Theor. Appl. Climatol. 83, 203–210 (2006) DOI 10.1007/s00704-005-0167-7 Department of Physical and Environmental Sciences, University of Toronto at Scarborough, Scarborough, Ontario, Canada Freeze thaw cycles in Toronto, Canada in a changing climate E. Ho and W. A. Gough With 7 Figures Received February 22, 2005; revised May 18, 2005; accepted May 27, 2005 Published online September 26, 2005 # Springer-Verlag 2005 Summary Freeze thaw cycles are examined in Toronto Canada. Using data from 1960 to 1989 for three Toronto area weather stations, trends in freeze thaw activity, the relationship to mean monthly temperature and projections of freeze thaw activity are examined. For downtown Toronto the annual frequency of freeze thaw cycles is decreasing significantly, most notably in the shoulder months of October and April. At the Pearson International Airport and the Toronto Island Airport similar annual trends were not found, however there was evidence of decreased freeze thaw activity in April and October. Polynomial curve fitting provided functional re- lationships between mean monthly temperature and freeze thaw activity. These relationships enabled the assessment of freeze thaw activity under synthetic warming conditions. The results of this analysis show that the warming of the magnitude typically projected for the rest of this century will not likely generate a significant change in the freeze thaw activity although there are indications that the freeze thaw season will contract. 1. Introduction Canada’s transportation network is estimated to have a value approaching $100 billion (Natural Resources Canada, 2004). Each year, Canada in- vests $19 billion in transportation infrastructure, and it is estimated that $1.7 billion of this is spent adapting to current climate conditions (Andrey and Mills, 2003). One aspect of climate that is important to Canada’s transportation network is the effect of changing temperatures on road infrastructure. Climate is certainly important to the structure of road pavement, where high tem- peratures can cause thermal warping, and fre- quent freezing and thawing results in a loss of pavement strength (Croney and Croney, 1998). Furthermore, increased frequencies of freeze thaw cycles (Hershfield, 1974; Schmidlin et al., 1987; Baker and Ruschy, 1995) are related to prema- ture deterioration of pavement structures, result- ing in increased surface roughness (Hershfield, 1978; Haas et al., 1999). Freeze thaw cycles play a key role in soil properties (Pikul et al., 1989; Hayhoe et al., 1992; Sharrat, 1993; Lehrsch, 1998), the weathering of rocks and minerals (Fraser, 1959) and in maple syrup production (Kozlowski and Pallardy, 1997). Freeze thaw cycles have been defined in a vari- ety of different ways (Baker and Ruschy, 1995). A common feature is that the daily maximum temperature must exceed a temperature threshold which is above 0  C while the daily minimum temperature must be less than a temperature threshold which is less than 0  C. The precise values for the thresholds have varied depending on the focus of each study. This definition is based on the generally observed diurnal cycle in air tem- perature, the direct response to radiative insola- tion. There are instances where this cycle is not