After years of inaction, the possibility of substantive
federal and international climate policy is finally in sight. With so much time
already squandered, insufficient action today will foreclose the ability to
prevent catastrophe tomorrow. If we are to avoid saddling future generations
with extreme economic and environmental hardships, emerging climate policy must
ensure a high probability of keeping future warming below dangerous levels.
Unfortunately, proposed federal climate legislation, which
aims at limiting temperature rise to 2–3°C above pre-industrial levels by
stabilizing greenhouse gases in the range of 450–550 parts per million (ppm) CO2eq, poses
significant and unacceptable risks. The best available scientific evidence now
indicates that a warming of 2°C is not “safe” and would not prevent dangerous
interference with the climate system. In addition, due to a number of climactic
processes that are not fully understood, equating a particular atmospheric
concentration of greenhouse gases with a specific temperature increase involves
a significant degree of uncertainty. As the consequences of overshooting a 2°C
threshold could include the displacement of millions due to sea level rise,
irreversible loss of entire ecosystems, and the triggering of multiple
climactic “tipping points,” the risk tolerance for overshooting a 2°C
temperature rise should be extremely low. Nonetheless, the risk of overshooting
a 2°C threshold is 50–82
percent at stabilization levels of 450–550 ppm CO2eq.
Thus, even the most aggressive federal climate change proposals seem, at best,
content to flip a coin in the hopes that future generations are not left with
few choices beyond mere survival. This is not risk management, it is
recklessness and we must do better.
In order to avoid dangerous anthropogenic interference (DAI)
with the climate system, sound climate policy must minimize the risk of severe
and irreversible outcomes. A policy objective of stabilizing greenhouse gas
emissions at 350 ppm CO2eq, would reduce the mean probability of
overshooting a 2°C temperature rise to 7 percent.
A 350 ppm CO2eq stabilization level is also consistent with that
proposed by leading climatologists, who have concluded that in order “to
preserve a planet for future generations similar to that in which civilization
developed and to which life on Earth is adapted . . . CO2 will need
to be reduced from its current 385 ppm to at most 350 ppm.” While
current CO2 levels exceed 350 ppm, a pathway toward 350 ppm is
possible though the rapid phase-out of coal emissions, improved agricultural
and forestry practices, and possible future capture of CO2 from
biomass power plants.
Article 2 of the United
Nations Framework Convention on Climate Change (UNFCCC) calls for
“stabilization of greenhouse gas concentrations in the atmosphere at a level
that would prevent dangerous anthropogenic interference with the climate
2 further provides that “[s]uch a [concentration stabilization] level should be
achieved within a time-frame sufficient to allow ecosystems to adapt naturally
to climate change, to ensure that food production is not threatened and to
enable economic development to proceed in a sustainable manner.”
With the United States and over 180 other countries as signatories, the
UNFCCC’s objective of avoiding DAI with the climate is widely viewed as the
international standard for protecting the global climate.
Determining the greenhouse gas stabilization level needed to
avoid DAI requires consideration of both the mean global temperature increase
at which resulting harm is judged to be “dangerous,” as well as the degree of
uncertainty that a “dangerous” temperature threshold will be exceeded at a
particular greenhouse gas stabilization level. While scientific analysis can
provide data on the potential impacts and risks associated with particular
levels of greenhouse gases and increases in global mean temperature, defining
what “dangerous” means is ultimately a political question. Policy makers must
weigh the relative importance of various impacts and risks to define what
constitutes “unacceptable risk.”
In the last 100 years, the Earth has warmed by over 0.7°C.
Unabated, current trends suggest that increasing emissions will continue to
raise the Earth’s temperature by 4–6°C (7.2–10.8°F), if not more, by the end of
Currently proposed federal climate policies aim to curtail this increase but
still fall far short of preventing DAI. Neither the United States nor the
international community as a whole has defined the point at which temperature
rise can be said to be “dangerous.” However, proposed federal legislation aims
at limiting temperature rise to 2–3°C above pre-industrial levels by
stabilizing greenhouse gases in the range of 450–550 ppm CO2eq, which
roughly comports with the European Union’s (EU) climate change objective to
“[limit] global warming to less than 2°C above the pre-industrial temperature
as there is strong scientific evidence that climate change will become
dangerous beyond this point.” While
the 2°C target set by the EU may have seemed acceptable when first proposed in
research now indicates that much smaller increases in global mean temperature
will result in substantial environmental and socio-economic consequences. As the
best available scientific information indicates that a 2°C mean global
temperature rise from pre-industrial levels is far in excess of what can
reasonably be considered “dangerous,” a 2°C temperature rise is not an acceptable
target from which to base climate policy.
Projected risks and damages from global warming are more
serious than believed even a few years ago. In 2001, the Intergovernmental Panel on Climate Change (IPCC)
used five Reasons For Concern (RFCs) in its Third Assessment Report (TAR) to
illustrate the temperature range at which impacts may be considered dangerous.
Relationships between the impacts reflected in each RFC and increases in global
mean temperature were portrayed in a “burning
embers” diagram, which reflected the severity of risk from rising
temperature through gradations in color from white (no or little risk) to yellow
(moderately significant risk) to red (substantial or severe risk).
Depending on the RFC, the IPCC predicted that substantial impacts or risks
(transition from yellow to red) would occur with a temperature rise 1–4°C above
Since the release of the TAR, scientific understanding of
the vulnerability of the climate to temperature rise has evolved considerably.
Based on new findings in the growing scientific literature since the TAR was
released, the burning embers diagram was revised in 2008 to reflect the
dangerous risks posed by smaller increases in temperature than identified in
the original TAR. In the
updated burning embers diagram, the IPCC now predicts that substantial impacts
or risks occur at or near current temperature levels for a number of RFCs.
As reflected in the updated RFCs, a 2°C temperature increase from
pre-industrial levels (or 1.4°C increase from 1990 levels) is well past the
point where severe and irreversible impacts will occur.
It is now estimated that a mean global temperature increase
of 1.5°C above pre-industrial levels has the potential to trigger irreversible
melting of the Greenland ice sheet, a process that would result in an eventual
seven meter sea level rise over and above that caused by thermal expansion of
the oceans, and potentially causing an additional sea level rise of 0.75
meters, as soon as 2100.
Specific consequences of a 2°C temperature rise from pre-industrial levels
include the loss of 97 percent of the world’s coral reefs and the transformation
of 16 percent of global ecosystems. Indeed, given increased confidence that a
1–2°C increase poses significant risks to many unique and threatened systems,
including many biodiversity hotspots, the updated burning embers diagram
indicates substantial impacts and/or moderate risks from warming that has
already occurred. At a
2°C temperature rise, approximately one to three billion people would
experience an increase in water stress, sea level rise and cyclones would
displace millions from the world’s coastlines, and agricultural yields would
fall in the developed world. In the
Arctic, ecosystem disruption is predicted upon expectations of a complete loss
of summer sea ice, with only 42 percent of the tundra remaining stable. This
would destroy the Inuit hunting culture, cause the extinction of the polar
bear, and result in large losses in global bird populations. Moreover, because
Arctic ice functions to reflect heat back into the atmosphere, its loss would
allow more sunlight to heat the Arctic Ocean and further accelerate the melting
of the Greenland ice sheet. As the devastating and irreversible impacts
resulting from a 2°C mean global temperature rise greatly exceed any reasonable
definition of DAI, the 2°C target originally proposed in 1996 and reaffirmed by
later EU policies is not an adequate objective for climate policy if DAI is to
Not only are the climate impacts expected from a 2°C
temperature increase far in excess of what should be considered “safe,” but
policies which propose greenhouse gas stabilization levels of 450–550 ppm CO2eq
present substantial risks of overshooting this target, thus exacerbating the
problem. Equating a particular atmospheric concentration of greenhouse gases
with a specific temperature increase involves a significant degree of
uncertainty. Climate sensitivity—the extent to which temperatures will rise as
a result of increasing concentrations of heat-trapping gases—depends on Earth’s
response to certain physical processes that are not fully understood.
Thus, due to uncertainty in climate sensitivity, scientists estimate that the
mean probability of exceeding 2°C where greenhouse gases are stabilized at a CO2eq
level of 550 ppm is 82 percent. The
mean probability of exceeding 2°C where greenhouse gases are stabilized at a CO2eq
level of 450 ppm is 54 percent with a 30 percent probability that global
average temperature would rise more than 3°C. At 400
ppm CO2eq, the mean probability of exceeding 2°C is 28 percent.
If greenhouse gas emissions were stabilized at 350 ppm CO2eq, the
mean probability of exceeding 2°C would be reduced to 7 percent.
Properly accounting for climate sensitivity in climate
policy is critical because, as dire as the projected impacts resulting from a
2°C mean temperature increase, increases above 2°C would result in apocalyptic
impacts. If a 2–3°C increase in mean global temperature occurred, feedbacks in
the climate system would cause a shift in the terrestrial carbon cycle.
Currently, land-based carbon acts as a sink for CO2, buffering the
effects of anthropogenic climate change. If CO2 concentrations
continue to rise, this sink will release stored CO2 through
increased soil respiration, further exacerbating climate change. The most
dramatic impacts will be a widespread loss of forests and grassland, including
the Amazon rainforest. The transition of these areas to savannah would trigger
wide spread implications for local population, global biodiversity, and the
global carbon cycle. At a
global increase in temperature of 3°C above pre–industrial levels, many
additional impacts in human and natural systems would occur in ways
exponentially more devastating that those predicted for a 2°C temperature
increase. Few ecosystems can adapt to such a large temperature rise: 22 percent
would be transformed, losing 7 percent to 74 percent of their extent.
An additional 25 to 40 million people would be displaced from coasts due to sea
level rise, an additional 1200 to 3000 million would suffer an increase in
water stress, and 65 countries would lose 16 percent of their agricultural
gross domestic product.
As much as the threats posed by global warming are
unfathomable, equally unfathomable is the degree of risk that current climate
policy appears willing to accept. None of the proposed federal climate change
legislation comes close to meeting the lower end of a U.S. emissions budget consistent with a 450 ppm CO2eq stabilization objective.
For example, the Lieberman-Warner proposal is consistent with a 500 ppm CO2eq
stabilization target, which has a 70 percent chance of resulting in temperature
increases above 2°C and a 50 percent chance of a greater than 3°C increase.
The United States gambles with proposed policies that only have at a 30 percent
chance of meeting a target that itself is too high to avoid severe and
irreversible impacts. Even California’s more aggressive emission reduction
targets, which call for emissions reductions to 1990 levels by 2020 and then to
80 percent below 1990 levels by 2050, is still only consistent with a stabilization
scenario in the 450 ppm range. This
target, however, provides only a fifty-fifty chance of averting the severe and
irreversible outcomes that would occur with a 2°C temperature rise from
pre-industrial levels. While a significant improvement from business-as-usual,
proposed climate policies are insufficient to minimize the risk of catastrophic
The assessment of what constitutes DAI with the climate
requires: (1) a consideration of climate sensitivity and its uncertainty; (2)
the range of possible temperature changes above which unacceptably large
negative impacts occur; and, (3) determination of a morally acceptable
tolerance for risk.
As set forth above, stabilization objectives in the 450–550 ppm CO2eq
far exceed levels that can be considered safe. Indeed, based on observed
impacts and expected future impacts likely to occur from existing emissions,
current climate conditions already constitute DAI. In order to avoid continued
DAI, CO2 concentrations must be reduced to a level no greater than
Current conditions already constitute DAI and are in
violation of the UNFCCC. As an initial matter, the rapid and significant
impacts observed to date are already serious enough to be deemed “dangerous.”
Atmospheric concentrations of CO2 have risen from a pre-industrial
concentration of 280 ppm to 383 ppm in 2007. Annual
mean global temperature has increased by 0.76°C relative to pre-industrial
times and is increasing at a rate of 0.17°C per decade.
Impacts from this anthropogenic interference with the climate has already
resulted in tens of thousands of climate-related deaths, species extinction,
ocean acidification and loss of coral reefs, and the significant retreat of
glaciers and sea ice. The
effects of this relatively small climate change are already being observed
around the world: sea level is increasing at the rate of 1.8 millimeters per
year, oceans have already acidified by 0.1 pH units (equating to a 30 percent
increase in acidity), glaciers are retreating worldwide, and local temperature
has increased in the Arctic by 1.8°C, which has accelerated the loss of sea ice
and permafrost. Across
the globe, species are changing their lifecycles and distribution in a direction
consistent with their expected response to climate change.
Unprecedented heat waves in large cities, intensifying drought in many regions,
and substantial and increasing damage due to extreme weather events are also
attributed, in part, to climate change.
In addition to the impacts already observed, additional
warming already “in the pipeline”—due to inertia in the climate system and
identified feedback loops—will result in further increases in temperature that
pose significant risks of severe and irreversible impacts. We are
already locked into anywhere from 0.3°C to 0.7°C additional warming relative to
late 20th century levels due to the eventual impacts of past historical
account of additional warming to which cannot be avoided, Ramanathan and Feng
noted there is a “high probability that the DAI threshold is already in our
Similarly, on the basis of paleoclimate evidence and ongoing climate change,
James Hansen and other leading climate scientists concluded the present CO2
levels of 385 ppm are “already in the dangerous zone” and that “[i]f
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, but likely less than that.”
In looking at dangerous climate change though the lens of
risk tolerance, Harvey concluded that, at a 10 percent risk tolerance, atmospheric
CO2 concentrations close to present levels “violates the UNFCCC” for
a range of assumptions of climate sensitivity.
Similarly, in Meinchausen’s analysis, only CO2eq concentration of
350 ppm is less than 10 percent likely to result in a temperature increase in
excess of 2°C. Given
the extreme consequences for future generations if a 2°C threshold is exceeded,
higher levels of risk tolerance are unacceptable.
In order to achieve a 350 CO2 ppm atmosphere,
atmospheric CO2 concentrations must be reduced quickly: “[i]ndeed,
if the world continues on a business-as-usual path for even another decade
without initiating phase-out of unconstrained coal use, prospects for avoiding
a dangerously large, extended overshoot of the 350 ppm level will be dim.”
While stabilization at 350 ppm CO2 requires a drawdown from existing
CO2 levels, a pathway to 350 ppm is possible. According to Hansen and his colleagues, “[a]n initial 350 ppm CO2 target may be achievable
by phasing out coal use except where CO2 is captured and adopting
agricultural and forestry practices that sequester carbon . . . . With
simultaneous policies to reduce non-CO2 greenhouse gases, it appears
still feasible to avert catastrophic change.”
While stabilization at 350 ppm present significant
challenges, the “[r]ealization 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.”
The decisions we make today on the national and
international level will determine the health and livability of the planet for
generations to come. It is critical that policymakers recognize the great
danger to humankind from policies that fail to set temperature thresholds at
levels necessary to avoid severe and irreversible outcomes and that accept
significant risks that a targeted greenhouse gas stabilization level will
result in temperature rises greater than expected. In order to ensure that
future generations are left with a functioning planet, climate policies must be
established to immediately execute an emission reduction pathway consistent with
stabilization at CO2 levels of no greater than 350 ppm.
is a unit of measurement used to compare the climate effect of all greenhouse
gases to each other. Currently, the effects of aerosol and land-use changes
reduce radiative forcing so that the net positive forcing from all greenhouse
gases is roughly equivalent to that of CO2 alone. See H. H.
Rogner et al., Introduction,
in B. Metz et al., Climate Change 2007: Mitigation. Contribution
of Working Group III to the Fourth Assessment Report of the Intergovernmental
Panel on Climate Change 102 (2007). However, assuming air quality improves
as projected, the net negative forcing from aerosols will no longer offset the
positive forcing of non-CO2 greenhouse gases. See Malte Meinshausen, What Does a 2°C Target Mean for Greenhouse Gas Concentrations? A Brief
Analysis Based on Multi-Gas Emission Pathways and Several Climate Sensitivity
Uncertainty Estimates, in Avoiding
Dangerous Climate Change 268 (2006).
Meinshausen, supra note 1, at 270.
 James Hansen et al., Target Atmospheric CO2: Where Should Humanity Aim? 2 Open Atmospheric Sci. J. 217, 226 (2008).
Because climate forcing from anthropogenic non-CO2 greenhouse
emissions is approximately offset by cooling affect of anthropogenic aerosol
emissions, future CO2 change should be considered as approximating
the net human-made forcing change, with several caveats. See id.
Operating under a different set of assumptions that appears to discount the
negative forcing of aerosols over time,, Meinshausen states that 550 CO2eq
roughly corresponds to 475 ppm CO2. Meinshausen, supra note
1, at 269. Under Meinshausen’s analysis, 500 CO2eq is
approximately equivalent to 450 ppm CO2 stabilization, 450 CO2eq
is approximately equivalent to 400 ppm CO2 stabilization, and
400 CO2eq is approximately equivalent to 350–375 ppm CO2
stabilization. Id. at 305. Meinshausen does not state the CO2
level equivalent to a 350 ppm CO2eq stabilization
level, but based on the ratio used for other CO2eq levels, a 350 ppm
CO2eq would equate to somewhat less than 350 ppm CO2. Id.
 See Hansen
et al., supra note 4, at 226–27.
Nations Framework Convention on Climate Change, art. 2, May 9, 1992, available at http://unfccc.int/essential_background/convention/background/items/1349.php.
H. Schneider & Janica Lane, An Overview of “Dangerous” Climate Change,
in Avoiding Dangerous Climate
Change 14 (2006).
 Id. Despite existing efforts to curb emissions by a number of Kyoto Protocol
signatory countries, anthropogenic CO2 emissions have been growing
about four times faster since 2000 than during the previous decade and at rate
above even the IPCC’s most fossil fuel intensive scenario. Global Carbon Project, Growth in the Global
Carbon Budget (2008), available at http://www.globalcarbonproject.org/carbontrends/index.htm.
 Union of
Concerned Scientists, How
to Avoid Dangerous Climate Change: A Target for U.S. Emissions 3
Press Release, Europa, Climate change: Commission sets out proposals for
global pact on climate change at Copenhagen (Jan. 28, 2009), available
Meinshausen, supra note 1, at
266. The EU has reiterated its 2°C objective as recently as January 2009
despite increasing scientific evidence that climate impacts would reach
catastrophic proportions with a 2°C global mean temperature rise. Id.
e.g., Joel B. Smith et al., Assessing
Dangerous Climate Change Though an Update of the Intergovernmental Panel on
Climate Change (IPCC) “Reasons for Concern,” Proc. of the Nat’l Acad. Sci., Feb. 26, 2009, at 1, available at http://www.pnas.org/content/early/2009/02/25/0812355106.abstract.
 IPCC, Climate Change 2001: Synthesis Report, Summary
for Policymakers 11 (2001). The five RFCs identified in the TAR
are: 1) Risks to Unique and Threatened Systems; 2) Risks of Extreme Weather
Events; 3) Distribution of Impacts; 4) Aggregate Impacts; and 5) Risks of Large
Scale Discontinuities. Id.
 Id. See also Smith, supra note 14, at 1, 5.
 IPCC, supra
note 15, at 11. The RFC’s assessed impacts from a baseline of 1990
temperature levels rather than pre-industrial levels. Because pre-industrial
warming until 1990 was 0.6°C, an impact resulting from a temperature rise of
1°C equates to a 1.6°C rise from pre-industrial levels. Id.
Smith, supra note 14, at 1, 5.
id. An updated burning embers diagram was omitted from the 2007 Fourth
Assessment Report due to opposition from the United States, China, Russia, and Saudi Arabia. Because the Assessment Report is a consensus document, these
countries were able to prevent the inclusion of an updated diagram despite the
insistence by New Zealand, small islands states, Canada, Germany, and the United Kingdom that inclusion of an updated burning embers was essential. See Andrew C. Revkin, Why 2007 I.P.C.C. Report Lacked ‘Embers’, N.Y. Times, Feb. 26, 2009, http://dotearth.blogs.nytimes.com/2009/02/26/why-2007-ipcc-report-lacked-embers.
supra note 14, at 3.
 Rachel Warren, Impacts of Global Climate Change at Different Annual Mean Global Temperature
Increases in Avoiding Dangerous
Climate Change 95 (2006). Unlike the IPCC’s RFC, Warren assessed impacts
from temperature rise from pre-industrial levels, not 1990 levels.
supra note 14, at 3.
Warren, supra note 22, at 98.
e.g., California Climate Change Center, Our Changing Climate:
Assessing the Risks to California 4 (2006), available at http://meteora.ucsd.edu/cap/pdffiles/CA_climate_Scenarios.pdf.
Meinshausen, supra note 1, at 268–69. Meinshausen operates under
assumptions that do not roughly equate CO2 eq with CO2
concentrations. 550 CO2eq roughly corresponds to a stabilization
of 475 ppm CO2 only. Id. at 269. Meinshausen notes that
500 CO2eq is approximately equivalent to 450 ppm CO2 stabilization,
450 CO2eq is approximately equivalent to 400 ppm CO2 stabilization,
and 400 CO2eq is approximately equivalent to 350–375 ppm CO2 stabilization.
 Id. See also Union of Concerned Scientists,
supra note 11, at 3.
Meinshausen, supra note 1, at 270.
Warren, supra note 22, at 98–99.
 Id. at 99.
 Id. at 96–97.
 Union of Concerned Scientists, supra note
11, at 16.
 Id. The American Clean Energy and Security Act of 2009 (ACESA), recently
introduced by Congressmen Henry Waxman and Ed Markey, aims to reduce emissions
from capped sectors by 83 percent below 2005 levels by 2050. See U.S.
House of Representatives, Discussion Draft, The American Clean Energy and
Security Act of 2009 3 (2009), available at
http://energycommerce.house.gov/Press_111/20090331/acesa_summary.pdf. It was
not evaluated in the Union of Concerned Scientists report. See Union of Concerned Scientists, supra note
11, at 16. While an improvement over Lieberman-Warner, which aimed to reduce
emissions from capped sectors to 71 percent below 2005 levels by 2050, the
ACESA would likely still be far from the lower end of a U.S. emissions budget consistent with a 450 ppm CO2eq stabilization objective. See id.
(finding that the most aggressive climate proposal, H.R. 1590 (Waxman), which
aims to reduce emissions to 80 percent of 1990 levels by 2050, was still at the
upper end of an emission budget aimed at stabilizing greenhouse gases at 450
ppm); The Lieberman-Warner Climate Security Act (S. 2191) (2008), available
Cal. Exec. Order S-3-05 (June 1, 2005). An emissions pathway whereby
developed countries would reduce emissions to 80 percent below 1990 levels as
envisioned under California Executive Order S-3-05 would cap atmospheric
concentrations of CO2 at approximately 450 ppm. See, e.g., U.N. Dev. Program, Human Development Report
2007/2008: Fighting Climate Change: Human Solidarity in a Divided World 46–50
(2007), available at http://hdr.undp.org/en/reports/global/hdr2007-2008/chapters/.
See L.D. Danny Harvey, Dangerous Anthropogenic Interference,
Dangerous Climate Change, and Harmful Climatic Change: Non-Trivial Distinctions
with Signifcant Policy Implications, 82 Climatic
Change 1, 14 (2007).
E. Trenberth et al., 2007: Observations: Surface and Atmospheric Climate
Change, in Susan Solomon et
al., Climate Change 2007: The Physical Science Basis, Contribution of Working
Group I to the Fourth Assessment Report of the Intergovernmental Panel on
Climate Change 253 (2007).
 Warren, supra note 22, at 94–97.
 Id. note 22.
 Id. at 95, 97.
V. Ramanathan & Y. Feng, On Avoiding Dangerous Anthropogenic
Interference With the Climate System: Formidable Challenges Ahead, 105 Proc. of the Nat’l Acad. Sci. 14245,14249 (2008); Hansen et al., supra note 4, at 226.
 Michael E. Mann, Defining Dangerous Anthropogenic Interference, 106 Proc. of the Nat’l Acad. Sci. 4065, 4066
Ramanathan & Y. Feng, supra note 43, at 14245, 14249.
et al., supra note 4, at 217–18.
 Harvey, supra note 36, at 20.
Meinshausen, supra note 1, at 268. Under the assumptions used in this
study, 350 ppm CO2eq is equivalent to CO2 concentration
of slightly less than 350 ppm. Id.
et al., supra note 4, at 227.
 Id. at 217, 229.
 Id. at 228.
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