Simulated Changes in the Frequency of Extremes and Regional Features of Seasonal/Annual Temperature and Precipitation when Atmospheric CO₂ Is Doubled

Following a transient increase in the atmospheric carbon dioxide to double the current level, and a subsequent maintenance at the doubled level, there is a climate shift toward a new equilibrium state. Changes in the mean temperature and precipitation, and changes in the occurrence frequencies of their extremes for the doubled carbon dioxide conditions have been assessed at the continental scale. There is a characteristic spatial pattern that involves a maximum annual mean warming in high northern latitudes and a minimum annual mean warming around Antarctica and in the northern North Atlantic. Under maintained doubled carbon dioxide, this interhemispheric asymmetry disappears except for an ocean–land asymmetry. A possible mechanism for this is considered in terms of changes in effective thermal capacity due to a reduction of overturning in the oceans that continues to decline after the atmospheric carbon dioxide stops increasing. It is also found that global warming becomes most noticeable in the occurrence frequency of high extremes in the annual mean temperature in the low latitudes, even though the temperature rise is largest in the high northern latitudes in autumn and winter. In addition, extremes of large (small) annual and seasonal total precipitation are recorded much more frequently in regions where the mean precipitation increases (decreases).