ABRUPT CLIMATIC CHANGE

climatic change is a statistically significant variation in the mean state of the [Page 2]climate whereas abrupt climate change is (1) a relatively sharp and sustained climatic discontinuity in the climatic record. Other definitions have used criteria such as (2) a change that is faster than the causal forcingfactors (which may be explained by the change being accentuated by feedback mechanisms within the climaticsystem) and (3) a change that is faster than the ability of humans to adapt (see adaptation of people). Abrupt climatic changes based on definition (2) are associated with critical physical and biological thresholds between two different states and have played an important part in the long-term evolution of climate and extinction events. However, ‘abruptness’ is a concept relative to the time-scale under consideration: abruptness on the geological timescale differs from abruptness from the human perspective in definition (3). During the past 20,000 years, there have been several major changes of short, 50–200 years duration or less, with temperature amplitudes of <1.0°C to 10°C. Although there are examples during the historic period, the concept is commonly associated with changes within and between glacials and interglacials (see glacial-interglacial cycle) as they experienced some of the most rapid and extreme climatic changes in the geological record.

The study of ice cores shows that, at any time, the Earth is in one of two basic states, glacial/interstadial or interglacial/interstadial, and that the ocean-atmosphere system may switch between these modes over relatively short time intervals. pollen analysis has shown that at the end of the Last Interglacial some temperate forests were replaced by boreal forests (pine-birch-spruce) possibly within 150 years, although the main climatic shift may have been even more rapid. terminations of glacials were usually more abrupt than their onset. The last deglaciation occurred in two main steps. The warming around 13,000 radiocarbon years BP was abrupt, large and possibly concentrated within a span of 200 years. Possible explanations have included an abrupt shift in the oceanic thermohaline circulation, albedo changes, increased carbon dioxide, volcanic ash and cloud. After a brief decline, there was a second abrupt warming about 10,000 radiocarbon years BP. ice cores and marine sediment cores have shown that this was associated with a rapid change of aerosol concentration, possibly over 20 years.

On the shorter holocene timescale, various mechanisms have been proposed for abrupt climatic changes. These can be divided into three types: natural causes, human causes and chaos theory. The natural causes include volcanic eruptions, which add large quantities of volcanic aerosols to the atmosphere (which reflects incoming solar radiation and is warmed by it), with the consequence that the Earth’s surface is cooled and precipitation may be increased because of slow fallout of aerosols. The eruption of Asama in Japan in AD 1783 and Laki in Iceland in 1784 cooled the Northern Hemisphere by 1.3°C, while the Mount Pinatubo eruption in the Philippines in 1991 reduced global mean air temperature by 0.5°C. A combination of the 11-year sunspot cycle, the quasi-biennial oscillation (QBO) and el niño-southern oscillation (ENSO) have also been shown to affect temperatures.

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