Are the heatwaves caused by climate change? 

Are the heatwaves caused by climate change?

I get a lot of questions about the connection between heatwaves and climate change these days. Particularly about the heatwave that has affected northern Europe this summer. If you live in Japan, South Korea, California, Spain, or Canada, you may have asked the same question.

The raindrop analogy
However, the question is inaccurate and I will try to explain this through an analogy. Let’s say I go for a walk with a friend and my friend feels a few drops of water that fall on her. She asks me if it’s raining. But as long as there was only few drops of water, it could also be something else. 

I tell her that we can get some more relevant information in order to get a more reliable answer. Look at the sky. Are there dark clouds on the sky above? And what does the weather forecast say? 

If there are dark clouds above and the weather forecast suggests showers, it’s a safe bet to say it is the start of the rain. The rain always start with a few drops, just the way a climate change starts with a few events. 

In the same way as with the observation of the first drops of of water, you could not be sure whether the heatwave is a freak event or the emerging pattern of climate change, if you don’t include other relevant information.

There is a range of different pieces of information which are relevant when it comes to the question about weather events and climate change: (a) statistical evidence, (b) physical processes connecting different aspects, and (c) attribution work.

(a) Statistical evidence
Heatwaves are becoming more widespread, last longer, and are getting more extreme (e.g. Keellings and Waylen, 2014). This trend has been predicted and reported in multiple reports, such as the IPCC SREX (2013), the Proceedings of National Academy of Sciences (e.g. Palmer, 2009), and European Academy Science Advisory Council (EASAC, 2013). 

Climate change is equivalent to changing weather statistics, and one line of evidence includes the nature of record-breaking events. We can find evidence in both the number[a] and the magnitude of new record-breaking values.

Coumou et al., (2013) observed an increase in the global number of monthly heat records that corresponded to what one should expect if the temperatures increased everywhere by the same rate as the global mean. They also found that local monthly records are on average five times as frequent as they would be in a stationary climate. In other words, four out of five new heat records would not have occurred without global warming.

Other types of evidence includes how often the events (e.g hurricanes) take place, their duration and intensity. Standard statistical tests can also indicate whether a particular event fits in with the expected range of outcomes. 

(b) Physical processes
Physical conditions and processes play a role both for the emerging pattern of precipitation,  the evolution of weather, and their statistical characteristics. Indeed, we expect the statistics of rainfall and temperature to respond to an altered physical situation

Earth’s climate has always changed, and there have always been physical causes for the changes. This means that the climate is sensitive to altered conditions, such as greenhouse gases.

It would be difficult to explain why increased concentrations of greenhouse gases had no effect on the global mean temperature or on the statistics of  extreme weather conditions while other types of forcing clearly have an effect. 

There is no shortage on explanations for why changes in the physical environment should cause more extreme events. Some of these are:

  • Greater temperatures are expected to make heatwaves more widespread in general.
  • Weaker winds circulating the pole make weather episodes such as blocking high pressure more persistent. This weakening is associated with a polar amplification and the retreat of the Arctic sea ice (Francis and Vavrus, 2012;Coumou et al., 2015).
  • Changes in the north-south temperature differences, for instance due to the polar amplification, can increase the prevalence of the phenomenon known as “quasi-resonant planetary waves”, which is associated with heatwaves (Petoukhov et al., 2013).
  • I have also reviewed the greenhouse effect and described how convection can be altered by higher concentration of greenhouse gases. This link with the hydrological cycle may explain why the rains seem to be concentrated over small area of Earth’s surface (Benestad, 2018)

    Diminished area of precipitation explains both more frequent flooding and more droughts, and dry conditions exacerbate the heat, as moisture restrain temperatures during evaporation. 

    We also expect more extreme rainfall in some locations, as higher surface temperatures boost the evaporation and increase the turn-around rate of the hydrological cycle. There are also indications of higher cloud tops (Witze, 2016) which allow the rain drops to grow further than before.

    (c) Attribution
    It is possible to reproduce extreme weather episodes in computer models, such as those used for weather forecasting. We can conduct experiments to see which effects greenhouse gases have for the outcome. In other words, the models can be used to simulate the same event with and without the present levels (Schiermeier ,2018).

    The World Weather Attribution (WWA) has carried out such experiments, and their efforts suggest that recent extreme events have become more likely with an increased greenhouse effect.

    Individual cases and emergent behaviour of many events
    The planetary system is extremely complex, with interactions between atmosphere, oceans, ice and land, and taking place over a vast range of temporal and spatial scales.

    It is hard to say that one aspect is directly connected to another, when there are so many interacting parts and such rich level of complexity. Understanding the difference between individual versus collective events is key to making sense of the situation.

    Nevertheless, complex systems tend to give rise to emergent behaviour (explained in Gavin’s TED-talk). And the statistical characteristics of a large number of outcomes is often predictable. In fact, statistics is remarkably predictable, and we can often attribute some probability to the causes of some event through standard statistical tests.

    What is causing what?
    On another level, there is also the more philosophical question of whether rain drops are caused by the rain or the rain is a result of many rain drops. Rain is a phenomenon that includes many collective events in the clouds. 

    The same way that extra information such as cloud observation and weather forecast give confidence in our interpretation of the first drops being the start of the rain, the statistical evidence and our understanding of the atmospheric physics provide relevant information for judging the connection between heatwaves and climate change.

    A more relevant question
    I think it makes sense to rephrase the usual question of whether climate change causes a particular event, since climate and weather are different aspects of the same earth system.

    The bottom line is whether we now are observing the first glimpse of a new normal, or if the world will return to its old state. In other words, the question should be whether the recent heatwave is a signs of a new type of weather patterns we can expect for the future. I think the answer to this question is “yes”, based on current information and knowledge. 


    [a] If data is independent and identically distributed (iid), then the probability of a new record-breaking event diminishes with the number of measurements (n) P(X > [x_1, x_2, ... x_{n-1}]) = 1/n. In this case, the expected number of records is E(n) = \sum_{i=1}^{n}(1/i). On the other hand, if you count many more records than E(n), then that is a sign that upper tail of the statistical distribution is stretching towards higher levels. In other words, it indicates that extremes are becoming more frequent.


    1. D. Keellings, and P. Waylen, "Increased risk of heat waves in Florida: Characterizing changes in bivariate heat wave risk using extreme value analysis", Applied Geography, vol. 46, pp. 90-97, 2014.

    2. T.N. Palmer, "Climate extremes and the role of dynamics", Proceedings of the National Academy of Sciences, vol. 110, pp. 5281-5282, 2013.

    3. J.A. Francis, and S.J. Vavrus, "Evidence linking Arctic amplification to extreme weather in mid-latitudes", Geophysical Research Letters, vol. 39, pp. n/a-n/a, 2012.

    4. V. Petoukhov, S. Rahmstorf, S. Petri, and H.J. Schellnhuber, "Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes", Proceedings of the National Academy of Sciences, vol. 110, pp. 5336-5341, 2013.

    5. R.E. Benestad, "Implications of a decrease in the precipitation area for the past and the future", Environmental Research Letters, vol. 13, pp. 044022, 2018.

    6. A. Witze, "Clouds get high on climate change", Nature, 2016.

    7. Q. Schiermeier, "Droughts, heatwaves and floods: How to tell when climate change is to blame", Nature, vol. 560, pp. 20-22, 2018.

    Superforest,Climate Change

    via RealClimate

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