Abrupt Climate Change Inevitable Surprises (2002) / Chapter Skim
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2 Evidence of Abrupt Climate Change
2 Evidence of Abrupt Climate Change
Pages 19-72
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From page 19... ...
Modern climate records include abrupt changes that are smaller and briefer than in paleoclimate records but show that abrupt climate change is not restricted to the distant past. INTERPRETATION OF PAST CLIMATIC CONDITIONS FROM PROXY RECORDS Paleoclimatic interpretation relies ultimately on the use of the present or recent instrumental records as the key to the past.
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From page 20... ...
For example, old air extracted from bubbles in ice cores and old water from pore spaces in seabed sediments or continental rocks provide direct indications of past compositions of atmosphere, oceans, and groundwater (see Plate 1~. Anomalously cold buried rocks or ice have not finished warming from the ice age and thus provide evidence that conditions were colder in the past.
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From page 21... ...
Water balance (precipitation Species assemblages minus evaporation Shell geochemistry Tree rings Temperature and/or Ring width or density of trees moisture availability stressed by cold or drought Variations in the isotopic Cellulose isotopic ratios ratio of water related to temperature Speleothems/cave Moisture availability Growth rate of formations formations Isotopic ratios of water Oxygen isotopic composition related to temperature or .
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From page 22... ...
Marine photosynthesis increasingly favors the light isotope of carbon as carbon dioxide becomes more abundant, and this allows estimation of changes in carbon dioxide concentration from the isotopic composition of organic matter in oceanic sediments. Similarly, the growth of ice sheets removes isotopically light water (ordinary water)
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From page 23... ...
During ice ages, the oceans were colder, but the water in them was also isotopically heavier because light water was removed and used in growing ice sheets. Shells that grew in water during ice age intervals contain heavier isotopes owing to cooling and changes in the isotopic composition of ocean waters.
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From page 24... ...
Such techniques include the identification of chemically "fingerprinted" fallout from particular volcanic eruptions, of changes in the composition of atmospheric gases trapped in ice cores, and of changes in cosmogonic isotope production or rock magnetization linked to changes in the earth's magnetic field. THE YOUNGER DRYAS AS AN EXAMPLE OF ABRUPT CLIMATE CHANGE Sedimentary records reveal numerous large, widespread abrupt climate changes over the last 100,000 years and beyond.
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From page 25... ...
The ice-core records provide a unique perspective that demonstrates the synchronous nature of the large, widespread changes observed. Annual-layer counting in Greenland ice cores allows determination of the age, duration, and rapidity of change of the Younger Dryas event with dating errors of about one percent (Alley et al., 1993; Meese et al., 1997~.
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From page 26... ...
The other curves are from the GISP2, Greenland ice core. The rate of snow accumulation and the temperature in central Greenland were calculated by Cuffey and Clow (1997)
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From page 27... ...
Variability in at least some indicators was enhanced near this and other transitions in the ice cores (Taylor et al., 1993) , complicating identification of when transitions occurred and emphasizing the need for improved statistical and analytical tools in dealing with abrupt climate change.
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From page 28... ...
indicate a broadly antiphased behavior between the high southern latitudes and much of the rest of the world, with southern warmth during the Younger Dryas interval (see Plate 2~. The record from Taylor Dome, Antarctica, a near-coastal site, appears to show a slight cooling during the Younger Dryas, although details of the synchronization with other ice cores remain under discussion (Steig et al., 1998~.
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From page 29... ...
, expressed as ratios to their average values during the approximately 2,000 years just prior to the Little Ice Age. Temperature is calculated as a deviation from the average over the same 2,000 years, from oxygen-isotopic data of ice (Stuiver et al., 1995)
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From page 30... ...
The ice cores show that much of the world must have changed nearly simultaneously to yield the observed changes in methane, Asian dust, and Greenland conditions, but we cannot say with confidence whether all events were simultaneous or some were sequential. A summary of much of the relevant terrestrial pollen information follows, organized by region.
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From page 31... ...
The documentation of the Younger Dryas event over much of North America demonstrated that it was not limited to the circum-Atiantic region (Peteet et al.,1997~. Central America and the Caribbean Marine evidence of the Younger Dryas event is recorded as an interval of increased upwelling or decreased riverine runoff from adjacent South American land in a core from the Cariaco Basin in the Caribbean (Hughen et al., 1996, 2000a,b; Peterson et al., 2000)
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From page 32... ...
and ice core (isotopic) evidence where the Younger Dryas cooling (11,500 - 13,000 BP)
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From page 33... ...
Further radiocarbon dating accompanied by high-resolution sampling is necessary. As noted above, ice cores from Peru and Bolivia show a strong late-glacial reversal (Thompson et al., 1995, 1998)
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From page 34... ...
This change suggested that reduction in formation of North Atlantic deep water was responsible for the Younger Dryas cooling observed on land (Oeschger et al., 1984; Broecker et al., 1985; Boyle and Keigwin, 1987~. Later work documented North Atlantic ice-rafting events that correlate with rapid climate oscillations in Greenland, not only during the glacial period but also throughout the Holocene (Bond and Lotti, 1995~.
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From page 35... ...
Evidence of rapid climate variability in the northwestern Pacific over the last 95,000 years has been observed (Kotilainen and Shackleton, 1995~. Even the eastern equatorial Pacific has yielded a Younger Dryas event determined from 6~80 and 6~3C records (Koutavas and Lynch-Steiglitz, 1999~.
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From page 36... ...
Changes into and especially out of the event were very rapid. ABRUPT CLIMATE CHANGES BEFORE THE YOUNGER DRYAS EVENT The 110,000-year-long ice-core records from central Greenland (Tohnsen et al., 1997; Grootes and Stuiver, 1997)
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From page 37... ...
North Atlantic deep water formation (e.g., Lehman and Keigwin, 1992; Oppo and Lehman, 1995; Bond et al., 1993; Bond and Lotti, 1995~. The geographic pattern of climate anomalies associated with the cold phases of the Dansgaard/Oeschger oscillations is thus quite similar to that of the Younger Dryas event.
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From page 38... ...
and climate anomalies similar to, but larger than, those of the cold phases of the non-Heinrich Dansgaard/ Oeschger oscillations (reviewed by Broecker, 1994; and Alley and Clark, 1999~. The panoply of abrupt climate change through the cooling into and warming out of the most recent global ice age and probably earlier ice ages has not been convincingly explained.
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From page 39... ...
Ice-core records from Greenland for this interval originally were interpreted as showing extremely large and rapid climate fluctuations, but flow disturbances are now known to have occurred and affected the records (Alley et al., 1995; Chappellaz et al., 1997~. Much work remains to be done on intact records from the Eemian, but it is increasingly clear from many paleoclimatic archives that although the Eemian included important paleoclimatic variability and ended abruptly, the warm period was not as variable as the periods during the slide into and climb out of the ice age that followed.
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From page 40... ...
Attention is especially focused on drought conditions in low latitudes rather than temperature in high latitudes. HOLOCENE RAPID CLIMATE CHANGE The relevance of abrupt climate change of the ice age to the modern warm climate or future warmer climates is unclear.
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From page 41... ...
The event is also important because it punctuated a time when temperatures were similar to or even slightly above more recent levels, demonstrating that warmth is no guarantee of climate stability. A less well-understood hydrologic event from wet to dry conditions, occurring roughly 5,000 years ago, also took place during a warm period.
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From page 42... ...
Ninos at 7,000 years ago, with the beginning of modern levels reached 5,000 years ago. Although there are other hints of important abrupt climate changes in the Holocene record, most of them have not been studied to the degree needed to place them in a coherent context (for example, examined at multiple sites)
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From page 43... ...
. In contrast with the abrupt late nineteenth to early twentieth century warming, timing of the onset of the Little Ice Age is difficult to establish in that the change manifests itself as a period of slow Northern Hemisphere cooling beginning at or before ca 1000 (Mann et al., 1999; Crowley, 2000; Crowley and Lowery, 2000; Briffa et al., 2001)
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From page 44... ...
Despite growing knowledge of the paleodrought record, causal mechanisms of changes are poorly understood (Woodhouse and Overpeck,1998~. Persistent oceanic temperature anomalies, perhaps related to ENSO or the North Atlantic Oscillation (NAO)
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From page 45... ...
Coverage gaps appear especially large in the oceans and southern latitudes, although broad gaps also exist elsewhere. RAPID CLIMATE CHANGES IN THE INSTRUMENTAL PERIOD Instrumental records from scientific monitoring programs offer the possibility of capturing directly the relevant data on abrupt climate change with greater accuracy and spatial coverage than are possible from the necessarily limited proxy records.
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From page 46... ...
It is important to the understanding of abrupt climate change that these patterns or "modes" of circulation and its variability be understood, particularly on the time-scale of decades to centuries. The abrupt changes surveyed here are smaller in strength than the extreme events of the paleoclimate record, yet they are nonetheless significant as human populations press the capacity of the environment, locally and globally.
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From page 47... ...
Thus, it is fortunate that temperature records are among the longest oceanic time-series and have the best spatial coverage. Data sets include sea-surface temperatures from ocean vessels, long coastal sea-level and temperature records, and shorter or more scattered time-series of temperature and salinity from surface to sea-floor.
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From page 48... ...
Natural variability of climate is now occurring in the context of global warming, so the discussion of abrupt climate change during the period of instrumental records must acknowledge the presence of anthropogenic and natural change, and the possibility of strong interaction between them. PATTERNS OF CLIMATE VARIABILITY Instrumental records show that the climate is characterized by patterns or modes of variability, such as the polar annular modes and ENS O of the equatorial Pacific, as described below.
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From page 49... ...
The possibility that mode shifts participated in or provide clues to the large, abrupt climate changes of preinstrumental times suggests common mechanisms or even common causes. Thus, the study of abrupt climate change should involve consideration of the preferred modes of the climate system.
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From page 50... ...
The coupled oscillation of the tropical ocean and atmosphere is important in global climate, with impacts that extend far beyond the tropical Pacific to the tropical Atlantic and Indian Oceans, to the Southern Ocean, and to middle to high latitudes in the Northern Hemisphere. There are speculations that greenhouse warming is sufficient to put the world into a warmer, near-perpetual E!
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From page 51... ...
Hence, regime shifts such as the one that occurred in 1976-1977 are difficult to diagnose in real time, let alone to predict. There are several different schools of thought as to the nature of the interdecadal PDO variability, which has shown both the abruptness and persistence to qualify under our definition of abrupt climate change.
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From page 52... ...
Tropical Variability in the Atlantic and Indian Oceans Tropical variability arising from feedbacks within the Atlantic and Indian equatorial regions also contributes to regional climate modes, although of smaller global impact than ENSO, probably because of the vast width of the Pacific relative to the Atlantic or Indian. Tropical Atlantic variability correlates strongly with forcing from ENS O and the AO.
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From page 53... ...
Indian Ocean SST is affected by ENS O and by an intrinsic Indian Ocean east-west mode of variability similar in mechanism but uncorrelated with the Pacific's ENSO. Extended Summer Drought The Northern Hemisphere's annular mode and the decade-to-decade ENSO-like variability discussed in the previous sections both affect Northern Hemisphere climate mainly during the winter season, and they involve the atmosphere's own preferred modes of month-to-month variability.
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From page 54... ...
The prevailing view is that drought is an inherently stochastic phenomenon, initiated and terminated by random fluctuations in atmospheric circulation patterns, and sustained over long periods of time by positive feedback from the terrestrial biosphere (Namias, 1960; Rind, 1982; Shukla and Mintz, 1982; Karl, 1983; Sud and Molod, 1988; Bravar and Kavvas, 1991; Xue and Shukla, 1993; Dirmeyer, 1994; Lare and Nicholson, 1994~. A few weeks of abnormally hot, dry weather are sufficient to desiccate the upper layers of the soil, reducing the water available for plants to absorb through their root systems.
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From page 55... ...
During the time covered by instrumental records, such shifts have occurred rather infrequently in the United States but more regularly in semiarid agricultural regions, such as the Sahel, northeast Brazil, and the Middle East (Nicholson et al., 1998; Street-Perrott et al., 2000~. If such dry regimes are sufficiently frequent or long, the cumulative loss of topsoil due to wind erosion makes it increasingly difficult for vegetation to thrive, and difficult-to-reverse "desertification" occurs (United Nations, 1980~.
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From page 56... ...
. A brief review of some of the main trends is provided here, showing that these trends often exhibit regional abruptness and are linked to modes of climate variability, again documenting the importance of instrumental records to the study of abrupt climate change.
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From page 57... ...
Single volcanic eruptions often have a widespread cooling effect (e.g., Grootes and Stuiver, 1997; Zielinski et al., 1997~. Such effects ordinarily would be considered "noise" in a climate state; however the compounding of such noise with longer climate trends, such as the Little Ice Age, can increase the impacts, as observed.
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From page 58... ...
, caused glaciers in Norway to advance (Hurrell, 1995) , and contributed to the thinning of the stratospheric ozone layer over middle and high latitudes of the Northern Hemisphere during winter and early spring (Thompson et al., 2000~.
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From page 59... ...
Sparse temperature records are supplemented by observations of a strong increase in ocean salinity and decrease in sea ice cover in the Nordic Seas (Kelley et al., 1982, 1987~. Good records from Danish stations on the west Greenland coast show the strongest and most abrupt arrival of warming at Upernavik, where a 4°C rise in surface Observed 60N 30N EQ 30S 60S -' , - .
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From page 60... ...
The Arctic region as a whole seems to amplify climate variability, as we are witnessing now. Relationship with the AO/NAO mode is not clear; the indexes of the mode were strongly negative in 1910-1920 and strongly positive in 1920-1930, indicating enhanced cyclone activity in the subpolar Atlantic through the 1920s with southerly winds at the entrance to the Nordic Seas.
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From page 61... ...
The close proximity of the northern Atlantic focus of this warming to the dominant sinking regions of the global deep-ocean circulation makes this episode particularly relevant to global climate. Abrupt Cooling and Freshening of the Subpolar North Atiantic, 1972-1996 Long-term measurements appropriate to climate variability studies are scarce in the ocean.
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From page 62... ...
The ocean has responded quickly, with warm, saline water invading the Labrador Sea to replace the cold, lowsalinity water characteristic of intense wintertime forcing. While possible feedback coupling of the atmosphere-ocean system remains to be determined, the direct correspondence of northwest Atlantic deep convection and cold-air outbreaks from Canada is not in doubt.
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From page 63... ...
responds to winter weather locally, while the deeper "NEADW" (Northeast Atlantic Deep Water) and "DSOW" (Denmark Strait Overflow Water)
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From page 64... ...
water (which forms the upper North Atlantic Deep Water) is enough to energize, strongly and quickly, the entire THC of the Atlantic, and its closely correlated meridional heat transport (Hakkinen, 1999; Cheng, 2000~.
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From page 65... ...
The increasing incidence of the positive phase of the AO/NAO appears to have driven warmer, more-saline Atlantic water into the Arctic and pushed back the boundary with fresher, colder waters of Pacific origin (Carmack et al., 1995, Morison et al., 1998~. Warming of the polar Atlantic layer has been so striking as to be invoked in possible future melting of the sea ice cover.
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From page 66... ...
A suggested link is proposed between this oceanic behavior and the Little Ice Age, one of the major climate events of the past 1,000 years in the northern hemisphere. Identifying global connections in the climate system is a particularly important goal of modern observations.
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From page 67... ...
Couplings Instrumental records show preferred modes of behavior of the earth's climate system. Furthermore, those modes might be coupled not only in time (such as the unknown relation between the few-year ENSO and fewdecade PDO variability)
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From page 68... ...
_ ~ . ~ ~ WORLD 0~N 04~: ND~H ATLANTIC ATLANTIC OCEAN WORLD OCEAN OTT ~ _ 195:Q 1970 aged 10{i0 1070 19901~950 1~070 again or Year Year :~ FIGURE 2.11 Time series for the period 1948 to 1998 of ocean heat content (1022T)
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From page 69... ...
Interpretation of sedimentary records to learn about past climate changes is improving rapidly. The unequivocal evidence of repeated large, widespread, abrupt climate changes in the past is striking.
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From page 70... ...
means that changes from before the deployment of widespread instrumentation will never be understood as well as more recent changes. Instruments have not yet observed any of the large, global abrupt climate changes (a statement that some day may require modification in hindsight with respect to human-induced changes)
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From page 71... ...
In reviewing the abrupt changes seen during the instrumental period, it is obvious how much more would now be understood if just a few well-placed modern instruments had been in place for an extended time. For abrupt climate change, special emphasis on the dynamics of freshwater in both atmosphere and ocean is needed.
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From page 72... ...
Regular coastal observational programs are hampered by overlapping jurisdictions and variable commitment to basic scientific questions. River systems are intimately connected with estuaries, and human modification again compounds natural climate variability of streamflow, water quality, and drainage patterns.
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