Survival Guide: Abrupt Climate Change
Shannon Buckley
Syd Partridge
INTRODUCTION:
Abrupt climate change is when the climate system experiences a shift in conditions that
occurs faster than society or ecosystems can adapt to it. They usually occur over the course of a
few decades or less. From paleo-records, researchers have found evidence supporting the
occurrence of abrupt climate change throughout the reconstructed past, many with global
impacts and others with regional effects. Currently scientists are working on prediction models
to apply human impacts on the climate system, but these models have much uncertainty
associated with them (Alley et al., 2003).
DEFINITION:
Alley et al. (2003) defined abrupt climate changes as events that occur when transitions
between climate systems are large and rapid, and consist of a trigger, amplifier and source of
persistence. Triggers of past abrupt climate change include orbital forcings, incoming solar
radiation, greenhouse gas concentrations, and freshwater influxes to oceans, among many others.
These initiate a change in the system, but must be followed by amplifiers to increase the
magnitude of change, such as positive feedback loops, and sources of persistence (such as a
weakening of the thermohaline circulation) that make it difficult for the climate system to
recover from a change (Alley et al., 2003).
LAST GLACIATION:
During the last glaciation, many abrupt climate change events have been recorded in
sediment cores, corals, sedimentary rocks and ice cores (Broecker 1997; Rahmstorf 2002). Some
of these events were periods of warming, Dansgaard-Oeschger (D/O) events, and others were
cooling events, Heinrich events (see figure 1). D/O events have shown warming of 5-10oC in the
Northern Hemisphere within less than a few decades and have been linked to cooling in the
Southern Hemisphere (‘a hemispheric ‘see-saw’’; Rahmstorf 2002). Some have attributed these
events to orbital forcings (Alley et al, 2003), while Rahmstorf (2002) relates it to changes in
thermohaline circulation. Overall, the forcings behind these events are still unclear and no
consensus has been arrived at. Heinrich events have been linked with greater confidence to
influxes of freshwater to the North Atlantic from the Laurentide ice sheet which resulted in a
near (if not complete) shut down deep water formation and subsequent weakening of the Atlantic
thermohaline circulation, causing cooling of the northern hemisphere (Rahmstorf 2002). They
also created a hemispheric ‘see-saw’ effect that was of greater amplitude than that experienced
during the D/O events (Rahmstorf 2002).
Figure 1: Reconstructed time scale of 10k to 60k years before present, with D/O warming events labeled by
numbers and Heinrich cooling events labeled by red squares. Source: Rahmstorf 2002.
Ocean circulation is responsible for much of the heat transfer around the world (see
figure 2 for map of ocean circulation model). The North Atlantic is one of the few regions of the
world where deep water is formed, and is responsible for the heat transport to the region (Clark
et al., 2001). So when the circulation is shut down, less heat is transported to the Northern
Hemisphere. The circulation has been found to be sensitive to freshwater fluxes (Broecker 1997;
Clark et al., 2002). The Younger Dryas event, which occurred around the transition between the
last glacial and current interglacial is thought to be triggered by the rerouting of the Mississippi
River into the St. Lawrence, providing large amounts of freshwater to the North Atlantic (Clark
et al., 2001). Other cooling events have been attributed to freshwater influxes from the
Laurentide ice sheet, as mentioned above.
Figure 2: Diagram of Ocean
Circulation, including points of deep
water formation in the North Atlantic
and Antarctic. Source: Rahmstorf
2002.
CURRENT INTERGLACIAL:
Warmer climates of the current interglacial, give rise to more stable conditions.
However, abrupt changes have occurred, including a brief cooling event around 8200 years ago
(8.2K) that has been linked to a weakening of thermohaline circulation from an influx of meltwater (Rahmstorf 2002). Broecker (1997) suggests that such an abrupt change during ‘stable’
warmer conditions could imply that a similar cooling could occur with the shutdown of the
thermohaline circulation under our conditions today. This possibility, however, is highly
questioned.
IMPACTS:
The most studied trigger for abrupt climate change involves a shift or complete shutdown
of the thermohaline circulation in the north Atlantic due to an influx of fresh water. Because it is
this event that is thought to be responsible for many of the abrupt climate shifts in our climate
record, the thermohaline circulation is the number one suspect for future abrupt climate change.
In this section, we will discuss some of the effects, uncertainties, and future predictions for
abrupt climate change resultant from thermohaline shutdown.
If the thermohaline circulation, which is a major mechanism by which heat is transported
from the tropics to the higher latitudes, were to completely shut off tomorrow, it would cause a
rapid cooling of the north Atlantic region. Europe and parts of North America could expect to
see a drop in temperature by as much as 5-6o F within a decade. This drop is very significant and
could lead to more severe winter storms, snowfall in June, and freezes in August (Masters,
wunderground.com). It is important to note, however, that the temperature drop would be
strictly regional, and the global average temperature would be expected to keep rising due to
greenhouse forcings even as regions around the north Atlantic cooled. Because of the projected
warming due to greenhouse gasses, it is thought to be impossible for a “Day After Tomorrow”
scenario to unfold (i.e. the temperature changes would not plunge the region, or the globe, into
another ice-age) (Gagosian, whoi.com). Along with regional cooling, it is possible that the
precipitation patterns across North America and Europe would be altered as well. Paleoclimate
evidence has suggested that past abrupt cooling of the north Atlantic has been linked with mega
droughts of the western U.S. (Alley et al. 1997). It is also important to note that
paleoclimatological studies have shown that abrupt thermohaline changes can and do cause
global changes in precipitation patterns, although present day climate models have yet been able
to reproduce these global effects. (Alley et al. 2003).
Because these changes would occur very rapidly, most likely within a decade, the
possible ecological and societal impacts could be devastating. Some of the possible
consequences would be massive crop failures resulting in food shortages, major economic strain,
and ecosystem damage do to elevated extinction rates (Alley et al. 2003). It is therefore
important to obtain a good grasp on the likelihood of future abrupt climate change.
UNCERTAINTIES:
Presently, Global Climate Models (GCM’s) have predicted that large changes to the
thermohaline circulation may occur in the next 100 years as a result of increasing freshwater
fluxes resulting from stronger precipitation patterns and increased glacial melt. It is important to
note, however, that there are still many flaws in the models with how they handle the
thermohaline circulation. For instance, resolution issues in GCM’s lead to an inconsistency
where the magnitude of the model’s Southern Ocean deep water formation is comparable to that
for the real ocean, but the location of the deep water formation is in the wrong place (Broecker
1997). Also, most models have issues with how they handle ocean salinity levels. In many
models, small perturbations away from present climate lead to large errors in salinity fields,
causing dramatic changes to the modeled thermohaline circulation (Weaver and Hillaire-Marcel
2004). Because of these issues, the reliability of these models ability to predict abrupt climate
shifts has come into question. Recent attempts to correct the salinity errors using “non-fluxadjusted” models have led to predictions of a more stable thermohaline circulation going into the
future. It is still not clear, however, how “believable” the current generation of coupled GCM
predictions are with regard to the thermohaline circulation.
The future uncertainty of abrupt climate change is not just due to imperfect models,
however. The inherent non-linear nature of the system implies that the system may be very
unpredictable near the thresholds of phase change (Alley et al. 2003). In short, it is to be
expected that the predictability of abrupt changes will always have more uncertainty than
predictions than that associated with longer term, slow climate changes.
FUTURE PREDICTIONS:
Past abrupt climate changes seem to have been more prominent during periods of global
climate forcings. Models likewise indicate that greenhouse forcings increase the likelihood of an
abrupt change (Alley et al. 2003). Given the large uncertainties associated with the dynamics
and the models of abrupt climate change, however, current predictions of the probability of
abrupt climate change vary drastically. Many model runs show a weakening of the thermohaline
circulation throughout the century by as much as 20-50% due to increasing freshwater flux,
however “not a single coupled model assessed by the 2001 IPCC Working Group I on Climate
Change Science predicted a collapse…during the 21st century” (Weaver and Hillaire-Marcel
2004). Other models, such as the “non flux-adjusted” models show a relatively stable circulation
over the same time period. In short, the uncertainty associated with abrupt climate change
remains very high. It is impossible to say with any degree of confidence that an abrupt change
will or won’t occur within the next century. Because the possible consequences are severe both
for societies and ecosystems, it is important to develop a better understanding of the probabilities
associated with abrupt climate change. As we speak, climate modelers and paleoclimatologists
are improving our understanding of past and future abrupt climate changes. Although there is no
consensus yet on future likelihoods, one thing can be said with certainty: as we increase the
concentration of greenhouse gasses in the atmosphere, the probability of future abrupt climate
change increases as well. (Broecker 1997).
REFERENCES:
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Alley, RB and co-authors, 1997. Holocene Climate Instability: A Prominent, Widespread Event
8,200 Years Ago, Geology, Vol. 26, No. 6.
Broecker, WS, 1997. Thermohaline circulation, the Achilles heel of our climate system: Will
man-made CO2 upset the current balance? Science, 278 (5343): 1582-1588.
Clark, PU and co-authors, 2001. Freshwater forcing of abrupt climate change during the last
glaciation. Science, 293 (5528): 283-287.
Clark, PU and co-authors, 2002. The role of the thermohaline circulation in abrupt climate
change. Nature, 415 (6874): 863-869.
Gagosian, R.B, 2003. Abrupt Climate Change: Should We Be Worried. Woods Hole
Oceanographic Institute website:
http://www.whoi.edu/institutes/occi/viewArticle.do?id=9986
Masters, J. The Science of Abrupt Climate Change. Wunderground.Com:
http://www.wunderground.com/education/abruptclimate.asp
Rahmstorf, S, 2002. Ocean circulation and climate during the past 120,000 years, Nature, 419
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