 At this point I'd like to introduce our next speaker Chris Field who's the Director of the Stanford Woods Institute for the Environment and the Melvin and Joan Lane Professor for Interdisciplinary Environmental Studies at Stanford. The title of his talk is Natural Climate Solutions, Important but Not Dominant, Pieces of the Puzzle. Thanks everyone for joining this morning. It's a pleasure to talk about one of the most consequential topics in our portfolio of approaches for dealing with climate change. The essence, the take-home message that I want to leave people with is in my title. Natural climate solutions can be a critically important piece of the puzzle. It's very unlikely they'll be a dominant component. Sally's already laid the foundations for many elements of that and I'll fill in some of the details. I'm going to start with the idea that we have a carbon budget and that the total amount of carbon that can be emitted from the beginning of the industrial revolution until the last time is emitted is really set and that the only way we can back down the warming scale is to remove carbon from the atmosphere. One important aspect of this that's not widely understood is that we know over time that the carbon that's emitted to the atmosphere is partitioned gradually between the atmosphere and the oceans. Arrhenius knew this in the late 19th century but a striking feature of the way the relationship between carbon dioxide and temperature plays out is that even as CO2 is partitioned increasingly from the atmosphere into the oceans we don't see a parallel decrease in the amount of warming. You can see that in the right hand result from Matthews and Caldera here where even with a very large pulse of CO2 into the atmosphere at the at the start of this model experiment and that both gradually partitions into the ocean over time you see over a 500 year period temperature staying essentially the same and it's because the same process that's removing CO2 from the atmosphere the gradual overturning the circulation of the ocean that's taking up CO2 is also limiting the ability of the oceans to remove heat so as we move through time a larger fraction of the CO2 was in the oceans but a smaller fraction of the heat is moving into the oceans and the consequence of that is that you have this effect of CO2 that leads to essentially permanent warming and the consequences this carbon budget. Let me just run through the details of that carbon budget one more time this one's for 1.5 c target but the fundamentals are the same no matter what budget level we talk about the overall budget but forever budget for a 66 probability of staying under 1.5 c is a little less than 2800 billion tons. Through 2019 we emitted a little over 2300 billion tons and that leaves about 485 forever if the ideas were going to have a 66 and a half percent chance of staying under 1.5 c. As Sally said the total emissions from fossil fuel combustion and land use in 2019 were around 43 billion tons everybody's good at math you can divide 43 into 485 come up that if we were to stick at our current emissions rate we would use up the remaining budget in in about 11 years and if we are going to solve the climate challenge staying anywhere close to 1.5 c by limiting emissions only we have to dramatically find the accelerator pedal on degarbonization and the question for today's session is to what extent can carbon dioxide removals provide a complementary series of strategies that allow us a little more time and if you look at the IPCC emissions scenarios what you can see is that there's of course a wide range of possibilities for staying within this budget and the overall emissions trajectory for the 21st century is going to be some mix of emissions from fossil fuel some mix of emissions and uptake from agriculture forestry and other land use the brown slices in the young figures across the bottom and potentially negative emissions from biomass energy from carbon capture and storage and and and what these scenarios illustrate is that there is of course a wide range of fossil fuel emissions that are consistent with staying under 1.5 but a higher longer lasting level of fossil emissions needs to be compensated by a larger longer lasting combination of natural sinks agriculture forestry and other land use and biomass energy with carbon capture and sequestration so that when you get out to the to the extremely high levels of persistence of fossil fuel combustion that are shown in in p4 here there need really massive amounts of of negative emissions here shown as coming from BEX but the negative emissions to offset the lasting fossil emissions a lot of the idea for exploring negative emissions has come from what tally describes it's a difficult to decarbonize parts of the economy and it is really important that we focus on those but it's also important that we not use the distant prospect of negative emissions as an excuse to avoid decarbonization in the near term and when we talk about the economics of decarbonization we need to be really careful to recognize that there's also an important moral hazard where we're simply putting off the action based on the idea the untested idea I should emphasize that we can effectively remove CO2 at a reasonable cost sometime in the future a lot of the motivation for thinking about natural climate solutions comes from the fact that fluxes in the natural carbon cycle are are really really large if you look in this figure at the emissions from fossil fuel you know they're on the order of eight to 10 billion tons of CO2 per year so since this on land is is more than 100 the movement of of CO2 into the water by solution is is also a very large number something like 80 so these natural fluxes that are at least 10 times the size of the endogenic fluxes has led to the question of well can't we tweak these in some way so that the balance between uptake and release on land or between uptake and release on the oceans is shifted in the direction of increasing storage and that's the whole idea of natural climate solutions which involves a wide range of technologies that go all the way from reducing deforestation and forest degradation to modifying the way we manage soils to be able to increase the amount of carbon that that agricultural and grazing soils contain when we look across this entire suite of candidate natural climate solutions it's important to recognize that they fall into two important categories some natural climate solutions are about slowing emissions and it's still the case that a significant fraction of annual CO2 emissions come from land use change especially deforestation in the tropics but reducing forest degradation improving nutrient management and croplands to decrease emissions of methane and nitrous oxide and protecting coastal peatlands are all about slowing emissions that's something we need to do to get to zero emissions independent of anything we do that creates natural sinks but then there's another set of natural climate solutions that are really about increasing sinks generating negative emissions and that includes techniques like regrowing forests where they're currently not forests have forestation putting forests where there had not historically been forests or various techniques that that might be used to increase the carbon stock in agricultural and grazing soils when we look at these techniques it's important to recognize that land use change is still an important source of CO2 emissions and it's this plot from the global carbon project shows land use change still constitutes something like 10 to 15 percent of total annual emissions and when we talk about decreasing those emissions we're still on the on the emission side of the budget not on the negative emission side so we contrast these two kinds of components natural climate solutions remember that the decreasing emissions part is bending us away from business as usual and the increasing sinks part is pushing us into the negative emissions very different implications of the two and we can think about them as constituting a portfolio but the implications of the carbon cycle are really quite different an important backdrop to the potential for natural climate solutions is that it's already the case that we're seeing large amounts of carbon uptake in the oceans and terrestrial ecosystems on an annual basis if you look across the the global map of of forests what you can see is that there are large amounts of deforestation in each of the tropical continents there are large amounts of of regrowth on each of the tropical continents and their their substantial forestry sinks on each of the tropical continents and if you add that up globally what you can see is that for every ton of carbon dioxide that's emitted to the atmosphere something like 30 percent is stored in these background sinks on land and an additional 20 to 25 percent is is stored in in the oceans these these natural sinks are an important backdrop to the prospects for natural climate solutions and most of our calculations about the potential for natural climate solutions are in the context of the expectation that these background sinks continue to occur and one of the real challenges as I'll show in a minute is the prospect of double counting of establishing programs to generate sinks that are in fact already being accomplished either on land or in ocean by these unmanaged processes so what is the the prospect for natural climate solutions on land and that's what I'd like to spend the the last few minutes on here there have been a number of estimates one of the most comprehensive ones was published in 2017 by Bronson Griscombe and colleagues and they identified a portfolio of 20 natural pathways with some potential to either decrease emissions or to increase carbon storage and they categorized these by the number that were potentially available at a carbon cost of less than $10 a ton less than $100 a ton and a total potential and what you can see from from their analysis is that the large majority of the potential fluxes are in forests especially in reforestation and the the idea that we could grow a lot more for us we have now has been a really important part of the dialogue over the last several years it was sort of brought into sharpest focused by a paper that was published in science in 2019 by bastin and and and colleagues and they used a clever but not very realistic AI technique to say well where does the earth have climate conditions that could potentially support for us and if you say well what would the difference in the biosphere be and the difference in carbon stocks if there were a forest everywhere that has a climate that's consistent with that and they ended up with a very large number that that if you could wave a magic wand and have forests everywhere that could support it it would add something like 752 tons of CO2 to the forest estate the magnitude of this number and the enthusiasm surrounding it was really stimulate a lot of our conversation about what we could do if if by waving a magic wand we could get a trillion more trees and those trees lived maturity and they they established stable permanent forests it could make a huge difference but that's a very very optimistic and a very non-mechanism-based way to think about the problem other groups that have taken a much more mechanistic approach including this report from the national academies on negative emissions that um Steve college chair Sally Benson was a member of the committee come up with a conclusion that if you really look across all of the mechanisms that are likely to be available on land the notions tend to end up with estimates on the order of five to 60 tons of CO2 per year little more than 10 percent of current total fossil emissions the debate over these issues led Chris Anderson who's going to moderate the discussion later today and a number of colleagues to argue that we really need to focus on this question of what the overall potential of natural climate solutions is and be realistic about it as we look to the future and one of the things that that's stimulated many people to do is try and figure out where we are in terms of actually being able to deploy natural climate solutions and I'm going to talk about a synthesis that that that I've been participating in recently was led by Connor Nolan and if you look at all the estimates of the potential for carbon storage in the terrestrial biosphere you end up with a huge range of possibilities all the way from substantially more than a thousand billion tons of CO2 to less than a hundred billion tons of CO2 as the maximum capacity the orange bar across this chart is the interquartile range for the expectation the requirement for negative emissions in the IPCC scenarios for the 1.5 C report one of the things that we realized in looking at all of these estimates is that when we think about natural climate solutions there are really two quite different conceptual models that drive people's thinking and you might understand those conceptual models as understanding the carbon storage capacity of the terrestrial biosphere as either operating like a silo we cut down a bunch of forests in the past and we can refill the biomass in this forest but but once it's refilled we're we're at capacity or you can think about the terrestrial biosphere as operating much more like a haystack and the the more carbon you dump on it the the bigger the pile of hay gets to be if you think about the carbon storage in the terrestrial biosphere is as fundamentally driven by things like the increasing CO2 concentration in the atmosphere by general phosphorus fertilization or intensive human management of things like forest plantations you tend to drift more toward this haystack model and the conceptualizations that are more tied in the idea of restoring us to the pre altered biosphere give you estimates that are on the order of a hundred billion tons of carbon potential storage and the ones that are more haystack oriented produce numbers that are in the thousand billion tons or greater and one of the things that's that's really interesting is if you look at estimates of carbon loss from all the human activities today you end up with with the indication that human activities have released something like 700 billion tons of CO2 from ecosystem to the atmosphere and the estimates of the potential gain from natural climate solutions have a mean of 300 to 400 billion tons but if you ask how much of a land sink has there been adding up all of the land sink that's occurred since about 1960 that number adds up to almost 500 billion so if we really are operating in a silo world where the total capacity is something like 750 billion tons and we have already restored 500 billion we don't have very much left if on the other hand we're in a haystack world with potentially unlimited storage that that may not be relevant when we looked at all the information and tried to figure out what the actual maximum capacity might be it's hard to imagine that if you look at the combination of biochemical constraints from nutrient and water availability and climate feedbacks if you look at the economic constraints about land tenure and the cost of intervention and if you look at political cultural social and economic constraints it's hard to imagine that during this century we could deploy natural climate solutions that are outside the range of something like one to 200 billion tons of CO2 total that's enough to really make a dent in the solutions to the climate challenge but it's not enough to abandon decarbonization it's not enough to abandon other approaches to CO2 removal and as is the case with almost every technology intervention we look at in in in the climate challenge we really need to look at a very broad portfolio and we need to think about ways that we can utilize natural climate solutions in a way that takes advantage of their co-benefits but looks hard at the implications for equity looks hard at the implications for ecosystem services and really opens the door to thinking very very hard about making natural climate solutions a part of a strategy to build a better world and not just to solve the climate problem that's where I want to stop and I would love to take any questions thank you Chris that was that's very informative as always very interesting so we do have a couple of questions from the audience can we start with Andrew Robertson would you like to ask a question yeah no problem everybody can hear me yeah so my question was really putting together your whole process we've got this sort of limited time frame where we need to start drastically reducing emissions reductions if we're going to still be emitting that 40 gigatons a year you know the 11 years you said at the beginning how realistic do you feel it is that we can ramp up those different nbs or ncs opportunities because obviously it takes time for plants to grow or people to change attitudes or implement new practices so if we need that if we need to fill it in the next 10 years or so a lot of those studies which are look at the ncs potential say that we can do this by 2030 or something how realistic do you actually think that that is well one of the things that's really clear is that we're not going to accomplish big changes in either emissions or in uptake in nature and unless we make a real commitment to it and we've spent an awful lot of time talking about decarbonizing energy in industry and a lot of time talking about deployment of natural solutions but we spent very little time investing in either investing either in the learning that's necessary to deploy at scale or in the deployment at scale I'm not optimistic that we'll see a dramatic set of changes in the next year things could change dramatically in only a few years but one of the most important enablers for that is the recognition that this time pressure was really extreme and when I think about the great potential of natural climate solutions to contribute it's really reassuring but when I think about the potential for discussion of natural climate solutions to be used as an excuse for delaying action and all the other things we need to do it's really concerning yeah I at that point I just said it's all a follow-up I guess looking at I was really interested and really loved that chart of all your the different potentials going from that thousand new terms to the very small all those different studies is that in that study do you standardize for the number of years so like each one is timed up by 30 years or something or is that yeah I just remember looking at it it's a great question the estimates that have been done have been done in two fundamentally different ways some calculate the size of the remaining storage potential and and that's the kind of a time independent remember how much carbon could the terrestrial ecosystem biosphere contain some are based on a rate of what's the maximum storage per year and and some have a number of years that you could apply that and in some cases we had to make an estimate of them the number of years and so in the in the griscombe paper that I showed the results from they they used a rate-based analysis that applied for 54 years and and when there was no indication of how long we're just we used that as an estimate but it's a big unknown for all these studies lovely okay thank you we have quite a few questions rolling in I'd like to go to Navin next Navin would you like to ask your question unmute yourself when you're able to hi thanks Chris for a great presentation so my question was you know whenever we talk about natural climate solutions most often we consider the earth as a homogeneous system right so there is so much of carbon to be taken out and you know so these are the solutions and we look at it with earth as a homogeneous system but given the you know if you look at the net primary productivity or carbon dioxide the spatial distribution across the earth will lend to different portfolio solutions being effective in different ways as a part as a as a as a function of the location the time right so your thoughts on on your studies on what you have observed from that point of view yeah thank you well yeah it is really important that the productive potential across the earth surface varies dramatically from the tropics and poles it's also important to keep in mind that at least historically the relationship between existing carbon stocks and net primary production isn't anything like linear and some of the largest carbon stocks are in high latitude ecosystems especially where carbon is frozen in soils and the net primary production is really low what we're really looking at in terms of the ability to generate additional carbon sinks is the balance between the uptake and the release and the release can either be from decomposition and respiration or it can be from disturbances and the disturbances can be anything from fire insect outbreaks to harvesting or clearing for agriculture and it is indisputably the case that the opportunities for storage very additional storage vary dramatically from place to place and they depend not only on this sort of natural cycle of of input and output and disturbance but also on any additional resources that might be applied and those additional resources could include things like pseudo fertilization from the atmosphere or nitrogen deposition from pollution or the addition of irrigation water in general my expectation is that we're unlikely to see very many natural climate solutions that are lucrative enough that we see massive inputs in terms of for example irrigation or fertilizer applications okay thank you we still have time for at least one more question lauri weber would you like to unmute yourself when you're able to and ask your question chris thank you so much for this and one thing that i find puzzling regularly is that there's often this juxtaposition of an either or solution it's either one or the other and i'm glad that you focus on the portfolio approach where we need it all and we're going to do it all so i have two questions for you you know our natural technologies are proven and they're deployable now and they're a least cost solution so i'm wondering if this doesn't tend to argue that we should be immediately deploying deploying this now some of us have been arguing for 25 years to immediately deploy it but that gives us some time to focus on the other technologies that as you point out are rather distant and untested in the geologic and bex arena and secondly both you and sally have focused on the cultural challenges of transforming the land use sector and i'm wondering if you see any lessons from how we've approached the cultural challenges in transportation and in fostering renewable energy that could be applied in the natural solutions sector great questions thank you i think we should be doing the best job we can for stewarding natural ecosystems always and in many cases there's a wonderful alignment between the kind of interventions that we want to make to do good stewardship and the kind of interventions that increase carbon stocks and that it comes into sharpest focus with any deforestation primarily in the tropics and the potential to decrease emissions by more than 10 percent and there are lots of reasons to think that in terms of the technology that that could happen really soon but it also has to happen in the context of of real people making real decisions and the reversal of the trajectory and in brazil after bolson are all became president is a is a good example of the kinds of difficulties we face but i totally agree that we should be deploying the natural climate solutions now when they work but i want to argue against the idea that that means that we have more time to work out solutions to the decarbonization that the time pressure for the whole climate challenge is really intense and we want to be sure that that we don't lose track of the intensity of the time challenge by the fact that there are compelling natural climate solutions that are available now they're not enough to to allow us to take the foot off the accelerator pedal for the solutions across the entire suite