What constitutes an abrupt climate change? What does the paleoclimate record say about how rapidly climate can change? How much can climate change during an abrupt climate shift? Were there ecological or social consequences associated with these abrupt changes? What causes an abrupt climate shift and how do changes evolve once set in motion? Are there climate thresholds which, when crossed, lead to rapid and dramatic, non-linear climate shifts? Are these climate threshholds known or knowable? What is the probability and likely outcome of provoking a rapid climate shift as a result of a global warming due to the present and projected build-up in the concentration of human-derived greenhouse gases?

Overview

Widespread climate changes in the distant past were larger and more rapid than those experienced during more recent historical times. For example, the cooling of the climate leading into the last "ice age," the peak of which occurred roughly 21,000 years ago, and the subsequent climate transition to a warmer, more modern world were punctuated by abrupt climate changes that were one-third to one-half as large as the change from an "ice age" to a warm climate [i.e., the roughly 11-13ºF (6-7ºC) transition from an "ice-age" to a warm climate, globally]. Paleoclimate records further indicate that during these abrupt shifts many aspects of the climate in many regions changed precipitously in the time span of a few years to as little as a single year.

Moreover, the current warm period since the peak of the last ice age (21,000 years ago) was previously thought to be very stable with none of the large climate shifts that so characterized "ice ages." Contrary to this once widely held notion, new evidence from deep-sea sediments and ice cores shows that this warm period was interrupted by a series of abrupt cooling events, each lasting several hundred years. One of the most prominent of these events occurred roughly 12,800 years ago, after Greenland had warmed to near-present conditions. Another smaller but significant abrupt cooling event occurred roughly 8,200 years ago when temperatures in Greenland were slightly above present-day temperatures. These and other recent, abrupt cooling events have been detected from Scandinavia to Africa, some of which occurred within a human lifetime. One such notable event 4,200 years ago (2200 BC) is shown to be synchronous with the collapse of the world's first human empire in Mesopotamia.

Thus, the paleoclimate record suggests that the climate system can respond to various climate forcings in a non-linear manner. In fact, these results document significant and consequential climate shifts during the time of human civilization, and highlight the characteristically abrupt aspects of climate change and their potential consequences. This raises the possibility that if humans alter the Earth's atmosphere rapidly enough, resulting in a global warming, an abrupt climate shift might be induced, with significant social and ecological consequences.

Abrupt Climate Changes and the "Younger Dryas" Event

Approximately 12,800 years ago, as the climate was warming following the Earth's last glacial maximum ("ice age"), an abrupt transition to cold conditions occurred, during which the surface temperature of the Northern Hemisphere dropped precipitously [nearly 27ºF (15ºC) in Greenland, for example] in a series of abrupt, decadal-scale jumps, some of which involved temperature changes on the order of 5ºF (3ºC). This abrupt climate cooling is known as the "Younger Dryas" event. Once the abrupt transition to a colder climate had occurred, the Northern Hemisphere, especially Europe and Greenland, experienced considerably colder conditions lasting about 1,300 years. Other parts of the world were affected as well. The termination of this cold event around 11,500 years ago occurred as an even more abrupt warming, most of which took place in a single 5-year period. The entire transition to a warmer, more modern climate took no more than 40 years. During this transition, snow accumulation in Greenland doubled in a single 3-year period, with 90% of that increase occurring in a single year. This abrupt transition to a warmer world led to a three-fold drop in wind-blown sea salt, a seven-fold drop in wind-blown dust, and a climate warming of 9-18ºF (5-10ºC) in Greenland, all in less than a decade. Within 30 years following this transition to a warmer climate, atmospheric methane (another greenhouse gas) levels increased, as a result of the creation of more wetlands globally. Conversely, the climate cooling associated with the onset of the "Younger Dryas" event resulted in a loss of wetlands worldwide, and a drop in the concentration of atmospheric methane. Numerous climate records from other parts of the world confirm these abrupt climate events recorded in the Greenland ice cores, and extend the signature of these events to other regions of the globe.

It appears that these abrupt climate shifts were caused and/or amplified by fundamental changes in the mode of operation of the coupled Earth system -- the interactions among the atmosphere, ocean, ice, and life. Changes in the rate at which freshwater is delivered to the North Atlantic Ocean may have played an especially important role in bringing about the changes. Warm, salty, surface ocean currents presently moderate the European climate by transporting heat from the tropics northward. These warm surface currents can be slowed or stopped if their salt content (density) becomes sufficiently diluted (and less dense) because of excessive rain, the melting of snow and ice, or large changes in river runoff into this region. This appears to have been the mechanism that triggered the "Younger Dryas" cooling event.

According to the 1995 IPCC (Intergovernmental Panel on Climate Change) report, the human-induced warming resulting from the continued build-up of greenhouse gases is projected to result in an increase in the melting of glaciers and an increase in precipitation in the North Atlantic basin. Just as paleoclimate records suggest that sufficiently rapid increases in precipitation or meltwater may trigger an abrupt reorganization of the ocean circulation, the question arises about whether this might happen in the future. Although climate models are not yet able to provide reliable estimates of either the probability or the impact of such abrupt climate events, they do provide some confirmation that similar changes could occur in the ocean circulation, suggesting that the probability of abrupt climate changes in the future is not zero. Proxy records of climate change also show that the abrupt climate changes of the past altered ecosystems substantially, and that considerably smaller and more recent abrupt climate changes have significantly affected human societies as well.

Abrupt Climate Shifts and Human Civilization

Deep-ocean sediments are used to estimate past changes in ocean temperature and circulation and climate changes on land based on analysis of their two main components: Carbonate fossils (shells of organisms that once lived at the sea surface) and mineral grains that come from land. The sediments typically accumulate at rates of 2-8 inches each millennium (1,000 years). In the North Atlantic, sediments accumulated since the end of the last ice age nearly 12,000 years ago show regular increases in the amount of coarse sediment grains deposited from icebergs melting in the now open ocean, indicating a series of 2-4ºF (1-2ºC) cooling events recurring every 1,500 years or so. The most recent of these cooling events was the Little Ice Age between 1500-1850 AD when European rivers and ports were choked with ice, and glaciers overran alpine villages.

These same cooling events are detected in sediments accumulating off Africa, but the cooling events appear to be larger, ranging between 5-15ºF (3-8ºC). The West African sediments additionally record the "African Humid Period," an interval between 16,000 and 6,000 years ago when Africa was much wetter due to a strengthening of the African monsoon by changes in summer radiation resulting from long-term variations in the Earth's orbit around the sun. During this period, the Saharan desert was dotted with numerous lakes containing typical African lake crocodile and hippopotamus fauna. A curious discovery from the marine sediments is that the transitions into and out of this wet period occurred within decades, not millennia as previously thought. While we understand how and why Africa was wetter during this period we do not understand why the transitions are so abrupt. This adds to mounting evidence that Earth's climate seems to reach certain thresholds, then switches abruptly (within a lifetime) from one operating mode to another.

Historical social consequences of these abrupt climate changes can also be assessed from the archeological record. Archeologists had long known of a large social disruption in Mesopotamia approximately 2200 BC (4,200 years ago) when the first known empire led by Sargon I of Akkad abruptly collapsed and splintered after reigning from Turkey to the Persian Gulf for several hundred years. Noting that this event was contemporaneous with one of the sharp cooling events detected in the North Atlantic and off Africa, a second sediment core from the Persian Gulf was analyzed for evidence of related changes in Mesopotamian climate during this time. Analysis of this evidence indicates that the supply of dust from the Mesopotamian region to the adjacent oceans, at roughly 2200 BC (4,200 years ago), was five times the amount of dust supplied to the ocean in more modern or recent time. Three hundred years later, around 1900 BC (3,900 years ago), the supply of dust that was being delivered to the oceans abruptly decreased, returning to modern concentrations. Geochemical analyses of a thin volcanic ash layer found at both the archeological sites, and in the deep-sea sediment core, indicate that this abrupt drying event coincided in time with the collapse of certain historic civilizations.

While it has long been held that abrupt climate changes were limited to glacial climates of the distant past, these and other results now document the occurrence of abrupt shifts in climate during the present, modern warm period, the interval encompassing the emergence of agriculture, the growth and collapse of civilizations, and the current exponential expansion of human population. These results, therefore, may have implications regarding any human-induced future climate warming.