Global Warming: Experts’ Opinions versus Scientific Forecasts

Policy Reports | Global Warming

No. 308
Friday, February 01, 2008
by Kesten C. Green and J. Scott Armstrong


More than 20 years ago, scientists began to express concern that human activities — primarily tropical deforestation and the burning of fossil fuels for energy — threaten to cause a rapid warming of the Earth by adding carbon dioxide (CO2) to the atmosphere.  Recognizing the problems that global warming might cause, in 1998 the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) established the Intergovernmental Panel on Climate Change (IPCC).2 The purpose of the IPCC was to provide a comprehensive, objective, scientific, technical and socio-economic assessment of the current understanding of human-induced climate change, its potential impacts and options for adaptation and mitigation.

“Climate change policies must be based on accurate, scientific forecasts.”

In 2007, the IPCC issued its Fourth Assessment Report.  The Assessment in fact consists of three reports and a “synthesis” report.  The first part was titled “The Physical Science Basis” and was authored by the IPCC’s Working Group One (WG1), a panel of experts on climate science, modeling and history.  This paper focuses on the first report.3 It included predictions of dramatic increases in average world temperatures by 2100, which might in turn cause such serious environmental harms as:  a global sea level rise that would threaten low-lying coastal areas, the spread of tropical diseases, an increasingly rapid loss of the world’s glaciers and ice caps, and a worsening of drought and flooding events across broad regions.4

Although the IPCC’s 1,056-page report makes these dire predictions, nowhere does it refer to empirically-validated forecasting methods, despite the fact these are conveniently available in books and articles and on Web sites.  These evidence-based forecasting principles have been validated through experiment and testing and comparison to actual outcomes.  The evidence shows that adherence to the principles increases forecast accuracy.  This paper uses these scientific forecasting principles to ask:  Are the IPCC’s forecasts a good basis for developing public policy?  The answer is “no.”

Three elements are necessary for governments to make rational policy responses to climate change:  Scientists must accurately predict (1) global temperature, (2) the effects of any temperature changes and (3) the effects of feasible alternative policy responses.  At any step in this process, the failure to obtain a valid forecast would render forecasts at the next step in the process meaningless.  This study focuses primarily, but not exclusively, on the first of the three forecasts required:  obtaining long-term forecasts of global temperature.  It finds that due to the unscientific method by which these forecasts were obtained, they cannot be relied upon.  [See the sidebar, “Three Forecasts Required for Climate Change Policies.”]

Read Article as PDF