How should we think about the relationship between climate change and day-to-day experience? Almost a quarter of a century ago James Hansen, the NASA scientist who did more than anyone to put climate change on the agenda, suggested the analogy of loaded dice. Imagine, he and his associates suggested, representing the probabilities of a hot, average or cold summer by historical standards as a die with two faces painted red, two white and two blue. By the early 21st century, they predicted, it would be as if four of the faces were red, one white and one blue. Hot summers would become much more frequent, but there would still be cold summers now and then.
And so it has proved. As documented in a new paper by Dr. Hansen and others, cold summers by historical standards still happen, but rarely, while hot summers have in fact become roughly twice as prevalent. And 9 of the 10 hottest years on record have occurred since 2000.
But that’s not all: really extreme high temperatures, the kind of thing that used to happen very rarely in the past, have now become fairly common. Think of it as rolling two sixes, which happens less than 3 percent of the time with fair dice, but more often when the dice are loaded. And this rising incidence of extreme events, reflecting the same variability of weather that can obscure the reality of climate change, means that the costs of climate change aren’t a distant prospect, decades in the future. On the contrary, they’re already here, even though so far global temperatures are only about 1 degree Fahrenheit above their historical norms, a small fraction of their eventual rise if we don’t act."