Kerry Emanuel: Reddit AMA on climate change and severe weather

I’m Kerry Emanuel, a Professor of Atmospheric Science at the Massachusetts Institute of Technology in Cambridge, Massachusetts. I do research on hurricanes and other types of severe weather, on climate change, and how climate change might affect severe weather. My research is mostly theoretical, but I also build computer models and occasionally participate in field experiments and build and use laboratory experiments. I have flown research aircraft into hurricanes, and wrote a book called “Divine Wind: The History and Science of Hurricanes”, aimed at a general reader and covering both the science of hurricane and how they have influenced history, art, and literature.

We discovered this conversation after it was concluded. Kerry Emanuel is one of the four leading scientists who wrote this open letter: ‘To Those Influencing Environmental Policy But Opposed to Nuclear Power’. He also wrote a short book for the informed layman called “What We Know about Climate Change” (recommended as an efficient and readable overview of the science)

Here I have cherry-picked a few of Dr. Emanuel’s answers. The questions are my paraphrasing, as the information is largely in his replies:

Q: …claims civilization as we know it will end with that 4°C

A: In my view, the only really good way to look at this is to view it as a problem of risk. By its very definition, risk is probabilistic. The consensus view of global temperature increase over the next century is a curve with a peak in the 2-4 C range, but a non-trivial tail at higher temperatures. The most probable outcome (at least on the 100 year time scale) has risks that are probably manageable, but as Marty Weitzman at Harvard has pointed out, we need to pay attention to the tail of the risk distribution, because the economic and societal risks can be very large there. Scientists by nature are conservative and do not like to talk about what might happen in the tail, but we do need to think carefully about tail risk as part of our overall assessment of the risk.

Q: …increasing hurricane risks…

A: In my view, at the moment, we in the U.S. deal so poorly with existing hurricane risk that climate change considerations take a back seat. We actively subsidize folks to live and build in hurricane prone regions, and we bail them out massively when disaster strikes. The subsidies come in the form of state-mandated caps on insurance premiums, cheap federal flood insurance, and federal disaster relief. We need to solve these problems regardless of whether climate change results in more frequent and/or intense storms. But there are two climate-related issues that we need to consider now: rising sea level (which is already affecting the magnitude of storm surges, which in practice do much of the damage in hurricanes and other coastal storms), and projections that the incidence of very intense hurricanes should increase in the 100-year time scale. These considerations may, for example, enter into calculations of how high and how strongly we need to build sea walls in certain places.

Q: …are weather forecasts improving? 

A: Weather forecasts have demonstrably improved over the past half-century or so, but as Lorenz demonstrated, there is a fundamental limit to how far out one can make a forecast. (We think this fundamental limit is at about 2 weeks.) But faster computers have allowed us to do something we could not do just 20 years ago or so, that is quantify the uncertainty in each individual forecast. This is done by running ensembles of computer models, or ensembles within just one model but starting from slightly different, but equally plausible, initial states. These slight differences in models or initial conditions typically amplify with time, but do so at different rates at different times and places. The divergence yields a measure of uncertainty.

Q: …aren’t there more hurricanes due to global warming?

A: We do see some signals in open-ocean hurricane statistics, but since only about 1 and 3 Atlantic hurricanes make landfall in the U.S., and these do damage over a tiny fraction of their lifetimes, the record of landfalling storms is too short to see any climate signals, save perhaps for El Nino-related signals. We do not expect to see a global warming signal in U.S. hurricane damage for some decades. [highlighted because this agrees with Roger Pielke Jr. analysis of US damage data. Ed.]

Second, there is some indication that hurricanes (and cloud clusters in general) dry out the atmosphere, and this could have climate impacts. But this is very early, tentative work.

It is very hard to attribute individual events, or even groups of events, to climate change. This is simply a matter of statistics. We usually need long records to detect climate signals. There are also natural, long-period fluctuations of the North Atlantic climate that modulate rainfall in places like England.

Q: How drastic do you predict climate change to affect the United States in the next 20, 50 and 100 years?

A: I think we have to avoid the idea of a prediction. We know enough about climate risk to assert that the level of risk is enough to be a serious issue, more so as time goes by.

Almost all studies that I am aware of show differences in hurricane response to climate change, among the various ocean basins where hurricanes occur. But there is almost no agreement in the magnitude or even the sign of these differences.

Q: What are the chances that Earth will enter a new Ice Age in the coming decades?

A: Nill. But the chances in 30,000 years are excellent!

Q: My question is, what is the most interesting cause and effect relationship you learned about in the course of your research, where it turned out that seemingly disparate things were actually closely related?

A: For me, the most exciting and robust finding of climate research to date is the determination of the ultimate cause of the great glacial cycles of the last 3 million years or so. There is now very strong evidence that the root cause of these cycles lies in periodic variations in the earth’s rotation axis and orbit around the sun. Such cycles obey very precise mathematical relations, and we can see these signals in ice core and deep sea sediment records.

Q: …you’ve surely run into people who think climate change is a “hoax” or people who are just misinformed … What essays or books would you recommend?

A: All I can say to this is that I try to get people to look at this as a problem of risk. But most risk problems we are used to dealing with (e.g. the risk that our house might burn down) confront problems that may develop in our own lifetimes. We are less used to thinking about risk to future generations. We have to intelligently weigh climate risks (and possible benefits) against the risks (and possible benefits) of any actions we might contemplate to deal with climate change. We have to get away from binary thinking… climate change will be either an apocalypse or nothing to worry about; solutions will either be a complete panacea or not work at all. I do think this is actually the way most people think about the problem of climate change. As usual, the extreme elements are the noisiest, though….

Q: where would you say you have seen the most change in YOUR views on climate change as more evidence has stacked up?

A: Back in the 1980s, I did not feel there was enough evidence to warrant much concern about climate change. But great advances in paleoclimate, analysis of in-situ and satellite observations, my own acquisition of some basic understanding of climate physics and, yes, climate models have all added up to very compelling evidence that we are changing climate and engendering serious risks in doing so.

Q: Does the data you are seeing suggest that everything..

A: It is a great human temptation to attribute just about everything to the cause-du-jour. I remember when, in the 1980s. everything under the sun was blamed on El Nino. But we have to stand back, fight that temptation, and look at the data. This says that precipitation extremes are likely to increase (and there is some evidence that they have, in some places), and that heat waves will become more common and cold waves less so. We think hurricanes might become more intense, but we do not know much about how many other phenomena — such as tornadoes and hailstorms — might be affected by climate change.

Q: To what extent was the severity of Hurricane Katrina affected by AGW? 

A: It is virtually impossible to attribute any one event in a chaotic system to any particular cause. We can say that had that exact same storm followed that exact same track, with exactly the same environmental winds but through the thermodynamic environment of the 1980s, its winds would have been perhaps 20 MPH less. But that is a very restricted statement.

Unfortunately, like energy policy, climate policy depends upon the ability to understand long term risk – to evaluate and choose amongst imperfect options. That’s just the way it is.