It is common to observe in the comments of older people some reports about climate change, such as: “in my time, it didn't go so many days without rain in this region”; “in my time, the winds weren't that strong and the weather wasn't even that dry”. Despite the influences of man on the climate, these reports can be representative of a natural climatic phenomenon, the Pacific Decadal Oscillation (ODP).
The ODP is a phenomenon very similar to the El Niño and La Niña (ENSO) events, as it is a variation of temperatures in the Pacific Ocean. However, unlike the two phenomena mentioned, the PDO (acronym for Pacific Decadal Oscillation in English) has a climate variation a little longer, lasting about 20 years, while ENSO usually lasts between 6 and 18 months.
There are two well-defined phases of the PDO: a positive – when there is a rise in Pacific temperatures – and a negative – when there is a decrease in temperatures. Such variations are related to factors such as marine currents, volcanism on the ocean floor and, mainly, solar activity. Thus, due to the fact that the Pacific Ocean occupies about a third of the Earth's surface, ODP variations directly influence the climate of the continents.
Effects of positive ODP: an increase in the number of occurrences and intensity of El Niño and a consequent decrease in La Niña. Thus, it is observed in the affected regions - among them the Brazilian territory - a greater number of dry years than rainy years (which helps to explain, for example, some periods of long drought in the Northeast and the decrease in winter intensity in the region South).
Effects of negative ODP: decrease in temperatures, increase in air humidity and a greater incidence and intensity of La Niña at the expense of the decrease and weakening of El Niño. The regions previously hit by long droughts now have a greater number of rainy periods, in addition to colder winters.
This explains, therefore, when some people claim that the climate was different in previous decades: it is probably the alternation between positive and negative PDOs.
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Pacific Decadal Oscillation vs. Global Warming
Normally, some meteorologists and, mainly, the media usually attribute climate variability, especially rising temperatures, to global warming.
However, many of the critics of what they call “global alarmism” have been refuting this conception, using as an argument, mainly, the Oscillation Pacific Decadal, which is associated with climatic variations not only in the present day, but in earlier periods as well, when there was no mention of warming of the Earth.
According to studies prepared by MetSul Meteorology, in the 1940s, the climate was under the effects of a positive PDO, with stronger and more frequent El Niños. From 1950 to 1976, however, the ODP became negative, resulting in extremely strong winters in southern Brazil and a colder climate in the Southeast and Midwest. In the 1980s/90s, the oscillation became positive again, allowing the occurrence of the strongest El Niños of the century, in 1983, 1997 and 1998.
From the 2000s onwards, the ODP again became negative and the influence of La Niña was greater than that of EL Niño. Proof of this was the occurrence of intense periods of rain in the Northeast, in the years 2000, 2001, 2006, 2008 and 2009, while in other years the droughts were milder. The trend, from now on, is that severe droughts and less severe winters are once again the climatic trend.
This, however, does not completely nullify the theories that warn about the influence of human action on the climate. According to specialists in climatology, it is necessary to consider both natural factors and anthropogenic elements to diagnose climate variations on the planet. Furthermore, it is necessary to consider microclimates, especially in cities. These are directly associated with human activities.
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* Image Credits: Giorgiogp2
By Rodolfo Alves Pena
Graduated in Geography
