An interview with Weatherunderground’s Dr. Jeff Masters:
Christine Shearer: How do you think about the relationship between climate, climate change, and daily weather?
Jeff Masters: Climate is what you expect; weather is what you get. I like to think of the weather as a game of dice. Mother Nature rolls the dice each day to determine the weather, and the rolls fall within the boundaries of what the climate will allow. The extreme events that happen at the boundaries of what are possible are what people tend to notice the most. When the climate changes, those boundaries change. Thus, the main way people will tend to notice climate change is through a change in the extreme events that occur at the boundaries of what is possible. If you want a longer explanation, think of the weather as a game of dice like craps or backgammon, where Mother Nature rolls two six-sided dice to decide the day’s weather. There are 36 possible combinations of the two dice, and rolls can range from two to twelve. Most often, an ordinary roll like six, seven, or eight comes up; seven is the most common, with a 6 in 36 probability. Rolls of six and eight are only slightly less common, coming up with a 5 in 36 probability. These rolls of the “weather dice” correspond to typical summer weather–high temperatures in the mid- to upper 70s on a nice summer day in New York City, for instance. It is much harder to roll an extreme event–snake eyes (corresponding to a record cold day, with a high near 65), or double sixes (a record warm day, with a high near 100.) These rolls only have a 1 in 36 chance of occurring–about 3%.
Now think about what happens if we take one of the six-sided “weather dice” and paint an extra spot on each side. The old die still rolls a one through six, but the new die now rolls a two through seven. The most likely roll increases to an eight, so we’ve shifted to a warmer climate, getting a typical summertime high of 78 degrees instead of 76. However, the increase in 78 degree days isn’t that noticeable, since we’ve only increased the likelihood of getting an eight on our “weather dice” from 5 in 36 to 6 in 36. But now look at what has happened to extreme events as a result of loading our “weather dice” in favor of higher rolls. Whereas before we had only a 3% chance of rolling a twelve on our “weather dice”–an extreme heat day of 100 degrees in New York City – we’ve now tripled these chances to almost 9%, since there are three possible combinations of the dice that total twelve or higher. Moreover, it is no longer possible to roll snake eyes, corresponding to a record cold day, but it is now possible to roll a 13–a previously unprecedented weather event. Temperatures higher than 106, New York City’s previous all-time high temperature, can now occur.