SUDDEN Climate Change - End Game?

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.

Conclusion:


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.

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