In this lecture, we'll look at why heat waves are happening more often. Spoiler alert, it's because of global warming. Europe, Asia, and Australia are already seeing more frequent heat waves. Heat waves can cause droughts, which can lead to wildfires and crop failures, and they can be extremely deadly. Did you know that the heat wave in Europe in 2003 killed more than 50,000 people? A study found that human induced greenhouse emissions made it approximately four times more likely that a summer like Europe had in 2003 would occur. Okay, so heat waves are already more common, but how do heat waves relate to global warming? This spell curve is showing us the odds of getting cold, average, or hot weather. The curve shows that in a normal climate, there are good odds for average weather. That's why it's the average. The chances of getting cold or hot weather are pretty low, making up the wings of the bell curve. Now, if the global average temperature goes up due to human cause global warming, the average temperature will go up, shifting the whole bell curve towards the hot end. This increases our chances for more hot weather, and the record hot weather now pokes out into dangerously warm territory. Think of global warming as the Earth on steroids. If a baseball player takes steroids, I bet he'll hit a lot more home runs. This means a warmer world is a weirder world with more extremes. Our chances of getting home runs or extreme temperatures just got bigger. This graph is showing that by increasing the range of temperatures or making the weather weirder, we've caused a world where we get average weather less often, but we get weirder weather more often. The rings of the bell curve got a little thicker. When we combine the effects of the increase in average temperature with the increase in weirdness, we get a lot more record hot weather. The dark orange area shows that we expect heat waves to be hotter and happen more often. It's likely that human influence on the climate has already more than doubled the probability of occurrence of heat waves in some locations. It's virtually certain that there will be more frequent hot temperature extremes over most land areas in the future. It's also very likely that heat waves will occur more often and last longer in the future. Another contributing factor is that we have also observed that nights are warming faster than days. This is a human fingerprint of global warming because it implicates the greenhouse effect. With the stronger greenhouse effect, less of the Earth's heat can escape to space at night. If you've ever spent a hot summer in a place without air conditioning, nighttime is our only chance to open the windows and cool things down. If nights are hot, this is one more ingredient for a nasty heat wave in the making. Finally, heat waves in a global warming world will put more heat stress on people in some regions, making heat waves even more deadly. This is because in a warmer world, on average, it's a more humid world. We know that the heat index, or what the temperature feels like outside, depends on how humid the air is. In the air is nice and dry, the beads of sweat on my forehead can evaporate easily, cooling me down. But when it's humid outside, my beads of sweat don't evaporate, leaving me hot and sweaty, yuck. This adds to human discomfort and makes heat waves even more deadly, especially in places without air conditioning. So that's how heat waves are related to global warming. However, one myth distorts the evidence for increasing heat waves. The myth is that heat waves have happened before, so you can't say that today's heat waves are caused by global warming. This myth uses the technique of a logical fallacy called a non-sequitor, which is Latin 4, it does not follow. It means the myth is simply jumping to conclusions. It's kind of like saying, well, humans were dying of cancer long before cigarettes were invented, it therefore follows that smoking does not cause cancer. Clearly this isn't good logic, just like saying because there were heat waves in the past, today's increasing heat waves can't be related to global warming. Remember, we are already observing more heat waves in some regions, and we expect to see more longer heat waves in the near future, and now you know the science behind why.