A recent Intergovernmental Panel on Climate Change report¹ highlighted the importance of understanding uncertainties when estimating the potential impacts of climate change. This paper examines the implications of using different techniques and methodological assumptions to develop climate change scenarios for two indicators relevant to human health-cooling degree days (CDD) and heat stress days (HSD). The choice of climate model, experiment, emission scenario, base climate period, base climate station(s), use of downscaling procedures, adjustment of climate variability, and selection of time period (e.g., 2020s or 2080s) for impact analysis are compared in terms of projected CDD and HSD for the Greater Toronto Area. The degree of variation in projected CDD and HSD provides a partial measure of the uncertainty associated with applying climate change scenarios in impact assessments.

¹McCarthy, James J., Osvaldo F. Canziani, Neil A. Leary, David J. Dokken and Kasey S. White (eds.) 2001. Climate Change 2001: Impacts, Adaptation & Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), Cambridge University Press, Cambridge,UK.