Christy says pockets of warm air over Canada and northernmost Europe helped push up the hemisphere's average. May. This is a drop from April when the temperature was 0.19 above average. Christy notes that temperatures in both hemispheres fell below norms starting in late 1991 as material from the Mount Pinatubo reduced the amount of sunlight reaching most of the earth. Northern Hemisphere temperatures have largely rebounded, Christy says, and have been above normal for 12 of the last 20 months. The Southern Hemisphere is probably staying cool because oceans cover a larger area than in the north.
Oceans are always slower to cool down or warm up than land areas.
April was warmest month since June 1991 April was the Earth's warmest month, after seasonal adjustments, since June 1991. The month was 0.21 degrees Celsius warmer than the 10-year average. The graph here shows how monthly temperatures since January 1990 have compared with the average for each month. The dip in temperatures that began in 1991 is blamed on the Mount Pinatubo volcanic eruption, which cooled the globe.