Posted by Olive Heffernan on behalf of Kevin Trenberth

Given that human induced climate change is with us, a looming challenge is to predict just what the climate will be. To date, there are no such predictions although the projections given by the Intergovernmental Panel on Climate Change (IPCC) are often treated as such. The distinction is important. A paper presented at the International Forecasting Symposium in New York City in late June 2007 by J. Scott Armstrong and K. C. Green is highly critical of IPCC procedures and “forecasts” for not being based on “evidence based” procedures as outlined in an earlier 2001 book of his. It is true that IPCC does not refer to Armstrong’s work as it has dubious relevance.

In fact IPCC does not do forecasts, as explained in my earlier post. The IPCC instead proffers “what if” projections of future climate that correspond to certain emissions scenarios. Armstrong has evidently read only chapter 8 of the IPCC Working Group I report and has therefore overlooked the fact that the other chapters address many of the things he is critical of.

In particular there is clear evidence (“warming is unequivocal”) that climate is changing in ways consistent with the climate forcings. Also, the projections are for all aspects of climate, not just global mean temperature. It has been said that “All models are wrong, but some are useful”. The Armstrong forecast of no change is not useful when the system is already changing in ways consistent with human influences on the composition of the atmosphere. Nonetheless, improvements in forecasting procedures are always welcome.

Bob Carter, a climate change doubter in Australia, has written a distortion of all this in the Courier Mail, issuing various attack against the science of climate change. Andrew Ash has written a rebuttal of these comments.

Another key point is that unlike forecasts based on past experience, weather forecasts are based on numerical weather prediction models and rigorous procedures, not empirical methods, although the latter are used to provide added value (e.g., based on known biases in the model). My own presentation at the same conference provides a description of weather and climate prediction.

The same atmospheric models are the atmospheric component of climate models and they are well tested and evaluated, although in climate models lower resolution is used.

The authors should recognize that IPCC does not make forecasts but rather makes projections to guide policy and decision makers. If those changes are considered undesirable, it can create efforts to change that outcome. Such mitigation is already happening in the U.S. Congress, in many states, and internationally under the Kyoto Protocol. Hence the projection will not be correct as actions are being taken to make it so. As such it is not a forecast of what will actually happen.

Kevin E. Trenberth

Head of Climate Analysis

NCAR