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Abstract

The mechanism of climate change involves not only an increase in surface temperature but also an adjustment of the global seasons through changes in the timing of the annual temperature cycle. By locating the temperature extrema in monthly temperature records using Lagrangian interpolation in regions where a single maximum and minimum occurs, we define the warming period as the interval between the dates of the temperature minimum and the temperature maximum. The length and mid-point of the warming period are objective measures which can be computed from reanalysis data and from climate model predictions. We have used the 40-year reanalysis dataset from the European Centre for Medium-range Weather Forecasts (ERA40) and the European Centre Hamburg Model (ECHAM5) simulations to investigate the global distribution of the length and mid-point of the warming period for the present climate (1958-2001) and under global warming (Intergovernmental Panel on Climate Change, Fourth Assessment Report, Special Report on Emissions Scenarios; IPCC AR4 SRES A1B scenario) for the period 2157-2200. The results show that the warming period is remarkably uniform in the extratropical temperate latitudes, where it is about 22 days shorter than the cooling period in the ocean and of similar length on land. In the polar region, sharp geographical differences in the length and mid-point of the warming period occur over the ocean and over land which we attribute to the presence of sea ice. Under global warming, these differences are eroded and the temperate regime is extended poleward by about 10 degrees in both hemispheres. This regime change is of particular importance to Canada, on which the discussion on global warming in high latitudes is focused.