Understanding future climate risks

27 January 2021

With global warming, extreme events like heatwaves and droughts of unimaginable proportions are becoming a possibility. Olivia Romppainen, co-organizer of an international conference on the subject, explains why we need a better understanding of these combined weather and climate events – so-called compound events.

Ms Romppainen, you originally expected about 50 participants in Bern, and then over 300 researchers joined the online version. Why so much interest?

Olivia Romppainen: On the one hand, virtual participation is much easier than travelling to Bern. On the other hand, at the moment there is a real need for more exchange in the field of science. After nine or ten months of the coronavirus pandemic, people are longing to meet again – even if it’s only virtually. I also notice my own need for these regular exchanges in order to develop and discuss ideas.

Don’t online conferences take place all the time?

No, not in our field, and many have been postponed. Of course there are the big conferences, but these are very impersonal and not designed for informal interaction. We, on the other hand, have created some opportunities, for example via a virtual coffee corner.

What did the conference cover in terms of content?

It demonstrated that there are two directions in research: There are researchers who investigate what very extreme events could look like – events that we’ve never experienced, but which would have catastrophic consequences. On the other hand there are researchers who study infrequent events that occur from time to time and have very large socio-economic consequences.

Can you give some examples?

Events that we have already experienced include the combination of droughts and heat waves. This can trigger a cascade of subsequent events. If forests are weakened by drought, they are more susceptible to pests and are further weakened by them. This can lead to the collapse of the inventory of a particular tree species. This was seen, for example, in the drought summers of 2018 and 2019 in Germany. The unlikely events could be, for example, heat waves of a duration and intensity that we have never experienced, but which are entirely plausible in terms of the underlying meteorological conditions. Possible, but very, very unlikely.

When researching compound events, is it mainly a question of how likely their occurrence is?

With the very extreme events, we now deliberately work with plausibility rather than probability. We ask ourselves: Is it plausible that something like this could happen? And if so, how should we deal with it? And: Can we somehow prepare for such a serious event? These are so-called scenario questions.

Is the growing interest in compound events driven by intellectual curiosity or by the interest of decision-makers in politics and administration?

The demand for our results certainly exists – not least because decision-makers simply ignored such events in their past analyses. They are aware that they exist, but it has simply been too time-consuming and complex to deal with them. It was already challenging enough to understand individual natural hazards and how to adapt to them and defend ourselves. This is why the storyline approach is very interesting. Firefighters, for example, can do well if we develop a scenario that they can use to practise their reactions. Such scenarios also involve complex interactions, and although they never happen exactly like as imagined, they still help people figure out how to coordinate and respond to such cascades.

What do such cascades of events look like?

For example, let’s say there is extremely heavy rainfall and hurricane force winds are blowing at the same time. What are the consequences for infrastructure facilities such as roads? What happens if they are blocked because of fallen trees and the fire brigades can no longer reach flooded buildings? Another sequence of events could be that first there’s a tropical storm and then very hot and humid conditions prevail. If such a storm now paralyses the entire electricity supply, people will be exposed to a heat wave without protection because they can no longer cool down adequately.

Can the consequences of such interlinked events be predicted at all?

The events in the atmosphere can, but it’s very difficult to predict the effects. It’s still relatively early days when it comes to modelling multiple compound events. But at our workshop, appropriate models were presented and these are now being developed and tested.

What about the pandemic: Do you include Covid-19 as an additional factor in your considerations?

No one explicitly took the pandemic into account in the analyses and modelling at the workshop. But it does show what a systemic risk can really look like. What the consequences are, for example, if supply chains that are important in one part of the world are interrupted in another. The coronavirus makes it very clear why we need to better understand complex systems. This also applies to the interactions between natural hazards that can occur simultaneously in different places on Earth. It’s about how the complex systems we depend on for our daily lives interact. The knowledge gained during the pandemic will greatly simplify communication about compound events in the future. We are no longer talking about abstract ideas; instead we have a concrete example. 

What research is being done on compound events at the University of Bern?

Numerous groups from the Oeschger Centre were represented at the workshop, and they are working on different aspects of compound events. For example, the interaction of heat and high humidity and the effects on human health are being researched. Another group is looking into the consequences of compound events for the oceans, and yet another at statistical prediction issues. They are also seeing how combined weather events can affect the availability of renewable energy. 

My own group is studying the temporally clustered occurrence of extreme events. For example, we want to understand the atmospheric processes behind the clustered occurrence of heavy precipitation, which can lead to flooding.

So is Bern a hub for compound events research?

Yes. There are only a few places where research is conducted with such thematic and methodological breadth. In general, one of the strengths of compound events research is the fact that it goes beyond the various disciplines and links knowledge from different sub-disciplines. This broad approach is one of the great assets of the Oeschger Centre for Climate Research at the University of Bern.

About Olivia Romppainen:

Olivia Romppainen is a professor of climate impact research at the Institute of Geography and co-head of the Mobiliar Lab for Natural Risks at the Oeschger Centre for Climate Change Research at the University of Bern. She organized the workshop on compound weather and climate events together with Jakob Zscheischler.

More information about the workshop and its talks can be found here.