The research on the changes in extreme temperatures was conducted at the Met Office Hadley Centre by Dr Robin Clark. This compared the occurrence of maximum temperatures in the pre-industrial climate to those that would be expected to occur in a world with a global average temperature +4 °C (7 °F) higher that the pre-industrial climate.
The Met Office climate model HadCM3 was run hundreds of times using a scenario of doubling of atmospheric CO2. Those model projections global average temperatures increases of between 3.5 and 4.5 °C, by the end of the century were selected. These were then used to give estimates of increases in the maximum temperature on hot summer days likely to result from an average 4 °C global warming. The estimates from across the projections were ranked from smallest to largest. The lower to upper estimates cover the central 80% range.
Temperature increases in several highly populated regions are very large and, based on impacts of recent heat extreme events, are potentially beyond limits of adaptation. Regional variations in increases are considerable and depend primarily on the likelihood and propensity for local land surface drying.
Ranges in estimates across the model simulations are also large and arise primarily from the model's handling of surface-atmosphere interactions and cloud-feedbacks.
On a location by location basis, there is at present, no evidence to discount the lower and upper estimates shown below, from occurring.
Lower estimate plot
Northern USA/Canada: Even the most benign projections give increases of 4 to 6 °C.
East China: 50% of projections giving increases of between 4 and 6 ºC over highly populated regions.
North America: Increases of at least 8 °C from at least half of the projections.
Upper estimate plot
Europe: Increases greater than 8 °C over European regions impacted by the extreme 2003 heatwaves.
North America: Increases of 10 to 12 °C can not be ruled out over Toronto, Chicago, Ottawa and New York.
Clark, R., S. Brown and J. Murphy, 2006: Modelling northern hemisphere summer heat extreme changes and their uncertainties using a physics ensemble of climate sensitivity experiments. J. Clim, 19, 4418-4435.
Murphy, J.M., D. M. H. Sexton, D. N. Barnett, G. S. Jones, M. J. Webb, M. Collins and D. A. Stainforth, 2004: Quantification of modelling uncertainties in a large ensemble of climate change simulations. Nature, 430, 7.