The Sun-Climate Effect: The Winter Gatekeeper Hypothesis (IV). The climate shift of 1997
Posted on August 22, 2022 by curryja | 130 Comments
by Javier Vinós & Andy May
“These shifts are associated with significant changes in global temperature trend and in ENSO variability. The latest such event is known as the great climate shift of the 1970s.” Anastasios A. Tsonis, Kyle Swanson & Sergey Kravtsov (2007)
4.1 Introduction
While the study of weather variability has a long tradition, the science of climate change is a very young scientific topic, as attested to by the 1984 discovery of the first multidecadal oscillation, the primary global climate internal variability phenomenon, by Folland et al. The impact of this fundamental feature of the global climate system was discovered ten years later by Schlesinger and Ramankutty (1994), after modern global warming had already been blamed on CO2 changes, illustrating the risk of reaching a consensus with insufficient understanding of the topic at hand. The Pacific (inter) Decadal Oscillation (PDO) was discovered three years later (Mantua et al. 1997; Minobe 1997). The Atlantic Multidecadal Oscillation (AMO) was not named until just two decades ago (Kerr 2000).
Prior to the 1980s, it was generally thought that climate changed so slowly as to be almost imperceptible during the span of a human lifetime. But then it became clear that abrupt climate changes took place during the past glacial period (Dansgaard et al. 1984), Dansgaard–Oeschger events demonstrated that regional, hemispheric, and even global climate could undergo drastic changes in a matter of decades. The problem was that modern climate-change theory was built around gradual changes in the greenhouse effect (GHE) and did not have much room for abrupt, drastic, global changes that could not be properly explained by changes in greenhouse gas (GHG) radiative forcing.
The first explanations for glacial Dansgaard–Oeschger events involved drastic changes in meridional transport (MT) by the Atlantic Meridional Overturning Circulation (AMOC). AMOC is part of the global conveyor theory that tries to explain the flow of heat through the Earth’s oceans. The AMOC explanation, better known as the salt-oscillator hypothesis (Broecker et al. 1990), falls short however, as there is no evidence that the AMOC has undergone the abrupt and drastic changes required to produce the events. The current theory on MT is based on what is known as the Bjerkness compensation hypothesis, where changes in one of MT components (the oceanic or the atmospheric) are compensated for by similar changes of the opposite sign in the other. Current interpretations of the Dansgaard–Oeschger phenomenon are based on rapid sea-ice changes taking place at the Nordic seas that abruptly released a great amount of ocean-stored heat under the sea-ice (Dokken et al. 2013).
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