Introduction to Decadal Climate Variability

Research on decadal climate variability (DCV) dates back to at least the 19th century, perhaps to as far back as when sunspots were first seen to vary in number, size and location on the Sun.  One anecdote suggests that Meton of Athens, an engineer in charge of the Athenian irrigation system, speculated in 400 B.C. about solar variability as the cause of rainfall variability in and around Athens. 

Observers have identified sunspots cycles of roughly 11 and 22 years, and fluctuations in lunar tidal strength of 18.6 years, as well as lower harmonics of these cycles. Because of the hypothesized influence of the 11- and 22-year solar cycles and the 18.6-year lunar cycle on earth’s climate, early researchers invoked the phrase “decadal climate variability” to describe climate “cycles” with 10 to 20-year repeat intervals.

Figure 1 shows perhaps the oldest definition of decadal climate patterns based on the 11-year sunspot cycle, dating back to at least Renaissance times.  This particular example is of sea-surface temperature (SST) anomalies in the tropical Pacific, overlaid with solar radiation fluctuations. They appear to be in synch during the 1960s and 1970s, but move out of phase in the 1980s and 1990s. Such phase shifts are why solar and lunar explanations fell out of favor around the mid-20th century.

Figure 1 - Overlaying solar radiation fluctuations with changes in sea surface temperatures in the second half of the 20th century suggests that a much longer record would be needed to draw a meaningful conclusion about whether the two are normally in or out of phase.

Another way to define a climate variation lasting approximately a decade is from the duration of “bursts” or groups of more frequent phenomena, such as El Niño-La Niña events, hurricanes and other tropical cyclones, extreme precipitation or heat.  The example shown in Figure 2 depicts the so-called Niño3 SST index, a measure of the El Niño-La Niña variability in the tropical Pacific.  In this time series, there are two bursts of El Niño events (red) from mid-1890s to 1905 or so, and from the mid-1970s to the late-1990s.  There is also one burst of La Niña events (blue) from approximately the early 1940s to the late 1950s.  Each of these bursts lasts approximately 10-20 years. 

Figure 2 - This figure shows the “Nino 3 Sea Surface Temperature Index,” with El Nino, higher temperatures, in red, and La Nina, lower temperatures, in blue. It shows two El Nino bursts, one from the mid-1890s to 1905 and another from the mid-1970s to the late 1990s, and one La Nina burst from the early 1940s to the late 1950s.

History is full of pivotal events related to climate -- precipitation, surface temperature, river flow, droughts, and floods that have affected the course of civilizations.  Decade-long droughts in the North American Great Plains led to out-migration of people, especially in the 1890s, the 1930s, the 1950s, the 1980s, and the years since 2001-02. The Pacific Northwest experiences decadal climate variation in precipitation, stream flow, fish catch, and forest fires, and the Southwestern United States and Mexico, in precipitation. Socio-economic-political instability in the Nordeste region of Brazil is linked to multiyear to decadal droughts.  The suffering caused by the Sahelian droughts in the 1970s-1980s-1990s is well-known. Long-term variability in numbers of hurricanes and other tropical cyclones in the Atlantic, Pacific, and Indian Oceans is also well-known. 

Scientists hope that by arriving at a better understanding of the concurrent cycles of the earth’s many subsystems, they will be able to provide more reliable forecasts of precipitation, temperature, and other climate events a season, a decade or longer in advance. Such information could be used to anticipate food shortages, avoid crop losses, protect habitats and reduce suffering and mortality.

Subsequent articles in this series will describe major DCV phenomena, their known societal impacts, and DCV forecast efforts.

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