The Crystal Ball: Forecasts Versus Projections in Determining Environmental Risk

By Dr. Don Wuebbles

When referring to future changes in weather and climate, and the resulting potential impacts on humanity, associated infrastructure, and societal sectors, scientists often refer to forecasts and projections.

The difference between these two terms can often be confusing and somewhat different than how these terms are often used in business, so let’s examine what is meant by each in determining environmental risk.

When scientists discuss the weather, they often provide a forecast. A forecast is a prediction of what the weather will be like near you over the next few days or as long as seven to ten days. Weather forecasts predict short-term atmospheric conditions (days to a week or so) based on current states (initial conditions).

Numerical models of the Earth system are often used in these analyses. These predictions are based on the most likely future outcomes using current data, trends, and historical performance for short-term conditions and potential societal impacts, e.g., due to heatwaves, major storms, extreme precipitation events, floods, droughts, or wildfires.

Scientists do not make forecasts about changes in climate, the long-term averages or statistics of weather. Since the climate is currently changing primarily because of human activities, e.g., especially due to increasing levels of heat-trapping gases called greenhouse gases from our use of fossil fuels, but because the climate couldalso be affected by natural forcings, e.g., from changes in the Sun or due to special events like large explosive volcanic eruptions, it is difficult to accurately predict the future changes in the climate over the coming decades.

Instead, scientists consider a range of possible futures based on trends and other factors, e.g., technology development or policy choices, affecting human activities and associated emissions.

These analyses could also consider the likelihood of a major natural event. Climate projections thus use a numerical model representing the complexity of the Earth system to examine potential future changes in climate to determine long-term trends (for decades or centuries) based on changing boundary conditions at the Earth’s surface these activities and/or events (e.g., from emissions and concentrations of long-lived greenhouse gases), showing probabilities of future weather conditions, including the probability of extreme events like those mentioned above.

Unlike predictions, projections are conditional on assumptions concerning, for example, future socio-economic and technological developments that may or may not be realized, making it useful therefore to consider a range of possible different futures.

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