Climate Shift Puzzle Solved
While looking over global weather data for the 20th century, climatologists began noticing a strange pattern.
The weather in many regions, particularly during June, July and August, changed abruptly around the late 1960s, and the changes rippled throughout the rest of the century.
Over the course of a decade, rainfall plummeted in the Sahel region of Africa, Caribbean, central Pacific and southwest Australia, but fell heavier in the Amazon and Philippines. Sea surface temperatures in the northern hemisphere turned cooler relative to the southern hemisphere seas. Some flows of the large-scale Hadley and Walker atmospheric circulations in the troposphere became weaker, others stronger, while some even reversed direction. The climatic shift was felt as far away as Greenland and the Antarctic.
Scientists believe even more weather deviations during the 1960s could yet be uncovered and that these changes are all linked. The imbalance of ocean surface temperatures between the north and south set up a chain of reactions that reduced tropical rainfall north of the equator while increasing precipitation farther south. The disruptions to rainfall affected tropical heating dynamics that in turn altered the Hadley and Walker atmospheric circulations.
Climatologists discount several possible causes for these shifts including increases in greenhouse gases, variations in solar activity, atmospheric ozone levels and desertification. Instead they find that a natural cycling of temperature fluctuations in the north Atlantic mainly triggered the chain of events. The cycle, which has been traced back to 1650, most recently caused temperatures to peak in 1940 then drop quite low around 1980. The heating of northern waters since then is more than can be attributed to global warming alone, and partially results from this natural cycle.
Another factor coincidentally influenced cooling of the northern hemisphere and explains why the weather changes were most evident from June thru August. In Europe and eastern North America especially, sulphur dioxide released into the air from fossil fuel burning rose dramatically after 1950. It served to increase cloud cover and reflect sunlight back into space, thereby further cooling the north Atlantic region. Due to the enhancing effects of solar radiation on sulphate aerosols, the influence on temperature was especially pronounced during northern summers.
Peter G. Baines and Chris K. Folland. 2007. Evidence for a Rapid Global Climate Shift across the Late 1960s. Journal of Climate. 20(12): 2721-2744.