How Can Regional Models Advance Climate Science?

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0:35I'm Steve Strogatz. And I'm Jana Levin. And this is The Joy of Why, a podcast from Quantum Magazine, exploring some of the biggest unanswered questions in math and science today. Hi, Jana. Hi. How are you? I'm good. How are you doing today? Yeah, good. We're back to it. We are, and our discussion is going to be about something I don't think we've ever talked about with each other, which is climate change. Yeah, I don't think we've had that conversation, surprisingly. Such an important topic in science and world politics and the future of our planet, you know, what our kids are going to be facing.

1:13We hear about it all the time, how all kinds of records are being broken in temperature, wildfires, droughts, sea level changes. Yeah, I live in a coastal city. I live on an island. Oh, yeah, I heard that. We're referring here to Manhattan, yes? It's true, though, right? But so I realized that I don't really know as much as I should about how the climate projections are being made. We hear all the time about climate models. But what goes into those models? And what is really the science of climate prediction? Also, what do we know about how things are changing around the world? and how are those changes affecting people?

1:55Yeah, I wonder a lot about how many models there are, if there are competitive models, but I think it almost surprises me how much they dovetail, how much they come together, but, you know, how do they get their data sets? Very difficult problem, actually. Well, so I sat down with our guest, El-Fati El-Tahir, who is a hydrologist, a climatologist, a meteorologist at MIT, he taught me so much. I mean, really learned a lot. And El-Fati, he's originally from Sudan. And, you know, that appreciation that he has from growing up along the Nile underscores the point that climate change does not just occur in isolation as a very social and cultural dimension that we really can't ignore both in our analysis and response to this issue.

2:43Yeah, sure. We want to live on this planet. I think so. Anyway, on the science, we are going to hear from El-Fati El-Tahir, MIT professor. So let me pass it over to him. Are you ready? Yeah, I'm ready. All right, here we go.

3:01Welcome to The Joy of Y, El-Fati. Thank you. I'm really pleased to have a chance to talk with you. I noticed that you hold degrees in all kinds of different fields, civil engineering, hydrology, meteorology, hydroclimatology. First, could you just tell us what is hydrology? I mean, I can guess what it is from the words, but how would you define it? Hydrology is the science of the water cycle. The way I think about it is the water cycle in its full extent, from rainfall to infiltration of water into the soils, into overland flow, river flow, then the evaporation into the atmosphere, and then the transfer of that water in the atmosphere from one region to another until it trains somewhere else.

3:50Observing it, understanding it, predicting it, is the subject of the science of hydrology. So the movement of water through its cycle from rainfall into the land, the plants, the atmosphere, back again. Okay, and how about hydroclimatology? In a way, you could think about it as long-term averages of the variability in the hydrologic cycle and how processes that happen in the atmosphere, leading to formation and fall of precipitation, are connected to evaporation, soil moisture, and conditions over land. Now, one of the distinguishing things in your work is this attention to the social considerations along with the climate ones.

4:35How necessary do you think it is to take that multidisciplinary perspective, bringing both the social and the scientific together? Yes, very important. The way I describe it is bringing the context over which climate change is happening at regional and local scales. I tell my students that climate change doesn't happen in a vacuum. It happens in a context. When we emphasize heat waves in Asia, Those are regions that are already experiencing lethal heat waves, and people are suffering because of that. If you look at disease in Africa like malaria, too many children died from malaria in places like Niger and other corners of the continent.

5:19You look at phenomenon, for example, like water availability and denial. Even without climate change, there is a conflict on water. There is too many people, too little water. There is already a system under stress. Understanding how climate change is going to impact society could not be developed thoroughly without understanding the context and the history. And so carrying these climate studies at usual scale allows us to not only look at what the physical climate models are telling us, but to interpret those in a context where social processes and where society is already engaged in addressing complex and acute problems.

6:03So we're going to spend quite a bit of time in our conversation about the Nile. And before we get into the science and engineering aspects of it, I understand that you have some personal connection to that part of the world. Yes. I was born like a few hundred meters away from the Nile in a city in Sudan called Omdoman. That's where I grew up. That's how I learned about the world early on, seeing that huge amount of water flowing down to Egypt every year, the flooding of the Nile, the flooding season and the dry season. For a while, I thought the whole world would look like that. You know? You have a river flowing, and then you have deserts, and I was more interested in the rivers than the desert.

6:50So for those of us who haven't been there, can you give us a little more description of the terrain? So Sudan is pretty much a desert region. The river flows from the south to the north across that desert and bringing huge amounts of water into Egypt. And the area right next to the river is utilized for agriculture for like thousands of years. So there is greenery, there is palm trees, there is agriculture. And that's basically how life in Sudan was shaped through centuries. Well, you've given us some indication already of how important the Nile is to Egypt, Sudan. It touches many other countries, doesn't it?

7:34Yeah, the Nile actually travels across 11 countries. There are two main tributaries of the Nile. The Blue Nile comes from the Ethiopian highlands. It comes with very strong flows and erodes very strongly into the Ethiopian highlands, and it brings a lot of sediment. So the color of the water itself will be darker. The White Nile, it travels long distances through South Sudan, through relatively flat landscape, and so the color is not as dark. They merge together in Khartoum, the capital of Sudan, and then they carry together as the Nile, and that travels until it discharges in the Mediterranean.

8:15In hydrology, the most important equation is the water balance equation. Other countries contribute to generation and to consumption of the water, but like 80 % of the water is generated in Ethiopia, and 80 % of the water roughly is consumed in Egypt. Once it became clear to you that you were interested in a life as a scientist, did you always think that you would come back to studying the Nile? having grown up, what you said, only a few hundred meters away from it? That actually is a very interesting question that I wonder about myself. I found myself going back to study the Nile intensively, but I was just naturally drawn to look at that system and to study.

8:59It didn't happen by plan. I remember the first paper I had about the Nile was actually after I finished my PhD. People started learning about this phenomenon called the Alnino phenomenon. Every few years, you get a warming pattern in the Pacific Ocean, and it impacts weather and rainfall around the world, and mostly in the tropics. I got my hand in that data, and I noticed that Alnino seemed to correspond to years in which you have droughts in the Nile. Oh, really? Drought? Huh. Yeah. That discovery did not happen through a computer or plotting a graph. It's just looking at the data. That took me to get the data, to do analysis, to do statistical significance testing and so on.

9:46And I published a paper in 1996 in which I argued that 25 to 30 percent of the interannual variability of the Nile is associated with this phenomenon, which is al-Niniuma-Ninia. When you have warming in the Pacific Ocean, you get drought in the Nile. And when you have cooling in the Pacific Ocean, you tend to have flooding in the Nile. The story of the seven years of flooding and the seven years of drought was mentioned in the Bible and in the Quran about, you know, a dream of that nature. And even if you don't believe in those religions, the natural variability of the Nile has been a subject of interest to civilizations for solving the fears.

10:27Wow. It's a really fascinating story. First of all, this timescale of seven years seems to be about right with what we think of for El Nino itself. Is that right? Yeah, it's around that. When you do spectral analysis, it peaks at that time scale. So you have the coherence between the two signals, the sea surface temperature in the Pacific Ocean, the river flow in the Nile are coherent at those time scales. But following that, there have been actually measurements of the water level in the Nile for almost 1 ,000 years. I wrote a paper in which I described that as the longest record of a geophysical phenomena.

11:07This is not like paleoclimatic records that you get from data, but people recording, measuring the level. And they were doing that for reasons that has to do with the taxation of the population. And so there is measurements from a device they call nylometers. I used in another paper that record to look at the occurrence of Al Nino and La Nina in the past. On the face of it, it seems pretty unbelievable that something happening in the Pacific, I don't know how many thousands of miles that is from the Nile, but it must be close to halfway around the world or something. And yet, somehow through what it's doing to what, the jet stream or something, it's affecting the weather in Africa?

11:50Mm-hmm. A simple picture you could think about is for rainfall to occur anywhere, you have to have air going upward. So when you have different rainfall pattern over the Pacific, that enhances the upward motion in the Pacific, and that air has to come somewhere else. And some of it comes over the Ethiopian plateau and inhibits rainfall over the Nile. That's a simple way of describing it. But it's quite an interesting geophysical, what we call teleconnection. Teleconnection, I see. Yeah, we call it the teleconnection when you have two phenomena that are so far apart having physical connection that explains some of the statistical association that you observe.

12:34Well, let's talk about now climate and its interplay with the Nile, and in particular, the use of regional climate models as opposed to global. So, I wonder if you could walk us through why it's important to look at regional scales, what regional scales actually are in terms of distances, that kind of thing. Yeah. My focus in climate studies has been consciously at the regional scale. We need to understand climate phenomena at regional scales for two reasons. To be able to inform society about how climate change is going to impact them directly, but also to be able to inform how those climate change impacts are going to be distributed.

13:21people need to know the details of that so that they could design their adaptation strategies. I started looking at regional models during the period of my PhD work, and my PhD topic was about how deforestation in the Amazon could impact the regional climate of the Amazon itself. And so that was the first time in which I built a model of the regional climate around the Amazon. because you are not focusing on the global scale, you focus on the regional scale, you can always have more detailed representation of the processes, more detailed representations of things like topography and coastlines and land cover.

14:08All are elements that are important in shaping the regional climate. And because of the high resolution, you could then connect directly to where people live. You could describe, for example, in a state like Massachusetts, where I live, how conditions could be different in the eastern side of the state versus the western side of the state. Oh, really? That fine scale? Yeah, we could go down now to models that have resolution of about 10 to 20 kilometers, while the typical global climate model would have resolution in the order of 100 kilometers. The downside to that is you always need to have information to constrain the models at the boundaries.

14:51That's the price you pay. And we could get that information to constrain the boundaries, either from archived observations, or we could get it from simulations that are performed with global climate models. The way I like to think about a regional climate model It's like a numerical laboratory that are set up for a certain region to study climate processes at that scale. And it allows to incorporate knowledge about details of the hydrology or the agriculture or the weather systems that have been gained and accumulated through time. We integrate that in our climate studies of that specific region.

15:34So let me just flesh out a few examples. You've already touched on some. You mentioned eastern and western Massachusetts. You mentioned the Amazon. That would be a much bigger region. There's the Nile River Basin. Would that be considered a region? Yes. We run our model on the surface of the Nile, which is like the Ethiopian highlands, the Ethiopian Plateau. The topography of that region is an important factor in generating the rainfall over the sources of the Nile. And if you try to simulate the formation of rainfall over the sources of denial using a global climate model, the resolution of the model would not be sufficient to represent the details of the topography that exist in reality.

16:18And so that's the kind of process that we try to incorporate and successfully do when we use regional climate models. I would like to ask one technical thing, being a mathematician and a modeler myself. For people who aren't used to what these models look like, can you just tell us in a little more detail? Like, are you solving systems of coupled partial and ordinary differential equations for motion of air, for humidity, for temperature? I mean, what is it that you're actually doing? Is it all in the computer? Is there some other way, an analog of a wind tunnel or something? I mean, how do you do your studies?

16:55So, climate models are numerical models. They are numerical solutions of a set of partial differential equations, coupled partial differential equations. The way I describe them to my students is you could think of seven principles described with seven equations in seven variables. And the principles are conservation of water mass, air mass, conservation of energy, conservation of momentum in three directions, and the state equation for air. the ideal gas law. So those are seven principles. You describe them with seven equations and you solve for seven variables which are pressure, temperature, humidity, density, and wind is three directions.

17:43So you have seven equations, seven variables. You solve them on a sphere for the atmosphere. You solve a similar set of equations for the ocean. You couple them. When you come then to describe impacts on things like disease, agriculture, and others, you have to develop other models that are tailored to describe those phenomena in accurate ways. Always you develop the models and you test them against the past climate. So we have a lot of data for the last number of years, at least 30 or more years. And we test the models against those. We calibrate them for those conditions that had been observed before we use them to project the future.

18:31And this process is an ongoing process. We are always coming up, not just my group, globally, scientists are coming up with better models. They are better representations, improving on their accuracy and they're reducing the uncertainty. And that's an ongoing process that happened in the past and will continue to happen in the future. I expect that you wouldn't just run one model at a time. You must have an ensemble because of parameter uncertainties. Is that right? Absolutely. There are like 40 centers around the world that are running models all the time. Each center has several models. And each model is run different simulations.

19:11So an ensemble of realizations for the same phenomena. It's a very sophisticated modeling system that had been developed through the work of hundreds and thousands of scientists. It's a very interesting and very exciting experiment and experience that had been performed by the climate modeling community for all these years. Rooted in mathematics, in good mathematics and good physics. Well, I have a lot of questions. I mean, seven parameters actually doesn't sound like a lot. In saying seven, these are fields, right? It's a velocity field and a humidity field and a temperature field across the whole sphere.

19:55So space is in there and time. These are spatiotemporal fields, seven coupled partial differential equations, all in space and time. So you can look at the global scale. You often hear about global climate change. But one of the really special things about Al-Fatih is he looks at a scale that is very regional and sometimes even local. Like, you know, what's the weather in Manhattan? Or what's the climate over Massachusetts or in Sudan or the Nile? And that's a really interesting math problem as well as important climate problem. Yeah, I was intrigued that he was saying, look, of course our models get ever better.

20:35And there are all of these centers. We have lots of different ways of modeling to try to have some kind of resonance, some way of improving those results. But it's still such a tricky problem. It's just so complex. And you study chaos, so you know how delicate this is. Yeah, that's an interesting issue. He's very aware of chaos theory, as everyone is who studies both weather and climate. But we're going to hear more from Elphati Elta here about climate change. So stick around and we'll be right back.

21:25Welcome back to The Joy of Why. We're here with MIT professor El-Fati Al-Tahir, and we're discussing climate models and the changing climate itself. For people who haven't had a chance to visit the Ethiopian highlands, I'm being one of them, it sounds like a beautiful part of the world. It's very green. It's a plateau. I mean, you tell us a little about it. Then you describe it, if UPA versus the surrounding countries, which is Somalia and Sudan, if UPA is elevated, the temperature is significantly cooler, and the rainfall-producing mechanisms are much more efficient. if you think about it, fundamentally from the geophysics, the mild water flow for all these southern spheres is because of the existence of that geological formation.

22:17It has also a lot of implications later on in our studies to impact of climate change and variability on things like crop production, but also things like disease transmission, mainly vector-borne diseases like malaria and dengue and so on. Again, a topic that I found myself teared towards studying and putting time on, mainly driven by my own experiences when I was living in that part of the world. Well, you mentioned your own experience. Did you yourself ever suffer from malaria or people in your family or friends? Actually, I have a story. When I was a student in the University of Khartoum, in Sudan, I experienced many incidents of being infected with the malaria parasite.

23:02And I used to go like spend time in the university and then by the weekend I would really catch malaria. I go home. My sister is a doctor and I bring her the injections and she gives me the injections. So the first thing after I got my tenure at MIT, I had a friend of mine who was a professor at the Harvard School of Public Health. I said, can we collaborate on this? We do a lot of good stuff with very sophisticated climate, hydrology modeling. can we bring in that into the study of malaria? He introduced me to a colleague of his and we started working together. We collaborated as a team of hydrologists, entomologists, and medical scientists, and we were able to develop very sophisticated models of how climate variability and climate change impact malaria transmission in African villages.

23:52And the thing about it is it wasn't just modeling work because we extended collaborations with Harvard, with Spastan Institute, they have local institutions in Africa, in Niger in particular, where we were able to collect a lot of very detailed data in the environment of villages in Niger on things like rainfall and temperature, but also on mosquitoes, numbers, different species of mosquitoes, but also data on prevalence of the parasite in the blood of children and so on. It's a very detailed model. I think one of the most sophisticated models of disease transmission. When COVID hit society years ago, I wish then that we had a model of the transmission of influenza or these viral diseases of the same level of sophistication.

24:43What we discovered is actually very, very interesting too is a lot of the climate studies will come with these like negative projections. what we learned is that in Niger, climate change is not likely to worsen the situation. The situation is not good, but climate change is not going to make it worse, which later on was not the case when we looked at the Ethiopian highlands. As the climate warms up, that opens the Ethiopian highlands for malaria, subjecting a population that has very little immunity to this disease. Is the work on the Nile and malaria a kind of case study in what the rest of the world can be expecting as far as impacts of climate change on disease transmission, on some social factors or whatever?

25:34That's an interesting question. So after I did my study on malaria, I had the opportunity to do research in Singapore. I shared with them that, you know, we have done this work on malaria in Africa. and it may have some relevance to a disease that's really a big problem in Singapore, which is dengue. They were a little bit skeptical initially, but we finally actually made significant progress in understanding the environmental side of how dengue is transmitted in Singapore. Singapore is a country with a very efficient public health system, very well funded, very well managed, But they're having a problem with dengue.

26:16They have houses that have been visited by the public health specialist, and they found evidence for the breeding of the Aedes mosquito, the mosquito that transmits dengue. In the case of the malaria, it's the Anophilus mosquito, which is a different species. So, a graduate student of mine made a discovery that when the Singaporean authorities went very strongly against the Aedes mosquitoes breeding inside homes, The mosquitoes were smart enough to find the niche where they would breed outside the home in small drains. And that's why their management of the problem had not been as efficient as it could have been otherwise.

26:56After a few years studying dengue in Singapore, I came to the U.S. and I was approached by the Department of Public Health in the state of Massachusetts because they have concerns about the mosquitoes that transmit dengue are creeping into the state of Massachusetts. And my main advice to them was the time to go after dengue is not when dengue flourishes and becomes a full-blown disease. When these mosquitoes are struggling to establish themselves, that's the time you go after them vigorously so that you eliminate them. What we learned about dengue in Singapore could help us develop strategies.

27:36Another aspect I could talk about is the regional climate of the American Midwest. If you look at the Midwestern U.S. and you look at the summer climate, you don't see warming of temperature as happened around the world, but you see actually cooling. And we attribute that to the changes in land use and land cover, the development of agriculture, more intensive agriculture, development of irrigation that happened during the 20th century, and increasing the rates of evaporation, the cooling process. Evaporative cooling is very efficient. And so as a result, we develop better understanding of the regional climate of the central United States, an understanding that should help us interpret the trends of global warming that are happening around the world.

28:23Well, those are great examples. And I think you're making the case pretty clearly that the lessons we learn from studying places like the Nile or Singapore are very relevant to things that may be happening closer to the U.S. Taking that as now established, let me just ask a little bit about some social factors as revealed in some of your studies. Things like population growth, increasing agricultural needs. Do you want to talk about some of that? But yeah, that's actually an important topic, I think. In a recent book, I coined a term describing an equation. I call it the equation of the future of Africa.