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:

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. 

Footnotes
^[a] If data is independent and identically distributed (iid), then the probability of a new record-breaking event diminishes with the number of measurements () . In this case, the expected number of records is . On the other hand, if you count many more records than , 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.

References

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. http://dx.doi.org/10.1016/j.apgeog.2013.11.008

2. 
   T.N. Palmer, “Climate extremes and the role of dynamics”, Proceedings of the National Academy of Sciences, vol. 110, pp. 5281-5282, 2013. http://dx.doi.org/10.1073/pnas.1303295110

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. http://dx.doi.org/10.1029/2012GL051000

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. http://dx.doi.org/10.1073/pnas.1222000110

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. http://dx.doi.org/10.1088/1748-9326/aab375

6. 
   A. Witze, “Clouds get high on climate change”, Nature, 2016. http://dx.doi.org/10.1038/nature.2016.20230

7. 
   Q. Schiermeier, “Droughts, heatwaves and floods: How to tell when climate change is to blame”, Nature, vol. 560, pp. 20-22, 2018. http://dx.doi.org/10.1038/d41586-018-05849-9

The NY Times Magazine has a special issue this weekend on climate change. The main article is “Losing the Earth” by Nathaniel Rich, is premised on the idea that in the period 1979 to 1989 when we basically knew everything we needed to know that climate change was a risk, and the politics had not yet been polarized, we missed our opportunity to act. Stated this way, it would probably be uncontroversial, but since the article puts the blame for this on “human nature”, rather than any actual humans, extensive Twitter discussion ensues…

Before we link to some of the more thoughtful responses, a quick reminder that a lot of people read the NY Times magazine (far more than follow any climate scientists on Twitter or Facebook), and that as David Roberts at Vox has pointed out, having differently-told climate stories – even if they are manifestly imperfect, might help broaden the conversation and basic awareness that this is a story worth delving into. Secondly, the last big NY Times magazine story I remember related to climate was the execrable profile of Freeman Dyson – a fascinating topic in theory, but one which focused on the least interesting thing about him – a barely warmed up stew of stale climate skepticism.

The article itself is supported by a lot of background work, some visually stunning photography by George Steinmetz and lesson plans hosted by the Pulitzer Center.

So, here is some of the more interesting commentary:

From Emily Atkin at New Republic:

“Losing Earth” is an impressive piece of journalism for several reasons. One is simply that it’s the Times’ longest-ever article—and it’s about global warming. This comes at a time when much of the news media is failing to live up to its responsibilities covering climate change, an issue that affects the entire population, hundreds of ecosystems, and every economic sector. Rich’s story, too, is proof that the climate story can be told in an engaging—fast-moving, human-centric, funny, and frustrating—way.

And the insights about human nature are worth pondering. “We’re a medium-term species,” he said in April. “We plan ahead, but only so far. We’re willing to sacrifice comfort in the present for security in the future, but within reason.” But the fossil fuel industry and Republicans know that, and have successfully exploited it for the last thirty years. “Losing Earth” is thus not the whole story of human’s failure to act on climate change. Its flaw is that it’s painted as such.

From Leah Stokes:

If you haven’t had the two hours necessary to devote to reading “Losing Earth” the full issue article on climate change in @NYTmag, let me summarize it for you: the central thesis is plain wrong. We are not all to blame for the climate crisis. https://t.co/bBCuuuXiks 1/

— Leah Stokes (@leahstokes) August 1, 2018

From Alex Steffen:

What is so off about this whole-issue @NYTmag piece on climate?

Let me share a few initial observations (thread).https://t.co/H3r1o5Sdwt

— Alex Steffen (@AlexSteffen) August 1, 2018

Both Climate Progress and Huffington Post have quotes from scientists (including Mike Mann, Bob Brulle, Jennifer Francis and David Titley) who are generally not supportive of the main conclusion.

Naomi Klein has her own idea of whose fault it was:

When I looked at the same period, I came to a very different conclusion: that what at first seemed like our best shot at lifesaving climate action had in retrospect suffered from an epic case of historical bad timing. Because what becomes clear when you look back at this juncture is that just as governments were getting together to get serious about reining in the fossil fuel sector, the global neoliberal revolution went supernova, and that project of economic and social reengineering clashed with the imperatives of both climate science and corporate regulation at every turn.

The failure to make even a passing reference to this other global trend that was unfolding in the late ’80s represents an unfathomably large blind spot in Rich’s piece.

At this point in a post, I’m supposed to summarise all of this and give my own informed opinion but… truth be told, I’m on vacation, and I haven’t got around to reading it all yet. So rather than demonstrate my own confirmation bias, let’s open it up. Maybe I’ll have something to say later this week…