TCS Daily

Climate Cycle or Climate Psychic?

By Robert E. Davis - May 12, 2005 12:00 AM

In light of the general hysteria over global warming, it's nice, once in a while, to be able to couch our current and ongoing climate changes into some larger perspective. We keep hearing about historically warm years, warm decades, or warm centuries, uncharacteristically long or severe droughts, etc. for which mankind's striving for a high quality of life is to blame, via the internal combustion engine and its by-product, carbon dioxide. But in reality, in most cases, we have a tragically short record of good observations to really determine how much of a record we're even close to setting.

For example, let's take one fairly recent climate discovery called the Pacific Decadal Oscillation, or PDO. In the late 1990s, some west coast fisheries researchers noted cyclical behavior in the annual salmon harvest and tied it to a Pacific Ocean climate anomaly. It turns out that when the Pacific Ocean off of Southern California and the Baja Peninsula is warmer than normal at the water surface, temperatures are typically lower than normal in the north central Pacific well south of the Aleutians. This state, called the positive phase of the PDO, is also linked to dry conditions in the Pacific Northwest and the northern Rockies and above average rainfall in the Desert Southwest (see Figure1). In the opposite situation, negative PDO, you simply flip the sea-surface temperatures and precipitation patterns.


Figure 1. Climate patterns associated with the PDO in its positive phase. The patterns are reversed when the PDO is negative (Source: MacDonald and Case, 2005).


What's interesting and potentially useful about the PDO is that it's behavior is quasi-cyclic, as the name suggests -- it oscillates from positive to negative back to positive every 50 to 70 years or so. Thus, the PDO should be a useful tool for forecasting water resources in the western U.S., where water is more precious than fresh salmon.

With the phenomenal accuracy afforded by hindsight, we now know that, sometime around 1977-78, our planet underwent an abrupt shift from one climatic state (generally cold) to another (warm), and much of the "action" was centered in PDO territory in the north Pacific Ocean. In the late 1970s, the PDO switched from negative to positive, and the snowpack in the northern Rockies hasn't recovered.

Of course, this climate shift was retrospectively blamed on increasing greenhouse gases, because such dramatic and abrupt shifts just couldn't be "natural." Presumably Nature, left to her own devices, does not cotton to wild mood swings.

But is global warming really to blame? Not likely, based on some new analyses by UCLA geographers Glen MacDonald and Roslyn Case. Using tree rings gathered from a hydrologically sensitive species of pine in California and Alberta (near two of the centers of high and low rainfall associated with the PDO), MacDonald was able to reconstruct the PDO all the way back to 993 A.D. Now this is a long climate record.

Their reconstruction (see figure 2) has two very important implications on our understanding of contemporary climate. First, the "great" climate shift of the late 70s that sent climatologists ballistic pales in comparison to many of the changes observed over the past 1000 years or more. Based on this graphic, the PDO is diving and leaping more than an Italian midfielder during the World Cup. It's awfully hard to see any evidence of global warming in the last 150 years of that record. Second, the 50 to 70-year quasi-periodicity of the PDO was not present in the 13th, 17th, and 18th centuries (also NOT related to greenhouse gases). It's also interesting and supportive of their analysis that a medieval mega-drought in western and central North America from about 900 to 1300 A.D. is evident in the PDO record which was negative over that entire time period, and, of course, unrelated to greenhouse gases.

Figure 2. Reconstructed PDO index (Source: MacDonald and Case, 2005).

Several things we thought we knew about the PDO we do not, in fact, know at all. The unprecedented great Pacific climate shift of the late 1970s linked to global warming was, in fact, precedented and unrelated to global warming. And the useful predictability of the PDO for predicting west coast drought may not be useful at all, since the PDO will, apparently on a whim, suddenly become non-periodic for centuries at a time (unrelated to global warming).

The biggest problem with all of these somewhat cyclical climate shifts is that no one knows for sure that a shift has actually taken place until many years AFTER the event, when it's too late to be useful. So be wary of global warming psychics warning us of unprecedented climate shifts -- in most cases, they are only unprecedented because of the short life span of most scientists. Remember one of the absolutely fundamental and too-often unstated tenets of science -- there's little point in studying anything that doesn't vary during a scientist's lifetime.

The author is Associate Professor Environmental Sciences, University of Virginia.


MacDonald, G.M. and R.A. Case, 2005. Variations in the Pacific Oscillation over the Past Millennium. Geophysical Research Letters, 32, L08703,doi:10.1029/2005GL022478.


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