 So paleoclimate is when we're trying to understand what happened to the climate prior to the period when we have instruments, so before the invention or the widespread deployment of thermometers and rain gauges and things like that. And we use indirect estimates of the climate. These are things like tree rings and ice cores and corals and sediments, various natural archives, physical or chemical or biological archives that tell us something about climate conditions in the distant past. Farther back then we have reliable instrumental measurements. We really only have widespread thermometer measurements around the globe for a little of our century, but we can extend these records further back in time by using these so-called proxy data. The ones I'm most interested in are ones with a kind of a finer resolution that can tell you about individual years or decades, for example, tree rings. And those tree ring records, if they're based on living trees, obviously they're limited by the lifespan of a tree, which might be a few hundred years, occasionally a thousand years or more. Back in the late 1990s, my co-authors and I sought to reconstruct the surface temperature record back in time, hundreds of years back in time, using these proxy data. Now what actually drove the research was our interest in the underlying patterns of past temperature variation, what was the history of El Nino in the past and its large-scale influence on the climate. What was the pattern around the world of the response to some of the largest volcanic eruptions of the past thousand years? So from the paleoclimate data, variations of climate in the last thousand years have been quite large in individual regions, but not necessarily coherent. So some regions are warm and others are perhaps cooler. If you take a large-scale picture, what you see is generally warm conditions at the beginning of the last thousand years, often called the medieval warm period or some name like that, and then coming down to a cooler period called the Little Ice Age, perhaps between 1450 and 1850 at its peak, and then after 1850, coming back up to warmer conditions in the 20th century and now into the 21st century. Now if you look at that curve, what it shows is that the modern warming spike is unprecedented as far back as we could go, a thousand years in our 1990s studies. But there were some temperature changes along the way, and there was a period of about a thousand years ago where temperatures over the northern hemisphere were relatively warm, and then temperatures cool as you slowly descend into what's sometimes called the Little Ice Age, sort of the 1300s through the 1800s. And so you have this long-term cooling from the medieval climate period into the Little Ice Age of the 17th, 18th and 19th century, followed by this abrupt spike, which takes you outside of the range of any of that previous variation. So our best estimates for the Little Ice Age is that there was a cooling effect in most parts of the world of a moderate level, but then superimposed on that original fluctuations, if there's changes in atmosphere or the ocean circulation, they can transport heat around and make some areas even cooler and some areas less cool. And so the phenomenon is probably a northern hemisphere-wide event, maybe a global event. Now when it comes to the medieval period, it used to be widely claimed that global temperatures were warmer than they are today. The evidence that we have now does not bear that out. Now what it shows is that there are some regions where temperatures were quite a bit warmer. The pattern of warming and cooling around the globe isn't uniform. There is a lot of redistribution of heat around the globe associated with changing ocean currents, associated with changing atmospheric wind patterns. And so when you look back in time, there's a very complex regionally diverse pattern of temperature changes. And what we find is that in certain regions in the North Atlantic and parts of Greenland, during the height of the sort of medieval climate period, may have been almost as warm as conditions today, if not even warmer, within the uncertainties, perhaps even warmer. But most of the globe was substantially cooler. And when you average over the globe or over the northern hemisphere, what you find was that temperatures then are not nearly as high as they are today, because what's different is the sort of coherent pattern of the warming. Comparing the warmth during the medieval period and the warmth during the modern period, they're actually really quite similar given our rather wide uncertainty errors. Our modern temperatures are well-known from the thermometer record, but our estimates of the medieval temperatures from these indirect proxies like truings, the uncertainty is much wider, and so making this comparison is quite tricky. On the basis of what's published, it's more likely that the modern temperatures are warmer than the medieval ones, but there's not something we'd be certain about. Study after study has not only reaffirmed our key conclusion about the unusual nature of the recent warming, more recent work has strengthened and extended those conclusions. The most recent report of the IPCC concluded that the recent warming is unprecedented not just in a thousand years, as we concluded a decade and a half ago, but at least 1,300 years and maybe even longer. I sometimes refer to what exists now when it comes to these paleoclimate reconstructions as not a hockey stick, but a hockey league, because there are dozens of these reconstructions and they don't all agree on all of the details. What was the coldest part of the Little Ice Age and what was the precise pattern of the medieval climate period? Different studies come to different conclusions because they use different kinds of data, different methods to take those data and form a climate reconstruction, but the one thing they all agree on is that the recent warming has no precedent as far back as we can go. One of the major misconceptions right now is that people argue that the climate has changed in the past and therefore what we see now might just be part of natural variability. This is not true and the changes we see now are so fast and so enormous that that doesn't compare to anything we actually have in the records, certainly not in the last two million years, but also if you go very far back, there was maybe one event 55 million years ago which was associated to a big increase in CO2, but even that happened at a much slower rate than the rate we are changing the climate right now. So ironically, when it comes to contrarians, climate change deniers who will a, wrongly claim that the medieval period was warmer than today because the science seems to definitively say otherwise now, but even granting them that if that were true. And then they say, and because that's true, it means that the warming today could be natural too. Well, in fact, we have a pretty good idea of what the natural driving factors were during the medieval period, volcanoes, changes in solar output, small changes, long term changes in earth orbital, the geometry of the earth's orbit around the sun. If the climate really were far more sensitive to the natural factors, it would imply that it's more sensitive to CO2 increases. It would imply just the opposite of what the skeptics or contrarians or deniers of climate want you to think. Looking into the past can tell us a little bit about how the system might react if we push it. But the way we push it right now is out of context of anything we can see in our records. We just look at the record, the observed record of the last 200 years, the variations in CO2 over that are tiny and get nowhere near these high values that we're expecting by the year 2100. So basically, if you look at the plots, if you just look at the temperature reconstructions of CO2 plots, you see all the variability and you have ice here and now you have ice down here and integrations here and it goes like this and today we're up here. So we are completely out of this range of variability. If you look at temperature, you know, the plot of changes in temperature and changing in CO2 concentration, they go together. When temperature is high, CO2 concentration is high and when it's low, CO2 concentration is low. A really good question to ask is, well, what was the climate like the last time CO2 was about 400 parts million and it turns out that our best estimate so that it was associated with what we call the Pliocene time period. So this was the mid Pliocene around about 3 million years ago. What was the climate like 3 million years ago? Well, this Pliocene world, one of the things we know about it is that sea level, for example, is probably about 10 meters higher than it is today. What does 10 meters mean? Well, we're talking about inundation of many of the world's major cities. We're talking about inundation of much of a lot of the crop plan, for example, that we use to feed the population today. A lot of the modern day population lives near the coast and those are obviously the ones that are going to be most greatly affected. A 10 meter sea level rise is serious, serious news for our planet. Let's make no bones about it.