Abrupt Climate Change
Analysis of many different types of data collected throughout the world
has shown that climate can change abruptly. The record is roughly 6°C
in 1–3 years recorded in Greenland ice cores (Steffensen et al
2008) and in a core through sediments in a German lake (Brauer et al
2008). Ice cores from Antarctica and Greenland show:
- Rapid climate changes, between multiple nearly stable states, can
and do occur. Look at the plot of temperature on
the Carbon Dioxide Problem web page. Notice the different periods of
relative constant temperature:
- 0 to 12,000 years ago at a temperature in Greenland of 0°C.
- 15,000 to 30,000 years ago at a temperature in Greenland of –8°C.
- 90,000 and 120,000 years ago at a temperature in Greenland of –4°C.
- 200,000 to 210,000 years ago at a temperature in Greenland of –2°C.
Thus we see five periods of nearly stable climate lasting tens
of thousands of years. Many others are seen in the plot, lasting
hundreds to thousands of years.
These are examples of multiple nearly stable climate states separated
by times of rapid (abrupt) change.
Here is an example from the period called The Younger Dryas seen
clearly in European Greenland data. This period is
named after a flower (Dryas octopetala)
that grows in cold conditions and that became common in Europe
during this time.
Central Greenland temperature over the past 20,000 years from analysis
of oxygen isotope ratios in Greenland ice cores.
From NOAA The
Younger Dryas cold interval as viewed from central Greenland.
Transitions in deuterium isotope concentration
(d) observed in Greenland ice cores sampled with a resolution of
The deuterium isotopes are closely related to temperature of the
oceanic area that supplied moisture that led to Greenland precipitation.
The data show that the atmosphere switched mode within 1 to 3 years
(horizontal grey bands) during these transitions and initiated
a more gradual change (over 50 years) of the Greenland air temperature,
as recorded by stable water isotopes.
From Steffensen et al 2008).
- Abrupt changes in the deep mass circulation (indicated in part by
comparison of Antarctic and Greenland ice core records) have taken
place, associated with large and rapid changes in climate.
- There are strong linkages between the physical and biogeochemical
aspects of the earth system. They show up in variations of key atmospheric
trace constituents such as CO2, CH4, N2O
and aerosols (small particles in the air, not the gas in an aerosol
- The earth system is characterized by both positive (self-reinforcing)
feedbacks which lead to rapid changes and negative (self-limiting)
feedbacks which lead to more-or-less stable periods of CO2 concentration
in the ice cores.
From Newsletter of the Global, Analysis, Integration,
and Modeling (GAIM) Taskforce of the International Geosphere Biosphere
Program, Summer 2003, page 13-14.
The possibility of abrupt climate change due to changes in the deep
circulation of the north Atlantic was first proposed by Wallace
Broecker of the Lamont-Doherty Observatory of Columbia University.
He is the one who wrote:
"... it is clear that Earth's climate system
has proven itself to be an angry beast. When nudged, it is capable
of a violent response." From: Broecker
Wallace Broecker of Lamont-Doherty
The Ocean and Abrupt Climate Change
The ocean and atmosphere transport heat from the tropics
to high latitudes. The ocean transport is important at the lower latitudes,
and the atmosphere is important at higher latitudes. See the zonal
average of heat transport from the Ocean
and Climate web page. The meridional overturning circulation in the
Atlantic plays an important role in the oceanic heat transport system
and abrupt climate change.
The Meridional Overturning Circulation in the
In winter, the surface waters of the North Atlantic Ocean cool and become
so cold and dense that they sink to the bottom. The sinking water carries
CO2 deep into the ocean, removing it from contact with the
atmosphere. The sinking water must be replaced by northward transport
of water at the surface. Because the Atlantic is connected to the rest
of the ocean through the Antarctic Circumpolar Current, the replacement
water must come all the way from the far South Atlantic.
The surface (red) and deep (blue) circulation
in the North Atlantic. The sinking of water in the Norwegian and
Greenland Seas helps keep the far North Atlantic ice free, leading
to a warmer Europe. From: Woods Hole Oceanographic Institution.
As the water moves northward along the surface, it passes the tropics
where it is warmed to nearly 28° C. Thus, heat transport in the Atlantic
is everywhere to the north, even in the southern hemisphere. All the
solar heat absorbed by the Atlantic, about one petawatt, is carried northward
to warm the northern hemisphere. The heat is lost when the warm water
cools keeps the far North Atlantic ice free, and it helps keep Europe
Northward heat transport for 1988 in each
ocean and the total transport summed over all oceans.
From Houghton et al., (1996: 212), which used data from Trenberth and
Abrupt Changes in the Meridional Overturning
Ice and ocean sediment core data show many instances of the northern-hemisphere
climate changing abruptly. The change seems to be related to changes
in the meridional overturning circulation. Here is what happened:
- Large numbers of icebergs were dumped into the North Atlantic by
continental glaciers. The icebergs carried sand and gravel which fell
to the bottom as the ice melted. The layers of sand and gravel are
seen in sediment cores (numbered 2 and 3 in the figure below).
- Ice cores on Greenland (number 1 in the figure below), show abrupt
cooling. The cold period lasts about a thousand years, then the air
over Greenland warms.
- During the cold periods, the polar front shifts southward. The front
separates relatively warm surface water in the Atlantic from ice-covered,
cold polar water. Ice covered water extends almost as far south as
the Mediterranean Sea.
Periodic surges of icebergs during the
last ice age appear to have modulated temperatures of the northern
hemisphere by lowering the salinity of the far north Atlantic and
reducing the meridional overturning circulation. Data from cores
through the Greenland ice sheet (1), deep-sea sediments (2,3),
and alpine-lake sediments (4) indicate that: Left: During
recent times the circulation has been stable, and the polar front
which separates warm and cold water masses has allowed warm water
to penetrate beyond Norway. Center: During
the last ice age, periodic surges of icebergs reduced salinity
and reduced the meridional overturning circulation, causing the
polar front to move southward and keeping warm water south of Spain. Right: Similar
fluctuations during the last interglacial appear to have caused
rapid, large changes in climate. The Bottom plot is a rough indication
of temperature in the region, but the scales are not the same. From
- The observed changes can be explained by the shut-down of the meridional
- Melting icebergs lower the salinity of the surface water of
the North Atlantic (icebergs are made of frozen fresh water).
- The fresh water never becomes dense enough to sink. Less salty
water is less dense that the cold salty water deep in the North
- If the water doesn't sink, warm water is not drawn to the far
Atlantic overturning circulation carries a tremendous amount of
heat northward, warming the North Atlantic region. It also generates
a huge volume of cold, salty water called North Atlantic Deep Water—a
great mass of water that flows southward, filling up the deep Atlantic
Ocean basin and eventually spreading into the deep Indian and Pacific
have found evidence for very different patterns of ocean circulation
in the past. About 20,000 years ago (bottom), waters in the North
Atlantic sank only to intermediate depths and spread to a far lesser
extent. When that occurred, the climate in the North Atlantic region
was generally cold and more variable.
Once and Future Circulation of the Ocean.
(Illustration by E. Paul
Oberlander, Woods Hole Oceanographic Institution)
- The surface circulation of the North Atlantic, including the
Gulf Stream, turns eastward, flows toward the Azores, and then
southward. The polar front is at the northern edge of this circulation.
- Water north of the polar front freezes. No heat is released from
the ocean to the atmosphere to warm Europe, and the northern hemisphere
- The expanded icy areas reflect sunlight, keeping the northern
- After many hundreds of years, the water in the North Atlantic
becomes salty and begins to sink, pulling warm water northward,
and the cold period ends.
- The circulation cannot turn on as soon as the surface water becomes
salty. It must become saltier than normal. Thus once the circulation
shuts off, it remains off for centuries. The delay is due to a
process called hysteresis.
The meridional-overturning circulation
is part of a non-linear system. The circulation has two
stable states near 2 and 4.
The switching of north Atlantic from a warm, salty regime
to a cold, fresh regime and back has hysteresis. This means
that as surface water in the north Atlantic (1 in the figure)
becomes less salty (2 in the figure) it quickly switches
to a cold, fresh state (3 in the
figure). For the system to switch back to the original
state 1, the surface water must become much more salty
than at 3 (4 in the figure). This type of behavior
is called a hysteresis loop.
- The controversy now is: Will this happen again?
- Will ice melting in Greenland and the Arctic, due to recent
global warming, produce enough fresh water to reduce or shut
down the meridional circulation?
- Some ocean-atmosphere climate models suggest it is possible,
but not likely.
Calculated changes in the Atlantic meridional overturning circulation
[strength of the circulation given in Sverdrups (Sv); 1 Sv = 106 m3/s]
for a simplified, coupled, ocean-atmosphere, numerical model for
100 model realizations. Radiative forcing is increased from years
1000 to 1140, equivalent to a doubling of CO2, and then
held constant. The warming pushes the model closer to the bifurcation
point, and transitions usually occur when the overturning is weakened.
Two individual realizations are highlighted by the black lines,
one in which the THC remains strong but highly variable, and one
in which the THC undergoes a rapid transition much later than,
and completely unrelated in time to, the forcing. Transitions occur
preferentially following a notable reduction of the meridional
From Alley et al. (2003).
If the circulation shuts down, climate will change. The northern hemisphere
is likely to get much colder, but not as fast as in the movie Day
Abrupt climate change is due to positive (self-reinforcing)
feedbacks that push the climate system into a new, stable state. Other
processes inhibit change and stabilize climate, they are negative
(self-limiting) feedbacks. Thus, knowledge of feedback is important
for understanding abrupt climate change.
Over the past 10,000 years, earth's climate has
been remarkably constant. This requires negative feedbacks that stabilize
climate. Over other time periods, external forcing such as the amount
of sunlight reaching high northern latitudes, pushes the climate system
out of equilibrium to the point where positive feedbacks dominate. The
positive feedbacks quickly change climate until negative feedbacks again
dominate, and the climate system settles into a new, but different state.
The forcing that pushes the climaye system out of equilibrium can be
external, such as changes in insolation or volcanic eruptions, or internal,
such as natural changes within the climate system. El
Niño is an example
of change due to internal forcing.
Animation of feedback leading to abrupt change.
From NOAA website Defining
Abrupt Climate Change.
Here is an animation of
two possible states of a system. Initially, external forcing shown by red
arrows is too weak to force the ball (which represents the climate system)
out of the original state. The ball stays in the valley on the left because
negative feedback due to gravity forces the ball back to the bottom of
the valley. Eventually, the external forcing is great enough to force
the ball to the pass between the two valleys. At the pass, the ball could
be stable. But if it moves slightly to the right or the left, positive
feedback takes over and rapidly pushes the ball back to the original
valley, or rapidly into the new, higher valley on the right. In this
example, the force of gravity causing the ball to move is proportional
to the slope of the ground (green surface). In the valleys and at the
pass, the slope is zero (level ground) and gravity does not move the
ball. On sloping ground, the gravity causes the ball to move downhill.
Please remember: positive feedback leads to instability
and rapid change. Negative feedback leads to stability and little change.
- Feedback must increase or decrease the rate of change the original
variable. If global surface temperature
is the original variable, feedback must either amplify or reduce change
in global surface temperature.
- If the process leading
back to the original variable does not change the original variable,
the process is a cycle, such as the carbon cycle. It is not a feedback.
In a very simple carbon cycle, phytoplankton grow, absorbing carbon
dioxide from the ocean. The phytoplankton then die, decay, and release
carbon dioxide back to the ocean. This is a cycle because it does not
change the concentration of carbon dioxide in the ocean.
- Positive feedback cannot continue indefinitely. It eventually pushes
the system into a new state dominated by negative feedback which reduces
the rate of change.
Examples of feedback include:
- Positive Feedback (self-reinforcing or amplifiers): More water vapor
in the atmosphere increases the greenhouse effect, which warms the
ocean, which leads to more evaporation, which leads to more water vapor
in the atmosphere. This is a positive feedback. The original variable
is water vapor, and the loop increases the rate at which water vapor
in the atmosphere changes. It increases faster and faster.
- Negative Feedback (self-limiting or source of persistence): More
water vapor in the atmosphere leads to more clouds. Clouds reflect
incoming solar radiation (energy), leading to cooling of earth's surface,
which reduces evaporation. This is a negative feedback. Again, water
vapor is the original variable, but this loop reduces the change in
the variable, leading to stability (persistence).
Much work is now being done to try to understand which feedbacks dominate
the climate system.
An example of a system that has two stable states (top), with negative
and positive feedback.
Small displacements of the system from stable state 1 (middle) have
negative feedback pushing the system back to state 1.
Large displacements (bottom) have positive feedback pushing the system
to state 2.
Argentine Foundation for Scientific Ecology.
To learn more about feedback, read this tutorial and
of feedback loops in the climate system.
Other Sources of Abrupt Change
The climate system has many feedback loops, including many that are
positive (causing amplified change). The climate system is non-linear.
Such systems can have abrupt changes when forcing becomes sufficiently
strong. For the earth's climate system, the forcing is coming from changes
CO2 in the atmosphere. If enough CO2 is put into
the air, the system may change in ways we cannot anticipate. The changes
in the meridional overturning circulation is one example of abrupt change.
But others are possible. Many smaller changes have already been observed.
They provide warning that other, larger changes are possible. Do we want
to wait for the "violent response"?
For more on changes in the north Atlantic, read the Great
Climate Flip-Flop, originally written for the Atlantic
Monthly, by William
H. Calvin which describes what we know about the role of the ocean
in the last ice age, and implications for the future.
For more on abrupt climate change, read Abrupt
Climate Change, a 2003 article in Science by a team of
oceanographers and climatologists. Other, more scientific, articles
are available in the Proceedings of the National Academy of Science
Volume 97, Issue
[If you are a Texas A&M University student, and if you have difficulty
downloading the paper, you will need to go through the library
site for e-resources. Click the radio button for E-Journal, then
type in the name of the journal, Science in this case.
Woods Hole Oceanographic Institution has a great web page on abrupt
Alley, R. B., J. Marotzke, et al. (2003). "Abrupt climate change." Science 299
Brauer, A., G. H. Haug, et al. (2008). An abrupt wind shift in western
Europe at the onset of the Younger Dryas cold period. Nature
Geoscience 1 (8): 520–523.
Steffensen, J. P., K. K. Andersen, et al. (2008). High-Resolution Greenland
Ice Core Data Show Abrupt Climate Change Happens in Few Years. Science 321
The last two abrupt warmings at the onset of our present warm interglacial
period, interrupted by the Younger Dryas cooling event, were investigated
at high temporal resolution from the North Greenland Ice Core Project
ice core. The deuterium excess, a proxy of Greenland precipitation moisture
source, switched mode within 1 to 3 years over these transitions and
initiated a more gradual change (over 50 years) of the Greenland air
temperature, as recorded by stable water isotopes. The onsets of both
abrupt Greenland warmings were slightly preceded by decreasing Greenland
dust deposition, reflecting the wetting of Asian deserts. A northern
shift of the Intertropical Convergence Zone could be the trigger of these
abrupt shifts of Northern Hemisphere atmospheric circulation, resulting
in changes of 2 to 4 kelvin in Greenland moisture source temperature
from one year to the next.
6 February, 2009