Climate change may dramatically affect the distributions and abundances of organisms.  With the world’s population size expected to significantly increase during the next 100 years, we need to know now how climate change might impact our food production systems.  In particular we need estimates of how future climate might alter the distribution of agricultural pests.  We used the climate projections from two general circulation models (GCMs) of global climate, the Canadian Centre for Climate Modelling and Analysis GCM (CGCM2) and the Hadley Centre model (HadCM3), for the A2 and B2 scenarios from the Special Report on Emissions Scenarios in conjunction with a previously published bioclimatic envelope model (BEM) to predict the potential changes in distribution and abundance of the swede midge, Contarinia nasturtii, in North America.
Results/Conclusions

The BEM in conjunction with either GCM predicted that C. nasturtii would spread from its current initial invasion in southern Ontario and northwestern New York State into the Canadian prairies, northern Canada, and midwestern United States, but the magnitude of risk depended strongly on the GCM and scenario used.  When the CGCM2 projections were used, the BEM predicted an extensive shift in the location of the midges’ climatic envelope through most of Ontario, Quebec, and the maritime and prairie provinces by the 2080s.  In the United States, C. nasturtii was predicted to spread to all the Great Lake states, into midwestern states as far south as Colorado, and west into Washington State.  When the HadCM3 was applied, southern Ontario, Saskatchewan, and Washington State were not as favourable for C. nasturtii by the 2080s.  Indeed, when used with the HadCM3 climate projections, the BEM predicted the virtual disappearance of ‘very favourable’ regions for C. nasturtii.  The CGCM2 projections generally caused the BEM to predict a small increase in the mean number of midge generations throughout the course of the century, whereas, the HadCM3 projections resulted in roughly the same mean number of generations but decreased variance.  Predictions of the likely potential of C. nasturtii spatial spread are thus strongly dependent on the source of climate projections.  This study illustrates the importance of using multiple GCMs in combination with multiple scenarios when studying the potential for spatial spread of an organism in response to climate change.