IPCC targets lead to (at least) 54% odds of catastrophic climate change

Sun, 05/03/2009 – 10:06 — maggie_zhou

See here for a more concise but somewhat dryer version of this analysis.

When regulating emissions of greenhouse gases (GHGs), nations around the world claim to use the GHG concentration stabilization target of 450 parts per million carbon dioxide equivalent (ppm CO2-eq) as recommended by the Intergovernmental Panel on Climate Change in 2007, as their guildline for setting domestic emission reduction targets.

IPCC’s 2007 Fourth Assessment Report asserted that Annex I (developped) countries need to reduce GHG emissions 25- 40% below 1990 levels by 2020, and 80-95% below 1990 levels by 2050, in order to stabilize below 450 ppm CO2-eq concentration, after a temporary overshoot by 50 ppm[19]. Such a stabilization pathway was said to provide a “reasonable chance” of averting warming beyond 2˚C above pre-industrial temperature that would lead to catastrophic consequences on human and ecological systems. Most countries that bothered with a climate legislation dropped the tougher end of the ranges, and touted targets such as 20% below 1990 level by 2020, and 80% below 1990 level by 2050, as aggressive climate policies that supposedly heeds the warnings of IPCC. The current US legislation under consideration, the Waxman-Markey bill, is even worse. (See my posts on that bill here and here.)


The IPCC report only reviewed studies published up until middle of 2006, therefore its conclusions are at least 3 years old. Even if such stabilization target is achieved, according to the widely quoted Meinshausen 2006 study[1] which IPCC itself based its estimates on, a 450 ppm CO2-eq stabilization concentration has a:
• 26–78% probability of exceeding 2˚C relative to pre-industrial
• 4–50% probability of exceeding 3˚C
• 0–34% probability of exceeding 4˚C
• 0–21% probability of exceeding 5˚C

The best guess in 2006 was that this stabilization level will have a 54% probability of exceeding 2˚C relative to pre-industrial. Would you board an airplane with a 54% chance of crashing?

Here is directly from the IPCC 2007 Working Group I report[20], chapter 10, Table 10.8. on page 826:

450 ppm CO2-eq corresponds to best estimate of 2.1°C temperature rise above pre-industrial global average, and “very likely above” 1°C rise, and “likely in the range” of 1.4–3.1°C rise.

In other words, the temperature with the highest probability of being realized is 2.1°C rise above pre-industrial global average, with roughly 50/50 odds for actually rising more or less than 2.1°C.

Yet again from the IPCC 2007 Working Group I report[20], chapter 10, Supplementary Figure S10.4. (page Sm.10-8). This graph uses a range of parameter choices, and shows that 450 ppm stabilization has a (lower) medium (neither likely, nor unlikely, but closer to unlikely than to likely) chance, or somewhat less than 50% chance, of staying below 2°C above pre-industrial average temperature.

As if that’s still not enough, reference [21] below shows an excerpt from the IPCC report, highlighting their emphasis that the conclusions in their report assumed linear responses in the climate system, and did not adequately consider the (now apparent and pervasive) non-linear responses, which leads to their underestimation of climate sensitivity (how quickly temperature rises in response to increased CO2). Of course, such fine prints in an academic publication is completely lost to policy makers, the media and the general public worldwide.


Since the release of the IPCC report, new data and analysis now proves the IPCC targets way too optimistic. The recent Copenhagen Climate Science Congress, attended by 2000 scientists, concluded: “Recent observations confirm that, given high rates of observed emissions, the worst-case IPCC scenario trajectories (or even worse) are being realized.” Unexpectedly high emissions from fast developing countries in the last few years mean that much faster global emission reductions are now necessary in order to keep the same cumulative emssions targets by 2100 used to model CO2-eq concentrations, without even integrating the warming potential of larger earlier emissions over the longer warming period. In other words, even if we still aim for the same stabilization target concentration, the emission reduction targets need to be stronger than 40% below 1990 levels by 2020, and 95% below 1990 levels by 2050.

Further, IPCC’s projections didn’t even take into consideration the major positive feedbacks from melting of heat reflective glaciers and ice sheets, and the release of methane (a GHG 25x more potent than CO2) from thawing tundra and under sea bed , due to the difficulty of modeling such dynamic processes and the large uncertainties[21]. Unfortunately, recent climate news is full of evidence of expedited methane release[2-7,18], and rapid loss of both Arctic sea ice and ice shelves buttressing land ice sheets[8-10]. the Arctic may become completely ice free in the summer as early as 2013, 80 years ahead of predictions! The loss of this major reflective feature of our planet’s climate system may well be the strong tipping point that thrusts us into the superfast trojectory of uncontrollable, irreversible runaway warming.

This, as well as the near-global retreat of Alpine glaciers, the fast shifting of climate zones leading to increased aridity in southern United States and other parts of the world, the dramatically increased incidence rates of extreme weather events on every continent in the past few decades, and the frequent bleaching of coral reefs under the dual stress of ocean warming and acidification (also a result of higher CO2), all mean that the Earth is already in a dangerous energy imbalance – absorbing more heat than it is radiating into space – at the current 387ppm, and that the level of warming already realized, which is around 0.8˚C above pre-industrial[11], is already too high, yet the current atmospheric CO2 concentration is still to bring another 2˚C delayed warming that is coming down the pipeline, when the system reaches equilibrium, due to delayed ocean thermal and ice sheet responses.[12,13]

It also means that even if the strongest emission reduction targets recommended by IPCC is achieved, we will not be able to stabilize below 450 ppm CO2-eq concentration. Once passed the climate point-of-no-return, we will not just get 2˚C warming, but 3, 4, 5, 6˚C and more due to the feedback loops. The scary thing is, no one knows how long we have until passing that point, or if it has already passed, but many prominant scientists say that crutial physical tipping points in the system may have already been passed, although, due to the above mentioned inertia, we may still have time to reverse the changes we caused before the climate state proceeds past the point-of-no-return.[14]


Evidence from paleoclimate data also clearly shows 450 ppm to be the wrong stabilization target. NASA’s James Hansen and other top international climatologists found in 2008 that the boundary condition between an ice-free planet (where sea level was over 220 feet higher than today) and the glaciation of Antarctica 35 million years ago, was around 450+/-100 ppm CO2. If we aim to stabilize below 450 ppm, we aim to cross that threshold in the opposite direction (de-glaciation). The authors state that the upper limit of safe greenhouse gas concentration is somewhere between 300-350 ppm CO2-eq.[12]

Hans Joachim Schellnhuber, head of the Potsdam Institute and climate adviser to German Chancellor and the EU, one of Europe’s leading climate scientists, also believes that operating well outside the historic realm of CO2 concentrations is risky as long as we have not fully understood the relevant feedback mechanisms. During the past 800,000 years where ice core data is available, and possibly much longer, CO2 concentrations oscillated periodically as a result of feedback responses to the Earth’s orbital changes, and that resulted in the glacial/interglacial cycles of temperature swings. During that period, at the peak of interglacial warmth of each cycle (which were also the peaks of CO2 concentration), CO2 never exceeded 300 ppm. (See Figure. I previously wrote about it here.)

Stabilization at or below 350 ppm CO2-eq provides a 93% probability of staying below 2°C above pre-industrial, with a best guess of 1°C and a likely range of 0.6–1.4°C above pre-industrial.[1,11] It is important to note that, the findings of IPCC suggest that a rise of 1°C in mean global temperatures and, correspondingly, sea surface temperatures, above pre-industrial levels is the maximum that should be aimed for if the global community wishes to protect coral reefs.[15] Coral reefs are extremely important marine ecosystems, and any suggestion that we could allow them to be wiped off this planet with an emissions target not even able to reasonably prevent a 2°C because we are too greedy to spend a fraction to a few percentage points of our GDP to preserve our planet[16], is pure madness.


Since we are already at 387 ppm CO2 now, we not only need to reduce our emissions as fast as humanly possible, we need to reduce the actual GHG concentrations already built up in the air by drawing down carbon, preferably with more natural methods such as aggressive reforestation and better land management. In other words, we need to achieve net negative emissions ASAP. Even if we achieved net zero emissions, we can not completely rely on natural processes such as the ocean to remove the excess carbon for us, because of the speed with which we must return below 350 ppm, and because dissolved CO2 in the ocean has already caused ocean acidification (the surface ocean is now 30% higher in hydrogen ion concentration over pre-industrial level), further aggravation will pose a grave threat to ocean ecosystems (particularly to any calcifying species such as corals, shell forming organisms, and some planktons, but also to water-breathing marine animals such as fish and zooplanktons).[17] Therefore we must try to alleviate it, instead of allowing acidification to intensify, which will progress down the water column into the deep ocean through water mixing and equilibrating, pushing up the so called “saturation horizon” below which calcium carbonate, the vital material for making shells and skeletons, becomes unsaturated and structures made from it are susceptable to dissolution.


1. Meinshausen, M. (2006): ‘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’, pp.253 – 280 in Avoiding dangerous climate change, H.J. Schellnhuber et al. (eds.), Cambridge: Cambridge University Press.

2. The methane time bomb Subsea methane are bubbling to the surface, dubbed “methane chimneys”, off the Siberian coast, as the Arctic region becomes warmer and its ice retreats.

3. Hundreds of methane ‘plumes’ discovered … bubbling from the seabed to the west of the Norwegian island of Svalbard

4. Bubbles of warming, beneath the ice “We are seeing thawing down to 5 meters,” says geophysicist Vladimir Romanovsky of the University of Alaska. “A third to a half of permafrost is already within a degree to a degree and a half [Celsius] of thawing.

5. Melting Ice-A Hot Topic? New UNEP Report Shows Just How Hot It’s Getting According to a report by United Nations Environment Programme, “Rising temperatures and the thawing of frozen land or ‘permafrost’ is triggering the expansion of existing- and the emergence of new- water bodies in places like Siberia. These are bubbling methane into the atmosphere with emissions so forceful they can keep holes open on the lakes’ icy surfaces even during sub zero winter months.”

6. A 2008 United Nations Environment Programme report: Methane from the Arctic: Global warming wildcard

7. Global-warming methane spiked in 2007 Pre-industrial concentrations of methane were at about 700 parts per billion, and levels rose gradually to 1773 parts per billion by the late 20th century, Rigby says. The rise in 2007 was about 10 parts per billion, a significant jump over such a short period.

8. Huge Ice Shelf Breaks From Antarctica, Fractures (Wilkins Ice Shelf, April 2009)

9. Arctic sea ice thinnest ever going into spring http://nsidc.org/arcticseaicenews/2009/040609.html http://www.nasa.gov/topics/earth/features/arctic_thinice.html

10. Ice melting dynamics, melting in action

11. IPCC AR4 WGI (2007): Solomon SD, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, and Miller HL (eds)], Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IPCC (Cambridge University Press, Cambridge, 2007), chapter 3, p 237.

12. Hansen, J., Mki. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231.

13. Ramanathan and Y. Feng (2008): On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead. PNAS 2008 105: 14245–14250

14. Hansen et al (in Ref 12 above) defines two distinctive concepts: “(1) the tipping level, the global climate forcing that, if long maintained, gives rise to a specific consequence, and (2) the point of no return, a climate state beyond which the consequence is inevitable, even if climate forcings are reduced. A point of no return can be avoided, even if the tipping level is temporarily exceeded. Ocean and ice sheet inertia permit overshoot, provided the climate forcing is returned below the tipping level before initiating irreversible dynamic change.”

15. IPCC AR4 WGII. Climate Change 2007: Climate Change Impacts, Adaptation and Vulnerability. WGII Contribution to the IPCC AR4 (Cambridge University Press, 2007), pp 12, 16 (Figure SPM.2.), 321, and 853.

16. See related blog on the cost of climate action/inaction.

17. Raven, J. A. et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide Royal Society, London, UK.

18. Methane rise points to wetlands The spike of methane in 2007 comes predominantly from biogenic sources, not from biomass burning. Taken together with wind direction to see where the gas is being produced, analysis implies the excess methane comes from the Arctic regions, rather than one further afield such as the additional output from Asia’s rapid industrialisation.

19. IPCC AR4 WGIII. Climate Change 2007: Mitigation of Climate Change. WGIII Contribution to the IPCC AR4 (Cambridge University Press, Cambridge, 2007), chapter 13, Box 13.7 on page 776.

20. IPCC AR4 WGI (2007): Solomon SD, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, and Miller HL (eds)], Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IPCC (Cambridge University Press, Cambridge, 2007)

21. reference #20, chapter 10, Table 10.8. on page 826 (highlighting mine):
It is emphasized that this table does not contain more
information than the best knowledge of S and that the numbers
are not the result of any climate model simulation. Rather it
is assumed that the above relationship between temperature
increase and CO2 holds true for the entire range of equivalent
CO2 concentrations. There are limitations to the concept of
radiative forcing and climate sensitivity (Senior and Mitchell,
2000; Joshi et al., 2003; Shine et al., 2003; Hansen et al.,
2005b). Only a few AOGCMs have been run to equilibrium
under elevated CO2 concentrations, and some results show
that nonlinearities in the feedbacks (e.g., clouds, sea ice and
snow cover) may cause a time dependence of the effective
climate sensitivity and substantial deviations from the linear
relation assumed above
(Manabe and Stouffer, 1994; Senior
and Mitchell, 2000; Voss and Mikolajewicz, 2001; Gregory et
al., 2004b), with effective climate sensitivity tending to grow
with time in some of the AR4 AOGCMs. Some studies suggest
that climate sensitivities larger than the likely estimate given
below (which would suggest greater warming) cannot be ruled
(see Box 10.2 on climate sensitivity).

Further Reading:

From Climate Code Red: “What’s up with emisions reductions of 25-40% by 2020?” (and 80% by 2050)

Climate Safety report: http://www.climatesafety.org/

My Submission for Massachusetts Democratic Platform testimony on the Environment
(I laid out proposals for climate target and broader solutions to environmental/economic/sustainability crisis)

Leave a Comment