Climate change and extreme events on Nature Soapbox Science

I wrote a post for the Nature Soapbox Science community blog on climate change and extreme events. If you want to take a look, it’s here. UPDATE (11/1/2012): well, I’d may as well just put the post here as well…

As I type, I have a massive chapter for the next full Assessment Report (due to be published in 2014) sitting on my desk to review and a couple of analysis routines churning their way through terabytes of climate model data. There’ll be hundreds of other people around the world focussed on similar things. The aim is to produce the 5th series of Assessment Reports since the IPCC was formed in 1988 to help decision makers, well, make decisions.

But the IPCC has been up to other things recently as well. In November 2011 it published a Special Report Summary for Policymakers on “Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation” (or SREX, the full report will be published in February 2012). Understanding how extreme events might change in the future is really important as it’s these things that will really impact people: heat waves, flash floods, hurricanes, droughts and sea level rise related inundation. This is far more useful to know than the quite abstract concept of global mean temperature change. This report looks like an advance in the IPCC procedure as it involved a far more integrated approach than usual IPCC outputs, having authors from climate science, impacts and adaptation backgrounds as well as disaster risk management experts.

Although it sounds obvious, one of the key conclusions of SREX was that the impact of extreme climatic events is greatest where vulnerability is highest. On the ground, this has manifested itself as higher fatality levels in developing nations and higher economic losses in developed countries. There’s a lot to think about here in terms of how developing nations move forward and how developed nations approach things sustainably to reduce exposure. That’s not really my area though.

From a scientific point of view, they also point out that analysing extremes is relatively difficult as they are rare and data from around the world are not always up to the job. That said, this depends a lot on the particular “extreme” being investigated – this has always struck me as slightly odd about the climate extremes community in that the only common theme is the statistics and not the science behind the phenomena.

Looking to projections, the IPCC SREX assign their highest confidence assessment (“virtually certain”) to increases in temperature extremes by 2100. This is because this is pretty much a direct response to the radiation changes forced by atmospheric greenhouse gas emissions. Everything else is a slightly more messy consequence of the temperature changes and these other fields vary much more amongst the 12 different models used in this analysis making their projections uncertain. However, it also looks likely that heavy precipitation events will increase in certain regions and that the maximum winds associated with tropical cyclones will increase whilst their total number will likely decrease.

Oddly enough, the emissions pathway that we take in the future (the IPCC analyses different sets of projections based on different socioeconomic and technological development assumptions) has little impact on extreme events in the next 30 years or so – they don’t appear to have an impact until the latter half of the 21st Century when inter-model variability masks most of the climate signal anyway. This highlights how making projections of extreme events is a difficult game. In that spirit, here are two of the key problems as I see them relating to my area of research on severe storms in Europe:

Loading the dice or getting new dice?

If we assume that climatic quantities have a normal distribution (which isn’t always the case, especially with precipitation) then you can view the extremes as the tails at either end of the distribution e.g. hot or cold. So climate change could be viewed as like loading dice – you start rolling more sixes (or getting more hot days). However, when the climate regime changes this analogy breaks down as, instead of just rolling more sixes, you start needing to roll sevens as climate records are broken (see the figure below). This poses a problem for climate models as, like a six sided die isn’t designed to roll a seven, climate models haven’t been designed (or at least haven’t been verified against) conditions that have never been observed.

The green curve represents the distribution of Swiss summer temperatures from 1864 to 2002. Clearly, 2003 does not align well with that distribution and is an example of an extreme breaking a previous record. This figure has been taken from the IPCC AR4, for more details click the image.

We’re gonna need a smaller box.

The second problem is that some important things – like severe storms, tornados and regional and local changes such as river catchment area precipitation changes – are too small for climate models to represent or resolve. The reason for this is that these computer models split the atmosphere (and oceans) into a 3D array of boxes. The important equations are solved in each box and then they pass information to neighbouring boxes as appropriate at each model time step. These boxes usually have horizontal dimensions of around 100-400 km to allow for a convenient computational time. However, storms and tornados work on scales of significantly less than 100 km so there’s no way that the models can tell us anything about these things. This problem is particularly acute in relation to the IPCC SREX as this analysis used a suite of climate model data from a project called CMIP3, which was completed in 2006 for the last IPCC assessment and, therefore, does not use the most up-to-date and highest resolution model data. (The data currently being prepared for the next IPCC Assessment Report called CMIP5 is, however, not yet complete so perhaps this criticism is a bit unfair.)

Is this good enough?

So does this mean that analyses using these model data are not useful or reliable? When faced with this question I struggle to get past the fact that, however much they can improve in the future, these models are still the best and only tool we have for making climate projections. Beyond that, we can take comfort in the fact that the very basic physics of climate science is really well understood – even very simple energy balance models can tell us useful things about the effects of increasing atmospheric greenhouse gas changes. What we’re talking about here are the details, albeit very important details, and in that respect our current analyses are consistent with the things that we’re pretty sure of.


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7 Responses to “Climate change and extreme events on Nature Soapbox Science”

  1. Peter Risdon Says:

    Your piece seems to assume all climate change will always produce more extreme events (rolling 7s). Why?

    • andyrussell Says:

      Because in a climate system that retains more energy (as a result of increased atmospheric GHG levels), this is the most likely outcome. For example, the IPCC SREX concluded that it is “virtually certain” that temperature extremes will increase by 2100.

    • andyrussell Says:

      Actually, I’m not sure I say “more extreme events” and if I did I probably didn’t mean to. It all depends how you define an extreme. In particular, if you move the distribution of events then what used to be an extreme (a 6 on the dice, say) is no longer extreme as the probability of it happening has changed. So in the period of transition it’s a bit more complicated to define the “extreme”. On the other hand, if we’re talking about temperature, then the other end of the distribution changes too (less 1s on a dice) so you might expect less cold extremes. Figure SPM.3 from the SREX that explains this better.

      The effect of changes in temperature distribution on extremes. Different changes of temperature distributions between present and future climate and their effects on extreme values of the distributions: (a) Effects of a simple shift of the entire distribution towards a warmer climate; (b) effects of an increase in temperature variability with no shift of the mean; (c) effects of an altered shape of the distribution, in this example a change in asymmetry towards the hotter part of the distribution.

      • Peter Risdon Says:

        Yes, I see this, although wonder whether we’d ever be in a period that wasn’t one of change, whatever we were doing with CO2.

        If there isn’t a trend to more extreme events, would it follow that the climate system is probably not storing more energy than before?

      • andyrussell Says:

        Well, all the palaeo evidence shows that big, natuaral global climate changes are relatively slow (even the “abupt” ones) and are punctuated by long periods of relative stability. The chaotic nature of the atmosphere means that you’d always get extremes within the distribution of events so part of the aim of the SREX was to look for changes. This isn’t easy as there is only a relatively short and incomplete data record. (However, that kind of analysis is much better than making assumptions that aren’t based on any evidence.) So naturally the SREX conclusions come with lots of caveats but still show some interesting stuff, for example:

        Confidence in observed changes in extremes depends on the quality and quantity of data and the availability of studies analyzing these data, which vary across regions and for different extremes. Assigning “low confidence” in observed changes of a specific extreme on regional or global scales neither implies nor excludes the possibility of changes in this extreme. Extreme events are rare which means there are few data available to make assessments regarding changes in their frequency or intensity. The more rare the event the more difficult it is to identify long-term changes. Global-scale trends in a specific extreme may be either more reliable (e.g., for temperature extremes) or less reliable (e.g., for droughts) than some regional-scale trends, depending on the geographical uniformity of the trends in the specific extreme. The following paragraphs provide further details for specific climate extremes from observations since 1950.

        It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, on the global scale, i.e., for most land areas with sufficient data. It is likely that these changes have also occurred at the continental scale in North America, Europe, and Australia. There is medium confidence of a warming trend in daily temperature extremes in much of Asia. Confidence in observed trends in daily temperature extremes in Africa and South America generally varies from low to medium depending on the region. In many (but not all) regions over the globe with sufficient data there is medium confidence that the length or number of warm spells, or heat waves, has increased.

  2. Tony William Powell Says:

    Always a hot topic and one I want to particualrly get involved in. However, I have weather and natural world interests covered by my UK blog. It is based on phenology research and this has to be an area of interest for anybody interested in the climate debate. Some of my trends are undeniable.

    Keep up the good work.

    Best Wishes

    Tony Powell

  3. Stormy weather ahead? at @ScienceLondon SciBar « Our Clouded Hills Says:

    […] So I was talking about some of my current research on future storminess. This was kind of based on a post I wrote for the Nature Soapbox Science blog a few months ago (and then re-posted here). […]

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