The 2007 IPCC report presents “projections” of climate in the coming decades. Policymakers and politicians are using the IPCC models to plan policy for regions and globally. However, what is the actual skill at forecasting the weather (even averaged over decades) in the coming years? The IPCC uses the term “projection” but it is being interpreted by almost everyone as a prediction if certain CO2 emission scenarios actually occur.
The actual skill at making long-term climate predictions, however, is illustrated by a statement on the website of the United Kingdom Meteorological Office with respect to seasonal prediction. It states
“Seasonal forecasting is a developing area of meteorology and, although these forecasts are not as accurate as our short-term forecasts, they do demonstrate some skill in predicting what may happen for a season (a three-month period) ahead.”
The obvious message from this, which is being almost completely ignored by policymakers, and was certainly ignored by the IPCC, is that seasonal forecasting is “a developing area of meteorology”.
However, how can longer term predictions be more skillful when the climate forcings and feedbacks become more complex the longer into the future we seek to forecast?
Of course, the multi-decadal climate forecasts must be less skillful. This was discussed in
Pielke, R.A., 1998: Climate prediction as an initial value problem. Bull. Amer. Meteor. Soc., 79, 2743-2746.
yet the IPCC community has chosen to ignore this issue. In my 1998 article I write
“One set of commonly used definitions of weather and climate distinguishes these terms in the context of predictions: weather is considered an initial value problem, while climate is assumed to be a boundary value problem. Another perspective holds that climate and weather prediction are both initial value problems (Palmer 1998). If climate prediction were a boundary value problem, then the simulations of future climate will “forget” the initial values assumed in a model. The assumption that climate prediction is a boundary value problem is used, for example, to justify predicting future climate based on anthropogenic doubling of greenhouse gases. This correspondence proposes that weather prediction is a subset of climate predictions and that both are, therefore, initial value problems in the context of nonlinear geophysical flow……
An important practical conclusion results if climate prediction is an initial value problem. This means that there are necessarily limits on the time into the future that we can predict climate, since the feedbacks between the ocean, atmosphere, and land surface are large and nonlinear. These limits have not been determined, yet climate “predictions” are routinely communicated to policy makers on timescales of decades and centuries. Second, in the context of predicting what the future climate would be in response to an anthropogenic forcing such as carbon dioxide input, there are, as of yet, undefined limits on what aspects of future climate we can forecast even if all the important ocean-atmosphere-land surface feedbacks were included and also accurately represented in the models. This leads to the conclusion that weather prediction is a subset of climate prediction. Societally useful (i.e., reliable, accurate, etc.) climate prediction requires that all of the feedbacks and other physical processes included in weather prediction be represented in the climate prediction model. In addition longer-term feedback and physical processes must be included. This makes climate prediction a much more difficult problem than weather prediction.”
The seasonal weather forecasters recognize that “Seasonal forecasting is a developing area of meteorology” as reported on the UK Met Office website. It is the IPCC community which has ignored the reality that multi-year and decadal climate forecasts is an even more daunting challenge.