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ICTP Colloquium on a Changed Climate

Clouds, Extreme Weather, & Global Changes

ICTP Colloquium on a Changed Climate
ICTP Colloquium on a Changed Climate

16/07/2019 - Trieste

“What is happening to our climate?” That is a question that is increasingly on the minds of experts, policymakers and the whole society. But what do the scientists who study climate have to say about it?

On Monday 8 July, three leading climate experts answered this question at a public colloquium in a crowded Budinich Lecture Hall at ICTP, each under the point of view of their own research fields.

The event was recorded and is available on ICTP's YouTube channel.

Sandrine Bony, a senior scientist at the French National Center for Scientific Research, opened the event with a look at a very relevant element in climate change: clouds. In fact, as she pointed out, clouds are crucial not only for the weather, as we can all experience during a gray weekend, but also for climate. Their effects are two-fold: they have both a cooling and a warming effect on climate. Clouds' cooling effect is because they reflect the Sun’s light back into space; the world would be considerably warmer without clouds. But they also induce warming, as clouds amplify the greenhouse effect of the atmosphere. How the fine balance between these opposing effects changes in the future will determine how clouds might amplify or even reduce potential future warming.

Professor Bony is trying to improve our understanding of this role of clouds. She underscored the uncertainty surrounding our knowledge of cloud processes. “The response of clouds to global warming has been one of the dominant causes of uncertainty – if not the dominant cause of uncertainty – in the prediction of the global warming, for a given change in greenhouse gas,” said Bony. “And it’s been like that for many decades.”

Professor Kerry Emanuel from the Massachusetts Institute of Technology, the event's second speaker, switched the focus of the talks from clouds to extreme weather events and how they are expected to evolve in our fast changing climate.

Starting from a discussion on tropical hurricanes, he pointed out how a change in occurrence, frequency and intensity of extreme events could affect society.

According to Emanuel, the predictability of these events will also change with changes in climate, and therefore extreme events may become more difficult to forecast.

Emanuel told the audience that although some of these phenomena may decrease in number, they are becoming more intense and dangerous.

“As the climate becomes warmer, hydrological extremes of both kinds actually become more probable,” said Emanuel. He added, “You experience an increase in the probability of both very heavy rainfall and long drought. We have what may seem like a paradox here: that the intensity of rainfall – that is, how hard it is raining, when and where it is raining – goes up fast, but the total global precipitation goes up slowly. The way nature resolves this paradox is to have it rain less frequently. So, in an average place, it rains harder when it rains, but it does not rain as often, and that spells a bigger problem both with floods and with drought.”

The final talk of the colloquium was given by Professor Bjorn Stevens, a director at the Max Planck Institute for Meteorology in Hamburg, Germany. According to Stevens, climate science gives rise to many challenges for science. In fact, every city and every place in the world experiences its own problems related to climate change, but these are local problems that are related to global questions.

“The one way that you could look at climate is that, in the past, climate was attached to a locale, and mostly we changed our climate by things we did locally,” said Stevens. “So to change climate we would move to a different location, migration was a key adaptation strategy.  Sometimes we could impact the climate locally by environmental modification. But now we are acting on a climate system globally (through greenhouse gas emissions), our actions no longer just impact the immediate locale, and these global changes are beginning to be competitive with the local changes.”

Stevens recounted how scientists who study the climate have to face new challenges, to try to predict how the climate is going to change on both a global and a local scale. “Something that is completely new in the field is the ability to do realistic climate simulations in a way that can coherently represent the entirety of these scales,” said Stevens. “For me, it’s a really exciting time as a climate scientist, because we are developing the tools that might give us an idea of what type of future we’re walking into.”

All of the speakers agreed that the field of climate science is far from a solved problem and there is still much uncertainty, therefore much new research is required. “I think those of us studying the climate system rapidly become humbled by what we don’t know about it,” said Professor Emanuel. “There is an awful uncertainty in this, and we look at our knowledge of the climate system and if we translate it into questions on human well-being it becomes a problem and a risk.

Professor Bony compared the current knowledge of climate to an incomplete puzzle. “Many of its pieces were placed many years ago. Now we are trying to put more pieces in the puzzle, that come from climate models, observations or many other sources of evidence. And it is really by combining all these sources of information and evidence that little by little we improve our physical understanding of what is going on.”

Fernando Quevedo, ICTP’s director, closed the session by asking the three speakers to identify what the field needs in order to discover breakthroughs.

“If there were to be a breakthrough,” said Emanuel, “it would occur by restoring the curiosity-driven basic science, focused climate science. We need to take brilliant young physics students, for example, and get them interested in the intellectual content of our field.”

This was a point on which all three of the speakers agreed. Professor Stevens added, moreover, that “to do that, we need to change the culture, and in climate science it is a bit difficult, because for decades we only have been allowed to really talk openly about what we know, because dishonest people would try to misuse expressions of doubt about a few things to caricature doubt about everything. And so, unlike many other fields, we have found it more difficult to talk about the things we don’t know. There are many things we don’t know, and they are really fascinating and cool, and it would be so much fun to talk about them. And that’s how you motivate people to come into the field!”


---- Marina Menga