I have just been working again on the HADCM3A2 scenario using GRASS. On taking a closer look at the model results I am surprised by dramatic consequences predicted for Mexico under this scenario in 100 years time. I subtracted the last fifty years mean monthly temperatures and mean monthly rainfall (measured as mm per day) from the means for 2000-2049 and 2050-2100 as simulated by this model. The red to green shaded figures show changes in rainfall with red as drier and green moister (note that the colour scheme is not necessarily strictly negative to positive). The blue to red maps show changes in temperature. The file name is in the top right hand corner prec50mx.1 means changes in the first fifty years in precipitation during the month of January, prec50mx.2 February etc. Note the scales on each map change depending on the absolute range.
So the fifty year scenario suggests a slight reduction in rainfall and moderate (0.5 C) warming.
Now look at the simulation after 100 years.
The reductions in rainfall are potentially severe and warming may be between 3 and 5 C. Even the cooler areas are positive warming, not cooling. This is the most extreme of the new IPCC SRES scenarios and one that is fundamentally similar to the old “business as usual” type scenarios.
The most surprising element of the HADCM3 model results, (that is also shown by the model under lower carbon emission scenarios) is the tendency towards drier conditions in Southern Mexico and Central America. Not all GCMs show the same regional pattern and the level of resolution of these models is still very low. Thus the results should be taken as representing one of many credible futures (see post on the ensembles project). A full treatment of the uncertainties involved in predicting the impact of climate change on the distribution and abundance of organisms should take into account as many credible futures as possible. However the prospect of a hotter, drier climate is very disturbing for conservation as many of the most vulnerable organisms are found under cooler, moister conditions.