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(WAIS), would one speak of an abrupt climate Abrupt Change change.

Stefan Rahmstorf, Potsdam Institute for Climate Paleoclimatic Data Impact Research, Potsdam, Germany A wealth of paleoclimatic data has been recovered 0003 ^ Copyright 2001 Academic Press from ice cores, sediment cores, corals, tree rings, doi:10.1006/rwos.2001.0269 and other sources, and there have been signiRcant advances in analysis and dating techniques. These Introduction advances allow a description of the characteristics of past climatic changes, including many abrupt 0001 High-resolution paleoclimatic records from ice and ones, in terms of geographical patterns, timing and sediment cores and other sources have revealed affected climatic variables. For example, the ratio of a number of dramatic climatic changes that occur- oxygen isotopes in ice cores yields information red over surprisingly short times * a few decades or about the temperature in the cloud from which the in some cases a few years. In Greenland, for snow fell. Another way to determine temperature is example, temperature rose by 5}103C, snowfall to measure the isotopic composition of the nitrogen rates doubled, and wind-blown dust decreased by an gas trapped in the ice, and it is also possible to order of magnitude within 40 years at the end of the directly measure the temperature in the borehole last glacial period. In the Sahara, an abrupt with a thermometer. Each method has advantages transition occurred around 5500 years ago from and drawbacks in terms of time resolution and relia- a relatively green shrubland supporting signiRcant bility of the temperature calibration. Dust, carbon populations of animals and humans to the dry de- dioxide, and methane content of the prehistoric at- sert we know today. mosphere can also be determined from ice cores. 0002 One could deRne an abrupt simply On long timescales, climatic variability through- 0004 as a large and rapid one * occurring faster than in out the past two million years at least has been a given time (say 30 years). The change from winter dominated by the Milankovich cycles in the earth’s to summer, a very large change (in many places orbit around the sun * the cycles of precession, larger than the glacial}interglacial transition) occur- obliquity, and eccentricity with periods of roughly ring within six months, is not an abrupt change in 23000, 41000, and 100000 years, respectively. Since climate (or weather), it is rather a gradual transition the middle Pleistocene transition 1.2 million years following the solar forcing in its near-sinusoidal ago, the regular glaciations of our planet follow the path. The term abrupt implies not just rapidity but 100000-year eccentricity cycle; even though this has also reaching a breaking point, a threshold * it only a rather weak inSuence on the solar radiation implies a change that does not smoothly follow the reaching the earth, it modulates the much stronger forcing but is rapid in comparison to it. This phys- other two cycles. The prevalence of the 100000-year ical deRnition thus equates abrupt climate change cycle in climate is thus apparently a highly nonlinear with a strongly nonlinear response to the forcing. response to the forcing that is still poorly under- In this deRnition, the quaternary transitions from stood. The terminations of glaciations occur rather glacial to interglacial conditions and back, taking abruptly (Figure 1). Greenland ice cores show that a few hundred or thousand years, are a prime the transition from the last to the warm example of abrupt climate change, as the underlying Holocene climate took about 1500 years, with cause, the earth’s orbital variations (Milankovich much of the change occurring in only 40 years. cycles), have timescales of tens of thousands The ice ages were not just generally colder than 0005 of years. On the other hand, anthropogenic global the present climate but were also punctuated by warming occurring within a hundred years is not abrupt climatic transitions. The best evidence for as such an abrupt climate change as long as it these transitions, known as Dansgaard}Oeschger smoothly follows the increase in atmospheric (D/O) events, comes from the last ice age (Figure 2). . Only if global warming triggered D/O events typically start with an abrupt warming a nonlinear response, like a rapid ocean circulation by around 53C within a few decades or less, fol- change or decay of the West Antarctic lowed by gradual cooling over several hundred or

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thousand years. The cooling phase often ends with Eemian, is so far inconclusive. During the present an abrupt Rnal temperature drop back to cold (&sta- interglacial, the Holocene, climate was much more dial') conditions. Although Rrst seen in the Green- stable than during the last glacial. However, two land ice cores, the D/O events are not a local feature abrupt events stand out. One is the 8200-year event of Greenland climate. Figure 3 shows that subtropi- that shows up as a cold spike in arctic ice cores and cal sea surface temperatures in the Atlantic closely affected the North Atlantic region. The second ma- mirror the sequence of events in Greenland. Similar jor change is the abrupt desertiRcation of the Sahara records have been found near Santa Barbara, Cali- 5500 years ago. There is much evidence from cave fornia, in the Cariaco Basin off Venezuela, and off paintings, Rre remains, bones, ancient lake sedi- the coast of India. D/O climate change is centered ments, and the like that the Sahara was a partly on the North Atlantic and regions with strong atmo- swampy savannah before this time. The best evid- spheric response to changes in the North Atlantic, ence for the abrupt ending of this benign climate with little response in the Southern Ocean or Ant- comes from Atlantic sediments off north-eastern Af- arctica. The &waiting time' between successive D/O rica, which show a sudden and dramatic step-func- events is most often around 1500 years or, with tion increase in wind-blown dust, witnessing a dry- decreasing probability, 3000 or 4500 years. This ing of the adjacent continent. suggests the existence of an as yet unexplained 1500-year cycle that often (but not always) triggers aD/O event. The second major type of climatic Mechanisms of Abrupt Climate event in glacial times is the Heinrich (H) event. Change Heinrich events involve surging of the through Hudson Strait, occurring in the The increased spatial coverage, quality, and time 0008 cold stadial phase of some D/O cycles. They have resolution of paleoclimatic data as well as advances a variable spacing of several thousand years. The in computer modeling have led to a greater under- icebergs released to the North Atlantic during standing of the mechanisms of abrupt climate H events leave tell-tale drop-stones in the ocean change, although many aspects are still in dispute sediments when they melt. Sediment data further and not fully understood. suggest that H events shut down or at least drasti- The simplest concept for a mechanism causing 0009 cally reduce the formation of North Atlantic Deep abrupt climatic change is that of a threshold. Water (NADW). Records from the South Atlantic A gradual change in external forcing (e.g., the and parts of show that the cold Heinrich change in insolation due to the Milankovich cycles) events in the North Atlantic were associated with or in an internal climatic parameter (e.g., the slow unusual warming there (a fact sometimes referred to build-up or melting of continental ice) continues as &bipolar see-saw'). until a speciRc threshold value is reached where 0006 At the end of the last glacial, a particularly inter- some qualitative change in climate is triggered. Vari- esting abrupt climatic change took place, the so- ous such critical thresholds are known to exist in called event (12800}11500 years the . Continental ice sheets may have ago). Conditions had already warmed to near-inter- a stability threshold where they start to surge; the glacial conditions and continental ice sheets were thermohaline ocean circulation has thresholds where retreating, when within decades the climate in the deep water formation shuts down or shifts location; North Atlantic region switched back to glacial con- methane hydrates in the seaSoor have a temperature ditions for more than a thousand years. It has been threshold where they change into the gas phase and speculated that the cooling resulted from a sudden bubble up into the atmosphere; and the atmosphere inSux of fresh water into the North Atlantic itself may have thresholds where large-scale circula- through St. Lawrence River, when an ice barrier tion regimes switch. holding back a huge meltwater lake on the North For the D/O events, Heinrich events, and the 0010 American continent broke. This could have shut Younger Dryas event discussed above, the paleocli- down the Atlantic (i.e., the matic data clearly point to a crucial role of Atlantic circulation driven by temperature and salinity differ- ocean circulation changes. Modeling and analytical ences), but evidence is controversial. Alternatively, studies of the Atlantic thermohaline circulation the Younger Dryas may simply have been the last (sometimes called the &conveyor belt') show that cold stadial period of the glacial following a tem- there are two positive mechanisms leading porary D/O warming event. to threshold behavior. The Rrst, called advective 0007 Does abrupt climate change occur only during feedback, is caused by the large-scale northward glacial times? Evidence for the last interglacial, the transport of salt by the Atlantic currents, which in

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turn strengthens the circulation by increasing den- cools the northern hemisphere while warming the sity in the northern latitudes. The second, called southern hemisphere, explaining the &bipolar see- convective feedback, is caused by the fact that saw' response in climate. It should also be noted oceanic convection creates conditions favorable for that the initial transient response can differ from the further convection. These (interconnected) feed- equilibrium response as the oceanic heat storage backs make convection and the large-scale ther- capacity is large. The patterns of these abrupt cha- mohaline circulation self-sustaining within certain nges differ from the longer-timescale (many thou- limits, with well-deRned thresholds where the circu- sands of years) response to the Milankovich cycles lation changes to a qualitatively different mode. because for the latter the slow changes in atmo-

0011 Three main circulation modes have been identi- spheric greenhouse gases (e.g., CO2) and continental Red both in sediment data and in models (Figure 4): ice cover act to globally synchronize and amplify (A) a warm or interglacial mode with deep water climatic change. forming in the Nordic Seas and large oceanic heat While the threshold behavior of the Atlantic 0014 transport to northern high latitudes; (B) a cold or ocean can dramatically shape and amplify climatic stadial mode with deep water forming south of the change, the question remains what triggers the shallow sill between Greenland, Iceland, and Scot- mode switches. As mentioned above, D/O switches land and with greatly reduced heat transport to high appear to be paced by an underlying 1500-year latitudes; and (C) a &switched off' or &Heinrich' mode cyclicity that is as yet unexplained. This could either with practically no deep water formation in the be an external (astronomical or solar) cycle or an North Atlantic. In the last mode, the Atlantic deep internal oscillation of the climate system, perhaps circulation is dominated by inSow of Antarctic Bot- also involving the Atlantic thermohaline circulation. tom Water (AABW) from the south. The irregularity in D/O event timing is probably the 0012 Many features of abrupt glacial climate can be result of the presence of stochastic variability in the explained by switches between these three circula- climate system as well as the presence of longer- tion modes. Model simulations suggest that the cold term trends such as the slow build-up of large conti- stadial mode is the only stable mode in a glacial nental ice sheets. climate; it prevails during the cold stadial periods of The ocean circulation change during Heinrich 0015 the last glacial. D/O events can be interpreted as events, on the other hand, can be explained by the temporary incursions of warm Atlantic waters into large amounts of fresh water entering the North the Nordic Seas and deep water formation there, Atlantic at these times in the form of icebergs. i.e., a switch to the warm mode causing abrupt Simulations show that the observed amounts of climatic warming in the North Atlantic region. As fresh water are sufRcient to shut down deep water this mode is not stable in glacial conditions, the formation in the North Atlantic. The nonlinear dy- circulation starts to gradually weaken and temper- namics of ice sheets provide a plausible trigger atures start to decline again immediately after the mechanism. Ice sheets may grow for many thou- incursion, until the threshold is reached where con- sands of years until their base melts owing to vection in the Nordic Seas stops and the system geothermal heating, when the ice sheet becomes reverts to the stable stadial mode. Heinrich events unstable and surges. can be interpreted as a switch from the stadial mode Thresholds in ocean circulation and continental 0016 to the Heinrich mode, i.e., a shutdown of North or sea ice dynamics are not the only mechanisms Atlantic deep water formation. As this mode is that can cause abrupt climatic changes. In the deser- probably also unstable in glacial conditions, the tiRcation of the Sahara in the mid-Holocene, prob- system spontaneously reverts to the stadial or to the ably neither of these mechanisms was involved. warm mode after a waiting time of centuries, the Rather, an unstable positive feedback between veg- timescale being determined partly by slow oceanic etation cover (affecting and evapotranspira- mixing processes. tion) and monsoon circulation in the atmosphere 0013 This interpretation is consistent with the observed appears to have been responsible. patterns of surface temperature change. The warm- It is almost certain that there are further nonlin- 0017 ing during D/O events is centered on the North earities in the climate system that could have caused Atlantic because this is where the change in oceanic abrupt climatic changes in the past or may do so in heat transport occurs; the warm mode delivers heat the future. We are only beginning to understand to much higher latitudes than does the cold mode. abrupt climate change, and the interpretations pre- A switch to the Heinrich mode, on the other hand, sented here * while consistent with data and model strongly reduces the interhemispheric heat transport results * are not the only possible interpretations. from the South Atlantic to the North Atlantic. This ReSecting the state of this science, they are current

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working hypotheses rather than established and convection in the Labrador Sea, one of the two well-tested theory. main NADW formation sites; a complete shutdown of NADW formation (i.e., similar to a switch to the Risk of Future Abrupt Changes Heinrich mode); and a shutdown of AABW forma- tion. A transition to the stadial circulation mode has 0018 The prevalence of abrupt nonlinear (rather than so far not been simulated, perhaps because convec- smooth and gradual) climatic change in the past tion in the Nordic Seas is strongly wind-driven and naturally leads to the question whether such changes is more effectively switched off by increased sea ice can be expected in the future, either by natural cover than by warming. causes or by human interference. The main outside A shutdown of Labrador Sea convection would be 0021 driving forces of past climatic changes are the a signiRcant qualitative change in the Milankovich cycles. Close inspection of these cycles circulation, but would probably affect only the sur- as well as modelling results indicates that we are face climate of a smaller region surrounding the presently enjoying an unusually quiet period in the Labrador Sea. Effects on ecosystems and Rsheries climatic effect of these cycles, owing to the present have not been investigated but could be severe. minimum in eccentricity of the earth’s orbit. The A complete shutdown of NADW formation would next large change in solar radiation that could trig- have wider climatic repercussions. Temperatures in ger a new ice age is probably tens of thousands of north-western Europe could initially rise several de- years away. If this is correct, it makes the Holocene grees in step with global warming, then abruptly an unusually long interglacial, comparable to the drop back to preindustrial values (the competing

Holstein interglacial that occurred around 400 000 effects of raised atmospheric CO2 and reduced years ago when the earth’s orbit went through oceanic heat tranport almost balancing). If CO2 a similar pattern. This stable orbital situation leaves levels decline again in future centuries as expected, unpredictable events (such as meteorite impacts or European temperatures could remain several degrees a series of extremely large volcanic eruptions) and below present as the Atlantic thermohaline circula- anthropogenic interference as possible causes for tion is not expected to recover. Most models indi- abrupt climatic changes in the lifetime of the next cate that for the present climate (in contrast to the few generations of humans. glacial), both the warm mode and the &switched off' 0019 SigniRcant anthropogenic warming of the lower mode of Atlantic circulation are stable. Global ef- atmosphere and ocean surface will almost certainly fects of a shut-down of deep water renewal include

occur in this century, raising concerns that nonlinear reduced oceanic uptake of CO2 (enhancing the thresholds in the climate system could be exceeded ) and accelerated (due and abrupt changes could be triggered at some to a faster warming of the deep oceans). point. Processes that have been (rather speculatively) The probability of major climatic thresholds being 0022 mentioned in this context include a collapse of the crossed in the coming centuries is difRcult to estab- West , a strongly enhanced green- lish and largely unknown. Currently, this possibility house effect due to melting of or trigger- lies within the (still rather large) uncertainty range ing of methane hydrate deposits at the seaSoor, for future climate projections, so the risk cannot be a large-scale wilting of forests when toler- ruled out. ance thresholds are exceeded, nonlinear changes in monsoon regimes, and abrupt changes in ocean cir- See also culation. 0020 Of those possibilities, the risk of a change in Climate: Past Climate From Corals. Deep Sea Drill- 0023 ocean circulation is probably the best understood ing: Results. Ocean Circulation: Thermohaline Circu- and perhaps also the least unlikely. Two factors lation; Water Types and Water Masses. could weaken the circulation and bring it closer to a threshold: the warming of the surface and a dilu- Further Reading tion of high-latitude waters with fresh water. The latter could result from an enhanced atmospheric Abrantes F and Mix A (eds) (1999) Reconstructing Ocean water cycle and precipitation as well as meltwater History * A Window Into the Future. Plenum, New runoff from Greenland and other . Both York. warming and fresh water input reduce surface den- Broecker W (1987) Unpleasant surprises in the green- house? 328: 123. sity and thereby inhibit deep water formation. Model simulations of global warming scenarios so far suggest three possible responses: a shutdown of

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Clark PU, Webb RS and Keigwin LD (eds) (1999) Mecha- Sachs JP and Lehman SJ (1999) Subtropical North Atlan- nisms of Global Climate Change at Millennial tic temperatures 60,000 to 30,000 years ago. Science Time Scales. Washington DC: American Geophysical 286: 756}759. Union. Stocker T (2000) Past and future reorganisations in the Clark PU Alley RB and Pollard D (1999) Northern hemi- climate system. Quarterly Science Review (PAGES sphere ice sheet inSuences on global climate change. Special Issue), 19: 301}319. Science 286: 1104}1111. Taylor K (1999) Rapid climate change. American Scientist Houghton JT, Meira Filho LG, Callander BA et al. (1995) 87: 320. Climate Change 1995. Cambridge: Cambridge Univer- sity Press.

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LGM O 18 d III II I V IV

Eemian Holocene _ 500 _450 _ 400 _350 _300 _ 250 _200 _150 _100 _50 0 Age (1000y BP) a0269fig0001 Figure 1 Record of d18O from marine sediments (arbitrary units), reflecting mainly the changes in global ice volume during the past 500 000 years. Note the rapid terminations (labeled with roman numbers) of glacial periods.

_ 34 1 0 20 19 17 12 16 14 8 15 11 3 _ 18 10 76 4 38 13 5 2 9 ∆T O (ppm) 18

d _ 42 _ 20

100 80 60 40 20 0 Age (1000y BP) a0269fig0002 Figure 2 Record d18O from the GRIP , a proxy for atmospheric temperature over Greenland (approximate temperature range *T is given on the right). Note the relatively stable Holocene climate during the past 10 000 years and before that the much colder glacial climate punctuated by Dansgaard}Oeschger warm events (numbered). The timing of Heinrich events 1 to 6 is marked by black dots.

22 20 18 _ 16 35 12 8 11 10 18 13 d Temperature (°C) Temperature 9 _40 GISP2 O (ppm)

_45 60 50 40 30 Age (1000y BP) a0269fig0003 Figure 3 Sea surface temperatures derived from alkenones in marine sediments from the subtropical Atlantic (Bermuda Rise, upper curve) compared to d18O values from the GISP2 ice core in Greenland.

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3 Depth (km) 4 "COLD" 5 (B) 0

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3 Depth (km) 4 "OFF" 5 (C) 30°S 0° 30°N 60°N 90°N a0269fig0004 Figure 4 Schematic of three major modes of Atlantic ocean circulation. (A) Warm or interglacial mode; (B) cold or stadial mode; (C) &off' or Heinrich mode. In the warm mode the Atlantic thermohaline circulation reaches north over the Greenland}Ice- land}Scotland ridge into the Nordic Seas, while in the cold mode it stops south of Iceland. Switches between circulation modes at certain thresholds can pace and amplify climatic changes.

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