Forecasting skill fades after two weeks. So how can scientists predict future climate? | KNKX

Forecasting skill fades after two weeks. So how can scientists predict future climate?

Jun 12, 2020

We’re in for another cool, wet weekend. Rain and rain showers dominate the forecast through Monday night. High temperatures won’t get past the mid-60s.

This is the kind of forecast most of us have come to rely on as we plan our activities, using radar viewers and other online tools to know what’s coming our way, sometimes down to the hour.

But that skill declines after about two weeks. Most people won’t look beyond a 10-day forecast when making plans, knowing it could easily change after a week or so.

So how can scientists predict climate?

That's one of the number one questions I get when I give public talks,” says KNKX weather expert Cliff Mass. “If we can only forecast the weather a week or two ahead of time, how can we forecast seasonally — for the upcoming season? And how can we forecast climate and climate change decades ahead?”

Mass says these longer-term analyses are a completely different kind of forecasting. Weather forecasting provides detailed predictions of specific configurations, such as where a front or low-pressure zone will be and what the high temperature will be on a given day.  

When we forecast seasonally or we forecast climate change, we don't do that,” Mass says.

“I'm not going to tell you that a low-pressure center is going to be right over Seattle on Oct. 5, 2061," he adds. "But what I will do is try to tell you whether the temperatures that winter in 2061 will be warmer or cooler than they are today or where they'll be wetter or drier.”

Mass works on these predictions for the Pacific Northwest with regional climate simulations that run many different high-resolution computer models at once. These so-called "ensembles" produce comparisons of many different variations, to get an idea of what is coming.

Mass says they do this by looking at the average or mean state of the atmosphere in the future. For climate, this is based largely on projections of how much carbon dioxide and other greenhouse gases will be in the air.

“If there's a lot of CO2 in the atmosphere, that is a warming effect," he says. "And the models know how to distribute that around the planet so we can forecast the mean state.”

The weekly KNKX feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and repeats twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX’s Environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington Professor of Atmospheric Sciences and a renowned Seattle weather prognosticator.  You can also subscribe to a podcast of “Weather with Cliff Mass” shows on AppleSpotify and Google