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Death of the PDO: Has the 30-year heat wave ended?

Posted: Friday, August 29, 2008

One of Mark Twain's more famous one-liners says that "Reports of my death have been greatly exaggerated."

Grim prediction by some climatologists that North Pacific sea surface temperatures (and hence Alaska weather) will be cooler for the next several decades may likewise turn out to be exaggerated. But then again, maybe not.

The death in question is that of the warm phase of the "Pacific Decadal Oscillation," or PDO. The PDO is long-term cycle of North Pacific ocean temperatures that oscillates between warm and cold phases every few decades, with a complete cycle of 50 to 70 years. The PDO turned strongly warm in 1977, and it had a big effect on Alaska and the Pacific Northwest. Post-1977 annual air temperatures at the Kenai airport, for example, have increased by 2.5 degrees Fahrenheit. The warming effect has been especially strong in the winter, with December and January temperatures increasing by 4.9 and 6.8 degrees, respectively.

After 1977, warmer ocean temperatures brought increased salmon runs and more halibut to Alaska, but the shrimp fishery was hammered because shrimp like colder water. The warm North Pacific has also brought more southern fish to Alaska, such as various shark species.

Nathan Mantua, a climatologist at the University of Washington, designed a statistical index that summarizes the monthly ocean temperatures from dozens of stations and buoys around the North Pacific (see graph). He revises this index every few months and displays it on his Web site at http://www.jisao.washington.edu/pdo/.

Mantua deliberately removes any long-term trend from this index, such as might be due to global warming, because he wants to emphasize year-to-year changes and turning points, which can have fairly immediate implications for fish stocks, winter fuel demand, and other weather-dependent economic factors.

Mantua's PDO graph does indeed show a recent downturn in the PDO, beginning in August 2006, with only a slight recovery last summer in June, July and August, before substantially deepening through last winter and this summer, with the lowest point so far in July of this year.

You can see on the graph that there have been two previous substantial downturns of the PDO (1988-90 and 1998-2000) similar to the present downturn, and they were followed by strong recoveries. These past recoveries provide hope for a recovery from the present downturn. On the other hand, if the warm and cool phases really are 25-35 years long, we are quite ripe for a downturn at 31 years since the 1977 warm upturn.

Just how well established is the PDO cycle? If it was a well-understood cycle like ocean tides with centuries of observational data and solid physical theory, we could feel confident about predicting the next turn of this climatic tide. Unfortunately, climate cycles are much more complex than ocean tides; they have a much shorter historical data record, and the physical causes are simply not known.

Mantua's PDO index starts in 1900 because prior to 1900 there weren't enough meteorological stations around the North Pacific to reliably calculate the index.

To get a handle on the PDO prior to 1900, investigators have used a variety of substitute or "proxy" records to estimate older PDO patterns. The best proxies are tree-ring records because tree-ring widths provide a rough annual record of temperatures in moist climates and of precipitation in dry climates. PDO tree-ring records extend back to 933 A.D. Other proxies include drought/flood records from China (to 1470 A.D.), corals from Marianas (to the 1840s), and geoduck clams from Puget Sound (to the 1870s).

The proxy records indicate that the PDO has been a fairly stable 50- to 70-year cycle for the last 200 years or so, but prior to that it was much more variable. During various times from the 1200s to 1700s, the PDO weakened or shifted to a shorter time scale of two to three decades. During a severe extended drought in the western U.S and Canada between 933 and 1300 A.D., the PDO was strongly negative; the long-term cycle was still present with varying degrees of negativity and rarely turned positive.

Given the variable track record of the PDO over the last millennium, most (but not all) climatologists are hesitant to make any strong proclamations about the "death" of the post-1977 PDO warm phase and the onset of several decades of cooler North Pacific weather, especially in the face of global warming, because there simply aren't any well-tested models that can predict the PDO several years in advance, let alone several decades.

But, setting scientific caution aside, what are the consequences if the PDO has in fact shifted gears down into a cool phase? Old-timers on the Kenai will remember the cooler and wetter summers of the PDO cool phase of 1950s to mid-1970s, as well as the substantially colder winters. Global warming may ameliorate such temperatures, but weather like that of last winter and this summer is typical of a PDO cool phase.

If we have indeed entered a PDO cool phase, winter fuel demand will increase, which will put more pressure on fuel oil prices. Interest in building super-insulated homes with solar panels and wind generators will likely increase dramatically.

There will still be warm El Nino periods but they won't be as warm as those of recent decades, and the La Nina periods, such the present time, will probably be colder than recent La Ninas.

Politically, global warming skeptics will cheer and say that global warming has been disproved. In the long run, however, the skeptics will probably have to eat serious crow when the PDO shifts into its next warm phase in several decades and global temperatures really start cooking, like a superheated version of the 1977 warming.

Environmentalists will worry that the public and the politicians will lose interest in reducing greenhouse gases if the weather turns cooler for several decades. The issue of "peak oil" and ever-increasing oil prices, however, will no doubt keep people well-focused on reducing their carbon-based energy consumption and hence their greenhouse gas emissions, regardless of their opinions about the reality of global warming.

Ed Berg has been the ecologist at the Kenai National Wildlife Refuge since 1993. Climate cycles will be among the topics of his one-credit "Cycles of Nature" course taught at the Kenai Peninsula College campuses in Soldotna and Homer, starting Sept. 9 and 11, respectively.

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Previous Refuge Notebook articles can be viewed on our Website, http://kenai.fws.gov/. You can check on new bird arrivals or report your bird sighting on the Kenai National Wildlife Refuge Birding Hotline at 907-262-2300.



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