 In this lecture, we're going to look at the difference between weather and climate. Weather changes all the time. Climate only changes when something makes it change. Global warming doesn't stop weather from happening. It doesn't even stop cold weather from happening. To understand the difference between weather and climate, let's visit a weather station. This is a historical weather station in England. Weather stations like this have been used to measure temperature for over a century. There are more than 10,000 weather stations around the world. The equipment has changed a bit over time, but the job they do is the same. We're mostly interested in temperature, which is measured here. This is a Stevenson screen. Inside there are two thermometers, measuring minimum and maximum daily temperatures. The screen keeps the sun and wind off the thermometer so that it provides a reliable measure of air temperature. What do weather station records tell us? People tend to remember record breaking weather, so we can look for records in the temperature data. We can count record breaking hot days and record breaking cold days, days which were either hotter or colder than any other day before that date. But once a record is set, a new record needs to be even more extreme. So as our weather history has grown, new records have become rarer. Just counting records is misleading. Instead, we can compare the number of hot and cold records in any decade. If the number of hot and cold records is about equal, then the weather is not changing. If we see more hot records than cold records, then it's getting warmer and vice versa. Here is a comparison for the number of hot and cold records counting 2,000 weather stations in the United States. The number of records is given for each decade from 1950 to the present. For the first few decades, the number of hot and cold records are about equal. Over the last few decades, the proportion has changed with almost twice as many hot records as cold records. That suggests that the United States has been getting warmer. Here is a similar comparison for Australia, counting record hot or cold months. Records have been counted for both daytime and nighttime temperatures. Again, we see that the proportion of hot records has increased. This is a very simple way of analysing historical weather data. In practice, scientists have better ways of detecting warming. But even these simple comparisons strongly suggest that it has been getting warmer. We call this global warming or climate change. What is climate? Climate is how likely you are to get different kinds of weather. Different parts of the world have different climates. They're colder or warmer, wetter or drier. But there's normally a mix of weather that is likely to happen in one place. Alaska is cold, Arizona is hot, New Zealand is wet and Egypt is dry. Each place has its own climate. If the climate changes, different kinds of weather happen more or less frequently. If the climate gets warmer, you're more likely to get exceptionally hot days and less likely to get cold days. You still get cold days. Some places will even set cold records, but they happen less often. Here's an illustration. I've got some dice with weather symbols. If I roll them, I get different kinds of weather. Sometimes I get three suns, a very hot day. Sometimes I get three snow clouds, a very cold day. Now, suppose I were to rig the dice so that they roll suns more often than snow. I'll still get the occasional very cold day, but less often than before. But I'll get more hot days. Global warming is like rigging the climate dice. People naturally find us confusing. What we experience is weather. You need a long memory and the right lifestyle. For example, someone who lives off the land to notice climate change. Most of us experience weather on a day to day basis, whereas climate change happens over the course of decades. Unless you live off the land, you probably won't notice the change in climate. But a small change in global climate can affect us through the water cycle, through extreme weather, and through agriculture. A common misconception is confusing weather and climate. You've probably heard someone saying something like, it's cold outside, whatever happened to go global warming. But cold weather doesn't disprove global warming. What is the fallacy here? It's a fallacy of false expectations. Global warming is a change in the climate. It doesn't mean that we'll never get cold days, but warm days will become more common and cold days will become less common. More importantly, climate change is gradual. The most noticeable impacts are going to be indirect. For example, through effects on the water supply, extreme weather, and agriculture.