 Okay welcome back everybody. The final speaker today is Steve Bacala, the Frederick D. Petrie Professor in Ecology and Evolutionary Biology at Princeton University and the former director of the Princeton Environmental Institute. The title of Steve's talk is The Interplay Between Human Interactions and Earth System Responses. Over to you Steve. All right thank you everyone. The title I was given is The Interplay Between Human Actions and Earth System Responses and I've added constraints on natural climate solutions. There I'm going to, I took this mandate quite broadly and I'm going to talk about all kinds of global feedbacks. Now the first thing I want to do is, is to just make very clear that I'm not going to be talking about stopping tropical deforestation even though I think of that as part of natural climate solutions. That's a win-win-win-win-win right? It's good for biodiversity and it's good for the climate and you can see there's five and a half gigatons of it per annum and that's just all good. But it's also true that however problematic natural climate solutions are, they're pretty essential if we were to try to use existing technology to solve the carbon and climate problem by going all the way to net zero. The figure on the left there is a one of many, many possible figures showing a trajectory to net zero. This is about a two-degree scenario and it's one from UNEP and the blue underneath is negative emissions that you need. Of course negative emissions at scale the only option we have currently is the set of natural climate solutions and these have got to be one-tenth to one-fifth of current emissions that must be negative at mid-century and that's true whether or not you look globally or you look in the United States. And so however limited natural climate solutions are we really do need them right? So as you've heard in many previous talks, many have estimated enormous potential. All right the top bar there from the Griscombe et al study goes all the way to 10 gigatons of CO2 per annum. If you look in the fine print that requires that people globally become almost entirely vegetarian but nonetheless it's a big number. The second paper listed here is the Bastian et al paper in science that said we could probably reforest an additional billion hectares of land using natural climate solutions and it admittedly all these are offered with caveats right? So Bastian et al said this was an upper bound and Griscombe et al said this was an upper bound but these are the numbers that stick in people's minds. We've already heard about indigo ag and the Taratun initiative. David Perry's famous video on which he's talking about storing a Taratun in agricultural soils. I've got a different take on indigo at the end that comes from just the last few weeks though that I'll share with you a little bit later on in the talk. I think there's some real opportunity there. Okay so I'm not going to cover permanence, additionality and leakage even though I know they're important but it was because I knew everybody else would or a lot of other speakers would talk about them and I will just kind of dog paddle over the deep waters of biophysical feedbacks that might limit natural climate solutions, competition for land among forest planting, food production, biodiversity preserves and biofuels, links between food prices suffering and violence, incomplete knowledge to realize the full potential for negative emissions by agricultural soils and this recapitulates a lot of what David Lobel just said, this number four in particular. Number five, the potential danger of popular net zero policies like an economy-wide price on carbon and safe levels of natural climate solutions deployment given current knowledge. So let's start with biophysical feedbacks. The sort of most famous one I think of is due to a guy named Gordy Bonin at the National Center for Atmospheric Research but it actually traces much earlier than that. One of the things that people don't talk about much because it sounds dangerous to talk about is that if you take a climate model and all of a sudden snap your fingers and remove the boreal forest, the planet cools a lot and it's because you turn the surface from kind of dark green to white over large portions of the air and that white surface reflects a lot of energy back into space and it gets a lot colder especially locally and especially in the spring. All right it gets way colder and this is a reflection of the fact that boreal forest if you do forest planting in the far north it actually warms the planet from this change in reflectivity more than it cools the planet from absorbing carbon dioxide so that's a big biophysical feedback that limits where we can can do forestry. These forces sort of balance not completely but they're closer to balanced in the temperate zone and the tropics it's all wind they absorb CO2 and the strong evaporative cooling dominates there. These kinds of climate effects go beyond the boreal forest but not really at global scales so that there are two other scales in which these have been talked about. Turns out that if you do a half continent scale push for reforestation, same is true incidentally if you stick up wind turbines on a half continent scale you all of a sudden have when the forest starts a change in the height of the stuff that's interfering with airflow on the planet and that causes the air to tumble and it sets up climate anomalies that are can think of them as a degree Fahrenheit cooler and warmer on either side of this divide that extend quite a ways and so there are these sort of regional effects that come from continental scale large-scale reforestation efforts that are on the order of a degree Fahrenheit as a maximum and there are also implications for rainfall so these are these are there but they aren't in any way showstoppers and those don't really affect things globally. It used to be thought that patches of forests that were like 60 miles across in a say a tropical grassland would also set up convection cells set up whether much the way the land sea breeze works on on the coast because of the difference between the temperature of the water and the temperature of the land but attempts to measure those impacts have have not not verified them so so there are these important biological global biogeochemical and climate climate scale feedbacks but they aren't you know they aren't they're a whole story by any means. Now this is Rob Jackson's work another way in which reforestation has a has a feedback that's really significant is that the trees suck up all the water and don't allow it to they return it to the atmosphere and don't allow it to go down the rivers and irrigate agriculture. The figure on the left shows what happens when you plant a plantation and how much less runoff there is. This is a big deal in some parts of the world already in South Africa reforestation efforts on the tops of the table lands have taken all the water out of the streams in the bottom and caused a strife between farmers and and foresters. A second big and important limit is competition for land among forest planting, food production, biodiversity preserves, and biofuels. We've got at least three big environmental problems that interact and compete for land. There's the climate problem where you know natural climate solutions and biofuels compete for land. We've got the food problem where we're expecting a two-thirds growth in food demand and that's got pressure for to maintain the existing agricultural land and to expand it to more. And we've got the biodiversity problem in which already the majority of threatened species are threatened because of agricultural conversion. So every one of these environmental problems wants the same land and and if you look at the solutions literature there are often you know separate solutions promise large amounts from the from the AR5 of the IPCC. We get a lot of scenarios calling for something like 600 million hectares of new bioenergy. I mentioned before already that Bastian et al talked about 900 million hectares of new forest. Tillman et al a many many I just I just put Dave's paper up here because he's a friend but there are many papers that say we need all our agricultural land in 2050 to meet the growth in food demand and we have a lot of evidence this from the millennium assessment that most other rain-fed land today is already forested and habitat loss to agriculture is already the dominant cause of biodiversity loss. So you can see what the constraints how the constraints come from many different places and particularly constraints on land for new forest planting. There's this myth in the literature of abandoned and marginal land. I chaired a national academy committee that looked into negative emissions. We issued a report in 2018 and we looked at all of its claims trying to figure out where this land was and the high-publish estimates count land that currently sustains half of humanity and so a better assumption is you know better rule of thumb is that approximately no new land is available for re and aforestation except perhaps for land that comes from beat substitutes. I've actually I think this is really very interesting and and if you haven't talked about it already certainly warrants more talk. I also want to talk a little bit about links between food prices suffering and violence. The graph here is an index is a relationship between an index of food price shocks and how much violence there is how much social unrest there is. This is part of the so-called high-end that is high sample size political science literature now and where they use econometric methods to to try to figure out large-scale human behavior. Saul Schang at Berkeley is one of the real pioneers in this area. This is really well established now. There are all these studies that go all the way back to the French Revolution. You know a good case study is the Arab Spring where food prices all of a sudden doubled in part because of land conversion to biofuels and in part because of extreme weather events and the countries that rebelled had populations with disproportionate income spent on staples like 50 percent which then doubled in price so people couldn't feed their families and historically when this happens people go crazy. Now these events are multi-causal but the high-end literature allows you to compute a fraction of the attributable risk for a particular event of violence and a lot of the violence that occurs after after a food price spike is in fact caused by the food price spike. Integrated assessment models that predict solutions that that sort of people use to come up with solutions to the climate models don't include these sorts of feedbacks. We you know our science is end up to it so we ought to be really careful and humble about calling for large-scale land conversion that could for instance impact food production. We heard a lot from David about incomplete knowledge to realize the full potential of negative emissions in agricultural soils. I've got one graph here this one is is one that we took a look at during the negative emissions technology was a report to that that academy study and this show carbon stock or you can think of it as current carbon stock in the horizontal axis sequestration rate on the vertical axis you can think of this if you're a mathy person is a differential equation dc dt versus c it equilibrates out there to the right at about a hundred years the thing I want to call your attention to is how much scatter there is around the mean despite hundreds of experiments many lacking lasting several decades which is in her QE and effort we still don't have a comprehensive understanding of what kinds of agricultural practices store carbon and how much and it's extra hard because the high variability means we have to know the mean all right to to be able to plan a strategy and so as a result using everything sort of all in we we estimated in the the negative emissions technology report an upper bound including biochar of three gigatons of co2 per year and 90 gigatons total capacity for for croplins and pastures globally we're going to turn briefly to indigo it's easy to beat up on indigo i i uh they kind of came across my radar in the spring after the coven started the lockdown so i called them up because i you know i i've been an edf trustee for years and was interested in the fact that like the gas companies are currently in europe where they face a need to comply with a clean gas standard to stay in the game all right to to to maintain their markets all of a sudden they've gotten religion on measurement all right and they and they've all started measurement programs they've got a big um uh uh uh measurement release planned in a couple of years and and they're all muscling up to be able to convince skeptics about what their methane emissions are and it struck me that indigo was in exactly the same position their business model depends on being able to convince ultimately a skeptic because a group could go out and measure it the same way that edf went out and uh you know we all went and measured uh methane emissions across the united states to verify what claims were and so um long story short they're now actually working with edf and they're really muscling up on their measurement program i think this is important because one of the things the national academy report called for was a herculean uh uh experimental program to try to extend these practices to as many lands as possible and figure out what they actually do and it struck me that here's a private sector actor that has a you know a selfish incentive to want to be able to do this well and so um they're muscling up to do it you know and it's it's not just co2 it's all the gases and so more power to them i'm going to be a cheerleader and i urge all of you who have expertise work with them not against them here final thing um uh well two two more things potential changer from popular net zero policies like an economy-wide price on carbon um this slide is a little bit out of place part of it is an advertisement uh i've been um working with a thing called princeton's net zero america project where we've tried to do the most comprehensive assessment that's been done yet about what it would take for the us to go to uh net zero over a 30-year period these are a bunch of different scenarios and what this shows is the demand for biomass and there's a bunch of scenarios there that are all capped at the horizontal red line the first one on the left is business as usual so that's no no climate mitigation all the other ones go to zero emissions net in 2050 every single one of them bangs up against the cap that we artificially imposed which included no agricultural land conversion you are allowed to use lands currently that go into corn ethanol to produce biomass and you can see that a lot of it is going into that light blue stuff which is hydrogen production from biomass with carbon capture and storage so that you're getting a twofer a hydrogen fuel and a negative emission near the end there is enormous pressure to get fuels and actually to get negative emissions to squeeze the economy down to zero if you double the cap as you do in this scenario in the middle the one that says b plus you just bang up into the cap and so that's tens of millions of hectares of land conversion and so when price this is just more advertisement for this thing is um the study is granular down to the county level and so this is a map of biomass use across the country now if you if you include the natural climate solutions here which we which I don't have yet slides for there will be enormous pressure on those as well and what's to stop a price on carbon from causing large-scale dangerous land use change including over reliance on natural climate solutions and so what are safe levels these are my numbers I'll just uh say here in 2050 I say okay zero here for fossil cement everything else zero approximately for the f gases and we've still got eight gigatons from agricultural methane and n2o which needs to be balanced by well the current technology is natural climate solutions unless we develop something like direct air capture and the point is that that negative almost all this positive eight is agricultural and so the natural climate solutions that we would need to zero the balance what they do is primarily make the land neutral carbon neutral right greenhouse gas neutral and so you should think about natural climate solutions as not the solution to the global warming problem but the but but the solution to the land greenhouse gas problem maybe we could make the land to neutral using them same was true in the United States even this scenario is pretty optimistic the one that we uh the the upper bounds that we came up with in the negative emissions technology report uh were a single gigaton globally possible from a forestation and reforestation primarily because we're concerned about land use change gigaton and a half from forest management and as I said before three gigatons from agricultural soils all land so that's a five gigaton of co2 total and to finish biophysical feedbacks half a degree and no forestry in the north is is what the answer is competition among land for land among all the different environmental problems the right rule of thumb is that no new land will be available for for new forests there will obviously will be some particularly if we get meat substitutes working to change diets number three links between food prices suffering and violence are strong and should make you suspicious about economics models that predict uh how we could deploy ncs's incomplete knowledge uh to realize the full potential of negative emissions by agricultural soils could be corrected by simply doing the you know applying the scientific method globally until we understand it potential danger of popular net zero policies like an economy wide price on carbon is what stops dangerous land use conversion all right we need policies that would do that too safe deployment of ncs's given current knowledge a lot less than 10 gigatons of co2 per year that's it thank you thanks Sarah thanks Steve fantastic presentation um so we'll go to Shafiq Jaffer who has a number of questions Shafiq would you like to unmute yourself and ask you a question thanks Jenny Steve um I think you raised a couple important points uh that I just wanted to touch on one was this unintended consequences and kind of how do we start to put it into the models how do we start to actually build in some of this knowledge we already have but are more in the social political science literature into more of the physical chemistry science models right how do we start to bring these together I think we're close from what you showed yeah so so the the answer there is I think this is an area where scholarship is actually coming rapidly to to uh to capacity the you know in economics I sort of divide economics between modelers who who basically believe that the world is a modeler and if we only changed laws enough so it would be exactly like the model that everything would be okay and empirical people who I think of as econometricians who who try to describe the world with statistical relationships right and and the the integrated assessment modeling comes very much out of the first tradition and the second tradition is what's producing the high end stuff so so if you haven't met him uh talk to sol shang Solomon shang at at at Berkeley but there are many others working in this area now and so I do believe that a that an empirically based set of planning tools is in the works and you know there's a big group Michael Greenstone um and sol shang are are two big pieces of out I think they call themselves the climate solutions a lamp or something like that at University of Chicago and so this is going to fix itself we'll have a separate tradition to throw at this and the second thing that you mentioned which uh kind of was in passing but I think it brings up the big point is this lifestyle change of trying to move away from very intense agriculture for things like protein right meat and things like this which is extremely impactful on the environment how do you see that kind of I mean you mentioned it a little bit which is kind of how do we uh get the naysayers to understand how do we get people who are very sceptic that this is a big part of the problem going forward well you've hit on another area of optimism for me you're gonna think I'm just an optimist but but the thing is that that I've been really pessimistic about this for a long time because historically how hard diets are to change right so if we only decided that we were all going to go vegan all of a sudden we can free up a huge amount of land you know the majority of the calories produced by land in wealthy developed countries goes to feed animals not not people and and um globally we have an enormous amount of rain fed pasture land that we could and cropland that we could abandon if we only all want to eat but you can't make anybody do it right unless they're born hindu or something like that right so so what's changed in just the last couple of years as evidenced by the nervousness of the meat industry who are running all kinds of ads to try to stop it is the uh is the alternative meats industry and the forecasts for the growth of meat consumption have just slashed all these different groups bloomberg all these characters who are just business geeks have now have now slashed the growth of demand for meat and I think this is a real perhaps opportunity to free some land up for us and it it's because it's something you can get people to do and from the standpoint of kind of educating the people on this I mean we get all sorts of numbers out there right but how do you actually convince people that these numbers are are real and to get that transition to occur faster because this is going to be a very slow transition to meatless if this goes even well so so um there are a lot of groups working on doing this right now I I run a center at at Princeton called the carbon mitigation initiative and we have a big project that's trying to um to try to quantify exactly how much you could do for the carbon problem if you um if you um pushed meat substitutes along you know long realistic growth paths and then used the the land dividend for reforestation and so I think there's going to be a lot of scholarship that comes out of this the calculation isn't hard to make right it's just not a tough calculation so so I expect there to be a lot of uh of this sort of thing in the literature in even a year or two that's great already is a little bit thank you thanks we have a question from Ajay Mehta Ajay would you like to go ahead and ask your question yeah thanks uh Jenny uh Steve a really interesting presentation especially the when you start talking about the impact of carbon price uh potentially having a negative impact could you expand a little bit further on that is there a particular trigger point in terms of pricing where you start seeing some of this negative feedback loop or could you just say a little bit more about that yeah I mean what I worry about is um if we if we decided to take net zero seriously and all of a sudden carbon prices in some societies got high um that is you know two hundred dollars a ton or three hundred dollars a ton say mid-century and you're allowed to do international trading uh this starts to really look attractive to people who are running marginal farms all over the world and and I can easily imagine uh a rush to convert those farms to uh to to a forestation reforestation efforts and and a resulting food price spike that would be dangerous that would be destabilizing and so I think we have to um figure out how to put guard rails on on on uh on some of these policies I think it's one of the reasons one of many reasons why um more and more um net zero solutions mixes don't rely on any single instrument but have a diverse portfolio of policies to keep anyone from having a gigantic unintended concave up negative consequence thank you great thank you we're going to try and squeeze in one more question from Arun Majumdar Arun would you like to unmute yourself and ask a question sure hi steve hey um so I have a question uh since your talk is about human action and graph and and carbon if you look at the carbon emissions from fires um by it's about roughly two to three gigatons of carbon per year and roughly half of that is from grasslands in Africa and which is largely human based and although the net emissions is low because the turnaround time is roughly two years or so my question to you is that hey is there a way to kind of modulate that in a way that you can try to get negative emissions or actually prevent uh the carbon emissions in the first place but if you can tweak it is there a way for negative emissions out there well um the the first thing is that um as as you point out the the the problem is not individual fires because the stuff grows back right and so that it so it can net itself out to zero so what what you would need to do to create a negative emissions is to increase the time between fires and then when you have a landscape that is um has fires in heterogeneous years um you can think of it as as a you know then there's a distribution of patch ages and that distribution gets older it's an older mean if you can reduce the fire frequency and that means the land is a whole game's weight right and so there's no question about it um one could do this if one decided to for instance to reforest with a fire resistant species all right you could definitely do this right you could breed plants to to burn less and you could decrease the fire frequency you know conifers are notorious they're full of monotropines and all kinds of stuff but you know they're designed a lot of them even to have to create crown fires to burn out their competitors sometimes and so broadleaf trees are really a lot harder to burn you know the the northeastern deciduous forest hasn't as a whole separate catastrophic you know crown fire burns since the ice age right so and you look at any boreal forest and any any conifer-laden forest and it it burns at a much higher frequency interesting idea