 For about four of the last five years, this was the view out my window in Uganda looking out at the rain Often falling on this forest here in the middle of a continent in Africa thinking about where does all this rain come from? It kind of plays in your head when you're looking out the window. Where does this rain come from? We're in the middle of a continent. Yeah, how come there's so much moisture in the air? So a good question. Do forests attract rain? So I'll try and explain why I'm enthusiastic about this question It's not very hard to justify an interest in water. Of course, water is absolutely basic Do you want to move around a bit? I'm worried people can't see if I'm waving my arms there Water is absolutely essential for lives and for livelihoods. Life on the planet requires good water supplies Declining access to water is associated with as a threat to food security and of course the human health You need water to live well. If you look up the official statistics There's been 11 million people killed in drought, 7 million people killed in floods in the last century Or just a little bit more. And many, many people, 5 billion people needing emergency aid So reliable access to water is a really key thing, whatever you're interested in If it's the environment, if it's people, if it's livelihoods, if it's food security, it's fundamental So bringing this back to forests, what do we know about forests? We know that they're a major source of atmospheric moisture You can read the literature and find out that deforestation is implicated in a declining amount of cloud cover A decline in reliable rain and in various parts of the world there are good correlation studies Showing that a declining forest cover leads to a declining amount of rainfall I should say that I know I'm talking to a kind of a cross-disciplinary group So if I start using jargon or I start talking too quick because I've had too many coffees this morning Which may have happened, do interrupt because I'd like it to be informal If I see that people are lost then I'm not doing my job properly So please interrupt So what do we know a bit more about forests and the evaporation? We know that a closed canopy tropical forest will generally evaporate amount of water Which is equivalent to about 1 to sometimes even more than 2 metres a year So this is a huge amount of water, so you can imagine everything flooded in 1 metre depth of water And that's the amount of water going into the atmosphere from most tropical forests every year Huge amounts We know that forests are much more efficient at evaporating water into the atmosphere Maybe around 10 times what you would get from grassy or shrubby vegetation And even more than you would get from open water So if we talk about a lake versus a forest we're getting about twice as much from the forest per unit area as we are from the open water Why is that? Well if you really look at a tree it's kind of a machine for evaporating water It really has this high leaf area that all these things hanging out on the ends of the branches These leaves, biologists here, leaves on the end of the branches You have these tall things sticking up to carry it in the air It's the same idea why you hang your clothes up on a clothesline to dry rather than putting it on the ground You call it a clothesline effect, your clothes dry quicker flapping around in the wind on a clothesline And the same way that leaves on a tree evaporate water more effectively below shrubby vegetation on the ground It's the same idea So why are trees so special? Well forests themselves have a lot of water in the soil they have The soil that's very good at accumulating moisture These deep organic soils Some experts on that here I think Trees also have incredibly deep roots in some cases So they're able to access moisture deep in the soil profile Even when the soil is very dry on the top They may be able to access moisture deep on the ground To keep this perspiration going in the atmosphere You also find that trees like most other plants have a big store, a big reservoir of moisture in the trunk So it can keep transpiring even when the amount of water it's releasing is much higher than it can pull in through the roots You find that the tree will actually shrink through the day As it pushes the water out, there's a store in there So it can keep transpiring water a long part of the time that herbaceous can And then the stem is replenished at night or gently So it averages it out And you also find that forests are a big source of aerosols That's a technical word for particles Any kind of particles in the atmosphere that can encourage water to condense Now if you want to have clouds, if you want to have water condensing out of the atmosphere It really helps to have impurities in the air It sounds strange but that's what you need And forests are a really good source of these impurities Often it's biological particles And here, for example, an image of the Amazon Basin in the afternoon You see the cloud is starting to form here over the forest They're not here over the ocean so much And a lot of this has to do with the chemistry of these particles that happen in the atmosphere There's a lot of research being done on that Okay, I thought I'd talk you through a few just recent articles that are not so controversial Before I get to the more controversial stuff There's been a lot of work recently in talking about the amount of evaporation from land That returns to land So how much of the rainfall on land is really dependent on moisture that's evaporated from land surface From forests and from other land cover There's a nice thesis on that too Van der Ents thesis about that Saying more than half of the evaporation that comes off land that returns to this rain So we're talking about really considerable amounts of moisture There's also a nice study in nature just a year ago or so That showed that the amount of moisture that's coming off the land surface is really dependent on vegetation Vegetation is responsible for more than 80% and perhaps as much as 90% Of all the moisture that's evaporating off the land surface This was kind of a stunning study at the time Because people before that thought the percentage was much lower Maybe around 20, maybe 25% Now we know that it's much, much more So we're really dependent on vegetation for the amount of moisture That's evaporating off the land There's also been some nice advances in technology In terms of how we can actually monitor this With satellites that can actually look at rainfall Models that can actually tell us where air is coming from And telling us that really rain depends on tree cover Meaning where the wind is blowing from How much forest cover the wind is coming through Effects very much the rainfall you get on a given day Where is the wind coming from? How much forest does it come through? If it's a forested area you will generally get twice as much rainfall As the wind has come across non-forest areas over the last 10 days Am I speaking too quick? I start thinking I'm speaking too quickly Too much coffee, doesn't it? So we know that where the wind is coming from We have good data on this, this is not controversial So I put this picture in here just to remind myself I'm not just talking about super wet forests You get the same results even in deserts That actually forest cover or plant cover It's really the primary source of evaporation Even in the arid parts of the world We're not just talking about wet human regions And that's because trees like this in the Kalahari Desert With roots down to almost 70 metres Are able to access brown water They are a major source of evaporation Even in these really arid parts of the world If we look at the Amazon basin, it's a nice example A lot of people study the Amazon basin We know how much water is coming out of the mountain of the Amazon Every second, one of the estimates here Is 9,000 cubic metres of water every second Huge amounts of water coming out of this region Out of the river Amazon Now if you think about it over a long period of time What comes out has to be going in There has to be a balance So we can think of it that all this water that's flowing out Also has to be flowing in from somewhere Somehow it has to be coming in So we're talking about huge amounts of moisture So how does it get inland? This was my fun bit, Annie She wanted a prize for a question So I read a paper recently Where I could use an article in science Talking about Swedish researchers Who like to quote Bob Dylan tracks In scientific papers So if anybody can name me the Bob Dylan track That's the answer to this question You get a prize How does it get inland? Blowing Blowing in the wind So to actually understand how rain gets inland We have to understand how wind Is actually carrying all this moisture inland It's a pretty basic idea But kind of important to what I'll be talking about So if we go back to how climate scientists look at this We have a fairly simple idea Of how actually wind carries moisture inland Here's a cycle, we call it the sea breeze cycle It's a thermal explanation Basically as land warms up You get the air over the land area here Warming up, the air rises It's the same idea as hot air and blue Spans, it's lighter The lighter air rises As it rises the moisture in the air condenses You get rainfall here Air has to circulate This is a problem to the atmosphere You can't have all the air ending up So it circulates back to where it's cooler And goes from the sea to the cool place You get this cycle as the air cools Comes back, warms, cools Carrying moisture off the sea inland So that's a pretty much the standard idea And it's based on our community's principles So we're going way back to 250 BC This is a fairly basic idea Way back in time I didn't think of asking that as a question So we can also see this at a larger scale The early ideas of how our atmospheric circulation works Is basically the same idea We get an equator in the bottom of the poles So here is where the air is warmer Here is where the air rises Here is where the air drops You get a circulation going like this So most of the rainfall is here in the equatorial zones The air dries As it rises, it rains The air dries, comes back, cools So you get a cycle like that And that's pretty much, even though it goes back to Edmund Haley The guy with the comet, by the way Just at the end of the 17th century Obviously it got a little bit more complicated People like Hadley came along People came along and added the circulation of the Sorry, the rotation of the earth And the formation of the cell systems It's a much more sophisticated idea But it's basically the same idea The engine, the energy in this circulation It's basically that it's hotter in the equator And colder towards the poles It's just a thermal explanation Of how this all operates Not too quick So what do we know about climate from our models? We have these really, really sophisticated, impressive models We can talk about forest loss In global climate models The kinds of ones that the IPCC are using How is it that they represent forests And they have quite nice physical ways Of looking at the lead barrier index Decline, a declining looting depth As you go from forests to conditions For crops or whatever Changing the canopy roughness There's all these different variables But there are really, really big uncertainties If you look at these IPCC reports They're impressive But a lot of the documentation A lot of the discussion is about the uncertainties Because the uncertainties are really big It's really hard to do an experiment on the earth So a lot of this we really don't know Except that we're looking at physical principles If we look at these models And what they tell us about forest rainfall Of course it's really analytical Complex, many unknowns There's lots of strange things going on For example, I just told you about the thermal model But that doesn't explain why the Amazon Is often colder than the surrounding oceans I'm still getting huge amounts of wind going in If you look at models Without actually making them fit And just ask what the basic physical processes Would tell you, you'll find That the amount of water in the Amazon Should be about half what we see So that's the best fit models that we have We're actually not explaining The entire flow of the Amazon In current models There are acknowledged problems So this was a review Last year in science, I'm just trying to Show you this to show that there is an acknowledgement That there are problems with the current views And models So what are climate models missing? This was a review in science by experts Who were looking at the whole idea of these climate models What is it that the modelers still needed to work on? And they said rainfall over land Is largely determined by unresolved processes That's a pretty strong word, unresolved processes And they really highlighted that this was true They couldn't really model rainfall In a realistic way This is the main limitation In current representations of the climate system So these are climate models Talking about their models And they're acknowledging and have this huge problem This is a major roadblock To progress in climate science So they're really telling you that this is a problem They really are acknowledging They don't know how rainfall works That's about how I read it anyway This is a graph from A figure from a paper I did with Daniel a few years ago But the nice thing about this Is just to point out Coming back to my initial question Before I subtract rain, we're about to get there If we look at the globe And we look at transects across forested areas Or non-forested areas Going inland from the coast With the direction of the wind So rainfall generally will be coming in From coastlines with the prevailing wind Direction inland We find that if we look first at the orange sites here These are non-forested areas You get pretty much an exponential decline And this is what you expect The rain falls in an area Some of it recycles, but a lot of it goes away on rivers It drains out to the sea eventually So as you go further and further inland There's less water to recycle So it has to decline in some way These are best fit lines Of course the real date is a bit noisy These are the best fit So as you go inland Here it has to bottom kilometers You get a pretty dramatic decline in rainfall As you go inland But if we look in areas where we have Rainforest or boreal forest up here We get a different pattern We get more or less continuous rainfall This is no observation, it's just a fact But then it raises the question of which came first The forest or the rain So as an ecologist I kind of like these questions It's kind of fun, you know So if I go to Daniel and say Daniel do you understand this stuff So that's kind of how I was getting into this And I was reading the work Of these two colleagues Physicists In Russia Anastasia Makariva and Viktor Bushkoff And they'd written some really interesting papers And a few years ago I guess when I was still at C4 I was reading a lot about different aspects Of tropical rainforest I was writing a book about tropical rainforest And you know you have different chapters on different topics One of the chapters is on climate and weather and stuff So I just was reading a whole subset of different papers And I found some of their work And then I started to say If what they're telling me is true This is really interesting, I want to write about this So I went to various colleagues Who know about hydrology like Daniel And he helped me talk to other colleagues Who know about hydrology To really find out whether what they were saying is true Because they're saying that yes Forests do attract rain So this is a pretty profoundly different way This is not about temperature anymore They'd come up with a way to explain How forests actually play a role In environmental patterns So my role in this Was basically to be the bewildered guy on the sidelines Initially, I didn't know I didn't know if it was true But I do want to try and understand If what they're saying is true And I should say he's a physicist So a lot of this work has been published In really good physical journals This is an example of physical letters A They've been publishing this stuff In physical journals, peer reviewed The physicist accepts it's true The first one I was reading Hydrology and Earth science systems Trying to address environmental scientists But these authors Had a real trouble getting this out to climate scientists And now I just want to tell you a little bit About the controversy kind of as it currently stands I've actually been working I'll jump a few steps here Because we don't have all day to do this talk But I've actually been working with these Physicists to try and make sure That their message reaches the climate scientists For me, it's not a question of Being certain that they're right But if it's a viable theory If it's a viable hypothesis Then we should either be able to test it Or say that it's wrong It has to be something we can validly test Or there's a logical flaw in what they're actually presenting So it's really important in science That we present this to the people who feel Best place to judge it, the climate scientists So I worked with them To actually produce this paper I've been collaborating them with them And in a sense it's helpful that I'm not a physicist And I'm not a climate scientist Because I'm always trying to say A lot of it's about simplifying the message And making sure that we don't just write 17 pages of differential equations But actually give clear English language Statements about what it is that we're saying The problem with working with physicists Is they think the differential equations are the argument But for a lot of us as an audience We say, okay, I don't know I can't judge this But we do have to come up with testable ideas That we can actually bring data to To actually test the science behind this So we have this paper In atmospheric chemistry and physics Which is quite a nice journal Generally they have a pretty quick turnaround On most of their articles, it's a public online system You put up your draft paper, it's public The comments are there in public And we had the debate going on And it was quite controversial It had a lot more comments than normal from outside Generally it would take about three months to be approved This one was hanging for over two years Finally, we had a lot of discussions Backwards and forwards of various people But it was published with an editor's comment Which was a bit unusual And the editor said, in contradiction To common textbook knowledge The handling editor and the executive committee Are not convinced the paper is wrong Okay, so that tells you That they were pretty scared Or they were pretty worried about publishing it All scientific theories are Provisional, they're not right In an ultimate sense But this one is not We're not convinced it was wrong Okay, there's obviously something going on here So what I want to do is fill you in A little bit more on the background Okay, so it's a very basic idea Earlier I talked about the thermal explanation For what's going on in the atmosphere And if you did the ideal gas law At physics at school You'll see the pressure is related to temperature This is the basic idea Temperature goes up, the pressure goes up And the air will rise Because the volume expands This is how we understand it normally But there's also another one in here In the ideal gas law Which is the number of particles The number of molecules in the gas Now what happens when you get condensation You go from having more particles in the air To having fewer basically It's a very simple idea So you also get a change in pressure As the moisture condenses Now there is temperature release There is energy released in this process What most of the debate is about And I'm not really qualified to tell you What's right and wrong But I can say that there's no clarity About what really happens So I'm going to skip over the physics a bit I have articles I can share with you If you're really interested in the physics But I think what's really interesting For this audience is what it really means And if we look at the theory itself What does it tell us It tells us the areas with really high Levels of evaporation Draw winds into them Draw moist winds into them Draw air in And there's also a positive feedback Because if you draw in moist Moist air This is also providing moisture for rain So you can provide more moisture in that point It provides a positive feedback To actually keep the system running So this is really important in oceans With big tropical storms and the likes But that's not so interesting for this audience And on land what are the most powerful As I told you earlier, it's forests So it's really telling us that forests Are playing this really important role In sucking air in So just to try and talk you through A little bit what we think is going on With this new theory, this is not the thermal idea This is the idea that comes from the Russians Anastasia and Victor And the rest of us So we've published this this year What we think is going on If you look at this top one up here You see the ocean, natural forest The ocean and the forest Higher evaporation over the forest The pink here is dry air So we're starting in a position here Where this is a landscape That has dry air everywhere, right? Now the forests evaporate air more quickly So we go to a situation Where this bluer air here Is now more moisture in the air So here maybe clouds start to form Because you're getting more moisture Forming here, there's no wind in this yet There's no wind, we're just talking about Actually on its own This is generating more humidity And of course after a certain point This side starts to rain And as soon as it starts to rain We get condensation happening We get a reduction in air particles Particles in the air the moisture is condensing And then that leads to low pressure here And this draws in moist wind Off the oceans So we're actually carrying air in land This is the basic idea And we've actually looked at this In one day So you can imagine this is the morning No winds And then in the afternoon you get your big thunderstorms This is what happens And if we look at this in the Amazon Basin We look at wind directions And we look at pressure changes I don't have time to go into all the technical details But this pattern more or less fits So just to go back To what does this all mean With this idea where you get the most evaporation You get the wind being drawn towards there You get the lower pressure So here for example on a desert Or maybe an open landscape versus the ocean The evaporation here will be lower than open water So the air will be drawn this way Because condensation will happen here more regularly You'll get rainfall here more regularly The air will move this way Drawing the moisture off the land Because any evaporation coming off the land here Will be drawn towards the ocean If we have enough forest Then the evaporation over the forest Is greater than the evaporation over the open water Because the forest has a more efficient evaporation Per unit area So the pressure here will drop Because we get more regular rainfall And condensation occurring here And that will draw air in And if you look at the physics And I won't go into it But it also allows us to look at the Amazon Basin And say yes this is a sufficient process We look at the physics to actually draw The winds right in so we can actually water The whole Amazon Basin with this mechanism I'm trying to see if people are still awake This is kind of a fun A figure it's a bit of a messy data But of course we can't cut down Whole rainforest or whatever to test an idea like this What one point would be If you cut down the entire Amazon rainforest And make it into a parking lot You would expect to see a very rapid Deciding rainfall as you go inland That would be one way to test it But quite difficult ethically to do that So what we can do is look for parts of the world Where the forest switches on and off Which is going to the boreal systems The forest up in the boreal to the northern Latitudes Get all snowy, the trees more or less shut down There is no moisture travelling Through those trees, the ground is frozen There is no transpiration going on Switched off, you've turned a knob Going from transpiration to off So from having a much higher transpiration Than the surrounding ocean You've switched it off And what we see here Bottom graph here is distance inland This is the zero, this is in the Siberia, as you go inland Obviously there's terrain This affects the data So there's little changes in the landscape That have an effect on rain too But as you go inland During the summertime You see not much change, maybe a little bit A decline, it's a bit noisy But in wintertime we see Well I think it's reasonably convincing But anyway, we can do this many times In different places, but basically the pattern Fits what the theory would say When the forest is switched on You get much better transport inland When the forest is switched off You get much more rapid decline In rainfall versus the ocean So I don't really have time to go Into everything on this So just point out that This theory has certain evidence To support it I think This idea that rainfall declines Into interiors when there's no forest But not when there is forest It actually resolves this problem Of the paradox And the idea that runoff is I think I've lost something there The idea that runoff in models is too low Because we actually have a different mechanism That explains why a cold Amazon Still draws moist air in and produces High rainfall It explains why we get a decline In the effect over the boreal forest In wintertime And there's a whole list of other things Including power estimates of global circulation If you're really interested in this stuff Okay, so Probably if you're going back to my original question When I started coming to Daniel all those years ago About six years ago now saying Daniel Do you believe this? And going around different people who actually know about this stuff Whose opinions I respected Does this make sense? Because if it does make sense Then it's really important So of course I got various different responses from people And I've had a lot of email communication With climate scientists and the stuff And one of the main responses initially was dismissal No, no, no, no Hundreds of years, you know, we have tons of data We have incredibly complex models It can't be right So that was one response And of course that's a bit frustrating But in a sense it's also exciting if you think Well, maybe this isn't right Maybe the dismissal is Then you say to them have you looked at it Can you say where it's wrong? And people are often just saying well I'm busy So it's hard to get a good reason But a lot of people were dismissive Other people said yeah, yeah We've known about this for a long time We've known about this for a long time It's a tiny effect, it's not important And some of them even said it's in the models already At this point I'm starting to get a little bit curious Because I think well, they can't both be right Some people I talked to were kind of cautiously interested And I think that was a kind of made sense You know, people who are saying well the physics looks to be right It's in these physics journals, I can't see a mistake Yeah, it's interesting It seems a reasonable response But there were people in that category Even a small number Accepted that it's true, I think particularly Environmental kind of groups It's actually quite exciting to have this whole new Potential for arguing for forests That it actually really makes this huge difference To regional rainfall So I think particularly amongst environmental people They want it to be true, you know This is an amazing story if it's true Forests are really generating this huge amount of rainfall Are really necessary for maintaining rainfall A lot of discussion right now Is people who've accepted that it's true But are arguing about the magnitude How important is it? It's not that the thermal argument is wrong The temperature argument is not necessarily wrong But how do we balance this? If we don't have models that take it into account It's very hard so far to really talk about How significant this forest effect might be So I think that's where we are right now Is on the magnitude So just to come back To why it's interesting hopefully for you One aspect of this theory that's really important Is to highlight That we need forests to maintain Rainfall in continental interiors So if we're like living right in the heart Of the Amazon basin and people start Deforestation all around the Atlantic coast You could be in trouble with your Rainfall, your rainfall is likely to decline And it's even possible According to the equations We're just talking equations here It's even possible to go from a situation To a harsh rainforest to a more or less desert Just by switching Because there's a positive feedback involved We can go from a system that sustains itself With a positive feedback to a system where we Switched it off, I think that the positive feedback Is lost, we have no rain there anymore So this technically is possible So that's a pretty big risk I think for this audience there's also Real opportunities If you look at the equations it tells us That if we actually could plant forests Not the Amazon, across the Sahara That there would actually be Enough Power drawing in wind To actually water Those forests, you wouldn't need irrigation This is what the equations say, I'm not saying it's Necessarily true but it's an intriguing idea Or at least you wouldn't need irrigation everywhere all the time As long as you think about the right places To grow, the right weather systems to work with We could potentially use a system Like this to green the deserts Of course we would have to do it on a big scale For climate systems, so the magnitudes There are sort of hundreds of kilometres across We can't just do a few hectares and expect it to water itself We're talking about expanses Of hundreds of kilometres Earlier I was saying it's a whole new Value for forests, of course if this is true Then it means that people who live in the middle Of continents have to start talking to people Who live on the edge of continents Going back to where I was in Africa in Uganda Looking out the window at mountain grillers And rainforests, if the people in Kenya Or the people in Congo lose all their forests In the mountains in Uganda may also dry out So it provides a whole new regional value That we have to start thinking about Yeah, potentially And of course a lot of what I'm saying is Hand-waving, I mean I think it's really Interesting, it's really profoundly important But we don't really know that much about it We don't know that much about the ecology What the implications are for the management of forests When is it that you would lose these properties How do you maintain them? Huge policy implications potentially And this idea that you are dependent On one place for rainfall in another So I think there's Yeah, a lot of opportunities there I was going to stop now Because I didn't want to go on too long But you're welcome to talk to me more if you're interested We have a lot of publications or I have a lot of publications My own but also my colleagues Physicists, if you like the technical stuff I can share those too I do have a few more slides at the end If you're interested on the arguments That people put forward but I think I've already Yeah, enough people are Paying attention, yeah, okay, we'll see I'm ready for questions, thank you