Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”
When people think about the weather, two variables are first to come to mind—temperature and precipitation. Unless it’s sunny when it’s not supposed to be (or vice-versa), near-term temperature forecasts tend to be pretty good. What messes up your day is when it rains when it’s not supposed to, and what really screws things up is when there is a significant unforecast snow, or a lot more (or less) than there was supposed to be. If whatever oracle you consistently consult, like The Weather Channel or Channel 9, consistently blows the precipitation forecast, you’ll soon be looking elsewhere for your forecast, and if changing forecasters doesn’t help, you’re going to sour on the whole weather forecasting business
Climate forecasts made by climate models running under scenarios of increasing human emissions of greenhouse gases are blowing both their temperature and precipitation prognostications. They tend to predict far more warming to be taking place than is actually occurring, and when it comes to precipitation, the projections are all over the place—a characteristic dislexically summed up in the Second Assessment Report from the U.N.’s Intergovernmental Panel on Climate Change (IPCC)
Warmer temperatures will lead to a more vigorous hydrological cycle; this translates into prospects for more severe droughts and/or floods in some places and less severe droughts and/or floods in other places.
So, according to the IPCC, whatever happens to precipitation will have been correctly forecast!
In some areas of the U.S., it is actually possible to pin down specific climate model expectations for precipitation changes. Unfortunately (for the models), the actual observations show little if any correspondence to the magnitude, or even direction, of the modeled changes.