Host
Eugene Daniels
Podcast Content
Many sudden changes are easily identified since the last Ice Age, but let us look at two examples that have quite different causes and consequences. There are several more examples of abrupt climate changes over the past 50,000 years that go by rather mysterious names, the Heinrich, as well as Dansgaard/Oeschger events.
The Younger Dryas event is the best studied and understood example of an abrupt climate change. An event occurring approximately 12,800 years prior to present , called the Younger Dryas is the canonical example of abrupt climate change. Changes recorded in Greenlands climate in late Younger Dryas, measured from ice cores, suggest abrupt +10degC warming over several years.
Paleoclimate records suggest climate changes of this magnitude and rate occurred at numerous times in the past. This direct comparison to a variety of proxy records provides strong evidence that changes in ocean circulation played an important role in this abrupt event in climate change. The absence of direct evidence of an ocean cooldown led to the suggestion that this abrupt event was solely confined to Greenland, and calls this proposed mechanism into question.
However, at the time of CCSM3, the southward expansion of Greenland sea ice continued, even by the time of the YD event , and Greenland temperatures did not exhibit any sudden warming. In LOVECLIM, the sea ice extends south of Greenland in The YD, then recedes somewhat quickly northwards with a abrupt cessation in the meltwater fluxes at 12.2 kyr ago (Fig. The yellow shading indicates the later, more gradual end to the YD event, 300 years later, from proxies of the time series of tropical hydroclimates. The modeled onset time of The YD follows the evolution of prescribed influxes of meltwater to the north Atlantic, supporting the hypothesis that the triggering of this climate event was driven by changes in the AMOC.
In CCSM3, the North Atlantic acts as the coupling point linking AMOC weakening during a YD event in the North Atlantic with changes in summertime northern monsoon precipitation over the Indo-Pacific. For instance, a progressive increase in the North Atlantic oceans release of freshwater led to a sudden shut-off of the Atlantic Basin Thermohaline Circulation during The Younger Dryas Event. These events, which are also documented in the oceanic and continental records, involved sudden changes of Earths climate systems from one equilibrium state to another.
Gradual changes in external factors may cause the threshold to cross; the climate system is unable to revert back to a previous equilibrium state, and quickly transitions to the new equilibrium. One source of the effects of abrupt climate change is a feedback process, whereby an event that warms causes changes that increase further warming. An abrupt climate change occurs when the climate system is forced to transition at a rate determined by the climate systems energy budget, which is faster than the rate of change in the external forcing, although this can involve abrupt forced events, such as meteorite impacts.
A 2013 report from the United States National Research Council called attention to the abrupt effects of climate change, saying that even stable, gradual changes in the physical climate system could produce abrupt impacts elsewhere, such as on human infrastructure and ecosystems, if a critical threshold is crossed. In particular, such transitions indicate nonlinearities, so-called tipping points, in which small, gradual changes in a single component of the earths climate system can cause a larger shift across the whole system. Knowledge about past climate variability can also help to determine risks associated with changes to the Earths system from greenhouse gas emissions and changes in regional-to-global-scale land cover.
Confidence in identifying such changes increases when multiple data sources, in more than one region, document an event. Our analysis suggests that land-driven climate variability is a plausible mechanism that has driven rapid ecological changes and biological transitions throughout Earths history, but that the relationship is not strong enough to rule out other sources of divergence.
Events vary in duration, speed, spatial magnitude, and impact on the climate. All three events had worldwide environmental, climate, and biogeochemical consequences. The events occurred in the Last Glaciation, a time of global warming in which Earths climate system was in transition from glacial to interglacial mode. These events represented the extent of similarity in timing, since onset and re-establishment of a YD event was more abrupt in Greenland than it was in Palawan.
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