Forumers:
For your interest, the latest take on where we are headed, based on a new
paleo-climate paper from James Hansen.
The text below is from
http://www.climatecodered.org/2011/01/rethinking-safe-climate-have-we-already.html
Click on the link to access the references.
The main resource for this is:
http://www.columbia.edu/%7Ejeh1/mailings/2011/20110118_MilankovicPaper.pdf
Mark Kowal
Rethinking a "safe climate": have we already gone too far?
by David Spratt, 23 January 2011
It is hard to argue that anything above the Holocene maximum (of
around 0.5 degrees above the pre-industrial temperature) can preserve a
safe climate, and that we have already gone too far. The notion
that 1.5C is a safe target is out the window, and even 1 degree looks
like an unacceptably high risk.
NASA climate chief James Hansen says:
At current temperatures, no "cushion" left
to avoid dangerous climate change "... even small
global warming above the level of the Holocene begins to generate a
disproportionate warming on the Antarctic and Greenland ice
sheets."
As global temperatures rise to be 0.8 degrees Celsius warmer than the
pre-industrial level, is the planet already entering a zone of dangerous
climate change?
With Arctic sea-ice in a "death spiral", Greenland in
2010 melting at an unprecedented rate, a seemingly extraordinary number
of extreme climate events in the last year from the Russian fires to the
Pakistan floods, and 18 countries setting new temperature records, have
we already gone too far for a safe climate?
In a draft of a new research paper, NASA climate chief James Hansen and
his collaborator Makiko Sato has opened a new debate about what might be
the conditions for a safe climate; that is, one in which people and
nations can continue to live where and as they have been, with secure
food production, and in a bio-diverse environment.
Temperature stability during the Holocene
The period of human settlement over the past 10,000 years is known
as the Holocene, during which time temperatures and hence sea levels (the
two having a close correspondence) have been remarkable
stable. Temperatures over the period have not been more than 0.5C
warmer or cooler than the mid-line (see chart). The warmest part of the
Holocene (the "Holocene maximum") was about 8,000 years ago,
and according to Hansen, today's temperature is about, or slightly above,
the Holocene maximum:
"... we conclude that, with the global surface
warming of 0.7C between 1880 and 2000, global temperature in year 2000
had returned, at least, to approximately the Holocene
maximum."
Note, this is to the year 2000, and temperatures have increased
~0.15C in the last decade, so:
"Global temperature increased 0.5C in the past
three decades to a level comparable to the prior Holocene maximum, or a
few tenths of a degree higher."
That is, we are already a little above the Holocene Maximum. This
matters because Hansen's and Sato's look at climate history
(paleoclimatology) in this new research finds that it is around this
temperature level that the large polar ice sheets start to behave
differently. During the Holocene the Greenland and Antarctic ice sheets
have been relative stable, as reflected in the stability of the sea level
(see chart).
But once substantial melting starts, the loss of heat-reflecting
white sea-ice which is replaced by heat-absorbing dark ocean water
produces an "albedo flip":
"Summer melting on lower reaches of the ice sheets
and on ice shelves introduces the "albedo flip" mechanism. This
phase change of water causes a powerful local feedback, which, together
with moderate global warming, can substantially increase the length of
the melt season. Such increased summer melting has an immediate local
temperature effect, and it also will affect sea level."
Their conclusion is that:
"... the stability of sea level during the
Holocene is a consequence of the fact that global temperature remained
just below the level required to initiate the "albedo flip"
mechanism on Greenland and West Antarctica.
The implication is clear that "just above" the Holocene
Maximum lurks real danger. As Hansen and Sato say:
"...the world today is on the verge of a level of
global warming for which the equilibrium surface air temperature response
on the ice sheets will exceed the global mean temperature increase by
much more than a factor of two."
That is, warming at the poles will become more rapid and exceed the
ratio so far, of being twice then global average. This change, they say,
can be found in past warming events such as the Pliocene around 3 million
years ago, so that:
"... even small global warming above the level
of the Holocene begins to generate a disproportionate warming on the
Antarctic and Greenland ice sheets. "
To put it bluntly, we are on the edge of a precipice in terms of
large ice sheet losses and sea-level rises, and there is little
"cushion" left:
"Polar warmth in prior inter-glacials and the
Pliocene does not imply that a significant cushion remains between
today's climate and dangerous warming, rather that Earth today is poised
to experience strong amplifying polar feedbacks in response to moderate
additional warming."
Sea-levels are one devastating metric of "dangerous climate
change":
"Sea level rise potentially sets a low limit
on the dangerous level of global warming. Civilization developed during a
time of unusual sea level stability. Much of the world's population and
infrastructure is located near current sea level."
Whilst some suggest a linear (or flat line) increase in sea-levels
this century, Hansen and Sato argue forcefully that:
"... the fundamental issue is linearity versus
non-linearity. Hansen argues that amplifying feedbacks make ice sheet
disintegration necessarily highly non-linear. In a non-linear problem,
the most relevant number for projecting sea level rise is the doubling
time for the rate of mass loss. Hansen suggested that a 10-year doubling
time was plausible, pointing out that such a doubling time from a base of
1 mm per year ice sheet contribution to sea level in the decade 2005-2015
would lead to a cumulative 5 metre sea level rise by 2095.
"
Here Hansen repeats his view, first published in 2007 but widely
ignored, that a 5-metre sea-level rise is possible. In fact, recent
research by Blancon et. al published in Nature in 2009, examining the
paleoclimate record, shows sea-level rises of 3 metres in 50 years due to
the rapid melting of ice sheets 123,000 years ago in the Eemian, when the
energy imbalance in the climate system was less than that to which we are
now subjecting the planet.
So what evidence do we have of Hansen's and Sato view that sea-level
rises will be non-linear?
"The most reliable indication of the imminence
of multi-meter sea level rise may be provided by empirical evaluation of
the doubling time for ice sheet mass loss. "
Looking at recent research on mass loss in Greenland and
Antarctica:
"These data records are too short to provide a
reliable evaluation of the doubling time, but, such as they are, they
yield a best fit doubling time for annual mass loss of 5-6 years for both
Greenland and Antarctica, consistent with the approximate doubling of
annual mass loss in the period 2003-2008.
There is substantial variation among alternative analyses of the gravity
field data, but all analyses have an increasing mass loss with time,
providing at least a tentative indication that long-term ice loss mass
will be non-linear....
We conclude that available data for the ice sheet mass change are
consistent with our expectation of a non-linear response, but the data
record is too short and uncertain to allow quantitative assessment. The
opportunity for assessment will rapidly improve in coming years if
high-precision gravity measurements are continued."
Further evidence of our lack of "cushion" can be found by
looking at the warm Eemian inter-glacial peak 125,000 years ago, when it
is generally understood that:
"... temperatures in the Eemian... were less than
1C warmer than peak Holocene global temperature"
In fact, Hansen and Sato conclude that:
"... global temperature was only slightly
higher in the Eemian and Holsteinian interglacial periods than in the
Holocene, at most by about 1°C, but probably by only several tenths of a
degree Celsius.
Yet at these times:
".. some paleodata suggest rates of sea-level
rise perhaps as high as 1.6 ± 0.8 metres per century and sea level about
4-6 metres above present-day values."
A look at the Pliocene, three-to-five million years ago, leads to the
conclusion that:
"...in the early Pliocene, when sea level was
about 25 metre higher than today, was only about 1C warmer than peak
Holocene temperature."
Whilst atmospheric CO2 amount in the Pliocene is poorly known, a
typical assumption, based on a variety of imprecise proxies, is 380 ppm,
or less than today's level!!
So at today's level of carbon dioxide, and not much above the current
temperature, the world has experienced sea-levels five to 25 metres
higher than at present! From that, it is not hard to understand why
Hansen and Sato conclude that:
"...goals of limiting human-made warming to 2C
and CO2 to 450 ppm are prescriptions for disaster."
Summing up:
"Earth at peak Holocene temperature is
poised such that additional warming instigates large amplifying
high-latitude feedbacks. Mechanisms on the verge of being instigated
include loss of Arctic sea ice, shrinkage of the Greenland ice sheet,
loss of Antarctic ice shelves, and shrinkage of the Antarctic ice sheets.
These are not runaway feedbacks, but together they strongly amplify the
impacts in polar regions of a positive (warming) climate forcing...
Augmentation of peak Holocene temperature by even 1C would be sufficient
to trigger powerful amplifying polar feedbacks, leading to a planet at
least as warm as in the Eemian and Holsteinian periods, making ice sheet
disintegration and large sea level rise inevitable."
In a line:
"Earth today is poised to experience strong
amplifying polar feedbacks in response to moderate additional
warming."
We are perhaps already a few tenths of a degree above the Holocene
maximum, and the system seems to be in the early stages of rapid change.
It is widely expected Arctic sea-ice will be totally lost in summer with
a few years to a decade or so, perhaps at less than 1C or warming.
Very few scientists think Greenland would be stable in an Arctic with
little or no summer sea-ice, and opinion is split as to whether it is
past its tipping point already.
It is hard to argue that anything above the Holocene maximum (of around
0.5 degrees above the pre-industrial temperature) can preserve a safe
climate, and that we have already gone too far.
The notion that 1.5C is a safe target is out the window, and even 1
degree looks like an unacceptably high risk.