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| Did a long term cooling trend start in 2007? |
arcura |
05/21/08 |
SATELLITE INDICATES 23-YEAR GLOBAL COOLING
May 5, 2008
BY DENNIS T. AVERY
CHURCHVILLE VA—Now it’s not just the sunspots that predict a 23-year global cooling. The new Jason oceanographic satellite shows that 2007 was a “cool” La Nina year—but Jason also says something more important is at work: The much larger and more persistent Pacific Decadal Oscillation (PDO) has turned into its cool phase, telling us to expect moderately lower global temperatures until 2030 or so.
For the past century at least, global temperatures have tended to mirror the 20-to 30-year warmings and coolings of the north-central Pacific Ocean. We don’t know just why, but the pattern of the last century is clear: the earth warmed from about 1915 to1940, while the PDO was also warming (1925 to 46). The earth cooled from 1940 to 1975, while the PDO was cooling (1946 to 1977). The strong global warming from 1976 to 1998 was accompanied by a strong and almost-constant warming of the north-central Pacific. Ancient tree rings in Baja California and Mexico show there have been 11 such PDO shifts since 1650, averaging 23 years on length.
Researchers discovered the PDO only recently—in 1996—while searching for the reason salmon numbers had declined sharply in the Columbia River after 1977. The salmon catch record for the past 100 years gave the answer—shifting Pacific Ocean currents. The PDO favors the salmon from the Columbia for about 25 years at a time, and then the salmon from the Gulf of Alaska, but the two fisheries never thrive at the same time. Something in the PDO favors the early development of the salmon smolts from one region or the other. Other fish, such as halibut, sardines, and anchovies follow similar shifts in line with the PDO.
The PDO seems to be driven by the huge Aleutian Low in the Arctic—but we don’t know what controls the Aleutian Low. Nonetheless, 22.5-year “double sunspot cycles” have been identified. |
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