Here is a hard copy of the "sudden climate change" essay. Most of what I did was to take Jonathan Adams’ long and detailed Web Page piece and edit it (with his assistance and permission, of course). I added notes and lead-ins (most of what is in bold) and a brief conclusion.
Jonathan and I have been working for months on how climatic changes – usually very rapid – have affected the course of modern human history (the past 20,000 years or so). Mired in the past, you might say. Then, lo and behold, at dinner with you I realized that we are living in a time when another such change is upon us. (Yep, I’m a little slow on the uptake) Like I have always tried to teach the young folks, history does indeed apply to the present.
Climate change may be likened to tectonic stress, that builds up imperceptibly until the stress releases in the form of an earthquake. In the case of climate, we can see and measure some of the immediate effects, such as gradual temperature increase, but we cannot predict the :"earthquake" that might ultimately result.
What I am sending you may just be the raw data that you summed up in your book, in which case you are familiar with it. But I was struck by how the possibility of sudden, unpredictable climate change raises the stakes on the current political negotiations for CO2 emissions standards. It gives the lie to the gradualist, we-have-plenty-of time position. Many people (particularly in a New England winter) think a little warming might not be a bad thing.
I don’t know to what degree this is now a part of the public discussion on global warming, but it certainly should be. The conclusion I wrote on the last page of this piece could easily be expanded into a longer, popular article, using the rest of the data as back-up. You and Dick Russell are certainly in a position to make good use of this material. If I can be of any help in that regard, please let me know.
SUDDEN CLIMATE CHANGE THROUGH HUMAN HISTORY
by Jonathan Adams and Randy Foote
The tendency of climate to change very suddenly (often in just a few decades) and then reverse has been one of the most surprising lessons of recent study of the last 130,000 years, and its implications for biogeography and for the evolution of human cultures and biology have barely begun to be considered. Sudden stepwise instability is also a disturbing scenario to be borne in mind when considering the effects that humans might have on the climate system through adding greenhouse gases. Judging by what we see from the past, conditions might gradually be building up to a 'break point' at which a sudden dramatic change in the climate system will occur over just a decade or two.
Sudden transitions after 115,000 years ago:
The Eemian interglacial seems to have ended in a sudden cooling event about 110,000 years ago, recorded from Ice cores, ocean sediment cores and pollen records from across Eurasia. Following the end of the Eemian, a large number of other sudden changes and short-lived warm and cold events have been documented. These are most prominent in the ice-core record of Greenland and the pollen records of Europe, suggesting that they were most intense in the North Atlantic region.
A new detailed study of two Greenland ice cores (GRIP and GISP2), just published in Science (Taylor et al. 1997), suggests that the main Younger Dryas-to-Holocene warming (about 11,000 years ago) took several decades in the Arctic, but was marked by a series of sudden steps in warming, each taking less than 5 years. About half of the warming was concentrated into a single period of less than 15 years. A rapid global rise in methane production at the same time suggests that the warming and moistening of climate (causing more methane output from swamps and other biotic sources) was a globally synchronized change, with the water vapor content of the atmosphere as the most likely 'messenger' in this transition, by virtue of its effect as a greenhouse gas (see below). The detailed chronology of different environmental indicators suggests that changes in lower latitude temperature and dust flux from the continents preceded the
change in Greenland temperatures that relates closely to the northern thermohaline circulation. According to the Greenland ice-cores, conditions remained slightly cooler than present for a while; 'normal' Holocene warmth may not have been attained immediately however, instead taking a further 1500 years (up until around 10,000 calendar years ago) before it was reached.
It is not yet clear if the general pattern of the transition between the Younger Dryas and Holocene is representative of other rapid warming and cooling events in the past 110,000 years. Not all of these events have been studied in such detail as the Younger Dryas, but those transitions which have been well studied using high-resolution records seem to have occurred over only a few decades. The Younger Dryas is probably a time of human extinction, especially in Europe. It marks the end of the High Paleolithic (Cro-Magnon/Magdalenian culture). It is likely that much of Europe became largely depopulated during this time, with people still surviving primarily in coastal areas, where the ocean was a moderating influence.
Other sudden climate transitions since the start of the Holocene: Following the sudden start of the Holocene about 11,000 years ago, there have been a number of sudden, widespread climate changes recorded from the palaeoclimatic record around the world. The most striking of these is a sudden cooling event, about 8,200 years ago and giving cool, dry conditions lasting perhaps 200 years before a rapid return to conditions warmer (and generally moister) than the present. This event is detectable in the Greenland ice cores, where the cooling seems to have been about half-way as severe as the Younger Dryas-to-Holocene difference. This change again hit the European population hard, leaving a vacuum into which came new peoples, when the climate again warmed. This new population was proto-Indo-European, and likely brought into Europe the beginnings of the Neolithic, agricultural, culture, which had arisen in the Middle East in response to climate stress.
What was apparently the same event shows up in records from North Africa across Southern Asia, as a phase of markedly more arid conditions involving a failure of the summer monsoon rains. Cold and/or aridity also seems to have hit northernmost South America, eastern North America and parts of NW Europe. Other smaller, but also sudden and widespread, changes to drier or moister conditions have also been noted for many parts of the world for the second half of the Holocene, since about 5,000 years ago. One particularly strong arid event occurred about 4,000 years ago across northern Africa and southern Asia. However, different sources seem to suggest differing speeds and intensities for Holocene climate events.
According to chemical indicators of windblown sea salt in the GISP2 ice core, the Little Ice Age – which began in late Medieval times and ended in the early 1800's – may have been the most rapid and largest change in polar circulation during the Holocene (O'Brien et al., Science 270, p.1962-1964.) (C. Wake pers. comm.) The effects of the Little Ice Age are well-documented in the recent historical record, although climatic change has rarely been considered as a significant factor in History.
The mechanisms behind sudden climate transitions:
It is still unclear how the climate on a regional or even global scale can change as rapidly as present evidence suggests. It appears that the climate system is more delicately balanced than had previously been thought, linked by a cascade of powerful mechanisms that can amplify a small initial change into a much larger qualitative shift in temperature and aridity. At present, the thinking of climatologists tends to emphasize several key components:
North Atlantic circulation as a trigger or an amplifier in sudden climate changes: The circulation of the North Atlantic Ocean is presently seen as playing a major role in either triggering or amplifying rapid climate changes in the historical and recent geological record. The North Atlantic has a peculiar circulation pattern; the north-east trending Gulf Stream carries warm and relatively salty surface water from the Gulf of Mexico up to the seas between Greenland, Iceland and Norway. Upon reaching there, the surface water cools off and (with the combination of being cooler and relatively salty) becomes dense enough to sink into the deep ocean. The 'pull' exerted by this dense sinking water is thought to help maintain the strength of the warm Gulf Stream, ensuring a current of warm tropical water into the north Atlantic that sends mild air masses across to the European continent.
If the sinking process in the north Atlantic were to diminish or cease, the weakening of the warm Gulf Stream would mean that Europe had colder winters. However, the Gulf Stream does not give markedly warmer summers in Europe – more the opposite in fact – so a shutting off of the mild Gulf Stream air masses does not in itself explain why summers also become colder during sudden cooling events (and why ice masses start to build up on land due to winter snows failing to melt during summer). In the North Atlantic itself, sea ice would form more readily in the cooler winter waters due to a shut-off of the Gulf Stream, and for a greater part of the year the ice would form a continuous lid over the north Atlantic. A lid of sea ice over the North Atlantic would last for a greater proportion of the year; this would reflect back solar heat, leading to cooler summers on the adjacent landmass as well as colder winters. With cooler summers, snow cover would last longer into the spring, further cooling the climate by reflecting back the sun's heat.
The rapid result of all this would be a European and west Siberian climate that was substantially colder (because the warm Gulf Stream air was diverted away by the shutting down of the North Atlantic circulation, and by a high-pressure region formed over the sea ice lid) and substantially drier (because the air that reached Europe would carry less moisture, having come from a cold sea ice surface rather than the warm Gulf Stream).
After an initial rapid cooling event, the colder summers would also tend to allow the snow to build up year-on-year into a Scandinavian ice sheet, and as the ice built up it would reflect more of the sun's heat, further cooling the land surface, and giving a massive high pressure zone that would be even more effective at diverting Gulf Stream air and moisture away from the mid-latitudes of Europe. This would reinforce a much colder regional climate.
The other side of the Warming:
Assume that the Arctic ice began to melt. Ocean circulation modeling studies suggest that a relatively small increase in freshwater flux to the Arctic Sea could cause deep water production in the North Atlantic to cease. During glacial phases, the trigger for a shut-off was the sudden emptying into the northern seas of a lake formed along the edge of a large ice sheet on land (for instance, the very large ice-dammed lake that existed in western Siberia), or a diversion of a meltwater stream into the path of the Gulf Stream (as seems to have occurred as part of the trigger for the Younger Dryas cold event). A pulse of fresh river water would dilute the dense, salty Gulf Stream and float on top, forming a temporary lid that stopped the sinking and pulling of water that drives the Gulf Stream. The Gulf Stream could weaken or switch off altogether, breaking the 'conveyer belt' and allowing a sea ice cap to form, preventing the Gulf Stream from starting up again. Theoretically, the whole process could occur very rapidly, in the space of just a few decades or even several years. The result could be a very sudden climate change to colder conditions, as has happened many times in the area around the North Atlantic during the last 100,000 years.
And this is a not-unlikely result of what is called "Global Warming".
This presents the apparent paradox that global warming could actually create much colder climate in certain parts of the world. Clearly, the "gradualist" models of warming over the next few centuries – as publicized by the energy companies – could be very far off the mark. Even farther from reality is the wishful thinking that "warming" means more pleasant. Americans would no longer need to move to the Sun Belt, rather the Sun Belt would move to them. It just isn’t that simple.
The sudden switch could also occur in the opposite direction, for example if warmer summers caused the sea ice to melt back to a critical point where the sea ice lid vanished and the Gulf Stream was able to start up again. Indeed, following an initial cooling event the evaporation of water vapor in the tropical Atlantic could result in an 'oscillator' whereby the salinity of Atlantic Ocean surface water (unable to sink into the north Atlantic because of the lid of sea ice) built up to a point where strong sinking began to occur anyway at the edges of the sea ice zone. The onset of sinking could result in a renewed northward flux of warm water and air to the north Atlantic, giving a sudden switch to warmer climates, as is observed many times within the record of the last 130,000 years or so.
If the Gulf Stream switched off, it would not only affect Europe. Antarctica would be even colder than it is now, because much of the heat that it does receive ultimately comes from Gulf Stream water that sinks in the north Atlantic, travels in a sort of river down the western side of the deep Atlantic Basin and then resurfaces just off the bays of the Antarctic coastline. Even though it is only a few degrees above freezing when it reaches the surface, this water is much warmer than the adjacent Antarctic continent, helping to melt back some of the sea ice that forms around Antarctica. The effect of switching off the deepwater heat source would be cooler air and a greater sea ice extent around Antarctica, reflecting more sunlight and further cooling the region. However, the north Atlantic deep water takes several hundred years to travel from its place of origin to the Antarctic coast, so it would only produce a direct effect a few centuries after the change occurred in the North. It is not known what delay was present in the correlated climate changes between the north Atlantic region and Antarctica, but it is generally thought that other (relatively indirect) climate mechanisms, such as greenhouse gases in the atmosphere, linked these two far-flung regions and produced rather more closely synchronized changes.
The idea of Gulf Stream slowdowns as a mechanism in climate change is not merely theoretical. There is actually evidence from the study of ocean sediments that deepwater formation in the north Atlantic was diminished during the sudden cold Heinrich events and other colder phases of the last 130,000 years, and that the process 'switched on' rapidly at times when climates suddenly warmed around the north Atlantic Basin. Other direct observations from the last few decades also suggest that deepwater formation off Iceland can slacken slightly in response to a run of wet years around the Arctic Sea, with detectable effects on the European climate. It seems that during other relatively cold phases that do not approach the extreme conditions of the Heinrich events, such as the Little Ice Age event of the last millennium, deep water formation remained in place but that the sinking water was not as dense as it is at present and that a smaller volume was produced. Sinking more gently and in smaller quantities, it would have exerted less of a 'pull' on the Gulf Stream circulation, and hence there would have been a diminished heat flux northwards from the warm Equatorial Atlantic waters. During the colder glacial phases, deep water formation in the present areas between Greenland, Iceland and Norway would have ceased due to a thick cap of sea ice (though there is evidence it occasionally opened up to let Gulf Stream water through to the sea between Iceland and Norway, this did not result in much deepwater formation and so the pull and the northward heat flux seems to have been small). Instead, during the most intense cold phases the deepwater formation area seems to have moved to the south of the British Isles, at the edge of the extended sea ice zone. Even here, it seems to have been weaker than at present, producing relatively small quantities of rather dilute deepwater. This was probably because the whole surface of the Atlantic Ocean (even the tropics) was cooler; with less evaporation from its surface, even the water that did reach northwards was less briny (and thus less dense), so less able to sink when it reached the cold edge of the sea ice zone. An initial slowdown of north Atlantic circulation may sometimes have been the initial trigger for a set of amplifying factors (see below) that rapidly led to a cooling of the tropical Atlantic, reinforcing the sluggish state of the glacial-age Gulf Stream.
Broader changes in temperature and rainfall over much of the world are thought likely to have occurred as a result of a switching on or off of the North Atlantic circulation, and these changes would result in amplification by the feedback mechanisms suggested below. As evidence of such a broader link to global climate, over recent years changes in the monsoon-belt climates of Africa and Asia have also been observed to occur in association with decadal-scale phases of weaker north Atlantic circulation. By extrapolation, it is generally thought that bigger changes in the north Atlantic circulation would result in correspondingly larger changes in climates in the monsoon belts and in other parts of the world.
In addition to this relatively direct effect of deepwater on North Atlantic and Antarctic climate, other subtle effects on global climate would be expected to result from a sudden change in North Atlantic circulation, or indeed they may themselves trigger a change in the North Atlantic circulation by their effects on atmospheric processes. These include the interaction with global carbon dioxide concentrations, dust content and surface reflectivity.
Carbon dioxide and methane concentration as a feedback in sudden changes: Analysis of bubbles in ice cores shows that at the peak of glacial phases, CO2 was about 30% lower than during interglacial conditions. This is thought to be due to some change in plankton activity or ocean circulation patterns that occurs under colder climates, drawing more carbon down out of the atmosphere once climate began to cool. The lower carbon dioxide concentrations resulting from this would cool the atmosphere, and allow more snow and ice to accumulate on land. Relatively rapid changes in climate, occurring over a few thousand years, could have resulted from changes in the atmospheric CO2 concentration. The actual importance of carbon dioxide in terms of the climate system is unknown, though computer climate simulations tend to suggest that it directly cooled the world by less than 1 deg.C on average, but due to amplification of this change by various factors within the climate system such as the water vapor content, the resulting change in global climate could have been more than 2 deg.C
Another, possibly neglected, factor in rapid regional or global climate changes may be the changes in the albedo of the land surface that result from changes in vegetation or algal cover on desert and polar desert surfaces. An initial spreading of dark-coloured soil surface algae following a particularly warm or moist year might provide a 'kick' to the climate system by absorbing more sunlight and thus warming the climate, and also reducing the dust flux from the soil surface to the atmosphere (see below). Larger vascular plants and mosses might have the same effect on the timescale of years or decades. The recent detailed analysis of the ending of the Younger Dryas by Taylor et al. 1997, suggests that warming occurred around 20 years earlier in lower latitudes
Water vapor as a feedback in sudden changes. Water vapor is a more important greenhouse gas than carbon dioxide, and as its atmospheric concentration can vary rapidly, it could have been a major trigger or amplifier in many sudden climate changes. For example, a change in sea ice extent or in carbon dioxide, would be expected to affect the flux of water vapor into the atmosphere from the oceans, possibly amplifying climate changes that would otherwise have occurred anyway. Water vapor may well act as a global 'messenger', co-ordinating rapid climate changes, which seem to have occurred all around the world fairly simultaneously.
Dust and particulates as a feedback in sudden change. Particles of mineral dust, plus the aerosols formed from fires and from chemicals evaporating out of vegetation and the oceans, may also be a major feedback in co-ordinating and amplifying sudden large climate fluctuations. It is known that the atmospheric content of dust and sulphate particles changed very rapidly, over just a few decades, during sudden climate transitions in the Greenland ice core record. The drier and colder the world gets, the more desert there is and the higher the wind speeds, sending more desert dust into the atmosphere where it reinforces the cold and dryness. Conversely, a run of wet years in the monsoon belt could trigger revegetation of desert surfaces and a sudden decrease in the amount of dust blown into the atmosphere. Less dust could help make conditions still warmer and wetter, helping the climate system to move rapidly in particular direction (though dust and other particles might actually tend to warm the surface if they blow over lighter-coloured areas covered by snow or ice).
Could dramatic decade-timescale climate transitions occur in the near future?
It is difficult to say what the risks of a sudden switch in global or North Atlantic region climate might be, because the mechanisms behind past climate changes are incompletely understood. In any case the system will have been influenced by probabilistic events (due to the chaotic nature of the ocean-climate system, with runaway changes coming from miniscule differences in initial conditions), so it is not justifiable to talk in terms of what 'definitely' will or will not happen in the future, even though the public and policymakers are looking for certainties. All that one can reasonably do is set out what the current understanding is, acknowledging that this understanding is limited and may turn out to be wrong in certain key respects, and then talk in terms of probabilities of particular events occurring.
Despite the fact that most of our information is based upon a time when the earth was coveted with ice-sheets, there were at least some rapid climate transitions which occurred when ice sheet extent was no greater than at present, such as the apparently widespread late Holocene cool/arid event around 3,800 y.a., and another cool event around 2,600 y.a. (although the time taken for onset of these later Holocene changes in regional and global climates does not yet seem to have been determined in the literature). The Little Ice Age was another climate oscillation (fairly small by comparison with many of the events recorded in ice cores) which gave cooler conditions over the lands around the North Atlantic between about 700 and 200 years ago. Recently interpreted evidence from the GRIP2 (Greenland) ice core suggests that the most intense phases of the Little Ice Age came on and ended suddenly, over just a few decades. Other, much larger changes in climate seem to have occurred during previous interglacial phases. For example, a quite severe cold and arid event may have affected Eurasia (and possibly other parts of the world) during the Eemian Interglacial about 121,000 years ago. Whether the onset and ending of this event was as rapid as only a few decades is not known at present.
Other relatively sudden cool and arid phases (occurring against a background of similar-to-present conditions) seem to have affected some of the previous interglacials before about 200,000 years ago. Again, the speed with which these climate transitions occurred does not seem to have been discussed in the ice-core literature, but the possibility that these changes occurred over only a few decades must be considered a possibility.
Other smaller changes are observed in the detailed Greenland ice cap record, but it is important to note that not all the rapid changes observed in the Greenland ice cap correspond to large climate changes elsewhere. For example, a warming of 4 deg.C per decade was observed in an ice core from northern Greenland for the 1920's (Dansgaard et al. 1989), but this corresponded to a global shift of 0.5 deg.C or less. For this reason it is always desirable to have sources of evidence from other regions before invoking a broad, dramatic climate shift.
What this relatively recent climate shift does suggest though, is that the climate system tends to undergo most of its changes in sudden jumps, even if those changes are relatively small against the background of those seen during the Quaternary. This is further evidence that if and when the next climate shift occurs, it will not be a gradual century-on-century change but rather a sudden step-function that will begin suddenly and occur over a decade or two.
The various large full-interglacial climate changes during the Holocene and certain earlier interglacials (e.g. the Eeemian and the Holstein Interglacials in Europe) that show up in the Greenland ice cap do seem to correlate with genuinely large climate shifts in Europe and elsewhere, taking conditions from temperate to boreal or even sub-arctic. They offer a worrying analogue for what might happen if greenhouse gas emissions continue unchecked. Judging by its past behavior under both glacial and interglacial conditions, climate has a tendency to remain quite stable for most of the time and then suddenly 'flip' over just a few decades, due to the influence of the various triggering and feedback mechanisms discussed above.
Such observations show that even without anthropogenic climate modification there is always an axe hanging over our head, in the form of random very large-scale changes in the natural climate system; a possibility that policy makers should perhaps bear in mind with contingency plans and international treaties designed to cope with sudden famines on a greater scale than any experienced in written history. By starting to disturb the system, humans may simply be increasing the likelihood of sudden events which could always occur anyway.
Another source of evidence seems to underline the potential importance of sudden climate changes in the coming centuries and millennia: computer modeling studies of the (still incompletely understood) north Atlantic deepwater formation system suggest that it is indeed sensitive to quite small changes in freshwater runoff from the adjacent continents, whether from river fluxes or meltwater from ice caps. Some scenarios in which atmospheric carbon dioxide levels are allowed to rise to several times higher than at present result in increased runoff from rivers entering the Arctic Basin, and a rapid weakening of the Gulf Stream, resulting in colder conditions (especially in winter) across much of Europe. Whilst these are only preliminary models, and thus subject to revision as more work is done, they do seem to point in the same direction as the ancient climate record in suggesting that sudden shutdowns or intensification of the Gulf Stream circulation might occur under full interglacial conditions, and be brought on by the disturbance caused by rising greenhouse gas levels.
Gradualist arguments have assumed that Man could adapt to the effects of slow global warming, with the associated rising of sea levels and changes in agricultural growing patterns. It is likely, though, that earth’s climate does not change in such gentle rhythms. A better model than the gradualist one might be plate tectonics, where stress generally surfaces in the form of earthquakes, rather than gradual motion and shifting.
The evolutionary record is littered with sudden mass extinctions of dominant species. Often these extinctions have been caused largely by rapid climate shifts to which species were unable to adapt. And it has generally been the most dominant species that were the most vulnerable, because their dominance was based on their particular successful adaptation to the existing conditions.
The earth will always survive catastrophic change. So, too will Life. There have been past extinctions when 90% of all species died; the few that were left then repopulated the planet. This was the case with the rise of mammals, after the end-Cretaceous extinction of the dinosaurs.
Man has become dominant across Earth in a time of narrow climatic range, particularly since the Neolithic revolution, the rise of agriculture. Agriculture has allowed the remarkable exponential population increase of the past 5,000 years, relying upon a few crops that are adapted to the current climate – such as wheat, rice and corn. The daily newspaper provides many examples of the effects of normal climatic fluctuations upon Man’s food supply, (e.g. Ethiopia and North Korea) especially when "abnormal" climate is coupled with social instabilities.
Such climate fluctuations and social instabilities are only likely to increase with the coming man-made climate change.
To take, as illustration, two of several possible examples of Man’s vulnerability:
Population distribution: Upwards of one-third of the human population lives in coastal areas that would be threatened by rising sea-level. This is roughly 2 billion people. How long would it take to move this many people inland and create infrastructures capable of support?
Agriculture: Humanity has already overextended its food resources. Crops cannot pack up and move as people can. It may be possible in the gradualist scenarios that people could slowly change their agricultural patterns over time to accord with changed temperature or rainfall. It is doubtful that this could happen very successfully in a situation where there was radical change in a decade. Further, most of the world survives not based upon agri-business, but rather on settled, subsistence farming whose strength rests on the farmers having a long-developed understanding of their land and crops. Sudden change would negate this understanding.
A small-scale example of man’s inability to adjust to climate change can be seen in the steady desertification of much of the Sahel in Africa, where the Sahara has been advancing. This has led to severe dislocation, starvation and social instability. The climatic oscillations outlined above would be far more widespread and devastating than anything witnessed in Africa.
In sum, what has been called the gloom-and-doom warnings of the long-term effects of global warming may actually turn out to have been optimistic. The future could well be far more catastrophic than is generally projected.