 Thank you. It's wonderful to be here. And it's totally exhilarating to hear about the progress being made in the energy space, especially in the context of the climate challenge, which we really think of as foundational. Now, I want to provide a slightly different framing for the topic and present climate change as really the speed regulator for the rate of progress that we need in the energy transformation. Salih and Arun asked me to speak about the knowns and the unknowns in climate change and in the spirit of providing the take-home message first. I want to say the most important known is that we need to be working harder on this problem than we're working now. And the main unknown is whether or not we're going to do that. What I'd like to do is start with just an overview of where we are in addressing the climate challenge with the recognition that climate change is no longer a hypothetical about the future, but it's something we're living now. The iconic image of climate change is the atmospheric CO2 concentration at Montelot. It's now more than 40% higher than it was in pre-industrial times. We know it's higher than it's been in the last 800,000 years, probably the last several million years. We also know that the climate has been warming dramatically. 2016, the warmest year in the combined land and ocean record here, is just about 1.3 centigrade warmer than the pre-industrial record. And that 1.3 is important in the context of the latest IPCC report that talks about the need to stabilize warming at something like 1.5, given that we're currently warming at about 0.2 per decade. The most recent reports from the Intergovernmental Panel on Climate Change confirm that warming is unequivocal, and this is essentially a zero doubt that the overwhelming majority of the warming in the last several decades is a consequence of human actions. As I said, when I open, it's really clear that we're no longer in an era where dealing with climate change impacts is about hypothetical future conditions. It's really where we are now, the impacts of climate changes that have already occurred, or widespread and consequential, we've seen impacts on every continent and in the oceans. We also understand now that the impacts of climate change don't really express themselves as a consequence of average conditions. They really express themselves as a consequence of the way the world responds to extremes. Just a couple of examples, drive this home. This is a photograph of the Oroville Reservoir, the iconic indicator of what happened to California during its historic 2012 to 2016 drought. And Stanford colleague Noah Diffenbaugh led really spectacular work attributing a lot of this drought to the changes in climate that have already occurred. One of the interesting things about this Oroville Reservoir is that this is exactly the same reservoir that in February of 2017 had the spillway fail from having too much water go over it. And it shows how climate variability can be enhanced by climate change. Second kind of extreme event that we know with a high level of confidence is connected to climate change, not caused solely by climate change, but where there's a big climate change component is wildfires in the American West, where the amount of time between when the soil dries out in the spring and when you get rain in the fall really has a tremendous control over the risk for wildfire. We also know with a high level of confidence that the amount of flooding we're seeing with coastal hurricanes is influenced substantially by climate changes that have already occurred, whether it's the sea level rise averaging 6 to 12 inches around much of our coasts, or the heavier precipitation that comes from a warmer atmosphere. There's no question that we're accumulating these damages in a consistent and in a really pervasive way. When we think about future damages, the framing that I'd like to provide is one about risk of threshold. We've tended to think about gradual impacts, and there's no question that the warmer it is, the greater risk we see of impacts that are severe, that are pervasive, and that are irreversible. But when we think about setting the speed regulator on climate, the important thing to keep in mind is where we face thresholds, and I want to speak very briefly about three important ones. One concerns sea level rise, and you probably know that the amount of sea level equivalent that's trapped in ice sheets that are currently on land is about 80 meters. And one of the things we know with a high level of confidence is there at some point at which substantial fractions, especially of the Antarctic ice sheet, decompose. And they decompose, resulting in several meters of sea level rise over hundreds of years, but in a way that we don't think stops, even if the temperature were to be cooled back down, a genuine threshold. And it's a consequence of what the topography is under these ice sheets. If we look at recent simulations about how much sea level rise we might expect in the 21st century, the good news is that if we live in a world of ambitious mitigation, too sea or less warming, we're likely to see only a few inches, 11 centimeters in this simulation of warming. In a world of high emissions, continued high emissions, that goes up to about a meter. That's enough sea level rise to really be existential for some countries. But even more importantly, that one meter of sea level rise in 2100 is on a trajectory that's increasing rapidly. And one of the places where I feel the climate change science community has often fallen short, is in acting as if the world ends in 2100. If I take this exact same simulation now and extend it out to 2,500, you see a very different pattern, where in the world of ambitious mitigation, sea level rise is still only about a quarter of a meter, a manageable level with investments and adaptation. But the level with continued high emissions in the 21st century only is now over 15 meters, about 50 feet of sea level rise by 2,500, a level that's clearly existential for dozens of countries in small islands and for coastal regions around the world. We have a high level of confidence that a world of ambitious mitigation is safe in the context of this high amount of sea level rise and that a world of continued high emissions and sudden safe. We have a low level of confidence about exactly where that threshold is. Another area where there's an important threshold response is in a release of carbon from the biosphere. The world is covered with frozen soils and about 15% of the land area. The amount of organic matter of carbon that's frozen in these soils is about four times the amount that's in the atmosphere. And all the evidence we have is that that carbon is highly decomposable when the frozen soils thaw. The issue is if we reach a level of thawing such that the CO2 that's released and the methane that's released from these thawing soils sustains global warming, we could live in an environment even where zero emissions of greenhouse gases from human activities don't have much impact on the rate of warming because now releases of greenhouse gases from the natural world are driving sustained warming. We don't have a high level of confidence about where the threshold is, where releases of carbon from the frozen soils become self-sustaining. We have a high level of confidence that we're currently in the safe zone and that warming to foresee or more is very unlikely to be safe. A final threshold I wanna talk about concerns economic activity, this work from Marshall Burke at Stanford shows that with most climate responses, the typical pattern is that some activity is relatively climate insensitive. It made an increase with warming over part of the range and then it hits a threshold and falls off. And you can see the data here for changes in labor supply, labor productivity or corneal. Pattern very, very constant across a wide range of different kinds of impacts. And the implication of this is that cool parts of the world, warming doesn't have much historical impact. Hot parts, it has a lot. If we extend that out to the end of the century, the pattern is really gripping and it's that if you look here at expected changes in per capita GDP by the end of the century, we're looking at potential improvements as a result of climate change in the parts of the world that are currently cold and rich and we see a devastating negative impacts in the parts of the world that are currently hot and poor with the decreases in per capita GDP being on the order of 50 to 75% in Southeast Asia and Sub-Taharan Africa. Where does that put us in terms of the time scale of warming? And one of the most consistent and compelling results of recent climate change science is that warming from carbon dioxide is essentially permanent on the scale of many thousands of years. What that means is that there is a forever budget of the amount of CO2 that can be released to the environment to be consistent with holding temperature below whatever our target is and I'll do an example here for 1.5. Most recent analysis is that staying below 1.5 C requires a cap on total emissions of on the order of 2,800 billion tons of CO2 forever. Emissions through 2017, a little over 2,200 billion tons. And that means that the forever budget for a 66% chance of holding warming below 1.5 is on the order of 550 billion tons of CO2. In 2017, the amount of CO2 that was released globally was a little over 41 billion tons and I know you guys are good at arithmetic. If emissions remain at the current level, what that means is that we are committed to a warming of 1.5 in approximately 13 years and two months and I calculate that as March of 2031 and if there's a single message that's more important than any other and the material that I wanna provide today is that this lifetime budget on carbon emissions is a compelling motivation for finding the accelerator pedal on the work that all of you are doing to drive an energy transition. I wanna say just a couple things in closing about the unknowns. The most important unknowns are in what we do and not in the way the earth system responds. There's been a lot of discussion about policy levers and when they can be effective, when they're not. Economic growth is of course incredibly important both as an empower but as a driver of historical emissions. The rate at which technology is embraced and deployed is incredibly important and a topic that we don't often talk about that has a lot of leverage over emissions in the long run is the rate of human population growth and it's clear that some of the best opportunities for decreasing future emissions are to meet existing needs for family planning. There are some important feedbacks that we need to be aware of in the natural system as well. It could be that the sensitivity of climate to greenhouse gas emissions is higher than we think and that could result in more warm, more damages. The ecosystem feedbacks are incredibly important and figuring out exactly where those thresholds are that result in sustained emissions. Really the most important topic in climate science. It's clear that the expression of climate change is mostly in extreme events and the sensitivity of the climate system to extreme events, the way those interact with other things that humans are doing to the earth system. Still far from clear. In many cases it's not the extreme in itself that produces the damages but the cascading events where the flood leads to the release of a chemical that's been stored in some kind of containment facility and that leads to a disease outbreak and on and on. So understanding the likelihood and figuring out how to limit the risk of cascading impacts is really important. We also don't have an incredibly clear idea of what we can do in the adaptation space and how much climate change we can adapt to except for the very broad generalization that we can adapt effectively to small amounts of warming and not to large amounts. And then finally I wanna leave with the optimistic thought that we're really just beginning to explore where there are opportunities for building a better world as a consequence of ambitious deployment of renewable energy at the same time we're rethinking urban infrastructure and rethinking the global food system. And the concept that gives me the most hope in this entire space is the prospect of discovering co-benefits as a result of deploying not only the energy agenda but a human development agenda simultaneously. Thanks so much.