In this interview, Nicholas Lutsko, an assistant professor of climate science at the Scripps Institution of Oceanography, answers questions regarding climate sensitivity, explaining how scientists estimate changes in the global climate due to increasing levels of greenhouse gases.
Photo by Valdemaras D. from Pexels.
Generally, how do scientists estimate the effect that increased greenhouse gas emissions will have on climate?
I guess I would say we have three different ways of answering that, which we kind of combine to ultimately make our projections. So the first kind of workhorse technique I would say is just using climate models. So, you know, big computer programs which we can use to simulate how the climate system might evolve under different emission scenarios. The second thing of course is observations — trying to take observations of Earth's climate and using those to project into the future. For example, you could look at how much warming we've already observed versus how much CO2 we've emitted into the atmosphere and try to extrapolate from that. The third way is using paleoclimate data, which is where we have these things which are called proxies, things which we can measure that record something about the climate system in the past. An easy one is tree rings. The growth of trees typically depends on temperature, so if you measure the width of tree rings that can tell you about past temperatures. We can use paleoclimate data to go back millions of years and try to find past climates that look something like what we expect a future to look like, and try to infer something about the future.
Based on current emission trajectories, what are the current projections for climate change?
When we look at different emission scenarios, people often develop these business-as-usual scenarios — which are you know, the idea is if we didn't do anything to stop climate change, what would happen — and those tend to be pretty dramatic. So maybe by the end of the 21st century we might have 3 or 4 degrees Celsius of warming in the global mean.
But I think recently people are starting to say that we're not quite following the business-as-usual scenario. We're doing a little bit better. Still, probably by the end of the 21st century we should expect somewhere between maybe 1 and 4 degrees in warming compared to where we are now.
Can you describe the basic statistics we see with the correlation between carbon dioxide levels in the atmosphere and climate change?
Yeah. As climate scientists, we work a lot with global mean temperature. The reason for that is a lot of the impacts of climate change scale with global mean surface temperature. For example, precipitation goes up about 2% per degree of warming. So in terms of these sorts of statistics, one of the biggest questions is how CO2 emissions relate to increases in surface temperature. We have this quantity: climate sensitivity, which is if we double CO2 how much does earth warm up? And it looks like an equilibrium that is somewhere between 2.5 and 4 degrees.
You just mentioned a range of estimates for an increase in temperature. Where does uncertainty in climate sensitivity projections come from? Why don't scientists just know this much carbon dioxide will raise the Earth's temperature by this many degrees?
The large majority of the uncertainty actually comes from clouds. Clouds have a huge impact on Earth's radiation budget, and this on its surface temperature. So on the one hand they scatter radiation — if we have sunlight coming in, they reflect it and thus cool the Earth. They also, however, emit radiation which warms the earth a little bit, or for some clouds it can.
But they're very hard to study because they're hard to observe. I mean, how do you even define a cloud? They're also very hard to model, because they involve processes on scales which range from the micron scale to hundreds of kilometers. We can't really resolve them in models, and we have to try to approximate their effect on the climate system. And so they’re a main source of uncertainty in climate sensitivity.
Recently, I think there is now a consensus. It used to be that people weren't even sure if clouds would have a warming or cooling effect on global warming, whether they would make it worse or make it better, but now we're pretty sure that clouds are a positive feedback on global warming; they make the earth warm up more than it would otherwise. But there is still a lot of uncertainty even having said that.
That leads into the next question about climate feedbacks. The example I had in the question was actually with water vapor, but what role do other climate feedbacks play in amplifying or reducing global warming?
You could think of there being three main feedbacks in terms of climate sensitivity. We have feedbacks associated with temperature. We have the water vapor feedback, or for the clouds, and then feedbacks associated with ice.
The temperature feedback generally is a negative feedback; it decreases the amount of global warming that we have. That's basically because the atmosphere warms up instead of the surface.
The water vapor feedback is positive; it makes global warming worse because it traps radiation. And so the Earth’s surface has to emit more radiation to balance things out. Clouds as I said, we used to be unsure whether they were a positive or negative feedback, but it now seems pretty clear that they are a positive feedback.
The ice albedo feedback is the fact that ice reflects solar radiation and cools the Earth. As we warm up and we melt ice, there's less of that reflection. That's a positive feedback as well.
How have researchers reduced the uncertainty of their projections over time and how accurate have climate models from the past been in predicting global temperature increases as a result of greenhouse gas emissions?
Answering the first part - we actually didn't make progress for a long time. The first estimates of climate sensitivity were made in the late 70s and they were basically made by groups of scientists sitting around and kind of speculating on work with what climate sensitivity might be. 40 years later, and with tons of work, we've learnt tons about the climate system, but we haven't improved on their initial estimates very much.
The initial estimates were that climate sensitivity is somewhere between 1.5 and 4.5 degrees. Now, arguably we could say it's between 2.4 and 4.5 degrees, so we have reduced the uncertainty that much. In that report that I sent you, I'd say the main contribution is just pulling together all the different lines of evidence we have to rule out some of the very low climate sensitivities. This comes from cloud measurements, from climate models, from paleoclimate estimates — all these kinds of separate strands of research which were seeing that a very low climate sensitivity, less than about 2.5 degrees, is very unlikely. As a community, we've just seen a force of evidence agreed on that.
The second question you ask is actually really interesting. There's a paper which came out I think last year which showed that early climate models like those that were developed, I want to say in the 80s and the early 90s, we're actually very accurate in estimating the warming we've experienced over the last 20 or 30 years. Even though they were very basic compared to what we use now, they pretty much got the right story, which I think is pretty amazing. It should ensure some trust in what we're doing now, though obviously we shouldn't blindly trust models.
According to the Climate Action Tracker, current pledges and policies would limit global warming to about 2.7 - 3.1 degrees Celsius above pre-industrial levels by the year 2100. To some, these numbers seem kind of small. If you're not a climate scientist, then you may think: “what does it matter if the Earth warms by a few degrees?” Can you speak on what would be the impact of increases like this on the climate, and maybe even going further to the impact on people?
Generally I'm kind of against reducing everything down to the numbers in this way, setting a target of below 1.5 degrees of warming or below 2 degrees of warming. Given the uncertainty in climate sensitivity and in emissions, I think it's very difficult for us to say that actually. But I understand that people do like having numbers to hold on to as a goal.
In terms of the impacts, let's say of a world that is 2 to 3 degrees warmer, in many ways the most important impacts of climate change are on the extremes. That's when we hear about climate change — when people think about it. Like with all the Hurricanes that we're seeing this year this; you know, there's some variability there, but a lot of that is due to global warming creating really warm sea surface temperatures in the Gulf of Mexico. Something that I've been working on lately is heat stress, so you know parts of the tropics may become uninhabitable by humans just because your body won't be able to regulate its internal temperature. Parts of India and the Middle East might become uninhabitable.
Actually, you won't be able to use your cell phones. You can't use an iPhone above 35 degrees Celsius, and they start disintegrating above 40 degrees Celsius. The number of days per year that you can't use your iPhone will only increase.
Yeah, that’s really interesting.
I guess I would say, even though a two degree change in mean temperature might seem arbitrary or abstract, it's better to focus on the extremes, like on the effect summer heat waves have on droughts and fires in California, and heat stress.
Finally, there is sea level rise. From the Paleo evidence that we have, the last time Earth was this warm the sea level was about 25 meters higher than it is today. It takes a long time for the sea level to adjust, so I don't think we're going to see 25 meters of sea level rise during the 21st century, but we may have already locked in a meter or two. Already, hundreds of millions of people would probably be displaced by that, which I think will be one of the main crises of the 21st century.
Is there anything else you would like to talk about regarding your own research, or to provide information to people who are not experts in this field?
That's interesting. I guess there are a few different things. I mean first of all, as climate scientists, I think it's difficult for us to talk about this concept of uncertainty because it makes it seem like we don't know whether global warming is real or that we don't trust the science. But there is a lot of basic science that we know about global warming, and it's definitely real. It's more a question of is it going to be really bad, or really really bad?
You know if climate sensitivity is 3 degrees, which would be on the lower end, that's still really bad, if we continue on a bad emissions scenario. On the one hand, we have a kind of scientific duty to be honest about our uncertainties; but on the other hand, I think sometimes people are a little nervous about doing that because of climate skeptics, and because it makes it makes us seem less sure.
Another thing that comes up, I don't know if this is even worth talking about too much, but I sometimes get emails from people who are not scientists who are worried about the breakdown of society or something — and I don't think that's likely, at least in the next hundred years. I really think of climate change as a social justice issue. It's going to exacerbate current problems. Wealthy people will always be able to move and buy a new beach house, or run their air conditioning all the time. It's the people who can’t adapt who are most likely to suffer and who will suffer.
One other thing, which I forgot to say in the thing about clouds is, most of the uncertainty in clouds comes from low clouds, like the marine clouds that we see off the coast of San Diego. UCSD and Scripps is a great place to work on these clouds because you see them every day.
I really found it interesting how you were talking about the sort of absurdity in reducing targets to certain temperature goals. With things like the Paris agreement, which is built around 1.5 and 2 degree celsius goals, how do they calculate that? How do they say, basically, “these are the things we're going to do and it will limit global warming to this many degrees” when there is so much uncertainty in the exact measurements?
It's a great question. There's a lot of stuff that I think is kind of sketchy. They have these things called integrated assessment models, which are kind of like economic models connected to a very idealized climate model. With that, you can play around with things like looking at how changes in agriculture might affect global warming, and you can do cost-benefits analyses and things like that, which I think is how a lot of these targets get set.
The details of some of the emission scenarios I think are pretty wild. Under like strong mitigation scenarios for example, they often assume a huge amount of carbon drawdown. They assume that, for example, most of the UK's surface land area would be used to draw down CO2, when we also need that land to grow food and live on. I'm often surprised how sort of unrealistic it seems, these kinds of emission scenarios.
Anything else you'd like to add, or to end on with climate sensitivity as a topic?
I mean, it's kind of inspiring to work on because it's a trillion plus dollar question.
And it's also fun because it forces you to think really big picture about the climate system. You have to think about the ocean and the atmosphere and land, the poles and the tropics. I think it's really fun to work on because it's both very societally relevant, and also it’s taking a really big picture view of this complex system, which I think is a great intellectual challenge.