How can Earth's climate be predicted decades from now when weather can't be predicted 2 weeks from now?

January 23, 2014

Weather and climate occur on different scales of time and space, and depend on different aspects of Earth's environment. Weather describes atmospheric conditions at a particular time and place. Climate is the overall statistical characteristics of weather and environmental conditions, such as long-term averages and ranges of variability, for a given place and season.

Weather forecasters look at initial conditions and then make short-term deterministic predictions about future events (from minutes to days, to less than 2 weeks). Due to the fluid and chaotic nature of the atmosphere, it is impossible to make accurate deterministic predictions about weather events more than 2 weeks in the future. Climate forecasters look at the state of the key controlling parameters of the climate system and then make long-term probabilistic predictions about future conditions (from more than 2 weeks to years, to decades). People do this all the time with high reliability. For example, while you don't know what the weather will be a like on a given day in Washington, D.C., in the year 2020, you can be confident that temperatures will be warmer in the summer than in the winter.

Blossoms on the National Mall

It's a reasonably safe bet that flowering trees will flower on the National Mall in the springtime, and bear fruit in the autumn. Photo CC license by Axel Drainville.

Another way to understand the differences between deterministic and probabilistic predictions is to consider how other types of experts use them. For example, a doctor may have a hard time determining exactly when a particular overweight, middle-aged person who smokes cigarettes is going to die. But a life insurance agent can easily make a probabilistic prediction of the person's life expectancy based on the average life spans of millions of overweight, middle-aged people who smoked.