The spatial and temporal characteristics of northern Hemisphere surface air temperature variations

The spatial and temporal characteristics of variations in surface air temperature over the Northern Hemisphere landmasses during the present century are determined. The spatial patterns of change accompanying major warming and cooling episodes are mapped. The spatial representativeness of the Northern Hemisphere average is assessed and implications for proxy climate studies are considered. Finally, the seasonal breakdown of trends and correlations between monthly and annual data for the Arctic and Northern Hemisphere is considered. It is shown that, although generally similar, significant differences in the spatial patterns of change accompanying warming and cooling in the Northern Hemisphere average have occurred. In particular, the early 20th century warming was more coherent spatially (as well as more rapid and stronger) then either the cooling that followed or the recent warming. Various regions tend to vary consistently with the Northern Hemisphere average but the relationships are not as marked or widespread as some investigators have claimed or assumed. The annual Northern Hemisphere average is shown to be more strongly correlated with late spring and summer temperatures than with winter temperatures (although in the latter season the variability is greatest). Trends in temperature over the Northern Hemisphere have been strongest in April and May, whereas over the Arctic they have been strongest in January. This seasonal distribution may give an indication of the physical mechanisms underlying these climatic variations.