only data available pertaining to event risk. But that is far from the case; we often have much more information about risk.
This does not mean that there is no underlying change in the risk, and the priors we have in this case point to a significant increase in such risk. One would be foolish to make plans that have to deal with U.S. hurricane risk without accounting for the evidence that the underlying risk is increasing, whether or not actuarial trends have yet emerged at the 95 percent confidence level.
This is particularly so when one accounts for another form of prior information: theory and models. While some disagreement remains about projections of the weakest storms, which seldom do much damage, both theory and models are now in good agreement that the frequency of high category hurricanes should increase, as should hurricane rainfall and the flooding it produces.
Emanuel makes essentially the same argument as the RC post.
And Then There’s Physics
And Then There’s Physics has a relevant post entitled Emergence timescale for trends in U.S. tropical cyclone loss data. While not directly germane to the points I want to make, the article is worth reading and concludes
The emergence timescale of anthropogenic climate change signals in normalized losses will be around 200 years from now. So, Roger Pielke Jr is right.
In a post on previous thread, Dan Hughes argued that
Existence of potential is not evidence of existence of outcome.
The potential for increased atmospheric temperature to retain increased water is not evidence that the atmosphere contains increased water.
The potential for increased atmospheric water content is not evidence that extreme rainfall events will occur.
The potential for increased atmospheric temperature to lead to increased strength and frequency of topical storms is not evidence that topical storms have increased in frequency and strength.
Radiative energy exchange balance at the top of the atmosphere implies energy-exchange balance ( not limited to radiative exchange ) at all the interfaces between the sub-systems within the Earth’s climate systems. The potential for radiative energy exchange balance at the top of the atmosphere is not evidence of energy exchange balance at sub-systems interfaces.
There are two fundamental issues here regarding reasoning about the impact of AGW on extreme weather events:
- the utility of physical reasoning and models in assessing this impact
- the choice of a null hypothesis
When it comes to extreme weather events, climate models are inadequate at simulating them; exercises in attribution that turn the anthropogenic impact on and off are of little use if the models have inherent problems in simulating the extreme events of interest.
With regards to the physical reasoning, as an example I’ll tackle the canard whereby global warming increases hurricane intensity (for a review of this topic, see this previous post). Simple physical reasoning suggests that increased sea surface temperatures will increase hurricane intensity. But what about circulation changes associated with global warming, that might provide more or fewer El Nino events? That might change wind shear? That might change subtropical dust emissions? Etc. There is no way to reason through the complex changes that might occur to complex atmosphere and ocean circulation systems and their impact on extreme events. There is observational increase of hurricane intensity in the North Atlantic and the North Indian Ocean since 1980. In the Atlantic, with such a short data set, there is no way to sort out whether this increase is associated with the transition to warm phase of the AMO, or to AGW.
Recall the dueling papers on Climate Null Hypotheses by myself and Kevin Trenberth [link]. Depending on which null hypothesis you select as a default position when conducting research:
- Humans have no influence on extreme weather events
- Humans are influencing extreme weather events
you approach the problem in a different way. For #1, the null would be rejected if you find evidence of a human influence. In the absence of such evidence, #1 is not rejected. This is what RP Jr argued. For #2, the null would be rejected if there is evidence of no influence. RC and Emanuel essentially that the data is insufficient, so they argue from ‘physics’ and state that there is no evidence of absence.
To me, the ‘no evidence of absence’ argument is rather fatuous given that simple thermodynamical reasoning is not really useful in elucidating the impacts of AGW on extreme weather events.
So, how to answer the question Does global warming make extreme weather events worse? Just say we don’t know. There is no evidence even of a trend in most extreme events; attributing any trend to AGW is extremely difficult. We can say that it is possible that global warming will make extreme weather events worse, but the evidence is theoretical.
Bottom line is that RP Jr took a very defensible position in his essay. Kerry Emanuel’s response was interesting, but he puts forward a theoretical scenario of worsening extreme events with global warming. The theoretical scenario is convincing to people who adopt as a null hypothesis and starting point for their reasoning that AGW is worsening extreme weather events. It is not convincing to people looking for empirical evidence.