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Temperature Trends
Oceans account for 61% of the surface area in the Northern Hemisphere and 81% of the area in the
Southern Hemisphere. Over the ocean the temperature patterns generally resemble the land-based
record (these are discussed further in Unit 2-9).
The combined temperature record, Figure 1, shows a relatively stable period from 1860 - 1910 followed by a rise to about 1940. After this time, the record has relatively weak downward trend but with higher variability until about 1975 when the rather abrupt rise to the present value begins.
If we look separately at daily maximum and daily minimum temperatures, Figure 2, we find that a rise in daily minimum temperatures is accounting for most of the warming that has been observed, at least in the US and certain parts of the world. This could be attributed to both rise in greenhouse gases and increases in cloudiness, which will have a larger influence on nighttime minimum temperatures than daytime maximum values.
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