Prediction versus Projection – Forecast versus Possibility

Mike MacCracken
U.S. Global Change Research Program

In public discussion about weather and climate, the words scenarios, projections, predictions, and forecasts are often used interchangeably, as if they are completely synonymous. I believe that important distinctions must be recognized if scientists are to talk clearly among themselves and communicate effectively with the media and, through them, the public. The Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Assessment have tried to recognize the distinctions, in keeping with distinctions that much of society accepts in other areas of planning for the future. However, much confusion is being created by some in our field who are not always as precise as they should be in using these terms.

Here are my views of how we should be using the various terms:

• A prediction is a probabilistic statement that something will happen in the future based on what is known today. A prediction generally assumes that future changes in related conditions will not have a significant influence. In this sense, a prediction is most influenced by the "initial conditions" – the current situation from which we predict a change. For example, a weather prediction indicating whether tomorrow will be clear or stormy is based on the state of the atmosphere today (and in the recent past) and not on unpredictable changes in "boundary conditions" such as how ocean temperatures or even society may change between today and tomorrow. For decision makers, a prediction is a statement about an event that is likely to occur no matter what they do.

• Related to a prediction is a forecast, which I would suggest is a "best" prediction made by a particular person or with a particular technique or representation of current conditions. An example of a forecast is a statement by a weather forecaster that it will rain at 3:30 PM tomorrow – that is that individual's best judgment, perhaps drawn from a prediction that there is a 70% chance of rain tomorrow afternoon. For a decision maker, the credibility of the forecast depends critically on the credibility of the forecaster (or forecasting technique) as well as on the inevitability of the event. The recent development of "ensemble forecasts" (i.e., assembly of a set of forecasts that are each based on a separate technique or set of initial conditions) can be considered a step toward transforming forecasts into predictions.

• In contrast to a prediction, a projection specifically allows for significant changes in the set of "boundary conditions" that might influence the prediction, creating "if this, then that" types of statements. Thus, a projection is a probabilistic statement that it is possible that something will happen in the future if certain conditions develop. The set of boundary conditions that is used in conjunction with making a projection is often called a scenario, and each scenario is based on assumptions about how the future will develop. For example, the IPCC recently projected a range of possible temperature changes that would likely occur for a range of plausible emissions scenarios and a range of model-derived estimates of climate sensitivity (the temperature change that would result from a CO2 doubling). This is clearly a projection of what could happen if certain assumed conditions prevailed in the future – it is neither a prediction nor a forecast of what will happen independent of future conditions. For a decision maker, a projection is an indication of a possibility, and normally of one that could be influenced by the actions of the decision maker.

• The National Assessment was even more cautious in its statements, very carefully labeling the climate results that it used to investigate potential vulnerability as climate scenarios from the Canadian and Hadley models. This was done because the climate results were drawn from only two climate models (so did not represent the full possible range of climate sensitivities) and each model had treated only one particular emissions scenario. Scenarios are best thought of as "plausible alternative futures – each an example of what might happen under particular assumptions" [as explained in the National Assessment report]; scenarios are not predictions or forecasts because they depend on assumed changes in key boundary conditions (like emissions) and scenarios are not fully projections of what is likely to happen because they have considered only a limited set of possible future boundary conditions (e.g., emissions scenarios). For the decision maker, scenarios provide an indication of possibilities, but not definitive probabilities.

I believe that much of the confusion and debate about global warming is arising because not enough care is being taken in understanding these distinctions. For example, a Gallup survey of scientists about a decade ago is still cited as indicating that many scientists have low confidence in predictions for the 21st century. Such a result is not at all surprising given how scientists define and understand predictions. Of course, we cannot offer a confident prediction for the 21st century because it depends on energy technologies and political decisions as well as how the climate will respond. However, without contradicting the survey results, the IPCC can report international consensus on its projections of a temperature rise during the 21st century if emissions follow the reported emissions scenarios. Similarly, the National Assessment is not making forecasts or predictions of what will happen during the 21st century (either nationally or regionally), but is using model results to explore the possibilities and implications of what types of consequences could occur (and it develops a lexicon of relative likelihood for these outcomes).

While these nuances easily can get lost in public discussion, I think that it is nonetheless incumbent on all of us to make sure we use the terms precisely, carefully explaining (and re-explaining) what is being done, and the limits of what conclusions can and cannot be drawn. Elsewhere in society – for example, in military and financial planning – scenario-based projections are widely used and it is understood (except perhaps by naïve investors) that these are projections of what could happen and not predictions of what will happen. Although we might all wish we could provide reliable (and verified) predictions, the complexities of society and the climate are such that we are forced to rely on projections if we want to use our understanding to look forward into the future; otherwise we are limited to advancing blindly because we can rely only on mindless extrapolations of changes that have been observed in the past.

– Mike MacCracken
U.S. Global Change Research Program
mmaccrac@usgcrp.gov

Comments? thunder@ucar.edu

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