[statecom-discuss] why we need to REDUCE CO2 emissions BELOW present levels

[Nat Fortune]

something more important than presidential campaigns!

The article below is a preprint. It has not yet been accepted for  
publication, but the authors are some of the world's pre-eminent  
climate scientists.  The take home message: business as usual for  
another decade will lead to irreversible climate changes that will  
ultimately tip us into the climate of the dinosaur age.

The authors recommend that CO2 levels must be reduced by 35 parts per  
million (ppm) below present levels. Since we are adding about 2 ppm/ 
year to the atmosphere, that means going back 17 - 18 years, to 1990  
levels.

Nat Fortune

from http://arxiv.org/pdf/0804.1126v1

(Submitted on 7 Apr 2008)

(warning: parts of the article are highly technical)

> If humanity wishes to preserve a planet similar to that on which  
> civilization developed and to which life on Earth is adapted,  
> paleoclimate evidence and ongoing climate change suggest that CO2  
> will need to be reduced from its current 385 ppm to at most 350 ppm.


> Present policies, with continued construction of coal-fired power  
> plants without CO2 capture,
> suggest that decision-makers do not appreciate the gravity of the  
> situation.  We must begin to
> move now toward the era beyond fossil fuels.  Continued growth of  
> greenhouse gas emissions,
> for just another decade, practically eliminates the possibility of  
> near-term return of atmospheric
> composition beneath the tipping level for catastrophic effects.




Target Atmospheric CO2: Where Should Humanity Aim?

James Hansen,1,2* Makiko Sato,1,2 Pushker Kharecha,1,2 David Beerling,3
Valerie Masson-Delmotte,4 Mark Pagani,5 Maureen Raymo,6 Dana L. Royer, 
7 James C. Zachos8
1
NASA/Goddard Institute for Space Studies, New York, NY 10025, USA.  
2Columbia University Earth Institute,
New York, NY 10027, USA. 3Dept. Animal and Plant Sciences, University  
of Sheffield, Sheffield S10 2TN, UK.
4
Lab. des Sciences du Climat et lÕEnvironnement/Institut Pierre Simon  
Laplace, CEA-CNRS-Universite de
Versailles Saint-Quentin en Yvelines, CE Saclay, 91191, Gif-sur- 
Yvette, France. 5Dept. Geology and Geophysics,
Yale University, New Haven, CT 06520-8109, USA. 6Dept. Earth  
Sciences, Boston University, Boston, MA 02215,
USA. 7Dept. Earth and Environmental Sciences, Wesleyan University,  
Middletown, CT 06459-0139, USA. 8Earth &
Planetary Sciences Dept., University of California, Santa Cruz, Santa  
Cruz, CA 95064, USA.

*To whom correspondence should be addressed.  E-mail:  
jhansen at giss.nasa.gov

a few additional excerpts

Human activities are altering EarthÕs atmospheric composition.  
Concern about global warming
due to long-lived human-made greenhouse gases (GHGs) led to the  
United Nations Framework
Convention on Climate Change (1) with the objective of stabilizing  
GHGs in the atmosphere at a
level preventing Òdangerous anthropogenic interference with the  
climate system.Ó

  The Intergovernmental Panel on Climate Change (IPCC, 2) and others  
(3) used several
Òreasons for concernÓ to estimate that global warming of more than  
2-3¡C may be dangerous.
The European Union adopted 2¡C above pre-industrial global  
temperature as a goal to limit
human-made warming (4). Hansen et al. (5) argued for a limit of 1¡C  
global warming (relative to
2000, 1.7¡C relative to pre-industrial time), aiming to avoid  
practically irreversible ice sheet and
species loss. This 1¡C limit, with nominal climate sensitivity of 3Ú4¡ 
C per W/m2 and plausible
control of other GHGs (6), implies maximum CO2 ~ 450 ppm (5).

  Our current analysis suggests that humanity must aim for an even  
lower level of GHGs.
Paleoclimate data and ongoing global changes indicate that ÔslowÕ  
climate feedback processes
not included in most climate models, such as ice sheet  
disintegration, vegetation migration, and
GHG release from soils, tundra or ocean sediments, may begin to come  
into play on time scales
as short as centuries or less (7). Rapid on-going climate changes and  
realization that Earth is out
of energy balance, implying that more warming is Ôin the  
pipelineÕ (8), add urgency to
investigation of the dangerous level of GHGs.

  A probabilistic analysis (9) concluded that the long-term CO2 limit  
is in the range 300-500
ppm for 25 percent risk tolerance, depending on climate sensitivity  
and non-CO2 forcings.
Stabilizing atmospheric CO2 and climate requires that net CO2  
emissions approach zero (10),
because of the long lifetime of CO2.

  We use paleoclimate data to show that long-term climate has high  
sensitivity to climate
forcings and that the present global mean CO2, 385 ppm, is already in  
the dangerous zone.
Despite rapid current CO2 growth, ~2 ppm/year, we show that it is  
conceivable to lower CO2 this
century to less than the current amount, but only via prompt policy  
changes.

...

Humanity today, collectively, must face the uncomfortable fact that  
industrial civilization itself
has become the principal driver of global climate.  If we stay our  
present course, using fossil
fuels to feed a growing appetite for energy-intensive life styles, we  
will soon leave the climate of
the Holocene, the world of prior human history.  The eventual  
response to doubling pre-
industrial atmospheric CO2 likely would be a nearly ice-free planet.

  HumanityÕs task of moderating human-caused global climate change is  
urgent.  Ocean and
ice sheet inertias provide a buffer delaying full response by  
centuries, but there is a danger that
human-made forcings could drive the climate system beyond tipping  
points such that change
proceeds out of our control.  The time available to reduce the human- 
made forcing is uncertain,
because models of the global system and critical components such as  
ice sheets are inadequate.
However, climate response time is surely less than the atmospheric  
lifetime of the human-caused
perturbation of CO2.  Thus remaining fossil fuel reserves should not  
be exploited without a plan
for retrieval and disposal of resulting atmospheric CO2.

  Paleoclimate evidence and ongoing global changes imply that todayÕs  
CO2, about 385 ppm,
is already too high to maintain the climate to which humanity,  
wildlife, and the rest of the
biosphere are adapted.  Realization that we must reduce the current  
CO2 amount has a bright
side: effects that had begun to seem inevitable, including impacts of  
ocean acidification, loss of
fresh water supplies, and shifting of climatic zones, may be averted  
by the necessity of finding an
energy course beyond fossil fuels sooner than would otherwise have  
occurred.

  We suggest an initial objective of reducing atmospheric CO2 to 350  
ppm, with the target to
be adjusted as scientific understanding and empirical evidence of  
climate effects accumulate.
Limited opportunities for reduction of non-CO2 human-caused forcings  
are important to pursue
but do not alter the initial 350 ppm CO2 target.  This target must be  
pursued on a timescale of
decades, as paleoclimate and ongoing changes, and the ocean response  
time, suggest that it
would be foolhardy to allow CO2 to stay in the dangerous zone for  
centuries.

  A practical global strategy almost surely requires a rising global  
price on CO2 emissions and
phase-out of coal use except for cases where the CO2 is captured and  
sequestered.  The carbon
price should eliminate use of unconventional fossil fuels, unless, as  
is unlikely, the CO2 can be
captured.  A reward system for improved agricultural and forestry  
practices that sequester carbon
could remove the current CO2 overshoot.  With simultaneous policies  
to reduce non-CO2
greenhouse gases, it appears still feasible to avert catastrophic  
climate change.

  Present policies, with continued construction of coal-fired power  
plants without CO2 capture,
suggest that decision-makers do not appreciate the gravity of the  
situation.  We must begin to
move now toward the era beyond fossil fuels.  Continued growth of  
greenhouse gas emissions,
for just another decade, practically eliminates the possibility of  
near-term return of atmospheric
composition beneath the tipping level for catastrophic effects.

  The most difficult task, phase-out over the next 20-25 years of  
coal use that does not capture
CO2, is herculean, yet feasible when compared with the efforts that  
went into World War II.  The
stakes, for all life on the planet, surpass those of any previous  
crisis.  The greatest danger is
continued ignorance and denial, which could make tragic consequences  
unavoidable.