How hot could the planet get?

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We constantly hear how we've experienced record-breaking heat. The hottest day, the hottest month, the hottest year. 2025 was 1.4 degrees Celsius above. industrial levels. That's the third highest temperature since records began. More carbon dioxide in the atmosphere And so on the climate question from the BBC World Service, we're asking how hot could the Earth get? I'm Jordan Dunbar.

Our climate expert is Professor Kate Marvel. Hi, Kate. Hi there. So excited to be here. Kate, I'm just gonna go through your credits. You may want to prepare to bow or indeed blush because you're a climate scientist. an astrophysicist, a cosmologist, as well as a world expert in clouds. Yeah, so the sky actually isn't the limit for you. Sorry, how to do it. We've got loads of questions for you, especially about your new book, The Nine Ways to Feel About Our Changing Planet.

But first I really wanna dig into the science of climate change and how hot our planet might get. Because indeed you work at NASA. So

For this, we use climate models. Kate, what are climate models and how do they work? So a climate model is basically everything that we know about the physics and the chemistry of how the world works. translated into equations and solved on big supercomputers. So you can think of a a climate model as as just a collection of equations. But you can also think of it as very literally a world-building machine. Climate models run on stories.

you take a story. So for example, the story of recent history, and you translate that story into emissions of greenhouse gases and aerosols and other pollutants. and you tell that story to the climate model. And the climate model will spit out a world. It'll tell you what the temperature is, what the rainfall patterns look like. And this is really useful because you can tell the climate model counterfactual stories. You can say, what if we never existed?

What if we don't check our emissions? What was the world like back in the last ice age? And it will spit out the world. And so that's the way that I like to think about what climate models are showing us. And where do you get all of the let's go like characters for those?

climate stories, do they come from the past? Where do you get the ingredients for those to make the predictions? Oh, it's such a good question. So I am a total boring pedant on this. Climate models don't make predictions. They make projections. And what that means, the really crucial difference is predictions are saying this is what will happen. And a projection is conditional. A projection is if we do this, then this will happen.

And so when somebody says climate models predict that in the year twenty one hundred everything is gonna be completely apocalyptic, or climate models predict that in the year twenty one hundred everything will be fine. Those are conditional. Those are conditional on the stories that are told to the climate model. So for example, in that future, that apocalyptic future, that is conditional, that's a projection conditional on human beings not getting emissions under control.

I know that models broadly say that the more of these planet warming gases like carbon dioxide there is in the sky, the warmer our planet. gets. But it's actually a very complicated world that you're having to build. And there are some things that might accelerate climate change, but also there are other things

uh mechanisms, natural feedbacks that might be putting the brakes or even reversing climate change. Could you talk us through some of those processes, like plants say for instance, and how that works in a very complicated model like the ones you were describing? So my job, if I could describe it really succinctly, is to figure out how hot it's gonna get.

And when you say, you know, we don't know how hot it's gonna get, people are like, What, climate scientists? Like you guys had one job, you know, come on, how how you don't know that? And the reason, one of the reasons that we don't know exactly how hot it's gonna get is we've never done this experiment before. We've never kicked the earth this hard. And so we don't really know exactly how it's gonna react.

We do know because it is such basic physics that we have known since the 19th century that putting a bunch of greenhouse gases in the atmosphere increases the temperature of the planet. And the primary greenhouse gas that we're emitting is carbon dioxide. So about half of the carbon dioxide that human beings put in the atmosphere gets taken out of the atmosphere by stuff that photosynthesizes, basically plants and phytoplankton.

And the thing is there's no guarantee that they're going to continue doing that into the future. It's really hard as a forest to take carbon dioxide out of the atmosphere when you're on fire. So we know that as the climate changes, the ability of stuff to grow on the earth's surface is going to change. And what that means is that more of the CO2 that we emit in the future might be staying up there in the atmosphere, warming the planet.

So we don't know how much of our CO2 is going to stay in the future atmosphere. And a lot of that is because we don't know what plants are going to do. But then we also don't know how that CO2 is going to warm up the planet. And that's because we don't exactly know what kind of changes to the whole earth system that's going to provoke.

So one of the big wild cards there is clouds. We don't know how clouds are going to rearrange themselves in order to speed up or slow down the warming. We are fairly certain, though, that there is no kind of natural emergency break. our best estimate of what plants are going to do, what we call carbon cycle feedback. Is that that's gonna lead to more CO2 in the atmosphere? And our best estimate of what clouds are gonna do is a destabilizing feedback, something that makes global warming work.

And is that why you have so many different climate models? It's not like you just have a few, you have lots of different ones to try and bring in all these variables. Completely, yeah. So when we think about a climate model, which is just like a computer representation of reality, right? If that is completely perfect, if that is able to exactly represent reality.

literally the matrix, right? And that's impossible and that's creepy and and we don't really want that, right? And so climate models, like all scientific models, are necessarily approximations of reality. We know how clouds work. But the thing is clouds are formed when little tiny water droplets or ice crystals coagulate around dust grains or or little bits of sea salt or or whatever.

And in order to get that completely accurately, you would need a computer that is capable of tracking every single little dust grain and water droplet in the entire atmosphere. We don't have computers that powerful. And so as a result, we have to make kind of approximations. We have to say, when you know this happens, clouds do this.

And there are different climate modeling groups all over the world who are all making kind of slightly different choices, different physically defensible approximations. And when we compare all of those different climate models together, we get kind of an understanding of what is really solid, what we completely get, and what there's still uncertainty in. You've mentioned clouds a couple of times there, Kate. Why? Why are they so important here?

Clouds are I so I wish I didn't have to think about clouds. Um I am a Californian. I love perfect weather. I get really mad when the temperature drops below twenty degrees Celsius. Come to England. Clouds in the sky. Um so I don't wanna have to care about clouds, but I kind of have to. And that's because they're incredibly important in regulating the climate system. So the thing about clouds is that they have this dual effect.

They both cool the planet by blocking sunlight and they warm the planet because they have their own greenhouse effect. Right now the unit that scientists use to measure kind of the energy imbalance of the planet, how much it's out of whack, is watts, like in a light bulb per square meter. And our best estimate of what happens when we double carbon dioxide in the atmosphere, that's around four watts per square meter.

When we look at what clouds are doing, clouds are blocking, are cooling the planet by about 50 watts per square meter. At the same time, they're also warming the planet by about twenty five watts per square meter. So they do two things at once. They're incredibly powerful.

And they're really, really difficult to get right in a climate model because it's this scale mismatch. It's the fact that you have to understand what's happening at the very small scales, but then you also have to get what's happening at global scales.

So what do those projections tell us then about how hot the Earth might get? So I would say the number one thing we can take away from that is actually the biggest uncertainty in how hot the planet will get is what human beings are going to do. basically what we call the story about what human societies will choose to do, whether we will choose to increase our emissions, keep them flat, or decrease them. That is the largest reason that we don't know how hot it's gonna get.

But even when you just look at the physical science uncertainty, even when you just look at what we know about the physics of how the earth will respond. We are fairly certain that the net aggregate impact of these what we call cloud feedback. is going to be destabilizing. It's going to make the warming worse. Our best estimate of what plants are going to do, these carbon cycle feedbacks, is that's going to lead to more CO2 building up in the atmosphere. That's going to make warming work.

I feel like I'm in this really unique position as a scientist because, you know, I have this research question, how hot is it gonna get? And I don't want to know the answer to that research question. I don't want to do that experiment. I think we shouldn't. But Kate, if I may, push back a little bit on that. Surely we need to know the trajectory we're heading in in order to adapt.

Oh absolutely. You know, I am I am very pro science. I am very pro doing science, figuring out how the earth works. And I think you're absolutely right that that is relevant no matter what the future looks like, because no climate change is unfortunately not on the table. We're almost certainly going to blow past the 1.5 degrees Celsius target. We're almost certainly going to warm far more than that.

And even if we were to stop emissions tomorrow, the temperature of the planet would probably stabilize, but the ocean is not done catching up. So there is a lot of baked in sea level rise already. And so you are absolutely right that it is completely crucial to understand the physics of the Earth system, the processes that are underlying that.

in order to make decisions in the future. What I was saying is that worst case scenario, I don't want to do that experiment. I don't want to have experimental evidence of what the world looks like under the worst case scenario.

I just wanted to get in my head a couple of things straight from what you've been talking about. There's uncertainty because of all these different variables and how complex it is in these projections. Not predictions. Projections that we're making in the future. Never predictions.

But can you talk just very simply about the certainty we have of how the planet is warming? So I want to be very clear that uncertainty is not the same thing as ignorance. We don't know everything, but that does not mean we know nothing. And as a scientist, I like to talk about uncertainty because that's where my head is all day. That's my job. If we figured out all of science, I would have to find something else to do with my life. And I don't want to do that.

So there is uncertainty, but you're absolutely correct that there is complete and total certainty in some things. The fact that carbon dioxide is a greenhouse gas, methane is a greenhouse gas. and greenhouse gases warm the planet. This is incredibly well established science. We have known this stuff since the 1800s.

And the science has only gotten more solid from there. So I think we need to be aware that uncertainty is often weaponized against us. People often say, Oh, well, the the science is unsettled, therefore we don't have to do anything. The science does have big uncertainties in the future. You know, we've we've never done this experiment before, we don't know exactly what's gonna happen. But we are more sure that greenhouse gases cause the climate to change than we are that smoking causes cancer.

A reminder that you're listening to the climate question from the BBC World Service. Now's the time for a great deal on a new Honda. It's time to take With rugged capability and commanding style. It's time for powerful performance, plus, plenty of room inside. Start your journey in a brand new vehicle. Check out the Honda Ridgeline, pilot, password, and the phone. or C R V. See Dealer for financing details. If journalism is the first draft of history, what happens if that draft is flawed?

In 1999, four Russian apartment buildings were bombed, hundreds killed. But even now, we still don't know for sure who did it. It's a mystery that sparked chilling theory. I'm Helena Merriman, and in a new BBC series, I'm talking to the reporters who first covered this story. What did they miss the first time? The History Bureau. Putin and the apartment bombs. Listen on bbc.com or wherever you get your podcasts. I'm Jordan Dunbar.

Just to let you know, if you'd like to watch this episode or any of our others, you can find them on the BBC World Service YouTube channel.

Today, Gray and I are chatting to climate scientist Kit Marvel. Kit, I'm fascinated because we've been talking about climate modeling and variables and data. But you've written a book called Human Nature, Nine Ways to Fail. About our changing planet. Why did you want to link emotions with climate science? Because to me, one is so data-driven and kind of rational.

Yeah, I was really uneasy for a while because I did feel things about the earth. I have a conflict of interest because I live on the thing that I study. And I do care. I do care what happens to it. But for a long time I thought, oh, I shouldn't have these feelings because I'm a scientist. I'm rational. I am data driven. And as scientists, we are trained to say, oh, we don't have any feelings. We don't have any emotions.

Because supposedly that is supposed to make people believe us more if we are completely objective scientific robots. And then it kind of hit me. I realized that, oh, pretending we don't have any emotions, that doesn't make us more objective. That makes us liar. And I don't want to be a liar. And then I kind of realize that, oh, there is no tension between getting the science right and having feelings about what you're seeing and what you're studying.

you can feel sad or joyful or angry and at the same time you can get the math right. There's no tension for me between those two. And I think when we talk about climate change, oftentimes just talking about the data, just talking about the equations and the graphs. And don't get me wrong, I love equations. I love graphs. But we scientists have this tendency to be like, oh, just one more graph and and people will believe us.

And that's not how people work, right? People respond to emotions, people respond to stories. And so what I wanted to do was talk in in that sort of language in the hopes that it would resonate with more people. What was the turning point for you? Why did you suddenly decide, okay, I've got to do this, I've got to talk about this as well?

Um, I think one of the turning points was having a kid. When you're a scientist, it's very easy to compartmentalize. It's very easy to study climate model projections. of what's going to happen in 2050 because that feels very abstract. That feels very far away. And then when you have a kid and you realize, no, 2050 is his adulthood. then things start to become really concrete in a way that maybe you knew intellectually before, but now you feel very profoundly.

I think a lot of times, and I'm I'm sure you guys have run into this too, a lot of times climate coverage is really designed to try to make you feel one thing, right? You're supposed to feel really freaked out or really, really angry. or just really, really sad at at what we're losing, or maybe hopeful that there are things that we can do about this. And I got really frustrated at the implication that there was only one thing that you could feel about climate change.

Because fundamentally, this is the whole planet changing. This is everything that we know, everything that we love. And how on earth can you have only one feeling about that? That's that's really impossible. And so sometimes, you know, people are like, How do you feel about climate change? And it's like, oh, I don't know. I feel all the feelings and I feel them all really strongly.

In the book you've written about grief, could you tell us about some of the things that you've seen change and that you are grieving? Yeah, so I live in New York City now, but I am a Californian by birth and my heart is in California. I lived in Northern California, which is objectively the most beautiful place on the planet. But when you talk about California and climate change, what comes to mind? It's change, it's destruction, it's fires.

One of my most beloved parts of California is the sequoia groves. These trees are incredible. They have seen so much. They have been around for thousands of years. And we tend to think of them as indestructible. And they have been indestructible for their entire long history. They're very resistant to fire. In fact, they they need fire. Fire helps them grow, fire helps them spread.

But now the fires are changing. The summers are hotter, the winters are wetter, and there's more fuel and there's more fire weather so that the fuel can go up in flames. And this is creating conditions that make it very hard for these things that have survived for centuries and centuries to continue into the future.

And so knowing the connection between what's happening in the atmosphere, which can seem so far away and so abstract, and just seeing how it is affecting these things that you grew up thinking would be there forever. is very quietly devastating, I think.

Do your bike is about nine emotions? Can you talk me through those nine emotions. So I wanted to start out with wonder because that for me is the emotion that I feel the most every day. I'm a scientist. It is my job to find out new things about this planet. I used to be an astrophysicist. I used to be a cosmologist. It was my job to study the entire universe. And then the more you know about the universe, the more you realize how much most of it sucks.

All of the other planets that we know of are complete garbage. And the earth is the only good place in the universe. And so just realizing that and realizing how beautiful and perfect this planet is just sort of fills me with a sense of wonder every day. And then the second chapter is anger, because you see the world changing, you know how well established this science is, and you see people lying about that, and that makes me angry.

And then I talk about guilt, I talk about uh fear, I talk about grief. But then I also wanted to talk about maybe the more positive emotions. So surprise, pride. both in the sense of hubris, but also in the sense of we know what to do. And there is, I think, something important in feeling pride and the job well done.

And then hope, an emotion that I think all climate people have really complicated relationships with. And then I felt very strongly about ending the book on love, because that is the reason that any of this matters.

Can you talk to me about that complicated relationship you have with hope? What is it that's giving you? Yeah,'cause I was gonna say a lot of people look to the future, don't they? And they have fear, they have anxiety about what's to come. But hope, yeah. Is that what you see when you look to the future? We know what to do. And so if we want to avoid the worst, if we want to build a better future, we know what to do.

I actually find a lot of hope and comfort in how solid the science is. We know exactly what is causing climate change, and that means that we know exactly how to stop it.

Mae'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn sy'n yw'r hyn. Yeah, so um from a you know physical science perspective up in the atmosphere, it's really simple. Climate change is caused by greenhouse gases. Greenhouse gases are emitted by humans to stop climate change, stop doing that. Now, obviously, that is much easier said than done. But I think it helps to zoom in and to look at.

where we are where those emissions are coming from, right? Those emissions are coming from the way we get around, from transportation. They're coming from the way that we generate electricity. They're coming from the way that we use land and and farm and make our food. They're coming from the way we heat buildings and and cook that food.

And they're coming from the way we make stuff, industrial processes. And when you break it down to that level of granularity, you start to realize, oh, there is a solution for nearly every part of that puzzle. Electricity. Well, we have different ways to make electricity. When you think about fossil fuels, what they really are is solar energy, but stupid. So you can either

make electricity from solar energy directly with solar panels, or you can use that solar energy to make a plant photosynthesize. Eventually that plant gets eaten and the thing that ate it dies or the plant dies. And it's got the remnants of that last meal still in it. And then you wait hundreds of millions of years and then you dig up and you set it on fire. So we can either use sunlight in that really indirect way or we can use sunlight in in a direct way.

So we have solutions for generating electricity. We have solutions for transportation, so public transportation, electrified transportation, and also creating more walkable cities and areas. And I could go on and on and on, but for every single aspect, every single area where emissions come from, we have the vast majority of the solutions that we need to do that differently.

Kit, lots of people listening will think How can you be hopeful right now when politically we've seen governments pulling back on reducing climate emissions, corporations pulling back, they're no longer talking about, you know, moving to green energy, et cetera. And just the state of the world. How y how do you remain hopeful when all of those things are happening? You know, I think if I had to write this book all over again, given what's happening now.

I would double down on the hope chapter. I would write another chapter on hope. Because I think it is really important to keep that motivation, partly because. The world has changed so much since the beginning of my career. If you had told me when I first started in climate science that eventually solar, it won't be a boutique niche energy source for hippies. It's gonna be the cheapest way of generating electricity in all of history.

They're gonna be cheap electric cars and they're gonna be better than gas powered cars. Battery storage prices are gonna be plummeting. If you had told me all of those things, I would think, no, no, no, there's no possible way that that can be true. That's way, way, way too optimistic. It remains true that

The climate is a motivating issue. Maybe you have to talk about it in different ways. Maybe you need to find different language. Maybe you need to really get at people's emotions as opposed to just hammering them over and over with the facts. But people still care about their lives. People still care about the planet that they live on. Maybe not when it's put like that, but when you think about climate change, it is essentially

everything that everybody cares about, all wrapped up together. Professor Kate Marvel, thank you so much for joining us today. Yeah, thank you so much. Really appreciate it. Well thank you so much. And thank you for listening. If you have a climate question of your own, then email the climate question at B. We have a listener show coming up, so get them in now if you

Question answered. The production team were Nick Sindle, Diane Richardson, Simon Watts, Philip Bull, and Tom Brignall. I'm Grey Jackson. And I'm Jordan Dunbar, and we'll see you next week. If journalism is the first draft of history, what happens if that draft is flawed? In 1999, four Russian apartment buildings were bombed, hundreds killed. But even now, we still don't know for sure who did it. It's a mystery that sparked chilling theory.

I'm Helena Merriman, and in a new BBC series, I'm talking to the reporters who first covered this story. What did they miss the first time? The History Bureau. Putin and the apartment bombs. Listen on bbc dot com or wherever you get your podcasts.