Prediction vs Projection

Prediction vs Projection

Another way to look at weather versus climate is looking at it as a forecast challenge.

Schematic diagram illustrating the difference between weather prediction and climate projection

Weather forecasters have a strong grasp of the fundamental mechanics of their task. If you know the initial conditions, you can predict how the weather is going to evolve for some time going forward. This is why so much effort is expended obtaining and assimilating observations for numerical weather prediction. And by extension, a crucial step in the forecast process is to evaluate the model initialization by comparing the model analysis against current observations.

Making an ENSO forecast or a seasonal outlook of precipitation is structurally no different than making a daily weather forecast. Why? Because they both assume you have a good understanding of how the basic system works. And because you know how it works, if you know the initial conditions, you can predict how it's going to vary going forward for some time into the future. Whether that's a 24-hr forecast or a seasonal outlook, it's structurally the same. The objective of such a forecast is the same, too: to produce a prediction for a given location for a given time period.

But there's a limit to predictability. You've undoubtedly heard of Lorenz's Butterfly: the allegory of how the flap of the butterfly's wings over Brazil leads to a thunderstorm a week later. Small differences in the initial conditions, things we can't even measure, will grow and contaminate the solution for a forecast. As a result, forecasters can't predict if there will be snow on New Year's day or rain on the summer solstice next year. That's not because the weather model is no good, or we don't understand the physics well enough. It's due to the chaotic nature of the system.

When it comes to longer-term climate problems, those determined over decades to centuries, it's a different challenge. We're not interested whether in the year 2083 we're going to have El Nino or La Nina, so the initial conditions don't concern us. We're interested in how the statistics of the climate system will change in response to changing boundary conditions.

Using our knowledge of how the climate system works, we want to know how factors that change the flow of energy will change the climate statistics. If the sun gets brighter, incoming energy increases. If the amount of carbon dioxide in the atmosphere increases, it changes how the atmosphere absorbs infrared radiation and, hence, changes the flow of energy through the system. If forest, which is dark, is replaced by crops and pastures that are brighter, the surface albedo changes. These are all examples of changes in boundary conditions. The physics are the same as those in weather prediction, but we're solving a different problem.

To distinguish between the questions addressed by weather and climate models, scientists use different terms. Projection refers to how the statistics of the climate system will change in response to changing boundary conditions. Prediction refers to the short-term evolution of the climate system from an initial state under constant boundary conditions.