I am posting this simply because it is one of those rarities – an article in the NZ mediaon climate change
What climate tipping points should we be looking out for?
15
July, 2014
The
concept of a "tipping point" - a threshold beyond which a
system shifts to a new state - is becoming a familiar one in
discussions of the climate.
Examples
of tipping points are everywhere: a glass falling off a table upon
tilting; a bacterial population hitting a level where it pushes your
body into fever; the boiling point of water, or a cube of ice being
thrown into warm water, where it rapidly melts.
The
ice cube is a poignant example, because scientists now fear that West
Antarctica's ice sheets are also heading towards irreversible
melting.
Likewise,
the recent discovery of deep canyons beneath
the Greenland ice sheet
raises concerns regarding its stability.
The
history
of the atmosphere, oceans and ice caps indicates that, once changes
in the energy level which drive either warming or cooling reach a
critical threshold, irreversible tipping
points ensue.
An
example is a process called "albedo
flip",
where a small amount of melting creates a film of water on top of the
ice. The water absorbs infrared radiation and melts more ice, leading
to runaway melting of ice sheet. The opposite process occurs where
the freezing of water results in reflection of radiation to space,
leading to cooling and freezing of more water.
Other
examples are abrupt warming episodes during glacial states, termed
"interstadials",
for example the "Dansgaard-Oeschger"
warming cycles which occurred during the last glacial period between
about 100,000 and 20,000 thousand years ago, which caused large parts
of the North Atlantic Ocean to undergo
temperature changes of several degrees Celsius within short periods.
Other examples are points at which a glacial state ends abruptly to
be replaced by
rapid glacial termination.
Over
the threshold
An
increase in global temperatures can lead to a threshold representing
the culmination and synergy of multiple
processes,
such as release of methane from permafrost or polar ocean sediments,
retreating sea ice and ice sheets, warming oceans, collapse of ocean
current systems such as the North Atlantic Thermohaline Current and -
not least - large scale fires.
A
major consequence of warming of ice sheets is the increase in supply
of cold fresh melt water to adjacent oceans, such as the abrupt
cooling of the North Atlantic Ocean inducing rapid freezing events
(stadials), as represented by the "Younger
dryas" event (12,900-11,700 years ago),
or the rapid melting of Laurentian
ice cap about 8500 years ago
and related abrupt cooling events in Europe and North America.
Satellite
images of Greenland, July 8 and July 12, 2012. White shows remaining
ice; red shows melt; pink shows probable melt; grey shows ice-free;
dark grey means no data. Image / NASA
The
question is whether the post-18th century global warming trend may
culminate in a major tipping point or, alternatively, is represented
by an increase in disparate extreme weather events, as are currently
occurring around the world.
A
potential indicator of such tipping point may be represented by a
collapse of the North Atlantic Thermal Circulation, which would lead
to a sharp, albeit transient, temperature drop in the North Atlantic
Ocean, North America and Western Europe. Evidence for a weakening of
the North Atlantic deep water circulation by about 30 percent between
1957 and 2004 has been reported in
Nature
as well as by other researchers.
The
question of tipping points is of critical importance since it affects
future climate projections and adaptation plans. In this regard the
latest Intergovernmental Panel on Climate Change report
leaves the question of tipping points open.
The
crucial question
So
how likely is the current climate change trend to reach a tipping
point, and if so of what magnitude and on what time scale?
General
circulation climate models which attempt to delineate overall future
climate trends are limited in their capacity to predict the precise
timing, location and magnitude of abrupt climate and weather events
with confidence.
Since
the 19th century the
rise in the energy level of the atmosphere
has reached a level of more than 3 degrees Celsius when the masking
effects of sulphur aerosols are discounted. This degree of
temperature rise is just under the energy rise level associated with
the last glacial termination between about
16,000 and 10,000 years ago.
The
atmosphere-ocean system continued to warm following the peak El-Nino
event of 1998. Most of the warming occurred in the oceans, whose mean
temperature has risen by about 0.3C
since 1950.
The
current rise in atmospheric CO2 of about 2 parts per million
CO2/year, reaching 401.85
parts per million at the Mauna Loa Observatory in Hawaii in May 2014,
exceeds rates observed in the geological record of the last
65 million years.
An
atmospheric CO2 level of 400 parts per million is estimated for the
Miocene, about 16 million years ago, when mean temperatures have
reached 3 to 4 degrees Celsius above those of pre-industrial
temperatures. Economically
available fossil fuel reserves,
if used, are capable of returning the atmosphere to tropical state
such as existed during the early to mid-Eocene prior to the formation
of the Antarctic ice sheet about 32 million years ago.
The
evidence indicates that, since the mid-1980s, the Earth is shifting
from a climate state that favoured land cultivation since about 7000
years ago to a climate state characterised by mean global
temperatures about 2-3 degrees Celsius above pre-industrial levels.
At
this level, extreme weather events would render large parts of the
continents unsuitable for agriculture. The accelerated
melting of the Greenland
and west
Antarctic ice sheets
could lead to conditions akin to those of the Pliocene, before 2.6
million years ago, when sea level were between 5 and 40 metres higher
than at present, as estimated
by the US Geological Survey.
The
evidence indicates the climate may be tracking toward - or is already
crossing - tipping points whose precise nature and timing remain
undefined, depending on the extent to which ice sheet melting is
retarded due to hysteresis. The increase in frequency and intensity
of extreme weather events around the globe may represent a shift in
state of the atmosphere-ocean system. There is no alternative to a
global effort at deep cuts of carbon emissions coupled with
fast-tracked CO2 sequestration.
As
Professor Joachim Schellnhuber, Germany's climate advisor and
Director of the Potsdam Climate Impacts Institute, has
said:
"We're
simply talking about the very life support system of this planet."
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