Here's another conclusion of the latest IPCC report. Changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels, and an increase in the number of heavy precipitation events in a number of regions. So people often ask the question about whether particular extreme weather event like a drought or a heat wave or a hurricane was due to climate change. I think that's actually the wrong question to ask because an individual event can never be wholly attributed to climate change. As climate deniers like to point out, extreme weather events have been happening throughout history and what the real question is a question of probability. So the correct question to ask is, are extreme events occurring more frequently and if so, how much more frequently? Climate modelers have developed methods for doing attribution studies for specific extreme weather events. So in these studies, they asked the question, how much more likely was an event of this magnitude in the presence of climate change than in the absence of climate change? The Bulletin of the American Meteorological Society has been publishing an annual report that they entitle, Explaining extreme weather events from a climate perspective. So far, there have been six editions. They've examined 131 extreme weather events. For two-thirds of them, they concluded that there was an appreciably higher probability of occurrence of that event due to climate change. For the other one-third, they concluded that there was not an appreciably higher probability of occurrence due to climate change. So overall, this body of work really points to a very strong influence in the presence of climate change driving extreme weather events with two-thirds of at least the ones they looked at being driven to at least a certain degree by climate change. Another question that people often ask is what's the big deal if we see an increase in mean global temperature of two degrees centigrade which is about 3.6 degrees Fahrenheit? Why does it matter? Winters would be a little warmer and we'd like that. It's only two degrees Centigrade. What's the big deal? So it turns out, and this isn't necessarily intuitive, that a small change in the mean can have large effects on extremes. So let's take a look at this graph which illustrates the point. So here we see two curves with the normal distribution. One, they're calling the previous climate. So that would be the climate with no climate change, and that's the solid line. Then, the dashed line is the future climate or the climate with climate change. So we see a normal distribution. Suppose what we're graphing here is temperature distribution and it's a relatively small change in the mean just from here to here. Let's define hot weather as anything to the right of this line here. So that's an extreme on the hot side. In the non-climate change scenario, this area would be the area of hot weather, the area under the curve in that portion of the curve. In the climate change scenario, now it would be the area under the dashed line or this whole area. So it appears that the amount of hot weather would more or less double with this small amount of climate change with regard to the change in the mean, it would approximately double. Now, we could also define record hot weather and we're doing that out here. So anything to the right of this line would be considered record hot weather. You can see that under the previous climate or no climate change, it's like this very little amount would be record hot weather. But with climate change, i.e the future climate in this graph, it would be this amount which is probably like 5-10 times the area of this little amount with no climate change. So we'd see a lot more record hot weather. Now, let's turn to precipitation. So here we're looking at a map of the world that's divided up into grid cells. We're looking at 1986-2015 relative to a baseline of 1901-1960. The more blue or bluish green, the more of a positive change in precipitation and the more brown, the more of a negative change in precipitation or more drying. So the basic generalization of what we're seeing in this map is that dry areas are getting drier and wet areas are getting wetter. The reason for that is relatively simple. So with a warmer atmosphere with the warming that we're seeing in a dry area, what's happening is we're seeing the already dry surface of the Earth becoming even drier due to increased evaporation due to the increase in temperatures. What we're seeing in the wetter areas is again because of the warming temperatures, we're seeing more evaporation. But this time where in the wetter areas for various more object reasons, they're prone to more rain. So with more evaporation, more water vapor in the atmosphere, we're seeing even more rain. So again, dry areas are tending to get dryer, wet areas are tending to get wetter. Just to drive that point home. So this is looking at the contiguous 48 states of the United States and you could see that going from here, we're going from 1910 to about 2015 and you could see this increase in extreme one day precipitation events. You won't be able to read the details of the slide, but let me explain it. So here we're looking at average number of coastal flood events per year between 1950 and 2015 and it's divided into 1950-'69, 1970-'89, 1990-2009, and 2010-2015. This is for a number of different cities in the United States. So you can see the increasing trend of coastal flood events pretty consistently across the different cities. So this one's particularly striking, it's Annapolis Maryland. So we're going from this number of flood events all the way out to here per year. Wilmington North Carolina is another striking one. But just about all of them, we're seeing an increase in the frequency of these coastal flood events. Now, we're turning to drought, another type of extreme event and this is looking at the Southwestern United States between 1895 and 2015. If we focus on the period starting around 1980, we're looking at the y-axis is something called the Palmer Drought Severity Index. So this is drier going down, wetter going up. You could see a drying trend since around 1980 for the Southwestern United States. We all know that that's been happening, we're seeing the increasing wildfires which I'll talk about in another lecture. So we already talked about that the oceans are warming. In addition, the atmosphere above the oceans is warming. So we're seeing more water vapor in the atmosphere and all of that fuels hurricanes, it fuels more intense hurricanes. That was really driven home in the hurricane season in 2017 when we saw a two Category 5 hurricanes, Irma and Maria and a very strong Category 4 hurricane Harvey which caused record rainfall. So here you see a satellite photo of hurricane Maria, an extremely intense hurricane. We're not necessarily going to see more frequent hurricanes. The prediction is that we're going to see and are starting to see more intense hurricanes but not necessarily more hurricanes overall but the ones that do occur will be more intense.
