Dr. James Hansen speaks on reticence within the climate-science community

“Reticence is fine for the IPCC. And individual scientists can
choose to stay within a comfort zone, not needing to worry
that they say something that proves to be slightly wrong. But
perhaps we should also consider our legacy from a broader
perspective. Do we not know enough to say more?”

“Almost four decades ago Eipper (1970), in a section of his
paper titled ‘The Scientist’s Role’, provided cogent advice and
wisdom about the responsibility of scientists to warn the public
about the potential consequences of human activities. Eipper
recognized sources of scientific reticence, but he concluded
that scientists should not shrink from exercising their rights as
citizens and responsibilities as scientists.”

Environ. Res. Lett. 2 (2007) 024002 (6pp) doi:10.1088/1748-9326/2/2/024002

Scientific reticence and sea level rise
J E Hansen
NASA Goddard Institute for Space Studies, 2880
Broadway, New York, NY 10025, USA

Received 23 March 2007
Accepted for publication 3 May 2007
Published 24 May 2007
Online at stacks.iop.org/ERL/2/024002

I suggest that a ‘scientific reticence’ is
inhibiting the communication of a threat of a
potentially large sea level rise. Delay is dangerous because
of system inertias that could create a situation
with future sea level changes out of our control.
I argue for calling together a panel of scientific
leaders to hear evidence and issue a prompt
plain-written report on current understanding of
the sea level change issue.

Keywords: sea level, global warming, glaciology, ice sheets

1. Introduction
I suggest that ‘scientific reticence’, in some cases, hinders
communication with the public about dangers of global
warming. If I am right, it is important that policy-makers
recognize the potential influence of this phenomenon.
Scientific reticence may be a consequence of the scientific
method. Success in science depends on objective skepticism.
Caution, if not reticence, has its merits. However, in a case
such as ice sheet instability and sea level rise, there is a danger
in excessive caution. We may rue reticence, if it serves to lock
in future disasters.

Barber (1961) describes a ‘resistance by scientists to
scientific discovery’, with a scholarly discussion of several
sources of cultural resistance. There are aspects of the
phenomenon that Barber discusses in the ‘scientific reticence’
that I describe, but additional factors come into play in the case
of global climate change and sea level rise.

Another relevant discussion is that of ‘behavioral
discounting’ (Hariri et al 2006), also called ‘delay discounting’
(Axtell and McRae 2006). Concern about the danger of ‘crying
wolf ‘ is more immediate than concern about the danger of
‘fiddling while Rome burns’. It is argued in the referenced
discussions that there is a preference for immediate over
delayed rewards, which may contribute to irrational reticence
even among rational scientists.

I can illustrate ‘scientific reticence’ best via personal ex-
periences. The examples are relevant to the Intergovernmental
Panel on Climate Change (IPCC) process of assessing the state
of the science, specifically to the issue of possible sea level rise.

2. The court case

‘Scientific reticence’ leapt to mind as I was being questioned,
and boxed-in, by a lawyer for the plaintiff in Automobile
Manufacturers versus California Air Resources Board (Auto
Manufacturers 2006). I conceded that I was not a glaciologist.
The lawyer then, with aplomb, requested that I identify
glaciologists who agreed publicly with my assertion that the
sea level was likely to rise more than one meter this century if
greenhouse gas emissions followed an IPCC business-as-usual
(BAU) scenario: ‘Name one!’

I could not, instantly. I was dismayed, because, in
conversation and e-mail exchange with relevant scientists I
sensed a deep concern about likely consequences of BAU
global warming for ice sheet stability. What would be the
legal standing of such a lame response as ‘scientific reticence’?
Why would scientists be reticent to express concerns about
something so important?

I suspect the existence of what I call the ‘John Mercer
effect’. Mercer (1978) suggested that global warming from
burning of fossil fuels could lead to disastrous disintegration
of the West Antarctic ice sheet, with a sea level rise of several
meters worldwide. This was during the era when global
warming was beginning to get attention from the United States
Department of Energy and other science agencies. I noticed
that scientists who disputed Mercer, suggesting that his paper
was alarmist, were treated as being more authoritative.
It was not obvious who was right on the science, but
it seemed to me, and I believe to most scientists, that the
scientists preaching caution and downplaying the dangers of
climate change fared better in receipt of research funding.

Drawing attention to the dangers of global warming may or
may not have helped increase funding for relevant scientific
areas, but it surely did not help individuals like Mercer who
stuck their heads out. I could vouch for that from my own
experience. After I published a paper (Hansen et al 1981)
that described likely climate effects of fossil fuel use, the
Department of Energy reversed a decision to fund our research,
specifically highlighting and criticizing aspects of that paper
at a workshop in Coolfont, West Virginia and in publication
(MacCracken 1983).

I believe there is a pressure on scientists to be
conservative. Papers are accepted for publication more
readily if they do not push too far and are larded with
caveats. Caveats are essential to science, being born in
skepticism, which is essential to the process of investigation
and verification. But there is a question of degree. A tendency
for ‘gradualism’ as new evidence comes to light may be ill-
suited for communication, when an issue with a short time fuse
is concerned.

However, these matters are subjective. I could not see how
to prove the existence of a ‘scientific reticence’ about ice sheets
and sea level. Score one for the plaintiff, and their ally and
‘friend of the court’, the United States federal government.

3. On the ice

A field glaciologist, referring to a moulin on Greenland, said:
‘the whole damned ice sheet is going to go down that hole!’
He was talking about his expectations, under the assumption
of continued unchecked growth of global greenhouse gas
emissions. Field glaciologists have been doing a good job
of reporting current trends on the ice sheets. It is the
translation of field data into conclusions needed by the public
and policymakers that is at issue.

Ice sheet disintegration, unlike ice sheet growth, is a wet
process that can proceed rapidly. Multiple positive feedbacks
accelerate the process once it is underway. These feedbacks
occur on and under the ice sheets and in the nearby oceans.
A key feedback on the ice sheets is the ‘albedo flip’
(Hansen et al 2007) that occurs when snow and ice begin to
melt. Snow-covered ice reflects back to space most of the
sunlight striking it. However, as warming causes melting on
the surface, the darker wet ice absorbs much more solar energy.
Most of the resulting melt water burrows through the ice sheet,
lubricates its base, and thus speeds the discharge of icebergs to
the ocean (Zwally et al 2002).

The area with summer melt on Greenland increased from
450 000 km 2 when satellite observations began in 1979 to
more than 600 000 km2 in 2002 (Steffen et al 2004). A
linear fit to data for 1992-2005 yields an increase of melt
area of 40 000 km2 /year (Tedesco 2007), but this rate may
be exaggerated by the effect of stratospheric aerosols from
the 1991 volcanic eruption of Mount Pinatubo, which reduced
the summer melt in 1992. Summer melt on West Antarctica
has received less attention than on Greenland, but it is
more important. Satellite QuickSCAT radiometer observations
reveal increasing areas of summer melt on West Antarctica and
an increasing melt season length during the period 1999-2005
(Nghiem et al 2007).

The key role of the ocean, in the matter of ice sheet
stability, is as a conduit for excess global-scale heating that
eventually leads to the melting of ice. The process begins
with increasing human-made greenhouse gases, which cause
the atmosphere to be more opaque at infrared wavelengths. The
increased atmospheric opacity causes heat radiation to space to
emerge from a higher level, where it is colder, thus decreasing
the radiation of heat to space. As a result, the Earth is now out
of energy balance by between 0.5 and 1 W m?2 (Hansen et al

This planetary energy imbalance is itself now sufficient
to melt ice corresponding to one meter of sea level rise per
decade, if the energy were used entirely for that purpose
(Hansen et al 2005). However, so far most of the excess
energy has been going into the ocean. Acceleration of ice sheet
disintegration requires tapping into ocean heat, which occurs
primarily in two ways (Hansen 2005): (1) increased velocity
of outlet glaciers (flowing in rock-walled channels) and ice
streams (bordered mainly by slower moving ice), and thus
increased flux and subsequent melting of icebergs discharged
to the open ocean, and (2) direct contact of ocean and ice sheet
(underneath and against fringing ice shelves). Ice loss from the
second process has a positive feedback on the first process: as
buttressing ice shelves melt, the ice stream velocity increases.
Positive feedback from the loss of buttressing ice shelves
is relevant to some Greenland ice streams, but the West
Antarctic ice sheet, which rests on bedrock well below sea level
(Thomas et al 2004), will be affected much more. The loss of
ice shelves provides exit routes with reduced resistance for ice
from further inland, as suggested by Mercer (1978) and earlier
by Hughes (1972). Warming ocean waters are now thinning
some West Antarctic ice shelves by several meters per year
(Payne et al 2004, Shepherd et al 2004).

The Antarctic peninsula recently provided a laboratory to
study feedback interactions, albeit for ice volumes less than
those in the major ice sheets. Combined actions of surface
melt (Van den Broeke 2005) and ice shelf thinning from below
(Shepherd et al 2003) led to the sudden collapse of the Larsen
B ice shelf, which was followed by the acceleration of glacial
tributaries far inland (Rignot et al 2004, Scambos et al 2004).
The summer warming and melt that preceded the ice shelf
collapse (Fahnestock et al 2002, Vaughan et al 2003) was
no more than the global warming expected this century under
BAU scenarios, and only a fraction of expected West Antarctic
warming with realistic polar amplification of global warming.
Modeling studies yield increased ocean heat uptake
around West Antarctica and Greenland due to increasing
human-made greenhouse gases (Hansen et al 2006b).

Observations show a warming ocean around West Antarctica
(Shepherd et al 2004), ice shelves thinning several meters
per year (Rignot and Jacobs 2002, Payne et al 2004), and
increased iceberg discharge (Thomas et al 2004). As the
discharge of ice increases from a disintegrating ice sheet, as
occurs with all deglaciations, regional cooling by the icebergs
is significant, providing a substantial but temporary negative
feedback (Hansen 2005). However, this cooling effect is
limited on a global scale as shown by comparison with the
planetary energy imbalance, which is now sufficient to melt
ice equivalent to about one meter of sea level per decade (table
S1 of Hansen et al 2005). Yet the planetary energy imbalance
should not be thought of as a limit on the rate of ice melt, as
increasing iceberg discharge yields both positive and negative
feedbacks on planetary energy imbalance via ocean surface
cooling and resulting changes of sea ice and cloud cover.

Global warming should also increase snowfall accumula-
tion rates in ice sheet interiors because of the higher moisture
content of the warming atmosphere. Despite high variability
on interannual and decadal timescales, and limited Antarctic
warming to date, observations tend to support this expecta-
tion for both Greenland and Antarctica (Rignot and Thomas
2002, Johannessen et al 2005, Davis et al 2005, Monaghan
et al 2006). Indeed, some models (Wild et al 2003) have ice
sheets growing overall with global warming, but those models
do not include realistic processes of ice sheet disintegration.
Extensive paleoclimate data confirm the common sense expec-
tation that the net effect is for ice sheets to shrink as the world

The most compelling data for the net change of ice sheets
is provided by the gravity satellite mission GRACE, which
shows that both Greenland (Chen et al 2006) and Antarctica
(Velicogna and Wahr 2006) are losing mass at substantial rates.
The most recent analyses of the satellite data (Klosco) confirm
that Greenland and Antarctica are each losing mass at a rate of
about 150 cubic kilometers per year, with the Antarctic mass
loss primarily in West Antarctica. These rates of mass loss
are at least a doubling of rates of several years earlier, and
only a decade earlier these ice sheets were much closer to mass
balance (Cazenave 2006).

The Antarctic data are the most disconcerting. Warming
there has been limited in recent decades, at least in part due
to the effects of ozone depletion (Shindell and Schmidt 2004).
The fact that West Antarctica is losing mass at a significant rate
suggests that the thinning ice shelves are already beginning
to have an effect on ice discharge rates. Warming of the
ocean surface around Antarctica (Hansen et al 2006a) is small
compared with the rest of world, consistent with climate model
simulations (IPCC 2007), but that limited warming is expected
to increase (Hansen et al 2006b). The detection of recent,
increasing summer surface melt on West Antarctica (Nghiem
et al 2007) raises the danger that feedbacks among these
processes could lead to nonlinear growth of ice discharge from

4. Urgency: this problem is nonlinear!

IPCC business-as-usual (BAU) scenarios are constructs in
which it is assumed that emissions of CO2 and other
greenhouse gases will continue to increase year after year.
Some energy analysts take it as almost a law of physics
that such growth of emissions will continue in the future.
Clearly, there is not sufficiently widespread appreciation of
the implications of putting back into the air a large fraction
of the carbon stored in the ground over epochs of geologic
time. Climate forcing due to these greenhouse gases would
dwarf the climate forcing for any time in the past several
hundred thousand years, when accurate records of atmospheric
composition are available from ice cores.

However, the long-term global cooling and increase of
global ice through the Plio-Pleistocene provides an even more
poignant illustration of the implications of continued BAU
burning of fossil fuels. The global oxygen isotope record
of benthic (deep ocean dwelling) foraminifera compiled by
Lisiecki and Raymo (2005), repeated in figure 10a of Hansen
et al (2007) for comparison with solar insolation changes
over the same period, reveals long-term cooling and sea level
fall, with superposed oscillations at a dominant frequency of
41 ky. The long-term cooling presumably is due, at least
in part, to the drawdown of atmospheric CO2 by weathering
that accompanied and followed the rapid growth of the Andes
(Ghosh et al 2006) and Himalayas (Raymo and Ruddiman
1992), which was most rapid in the late Miocene. Changes in
meridional heat transport may have contributed to the climate
trend (Rind and Chandler 1991), but the CO2 amount providing
a global positive forcing seems unlikely to have been more
than approximately 350-450 ppm (Dowsett et al 1994, Raymo
et al 1996, Crowley 1996). The global mean temperature three
million years ago was only 2-3 ? C warmer than today (Crowley
1996, Dowsett et al 1996), while the sea level was 25 ± 10 m
higher (Wardlaw and Quinn 1991, Barrett et al 1992, Dowsett
et al 1994).

The Plio-Pleistocene record compiled by Lisiecki and
Raymo (2005) is fascinating to paleoclimatolgists as it clearly
shows the expected dominance of global climate variations
with the 41 ky cyclic variation of the tilt of the Earth’s spin axis,
increased tilt melting ice at both poles. When the planetary
cooling reached a degree that allowed a large mid-latitude
Northern Hemisphere (Laurentide) ice sheet, the periodicity
necessarily became more complex, because of the absence of
land area for a similar ice sheet in the Southern Hemisphere
(Hansen et al 2007). However, the information of practical
importance from the Plio-Pleistocene record is the implication
of dramatic global climate change with only moderate global
climate forcing. With global warming of only 2-3 ? C and CO2
of perhaps 350-450 ppm it was a dramatically different planet,
without Arctic sea ice in the warm seasons and with a sea level
25 ± 10 m higher.

Assuming a nominal ‘Charney’ climate sensitivity of 3 ? C
equilibrium global warming for doubled CO2 , BAU scenarios
yield a global warming at least of the order of 3 ? C by the
end of this century. However, the Charney sensitivity is
the equilibrium (long-term) global response when only fast
feedback processes (changes of sea ice, clouds, water vapor
and aerosols in response to climate change) are included
(Hansen et al 2007). Actual global warming would be larger
as slow feedbacks come into play. Slow feedbacks include
increased vegetation at high latitudes, ice sheet shrinkage, and
terrestrial and marine greenhouse gas emissions in response to
global warming.

In assessing the likely effects of a warming of 3 ? C, it is
useful to note the effects of the 0.7 C warming in the past
century (Hansen et al 2006a). This warming already produces
large areas of summer melt on Greenland and significant melt
3Environ. Res. Lett. 2 (2007) 024002 J E Hansen
on West Antarctica. Global warming of several more degrees,
with its polar amplification, would have both Greenland and
West Antarctica bathed in summer melt for extended melt

The IPCC (2007) midrange projection for sea level rise
this century is 20-43 cm (8-17 inches) and its full range is 18-
59 cm (7-23 inches). The IPCC notes that they are unable to
evaluate possible dynamical responses of the ice sheets, and
thus do not include any possible ‘rapid dynamical changes in
ice flow’. Yet the provision of such specific numbers for sea
level rise encourages a predictable public response that the
projected sea level change is moderate, and smaller than in
IPCC (2001). Indeed, there have been numerous media reports
of ‘reduced’ sea level rise predictions, and commentators have
denigrated suggestions that business-as-usual greenhouse gas
emissions may cause a sea level rise of the order of meters.
However, if these IPCC projected rates of sea level rise are
taken as predictions of actual sea level rise, as they have been
by the public, they suggest that the ice sheets can miraculously
survive a BAU climate forcing assault for a period of the order
of a millennium or longer. This is not entirely a figment of the
IPCC decision to provide specific numbers for only a portion of
the problem, while demurring from any quantitative statement
about the most important (dynamical) portion of the problem.
Undoubtedly there are glaciologists who anticipate such long
response times, because their existing ice sheet models have
been designed to match paleoclimate changes, which occur on
millennial timescales.

However, Hansen et al (2007) show that the typical 6 ky
timescale for paleoclimate ice sheet disintegration reflects the
half-width of the shortest of the weak orbital forcings that drive
the climate change, not an inherent timescale of ice sheets
for disintegration. Indeed, the paleoclimate record contains
numerous examples of ice sheets yielding a sea level rise of
several meters per century, with forcings smaller than that of
the BAU scenario. The problem with the paleoclimate ice sheet
models is that they do not generally contain the physics of ice
streams, effects of surface melt descending through crevasses
and lubricating basal flow, or realistic interactions with the

Rahmstorf (2007) has noted that if one uses the observed
sea level rise of the past century to calibrate a linear projection
of future sea level, BAU warming will lead to a sea level rise of
the order of one meter in the present century. This is a useful
observation, as it indicates that the sea level change would
be substantial even without the nonlinear collapse of an ice
sheet. However, this approach cannot be taken as a realistic
way of projecting the likely sea level rise under BAU forcing.
The linear approximation fits the past sea level change well
for the past century only because the two terms contributing
significantly to sea level rise were (1) thermal expansion of
ocean water and (2) melting of alpine glaciers.

Under BAU forcing in the 21st century, the sea level
rise surely will be dominated by a third term: (3) ice sheet
disintegration. This third term was small until the past few
years, but it is has at least doubled in the past decade and
is now close to 1 mm/year, based on the gravity satellite
measurements discussed above. As a quantitative example,
let us say that the ice sheet contribution is 1 cm for the
decade 2005-15 and that it doubles each decade until the West
Antarctic ice sheet is largely depleted. That time constant
yields a sea level rise of the order of 5 m this century. Of
course I cannot prove that my choice of a ten-year doubling
time for nonlinear response is accurate, but I am confident that
it provides a far better estimate than a linear response for the
ice sheet component of sea level rise under BAU forcing.

An important point is that the nonlinear response could
easily run out of control, because of positive feedbacks and
system inertias. Ocean warming and thus melting of ice
shelves will continue after growth of the forcing stops, because
the ocean response time is long and the temperature at depth
is far from equilibrium for current forcing. Ice sheets also
have inertia and are far from equilibrium: and as ice sheets
disintegrate their surface moves lower, where it is warmer,
subjecting the ice to additional melt. There is also inertia in
energy systems: even if it is decided that changes must be
made, it may require decades to replace infrastructure.

The nonlinearity of the ice sheet problem makes it
impossible to accurately predict the sea level change on a
specific date. However, as a physicist, I find it almost
inconceivable that BAU climate change would not yield a sea
level change of the order of meters on the century timescale.
The threat of a large sea level change is a principal element in
our argument (Hansen et al 2006a, 2006b, 2007) that the global
community must aim to keep additional global warming less
than 1 ? C above the 2000 temperature, and even 1 ? C may be
too great. In turn, this implies a CO2 limit of about 450 ppm,
or less. Such scenarios are dramatically different than BAU,
requiring almost immediate changes to get on a fundamentally
different energy and greenhouse gas emissions path.

5. Reticence

Is my perspective on this problem really so different than that
of other members of the relevant scientific community? Based
on interactions with others, I conclude that there is not such
a great gap between my position and that of most, or at least
much, of the relevant community. The apparent difference may
be partly a natural reticence to speak out, which I attempt to
illuminate via specific examples.

In the late 1980s, an article (Kerr 1989) titled ‘Hansen vs.
the World on the Greenhouse Threat’, reported on a scientific
conference in Amherst, MA. One may have surmised strong
disagreement with my assertion (to Congress) that the world
had entered a period of strong warming due to human-made
greenhouse gases. But participants told Kerr ‘if there were a
secret ballot at this meeting on the question, most people would
say the greenhouse warming is probably there’. And ‘what
bothers us is that we have a scientist telling Congress things
that we are reluctant to say ourselves’.

That article made me notice right away a difference
between scientists and ‘normal people’. A non-scientist friend
from my hometown, who had congratulated me after my
congressional testimony, felt bad after he saw the article by
Kerr. He obviously believed that I had been shown to be
wrong. However, I thought Kerr did a good job of describing
the various perspectives, and made it clear, at least between the
lines, that differences were as much about reticence to speak as
about scientific interpretations.

IPCC reports may contain a reticence in the sense of
being extremely careful about making attributions. This
characteristic is appropriately recognized as an asset that
makes the IPCC conclusions authoritative and widely accepted.
It is probably a necessary characteristic, given that the IPCC
document is produced as a consensus among most nations in
the world and represents the views of thousands of scientists.
Kerr (2007) describes a specific relevant example, whether
the IPCC should include estimates of dynamical ice sheet
loss in their projections: ‘too poorly understood, IPCC
authors said’, and ‘overly cautious-(dynamical effects) could
raise sea level much faster than IPCC was predicting’ some
scientists responded. Kerr goes on to say ‘almost immediately,
new findings have emerged to support IPCC’s conservative
position’. Glaciologist Richard Alley, an IPCC lead author,
said ‘Lots of people were saying we [IPCC authors] should
extrapolate into the future, but we dug our heels in at the IPCC
and said that we don’t know enough to give an answer’.

6. Our legacy

Reticence is fine for the IPCC. And individual scientists can
choose to stay within a comfort zone, not needing to worry
that they say something that proves to be slightly wrong. But
perhaps we should also consider our legacy from a broader
perspective. Do we not know enough to say more?

Confidence in a scientific inference can be built from many
factors. For climate change these include knowledge gained
from studying paleoclimate changes, analysis of how the
Earth has responded to forcings on various timescales, climate
simulations and tests of these against observations, detailed
study of climate change in recent decades and how the nature of
observed change compares with expectations, measurements
of changes in atmospheric composition and calculation of
implied climate forcings, analysis of ways in which climate
response varies among different forcings, quantitative data
on different feedback processes and how these compare with
expectations, and so on.

Can the broader perspective drawn from various sources
of information allow us to ‘see the forest for the trees’,
to ‘separate the wheat from the chaff ‘? That a glacier
on Greenland slowed after speeding up, used as ‘proof ‘
that reticence is appropriate, is little different than the
common misconception that a cold weather snap disproves
global warming. Spatial and temporal fluctuations are
normal. Moreover, short-term expectations for Greenland
glaciers are different from long-term expectations for West
Antarctica. Integration via the gravity satellite measurements
puts individual glacier fluctuations in a proper perspective. The
broader picture gives a strong indication that ice sheets will,
and are already beginning to, respond in a nonlinear fashion
to global warming. There is enough information now, in my
opinion, to make it a near certainty that IPCC BAU climate
forcing scenarios would lead to a disastrous multi-meter sea
level rise on the century timescale.

Almost four decades ago Eipper (1970), in a section of his
paper titled ‘The Scientist’s Role’, provided cogent advice and
wisdom about the responsibility of scientists to warn the public
about the potential consequences of human activities. Eipper
recognized sources of scientific reticence, but he concluded
that scientists should not shrink from exercising their rights as
citizens and responsibilities as scientists. Climate change adds
additional imperative to Eipper’s thesis, which was developed
with reference to traditional air and water pollution. Positive
climate feedbacks and global warming already ‘in the pipeline’
due to climate system inertia together yield the possibility
of climate ‘tipping points’ (Hansen et al 2006b, 2007), such
that large additional climate change and climate impacts are
possible with little additional human-made forcing. Such
a system demands early warnings and forces the concerned
scientist to abandon the comfort of waiting for incontrovertible

There is, in my opinion, a huge gap between what
is understood about human-made global warming and its
consequences, and what is known by the people who most need
to know, the public and policy makers. The IPCC is doing a
commendable job, but we need something more. Given the
reticence that the IPCC necessarily exhibits, there need to be
supplementary mechanisms. The onus, it seems to me, falls on
us scientists as a community.

Important decisions are being made now and in the near
future. An example is the large number of new efforts to make
liquid fuels from coal, and a resurgence of plans for energy-
intensive ‘cooking’ of tar-shale mountains to squeeze out liquid
hydrocarbon fuels. These are just the sort of actions needed to
preserve a BAU greenhouse gas path indefinitely. We know
enough about the carbon cycle to say that at least of the order
of a quarter of the CO2 emitted in burning fossil fuels under
a BAU scenario will stay in the air for an eternity, the latter
defined practically as more than 500 years. Readily available
conventional oil and gas are enough to take atmospheric CO2
to a level of the order of 450 ppm.

In this circumstance it seems vital that we provide the
best information we can about the threat to the great ice sheets
posed by human-made climate change. This information, and
appropriate caveats, should be provided publicly, and in plain
language. The best suggestion I can think of is for the National
Academy of Sciences to carry out a study, in the tradition of
the Charney and Cicerone reports on global warming. I would
be glad to hear alternative suggestions.

I thank Tad Anderson, Mark Bowen, Svend Brandt-Erichsen,
Jonathan Gregory, Jost Heintzenberg, John Holdren, Ines
Horovitz, Bruce Johansen, Ralph Keeling, John Lyman, Len
Ornstein, Maureen Raymo, Christopher Shuman, Richard
Somerville, and Bob Thomas for comments on a draft version
of this letter.

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