Russian Heat Wave - Round 3

Following on from one post that found no evidence that a changed climate was responsible (Dole et al. 2011) for the Russian heat wave of 2010, and another that found exactly the opposite by Rahmostorf and Coumou (2011), I'm going to look at whether it's possible to reconcile the two approaches.

1000 Earths

The approach taken by Otto et al (2012) was to model the Earth's atmosphere as many times as they could, looking for the proverbial needle in the haystack. The needles they were looking for were events that were similar to the 2010 Russian heat wave, and the reason they needed so many models is because they are looking specifically for rare events, so a large number of runs of the climate is necessary to find these statistical outliers. 20 years ago, the only way to do this would have been to convince a large institution to loan you their supercomputer, nowadays an alternative exists: convince a large number of people to download and run a small program using the idle time of their computers. This is a remarkable crowd sourcing achievement, allowing people to run the HadAM3P atmospheric model on their laptops and desktops (which are about as powerful as an early 90s supercomputer by the way). 

Possible clouds over the Earth in 2090
(running on my laptop)

So how does this help work out if the 2010 Russian heat wave is attributable to climate change? The crux of it comes down to two sets of simulations, one for the Earth as it was in 1960s, and one as it was in the 2000s, each containing over 1000 individual simulations (1600 and 1200 to be precise). The occurrence of anomalous events between these two decades can then be compared:

Magnitude and expected return time of
events similar to Russian heat wave.

What the above graph shows is the frequency of events (expressed as a return time, or the average length of time you would expect between events of a given magnitude) plotted against their magnitude, in terms of the temperature of the heat wave that was modelled. It does this for two different decades, with each blue or green circle representing a particular event in the model with corresponding return time and temperature. The Russian heat wave equivalent is shown as the dot that the arrows are pointing towards. There are two important things to take from this graph. First, the return time between an event of the size of the Russian heat wave has decreased from around 100 years to 33 year from the 1960s to the 2000s (as shown by the horizontal red arrow). Second, that the magnitude of a 33 year event has increased by around 1ºC from the 1960s to the 2000s.

Tying things together

How can this help us reconcile the two different findings by Dole et al. and Rahmstorf and Coumou? Otto et al. argue convincingly that what Dole et al. found was that the movement vertically by 1ºC was inside of natural variability, and therefore should not be judged to be caused by climate change. (Note, this is looking at the magnitude of the event.) Alternatively, Rahmstorf and Coumou found that the likelihood of this event had increased (by 5 times), which is equivalent to the horizontal movement of 100 years to 33 years in return time. (looking at expected frequencies this time.) Hence to say they are contradictory is to compare apples with oranges, because they were never analysing the same thing.

All three articles were insightful, the empirical arguments of Dole et al. and the theoretical arguments of Rahmstorf and Coumou both brought a different perspective to the same problem. However, this week's article by Otto et al. ties the both of them together in a most satisfactory manner. I can only hope that the results from the simulation currently running on my computer can help contribute to science like this in the not too distant future!

Britain gets flooded, America gets attacked by the "Polar Vortex"

It sounds like Hollywood has been drafted in to write the news for America!

The extreme weather has lead to impressive pictures from around Britain, and some staggering ones from the States.

Spot the difference!

Chicago from above

This blog tends to focus on the negative aspects of these extreme events, but will anything positive come out of this weather? The Guardian seems to think so: with wind power, water supplies and wildlife all set to get a boost. Not to mention the surfers:

Surf's up off the French coast. [Photograph: Gaizka Iroz/AFP/Getty Images]

Attributing extreme weather to climate change

Can this weather be attributed to climate change? (If you've been reading this blog you should know the answer already...) No, at least not yet. Attribution is a tricky business, although there's reason to think that a warming Arctic may lead to events like these (Cohen et al. 2010). From the article:

Further, and somewhat counterintuitively, the severe cold winter weather may be attributed to boundary forcing changes consistent with an overall warming planet. A warmer atmosphere can hold more moisture and as the interior of the NH [Northern Hemisphere] continents cool in fall, this can lead to increased snowfall. And as argued here, more extensive fall snow cover contributed to the extreme negative AO [Arctic Oscillation] observed during the winter of 2009-2010.

So if anyone claims that the current weather in the States in anyway disproves that global warming is happening, you can point them towards the above article. And I wouldn't be surprised at all to see attribution studies for the current extreme events in the next 6-9 months.

2010 Russian Heat Wave - An Alternative Approach

Following this month's earlier post on the same topic, I'd like to have another look at the Russian 2010 heat wave. The previous post focussed mainly on a paper which found that there was no reason to suspect global warming was to blame for the heat wave (Dole et al., 2011). It's worth repeating one of this paper's main findings: "We conclude that the intense 2010 Russian heat wave was mainly due to natural internal atmospheric variability".

Scientific consensus?

However, there's a difference of opinion expressed by other climate scientists, who find that there was an increased likelihood of the Russian heat wave of 2010 because of global warming (Rahmstorf and Coumou, 2011). The approach they take is appealing because of its simplicity: they use straightforward arguments based on the statistics of such events and a combination of Monte Carlo simulations and analytical solutions. The conclusions they draw are not hard to understand either: if you assume there is an underlying upward trend in the time series with some random noise added to this then you would expect there to be a higher chance of observing an extreme event towards the end of the time series than if the time series were stationary (i.e. no underlying trend). A simple visual representation of this can be seen in the graph below, which shows the average global (D) and Moscow (E) temperature record over the last 100 years.

Normalised (by the s.d.) temperature records for the Earth (D) and Moscow (E)

Using this line of reasoning they find that there is an 80% chance that the 2010 Russian heat wave was caused by the underlying warming trend, and they point out that "Our results thus explicitly contradict those of Dole et al.".

They also claim that "Fig. 4 clearly shows that the warming trend after 1980 has multiplied the likelihood of a new heat record in Moscow and would have provided a strong reason to expect it before it occurred.". This is again in contrast to the Dole et al. paper, which says that it was unpredictable. I think the resolution of these two points of view is fairly easy though: the ability to predict an event is far tougher than merely expecting that an event might happen.

In a final post on this topic in the new year I'm going to see whether these two different takes on the same event can be reconciled.

EDIT:

The final post is now up, take a look here for the final instalment.

2010 Russian Heat Wave

In 2010 large swaths of the earth were hit by extreme weather in the form of heat waves. The effects were particularly strong in Russia, where numerous records for the hottest day across the country were broken (or smashed in some cases). Over 50,000 deaths were reported in Russia (Khandekar et al., 2013), and according to the Guardian, temperatures were around 10C above their seasonal average and there were substantial damages to the wheat harvest, costing the Russian economy $15bn. The cause of the heat wave was a blocking anticyclone over Russia, that was maintained by "anomalous cyclonic conditions over the Mediterranean area" and a similarly anomalously strong monsoon circulation (Trenberth et al., 2012). This blocking high lead to a temperature inversion which trapped exhaust fumes close to the ground, leading to some pretty horrendous conditions in Moscow:

Moscow on the 17th of June and 10th of August

From this it's clear that the human costs of extreme weather can be devastating, even when the extreme weather is not in the form of a one off extreme event such as a hurricane or a flood. The question of attribution of these events is therefore hugely relevant to the debate on climate change, because if it were to be shown that these events were caused by a warming climate, then this would directly link anthropogenic influences to huge economic and human costs.

How can a single event be attributed to a warming climate?

It is notoriously difficult to attribute a single given event to climate change. Indeed, it may well be a futile task, here is part of a paper examining the role climate change had on the 2003 heat wave in Europe (Stott et al, 2005): 

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.

The paper goes on to examine the link between increased global temperatures and the risk of a given event happening. That is, assuming that there's no direct connection between the event and global warming, rather there's a possibility of increased statistical likelihood that the event was caused by climate change. They find that this was the case for the 2003 heat wave in Europe, through a method of comparing the temperature in 2003 against a number of simulations using the HadCM3 model for the same area and seeing if the value was outside of statistically significant levels.

Was there a greater risk of the 2010 Russian heat wave because of a changed climate?

In this paper: Was there a basis for anticipating the 2010 Russian heat wave? (Dole et al., 2011), the authors find that "the intense 2010 Russian heat wave was mainly due to natural internal atmospheric variability". They use a range of techniques to draw this conclusion, using CMIP3 models and also comparing the heat wave temperatures against historical temperatures for the region. Indeed, this provides the main evidence for their conclusion about the attribution of the event to climate change, making it differ slightly in its methodology to Stott et al. (The model simulations were more involved in trying to hindcast the event than as part of the study of attribution.) This means that their approach for attributing 'blame' is mainly empirical in nature, as opposed to the more model orientated methods used in Stott et al.

Conclusion

It's impossible to say that any one event was 'caused' by climate change. However, it is perfectly reasonable to apportion an amount of blame to climate change, as was done by Stott et al. for the European heat wave. Whereas for the Russian heat wave it is "very unlikely that warming attributable to increasing greenhouse gas concentrations contributed substantially to the magnitude of this heat wave". So is it fair to say that the 2010 heat wave had no link to climate change? On the face of this post's evidence, yes, but things aren't always clear cut in science, and in a future post I'm going to have a look at a paper that finds itself in explicit contradiction to the Dole et al. paper, and see if there's any way of resolving this apparent contradiction.

EDIT:

See this post then for the next in the series, and this post for the final instalment.