As climate scientists attempt to forecast the future pace of global warming, one of the more complex variables has proven to be the interrelation of clouds and climate change. In an interview with Yale Environment 360, physicist Kate Marvel discusses the double-edged effect clouds have on rising temperatures.

Clouds perform an important function in cooling the planet as they reflect solar energy back into space. Yet clouds also intensify warming by trapping the planet’s heat and radiating it back to earth. As fossil fuel emissions continue to warm the planet, how will this dual role played by clouds change, and will clouds ultimately exacerbate or moderate global warming? Kate Marvel, a physicist at Columbia University and a researcher at NASA’s Goddard Institute for Space Studies, is investigating the mysteries of clouds and climate change. And while she and her colleagues would like to offer definitive answers on this subject, the fact is that few now exist.



Kate Marvel

In an interview with Yale Environment 360, Marvel discusses what is known about the behavior of clouds in a warming world (they are migrating more toward the poles), explains why strict controls need to be imposed on geoengineering experiments with clouds, and talks about why she is confident that science and human ingenuity will ultimately overcome the challenge of climate change.

“The fact that we understand how the planet actually works and we keep continuing to ask these questions — that makes me optimistic about the future of our species,” says Marvel. Yale Environment 360: Why are clouds such a challenge when it comes to accounting for them in climate change models? Kate Marvel: It turns out that clouds are really important to the climate. They kind of explain why different climate models predict different possible futures. Some climate models are saying it’s going to get really hot, and some climate models are saying it’s going to get hot, but not as hot. And it turns out that differences in cloud changes, predicted by the models, are kind of the main reason for that. And clouds are hard to model because they’re simultaneously large and small. When you think about how clouds form, they’re the result of microscopic water droplets, or ice crystals, coalescing like grains of sand. But at the same time, they cover 75 percent of the Earth’s surface. So it’s really hard for a climate model, which is fundamentally large scale, to get those really small-scale processes right. So we know that climate models don’t handle clouds very well. We’ve known that for a while. But clouds are also very important in regulating the climate. They block a lot of sunlight, but they also trap a lot of heat coming up from the Earth’s surface.

“Low, thick clouds are really good at blocking sunlight, so they have a cooling effect.”

e360: So it’s opposing effects. What dictates which effect a cloud will have? Marvel: It depends on several aspects; what the cloud’s made of is very important. So clouds that are made of ice particles are less reflective, they reflect less sunlight than juicier clouds made of liquid water droplets. Something else that’s really important is cloud height. If you look up on a clear, summer day, and you see kind of wispy, cirrus clouds, those obviously are not very good at blocking sunlight; they let that sunlight stream through. But they’re very good at trapping the heat from the planet’s surface. So, high clouds in general have a warming effect, but, low, thick clouds — the kind that you see on a cloudy, gloomy day — those are really good at blocking sunlight, so those have a cooling effect. e360: Researchers have gained a deeper understanding of cloud behavior in a warming world. Talk a bit about what we’ve learned. Marvel: We had a paper in the Journal of Climate that showed clouds are moving in the way that we would expect them to. So if you think about those low clouds that block sunlight, they are going to be much more effective in the tropics, where there’s kind of more sunlight, than there is further toward the poles, where the sun’s less intense. And climate change is predicted to alter atmospheric circulation patterns. It’s predicted to push everything poleward. What we’ve seen is that clouds are following that particular trajectory. If you look at the sort of large-scale patterns of clouds, they are moving toward the poles. e360: A Nature paper published after yours found an upward motion to clouds. Did you also find that? Marvel: We found hints of it; in the Nature paper it’s much clearer because they made corrections that accounted for the fact that the satellite records are really patchy and they were able to really observe clearly that high cloud shift. And there have been other studies that have observed that when it’s warmer, high clouds shift upwards. That’s an effect that we think we understand fairly well. e360: What are the consequences of this poleward and upward motion of clouds? Marvel: Both of them give rise to what I think is the most confusing part of climate jargon. We call it a positive feedback, and you think “positive feedback” — that sounds great. I like getting positive feedback. But when climate scientists talk about a positive feedback, we mean some process that changes in response to warming, that accelerates that warming. So both of those changes are what we would call a positive feedback, meaning it enhances the warming. e360: And is this trend, poleward and upward, something that the models say will continue under a business-as-usual scenario? Marvel: Yes, we think so. e360: There was some discussion when these studies came out regarding whether this was a consequence of global warming or that these movements were a consequence of recovery from major volcanic activity. Where do things stand with that at this point? Marvel: That’s a really good point. You have to be careful about attributing things to natural external forces versus human-caused external influences. We’re currently working on a project that’s trying to sort of tease out those influences. But I don’t think there’s been a study published that really definitively attributes the observed changes to one particular external sourcing.

“One of the greatest things about science is that in science we can say, ‘We don’t know,’ or ‘We don’t know yet.’”

e360: Of course, climate deniers can grab hold of something like that and say, “See, they don’t really know.” Marvel: I think that’s one of the greatest things about science is that in science, we can say we don’t know, or we don’t know yet. I’m really suspicious of people who come along and say, “I have all the answers.” I think there’s this real tendency to say that, “Oh, well if you don’t know everything, you don’t know anything.” And I think that’s totally wrong. You know, the continued existence of science does not invalidate science. The fact that we’re still asking questions doesn’t mean that the method we’re using to answer those questions is wrong. e360: Most of our cloud observations come from weather satellites. However, there was a NASA satellite in the planning stage that was designed to look at cloud behavior. President Trump’s proposed budget zeroes out that mission. Should NASA projects that deal with climate change be a priority in the upcoming budget? Marvel: I certainly think so. I don’t really understand why on earth we wouldn’t want to look at our own home. We have this amazing satellite technology. We can observe all of us from space. We can measure things that we never dreamed we could measure and that’s kind of a miracle, I think. I also think it’s extremely cost-effective. Why would we not want to do it? I started my career as an astrophysicist, and then I realized that nowhere else in the universe is as good as here, right? This is the best planet. And I think it’s part of NASA’s mission to look at other worlds, but we also want to look down at our world as well. e360: In a recent commentary, a group of researchers, including two from the University of Washington, made the case for conducting small scale experiments using a geo-engineering technique known as cloud brightening. They want to spray saltwater into marine clouds making them more reflective of sunlight as a possible way of reducing global warming. What’s your take on such experiments? Marvel: With geoengineering, I’m always very concerned because if I want to do an experiment on human subjects, as a university researcher, I have to go in front of a review board and convince them that all of my human subjects have given informed consent to participate in this experiment. And with a lot of geo-engineering experiments, I worry about how that consent is going to be obtained. There are starting to be organizations thinking about this; there’s something called the Climate Geoengineering Governance Project, that’s starting to think about how would we design an international law regime to actually regulate this. And I think that’s something worth thinking about.

“People ask me, ‘Are you just depressed all the time? How do you keep going in the face of this?’”

e360: You’ve written that when it comes to climate change, “I refuse to give in to despair. I just don’t have time for that. I’m too busy doing science.” Tell me more about the roll-up-your-sleeves attitude you’ve got.