The Science

Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years. Tripati, AK., et. al. Science. 326:1394. 2009

Sun, 25 Jul 2010 18:03:24 -0400

Abstract: The carbon dioxide (CO2) content of the atmosphere has varied cyclically between ~180 and ~280 parts per million by volume over the past 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, the partial pressure of CO2 (pCO2) is much less certain, and the relation between pCO2 and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO2 during major climate transitions of the past 20 million years. During the Middle Miocene, when temperatures were ~3° to 6°C warmer and sea level was 25 to 40 meters higher than at present, pCO2 appears to have been similar to modern levels. Decreases in pCO2 were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago) and Late Pliocene (~3.3 to 2.4 million years ago).

Working Group comments: These scientists utilized radioisotope dating techniques and 28 models to determine the atmospheric carbon dioxide (CO2) concentrations and ocean pH millions of years ago. They found that in the Miocene and Late Pliocene periods data to “support a close coupling between pCO2 [atmospheric CO2 levels] and climate. Relative to today, surface waters appear to have been more acidic [lower pH] and pCO2 values higher during the early and Mid-Miocene (approximately 20-15 million years ago). This interval was characterized by global warmth, with little evidence of substantial (i.e., similar to modern) ice storage in Antarctica or Greenland.” There was only 1 time period that had higher atmospheric CO2 levels than current levels today, that being about 16-14 million years ago when sea levels were 25 to 40 meters higher than today. The authors state atmospheric CO2 levels “played an important role in driving” climate change millions of years ago and that when levels were similar to current levels, “global surface temperatures were, on average, 3-6 degrees C warmer than present.” The study’s lead author is quoted as saying “A slightly shocking finding is that the only time in the last 20 million years that we find evidence for carbon dioxide levels similar to the modern level of 387 parts per million was 15 to 20 million years ago, when the planet was dramatically different.”

Go here for a more detailed summary on the paper and background information. The paper was published in Science.

Lancet and University College London Institute for Global Health Commission Managing the health effects of climate change Lancet. 373: 1693. 2009

Fri, 5 Jun 2009 20:20:36 -0400

Executive Summary: Effects of climate change on health will affect most populations in the next decades and put the lives and well being of billions of people at risk. During this century, earth’s average surface temperature rises are likely to exceed the safe threshold of 2oC above preindustrial average temperature. Rises will be greater at higher latitudes, with medium-risk scenarios predicting 2-3 degree C rises by 2090 and 4-5 degree C rises in northern Canada, Greenland, and Siberia. In this report we outlined the major threats - both direct and indirect - to global health from climate change through changing patterns of disease, water and food insecurity, vulnerable shelter and human settlements, extreme climatic events, and population growth and migration. Although vector-borne diseases will expand their reach and death tolls, especially among elderly people, will increase because of heatwaves, the indirect effects of climate change on water, food security, and extreme climatic events are likely to have the biggest effect on global health.

Working Group comments: So begins this 40 page review of climate change science and impact on human health anticipated to be associated with a warming planet published in Lancet, one of the world’s most prestigious peer-reviewed medical journals. The report is comprehensive and an important contribution to the literature. We’ve heard that climate change will have a deleterious impact on the health of the planet; but what does that mean for you and me? Or more appropriately, what does it mean for our children and their children? They will be the recipients of the consequences of our dependency on fossil fuels and slow response to the likely catastrophic changes predicted to occur in the coming decades.

The report is likely too conservative. As we are all now well aware, greenhouse gas emissions and climate change are occurring at a rate exceeding that predicted by the IPCC 2007 report. The Lancet report “deliberately supports a conservative approach to” the consequences of climate change for two reasons; “first, even the most conservative estimates are profoundly disturbing and demand action” and “second, less conservative climate changes scenarios are so catastrophic that adaptation might be unachievable”.

The authors discuss that a great deal of suffering will be bestowed on the poor and developing countries; they have the least resources to mitigate and respond to climate change. “The damage done to the environment by modern society is perhaps one of the most inequitable health risks of our time. The carbon footprint of the poorest 1 billion people is 3% of the world’s footprint; yet, these communities are affected the most by climate change”. Moreover, because of rising sea levels and the fact that 30% of the world’s population lives along the coast, “more than a billion people could be displaced in environmental mass migration.”

“The rich will find their world to be more expensive, inconvenient, uncomfortable, disrupted and colourless; in general, more unpleasant and unpredictable, perhaps greatly so. The poor will die.”

So what will be some of the specific impacts on health that our children can look forward to? To list a few:

• Increase spread and transmission rates of vector-borne and rodent-borne diseases such as malaria. Increase in rates of dengue fever, arbovirus, schistosomiasis, fascioliasis, alveolar echinococcosis, leishmaniasis, Lyme borreliosis, tick-borne encephalitis, and hantavirus infections.

• Under nutrition and food insecurity. Climate change will markedly effect food production; for example, it has been shown that corn and soybean yields in the USA decreased by 17% for every degree rise in growing season temperature. Think of how much of our food chain is dependent on those 2 crops. One study suggests “that half of the world’s population could face severe food shortages by the end of the century because rising temperatures take their toll on farmer’s crop.”

• Higher temperatures will directly effect health, as exemplified by the 2003 heatwaves in Europe which caused up to 70,000 deaths. People with respiratory and cardiovascular diseases will be particularly at risk.

• Lack of clean water and good sanitation will contribute to human suffering. For example, more than a sixth of the world’s population is dependent on glacial water flow. “Increasing rates of glacial melting are predicted to lead to great reductions of water availability”.

• Increased likelihood for international tension and conflict over resources and energy.

The authors propose “a framework for responding to the health effects” of global warming utilizing “adaptation strategies, which in turn embeds mitigation strategies to improve human health worldwide”. The proposed response to the crisis includes “empower poor countries, and local government and local communities everywhere, to understand climate implications and to take action”, global cooperation of developed countries to develop an agenda for addressing the health effects of climate change, “climate change should be integrated into the entire discourse of our present and should be taken into consideration of all governance actions”, creation of accountability mechanisms, education and awareness campaigns by national institutes and university leaders worldwide, a move to a low-carbon economy and a call for a “major conference within the next 2 years” by health experts to “define the priorities for management, implementation, and monitoring the response to climate change.

All-in-all, a pretty bleak picture for the health of future generations.

As a physician, I have access to the Lancet and this article via my work. It is a shame that this important paper is not freely available on the Lancet website.

Tipping elements in the Earth’s climate system. Lenton, TH et al. Proceedings of the National Academy of Science. 105: 1786. 2008

Sat, 30 May 2009 14:01:02 -0400

Abstract: The term ‘‘tipping point’’ commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term ‘‘tipping element’’ to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.

This is an open access article and the full report is on the PNAS website here.

Working Group comments: “Human activities may have the potential to push components of the Earth system past critical states into qualitatively different modes of operation, implying large-scale impacts on human and ecological systems” so begins this article which describes the potential for large scale changes to the dynamics on Earth cause by greenhouse gas emissions and human-induced climate change. The discussion focuses on events which could be triggered this century and would undergo a qualitative change within this millennium, e.g. melting of the Greenland ice sheet. The potential events discussed in the paper are shown in this diagram and are ranked according to their probability of occurring. The authors state “the greatest (and clearest) threat is to the Arctic with summer sea-ice loss likely to occur long before (and potentially contribute) to Greenland ice sheet melt.” Why is there major concern about the melting of Arctic ice? Because “as sea-ice melts, it exposes much darker ocean surface, which absorbs more radiation - amplifying the warming” of Earth. In other words, a positive feedback loop is established that will increase the Earth’s temperature.

The authors conclude with the comment that “societies may be lulled into a false sense of security by smooth projections of global change. Our synthesis of present knowledge suggests that a variety of tipping elements could reach their critical point within this century under anthropogenic climate change.”

Take the time to read this paper and better understand the potential for large scale change that have the real possibility of occurring this century due to the burning of fossil fuel.

Global warming: Stop worrying, start panicking? Hans Joachim Schellnhuber. Proceedings of the National Academy of Science. 105:14239. 2008.

Sun, 26 Apr 2009 11:38:36 -0400

Working Group comments: The full text of this commentary is available at the PNAS website and is copied below (in italics). Hopefully it will be comprehensible. If the commentary is too long or too overwhelming, then just read the first 3 and last paragraphs. Some background on the terminology: anthropogenic = due to human activity (i.e., burning of fossil fuels); dangerous anthropogenic interference (DAI) = not precisely defined by scientists but the goal is to stay below temperatures that will bar ecosystems from naturally adapting to climate change, endanger sufficient food production and prohibit sustainable economic development; aerosols - small particles or droplets in the atmosphere originating from natural and man-made sources that reflect sunlight back into space and generally have a cooling effect on the planet (global dimming); burning embers diagram - see article by JB Smith posted on this site on April 22nd.
This commentary demonstrates that scientists do have disagreements over models and conclusions, but should not be taken as a rebuff that human-induced global warming is real, dangerous and requires immediate and comprehensive action to prevent DAI. In fact, the author concludes that we only have a “fair chance” of preventing DAI if the appropriate climate policies are immediately initiated and “this requires an industrial revolution for sustainability starting now”.

In their excellent Perspectives article in this issue (1), Ramanathan and Feng (R&F) sound a harsh wake-up call for those concerned about anthropogenic climate change: the authors maintain that the greenhouse gas (GHG) emissions of the past have already loaded the Earth System sufficiently to bring about disastrous global warming. In other words, the ultimate goal of climate protection policy, as stipulated by the United Nations Framework Convention on Climate Change (UNFCCC) (2), appears to be a delusion. So should we stop worrying and rather start panicking now?
The scientific evidence about climate change comes in thousands of parcels, yet the monumental reports of the Intergovernmental Panel on Climate Change (IPCC) (3) are the guideposts for both experts and stakeholders. The IPCC format, perfected by the late Bert Bolin, is a painstaking self-interrogation process of the pertinent scientific community. In this process, virtually every stone in the cognitive landscape is turned and the findings, however mundane or ugly, are synthesized into encyclopedic accounts. Unfortunately, such an approach is inherently tuned for burying crucial insights under heaps of facts, figures, and error bars. Therefore, R&F must be commended for dredging up one of the most inconvenient truths hidden in the IPCC tangle, namely the aerosol masking of global warming (AMGW).
Also, by construction, the IPCC vessel tends to steer clear of value judgments that might be easily converted into “policy-prescriptive” statements. The downside of this well-meaning attitude is that the 2007 report does not, for instance, make a systematic attempt to characterize what dangerous anthropogenic interference (DAI) with the natural climate system is all about. Again, all of the relevant information is implicitly contained in the IPCC tomes, most notably in chapter 19 of the Working Group II report (3) (see also ref. 4). Yet even that chapter shies away from updating the “burning embers diagram” (5), which provides a direct scientific way to gauge the political target of limiting global mean temperature (GMT) rise to less than 2°C (6) against avoided climate impacts.
R&F (1) also dare to pull out this big issue from the IPCC ocean. They enrich their take on the two topics, i.e., DAI and AMGW, by recently published evidence and add the insights up in three disturbing conjectures.
(i) Our planet is already committed to anthropogenic warming in the range of 1.4–4.3°C, where 2.4°C is the most likely amount. The main reason why only roughly a quarter (actually 0.76°C since the latter half of the 1800s) of that equilibrium temperature response to the current atmospheric GHG concentrations has been observed is the (predominantly) cooling effect of various aerosols that often accompany GHG emissions. Large scientific uncertainties remain regarding the forcing potential of the various aerosol species. There is certainty, however, that GHG concentrations (in particular, CO2 levels) will rise further in the medium-term future and that clean-air policies will remove that accidental antidote against global warming in the decades to come. Thus, the likelihood of global warming even beyond the 2.4°C margin in the 21st century is frustratingly high.
(ii) The resulting expectations for the planetary temperature clearly qualify for DAI, whether one refers to the emerging political consensus on a long-term climate stabilization goal as implicitly debated at the Conference of the Parties to the UNFCCC in Bali (COP13) or to the growing scientific evidence about critical thresholds for tipping vital Earth System components (e.g., the Greenland ice sheet) into different modes of operation.
(iii) No conceivable international CO2-reduction strategy (including the one hoped to transpire from the COP15 negotiations in Copenhagen next year) could possibly avoid that the planet will enter the DAI zone, where largely unmanageable climate impacts (like sea-level rise in the multimeter range) lurk. All we can do is to limit the warming in excess of the 2.4°C.
The first point made by R&F (1) is substantial and hard to dismiss. The tale about our dubious friend (namely “ordinary” air pollution as manifesting itself in atmospheric brown clouds laden with sulfates and nitrates) who luckily counteracts global warming by “global dimming” has been around for a while. Yet the authors succinctly summarize the state of the art and provide a convincing estimate of the still-hidden component of anthropogenic planetary temperature rise.
One needs to appreciate, however, that R&F's assessment mainly results from clever composition of a limited number of syntheses done by other researchers [such as the paper by Roe and Baker (7)] and by masterly back-of-the-envelope reasoning. The paper performs a basically static thought experiment by pulling back the aerosol veil while keeping all other factors fixed. In the real world, some aerosol emissions will be harder to reduce than others: whereas sulfate aerosols might face a quick decline, ammonium or nitrogen oxides might not. The AMGW might vanish quickly, as R&F suggest, but could also stick around for a while. Also, there will be intricate dynamic adjustment and feedback processes (involving, for example, the terrestrial and the marine carbon cycles) that can be captured only by simulation runs of fully fledged climate-system models. I will return to this crucial argument below.
Venturing into the 2+x°C-warming realm is risky, as the authors rightly emphasize in their second point, because large-scale nonlinear responses of the planetary machinery are likely to be triggered then (8, 9). These effects might even conspire to bring about—in the worst of all possible climate change science fictions—something like a runaway greenhouse effect. The present level of knowledge about the geobiosphere does not allow for rigorously ruling out such a cataclysmic accident, because we still do not possess genuine Earth models that could adequately simulate the hypercomplex dynamics involved. However, the research community keeps churning out relevant specific findings—for instance, about positive feedback processes such as methane release from Siberian thawing (10), teleconnections between tipping elements in the planetary system (11), or the volatility of the climate machinery as confirmed by empirical evidence about abrupt environmental transitions in the past (12). This topical information strengthens rather than weakens the R&F conjecture that we are heading toward DAI.
So the decisive question remains whether the authors' third point is correct: Are we really doomed to sail straightly into those stormy seas? My answer is a qualified “no.”
The R&F projections make two pivotal assumptions, namely (i) that the atmospheric GHG concentrations are at least constant or increasing throughout the next hundred years, and (ii) that clean-air policy operates much faster than GHG-emissions-reduction policy. Both assumptions can be challenged, as I will demonstrate by a quick model-based comparison of GMT rise in the R&F thought experiment, on the one hand, and a realistic (yet ambitious) climate protection scenario rooted in IPCC ground, on the other hand. The latter scenario anticipates that COP15 will adopt the long-term climate-stabilization goal endorsed by the recent Group of Eight (G8) Summit in Japan, namely to halve global emissions of the Kyoto-GHGs (CO2, methane, nitrous oxide, fluorinated gases) by 2050. 2000 is chosen as the reference year here, and a convergence to zero CO2 emissions by 2100 is postulated. By way of contrast, the R&F assessment explores the effects of an artificial freezing of the Kyoto-GHG concentrations at their 2005 levels, tacitly assuming that climate policy could not possibly do any better.
The comparison is achieved by appropriate parameter choices in a well established climate-carbon cycle model (13) that emulates the ensemble dynamics of the most advanced full-complexity models, and are summarized in Fig. 1. In Fig. 1A, GMT results without the “invisible R&F hand” that instantaneously suppresses the aerosol effects (more precisely eliminating all forcings except those of long-lived GHGs and tropospheric ozone) are depicted. Note that the median trajectory in the G8-mitigation fan (as generated by state-of-the-art carbon-cycle uncertainties) avoids the 2°C line and even bends down again after peaking toward the end of this century. This favorable outcome is mainly due to the oceans' capacity to keep on taking up big quantities of certain GHGs for quite a while, so the concentrations of the latter will fall again—instead of just remaining constant—under the specified mitigation strategy. Also, in contrast to the R&F scenario, land-use changes (affecting surface albedo) and volcanic activities can be assumed to keep exerting some cooling effects. For the sake of validation and consistency, a control model run is done with concentrations of all global-warming-relevant atmospheric substances (including the ordinary pollutants) fixed at 2005 levels, which yields the slowly rising solitary curve below the G8 bundle.
In Fig. 1B, the corresponding results for switching off aerosol effects in 2005 are presented. The control curve generated by fixing the Kyoto-GHG concentrations nicely illustrates the R&F calculations by asymptotically approaching their equilibrium global warming estimate (≈2.4°C). The really interesting result, however, is the elevated transient temperature fan produced by G8 mitigation plus abrupt sky cleaning: the median trajectory overshoots the political 2°C guardrail by ≈0.4°C in 2070, and closely approaches it again some 70 years later. This can still be perceived as a dangerous climate excursion because it may be sufficient to trigger the collapse of the Greenland ice sheet, but the overall environmental dynamics differs fundamentally from the disturbing R&F scenario. And this is not the end of the story, because the assumption of instantaneous aerosol unmasking is certainly an unlikely one: relevant measures will be implemented rather inhomogeneously in geographical space and political time. On the other hand, the G8-mitigation scenario used here (16) already contains substantial removal of cooling air pollutants in phase with the substitution of fossil fuels and technological innovation. Therefore, the more realistic temperature developments under ambitious climate-policy conditions sit between the two fans depicted in Fig. 1 A and B, respectively.
My conclusion is that we are still left with a fair chance to hold the 2°C line, yet the race between climate dynamics and climate policy will be a close one. The odds for avoiding DAI may be improved by aerosol management as suggested by R&F (taking the warming components such as black carbon out first), and even techniques for extracting atmospheric CO2 (like bio-sequestration) might eventually prove necessary. However, the quintessential challenges remain, namely bending down the global Kyoto-GHG output curve in the 2015–2020 window (further procrastination would render necessary reduction gradients too steep thereafter) and phasing out carbon dioxide emissions completely by 2100. This requires an industrial revolution for sustainability starting now.

Figure 1. Comparison of GMT development as resulting from fixing concentrations at 2005 levels [“concentrations stabilization scenario” (CSS)] and halving global Kyoto-GHG emissions by 2050 relative to 2000 levels [“mitigation scenario” (MS)], respectively. (A) Freezing of current air pollution and GHG levels in CSS, and concomitant gradual decrease of air pollution in MS. (B) The “invisible hand” of R&F (1) eliminates all forcings except those of long-lived GHGs and tropospheric ozone in 2005, i.e., aerosol cooling vanishes, in both CSS and MS. Climate sensitivity is chosen as 3°C throughout; other climate parameters (such as those affecting ocean inertia) are calibrated toward HadCM3; carbon cycle parameters are varied for representing the range of ten C4MIP models (14) by using MAGICC 6.0 (13). Historical observations of GMT are taken from HadCRUT3v (15).

References
1. Ramanathan V, Feng Y (2008) On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead. Proc Natl Acad Sci USA 105:14245–14250. Abstract/FREE Full Text
2. United Nations Framework Convention on Climate Change (1992) Available at http://unfccc.int/resource/docs/convkp/conveng.pdf. Accessed August 20, 2008.
3. IPCC (2007) Climate Change 2007: Contribution of Working Group I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ Press, Cambridge, UK, and New York) eds (I) Solomon S, et al., (II) Parry ML, et al., (III) Metz B, et al.
4. Parry M, Palutikof J, Hanson C, Lowe J (2008) Squaring up to reality. Nat Rep Clim Change 2:68–70.
5. Smith JB, Schellnhuber HJ, Qader Mirza MM (2001) in Climate Change 2001: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change, eds McCarthy JJ, Canziani OF, Leary NA, Dokken DJ, White KS (Cambridge Univ Press, Cambridge, UK, and New York).
6. Commission of the European Communities (2007) Communication from the Commission. Limiting Global Climate Change to 2 degrees Celsius—The Way Ahead for 2020 and Beyond, Available at http://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=com:2007:0002:FIN:EN:PDF. Accessed August 20, 2008.
7. Roe GH, Baker MB (2007) Why is climate sensitivity so unpredictable? Science 318:629–632. Abstract/FREE Full Text
8. Lenton TM, et al. (2008) Tipping elements in the Earth's climate system. Proc Natl Acad Sci USA 105:1786–1793. Abstract/FREE Full Text
9. Lenton TM, Schellnhuber HJ (2007) Tipping the scales. Nat Rep Clim Change 1:97–98.
10. Khvorostyanov DV, Ciais P, Krinner G, Zimov SA (2008) Vulnerability of east Siberia's frozen carbon stores to future warming. Geophys Res Lett 35:L10703, . CrossRef
11. Chang P, et al. (2008) Oceanic link between abrupt changes in the North Atlantic Ocean and the African monsoon. Nat Geosci 1:444–448. CrossRef
12. Steffensen JP, et al. (2008) High-resolution Greenland ice core data show abrupt climate change happens in few years. Science 321:680–684. Abstract/FREE Full Text
13. Meinshausen M, Raper SCB, Wigley TML (2008) Emulating IPCC AR4 atmosphere-ocean and carbon cycle models for projecting global-mean, hemispheric and land/ocean temperatures: MAGICC 6.0. Atm Chem Phys Disc 8:6153–6272.
14. Friedlingstein P, et al. (2006) Climate-carbon cycle feedback analysis: Results from the (CMIP)-M-4 model intercomparison. J Clim 19:3337–3353. CrossRef
15. Brohan P, Kennedy JJ, Harris I, Tett SFB, Jones PD (2006) Uncertainty estimates in regional and global observed temperature changes: A new data set from 1850. J Geophys Res Atmos 111:D12106, . CrossRef
16. Meinshausen M, et al. (2006) Multi-gas emission pathways to meet climate targets. Clim Change 75:151–194. CrossRef

Is the climate warming or cooling? Easterling, DR and Wehner, MF. Geophysical Research Letters. 6, L08706. 2009

Sun, 26 Apr 2009 11:04:39 -0400

Abstract: Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer‐term warming.
Working Group comments: The full paper is unavailable to the Working Group. The abstract indicates there will be years, or even decades, where global temperature may decrease. Variability is expected in a complex global climate system perturbed by natural and human-induced forces. However, the long-term outlook is hot and getting hotter.

Temperature increase of 21st century mitigation scenarios. Van Vuuren, DP, et al. Proceedings of the National Academy of Science. 105:15258. 2008.

Sat, 25 Apr 2009 20:24:51 -0400

Abstract: Estimates of 21st Century global-mean surface temperature increase have generally been based on scenarios that do not include climate policies. Newly developed multigas mitigation scenarios, based on a wide range of modeling approaches and socioeconomic assumptions, now allow the assessment of possible impacts of climate policies on projected warming ranges. This article assesses the atmospheric CO2 concentrations, radiative forcing, and temperature increase for these new scenarios using two reduced-complexity climate models. These scenarios result in temperature increase of 0.5–4.4°C over 1990 levels or 0.3–3.4°C less than the no-policy cases. The range results from differences in the assumed stringency of climate policy and uncertainty in our understanding of the climate system. Notably, an average minimum warming of ≈1.4°C (with a full range of 0.5–2.8°C) remains for even the most stringent stabilization scenarios analyzed here. This value is substantially above previously estimated committed warming based on climate system inertia alone. The results show that, although ambitious mitigation efforts can significantly reduce global warming, adaptation measures will be needed in addition to mitigation to reduce the impact of the residual warming.
This is an open access article and the full report is on the PNAS website here

Working Group comments. This paper utilizes climate models to determine the potential effect of “mitigation scenarios” on future greenhouse gas emissions and temperature change at the end of this century. In other words, the authors model the effects of moving to a less carbon intensive, lower greenhouse gas emitting civilization. They also modeled the cost of such endeavors. The models utilized selected a “cost-effective set of emission reduction measures. In general, most reductions are obtained by reducing energy-related CO2 emission (70-90% of reductions across the scenarios), followed by non-CO2 gases (15-30%) and CO2 from land use...Energy-related CO2 emissions are generally reduced by increases in energy efficiency and application of low/zero carbon energy technologies. In terms of timing, models aim to avoid drastic emission reductions that require (costly) premature reduction in capital; in other words, emission reductions are bounded by the inertia of capital replacement in the energy system”. In plain English: they assumed most of the reduction in greenhouse gas emissions would come from cutting CO2 emissions from the production of energy, i.e, the burning of coal, oil and natural gas and they took into account that this would probably not happen overnight. The figure above shows the temperature change in the year 2100 (relative to 1990) projected for each of the scenarios modeled. In the best case scenario, in which the authors modeled an “ambitious climate policy,” the temperature in 2100 will warm by 0.5 to 2.8 degrees Celsius (an average of 1.4 degrees Celsius). In a more middle of the road model, and potentially one that’s more plausible, the authors state that temperatures in 2100 will be 0.8 to 4.4 degrees Celsius warmer than in 1990. “The net present value of abatement costs from 2000 to 2100 range from 2 to 19 trillion 2000-US$ across the models” for the more middle of the road scenarios modeled. The authors conclude “our results show that even the lowest scenarios available in the literature, based on optimistic assumptions with respect to international cooperation in climate policy, lead to considerable increases in global mean temperature. These results show that adaptation measures will be needed in addition to mitigation to reduce the impact of residual warming.”

In other words, this paper shows that even under the most ambitious greenhouse gas reduction policy, the earth is going to warm up substantially and we’d better start planning now for the consequences.

Irreversible climate change due to carbon dioxide emissions. Solomon, S. et al. Proceedings of the National Academy of Science. 106: 1704. 2009

Fri, 24 Apr 2009 20:12:45 -0400

Abstract: The severity of damaging human-induced climate change depends not only on the magnitude of the change but also on the potential for irreversibility. This paper shows that the climate change that takes place due to increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop. Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1,000 years. Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from current levels near 385 parts per million by volume (ppmv) to a peak of 450–600 ppmv over the coming century are irreversible dry-season rainfall reductions in several regions comparable to those of the “dust bowl” era and inexorable sea level rise. Thermal expansion of the warming ocean provides a conservative lower limit to irreversible global average sea level rise of at least 0.4–1.0 m if 21st century CO2 concentrations exceed 600 ppmv and 0.6–1.9 m for peak CO2 concentrations exceeding ≈1,000 ppmv. Additional contributions from glaciers and ice sheet contributions to future sea level rise are uncertain but may equal or exceed several meters over the next millennium or longer.
This is an open access article and the full report is on the PNAS website here.

Working Group comments: figure 1 of the paper, shown below, demonstrates how long it will take for atmospheric CO2 levels to fall after reaching peak levels following the immediate cessation of further emissions. The model shows the kinetics of atmospheric CO2 dissipation after peak levels of 450, 500, 650, 750, 850, or 1200 parts per million (ppm) are obtained (top graph) following a 2% increase per year (“which is comparable to observations over the past decade”). For the corresponding peak CO2 level, the effect on global surface temperature (middle graph) and sea level rise (lower graph) are shown. A couple of points about the assumptions and data: 1. The model is highly idealized. Unfortunately, CO2 emissions will not abruptly cease - therefore this models presents an optimistic version of reality, 2. the middle graph underestimates the change in temperature because the model takes into account global averaged surface warming and “warming over land is expected to be larger than these global averaged values”, and 3) the lower graph underestimates the degree of sea level rise because the model used only accounts for sea level rise from thermal expansion (i.e., due to the warming of water) and does not account for the contribution of melting glaciers, ice caps or ice sheets to sea level rise.
So, let’s hear what the scientists state in this paper and what will be the consequences of further increases in atmospheric CO2 concentrations. The following in italics are direct quotes from the paper:
It is not generally appreciated that the atmospheric temperature increases caused by rising carbon dioxide concentrations are not expected to decrease significantly even if carbon emissions were to completely cease. Future carbon dioxide emissions in the 21st century will hence lead to adverse climate changes on both short and long time scales that would be essentially irreversible (where irreversible is defined here as a time scale exceeding the end of the millennium in year 3000).
Global average temperatures increase while CO2 is increasing and then remain approximately constant (within approximately +/- 0.5 degrees C) until the end of the millennium despite zero further emissions in all the of the test cases shown in figure 1. This important point is due to a near balance between the long-term decrease of radiative forcing due to CO2 concentration decay and reduced cooling through heat loss to the oceans.
Warming is expected to be linked to changes in rainfall, which can adversely affect the supply of water for humans, agriculture, and ecosystems. Precipitation is highly variable but long-term rainfall decreases have been observed in some large regions including. e.g., the Mediterranean, southern Africa, and parts of southwestern North America (clarification: this last sentence is referring to current observations).
The spatial changes in precipitation as shown in figure 3 (not shown here) imply greater challenges in the distribution of food and water supplies than those with which the world has had difficulty coping in the past. Such changes occurring not just for decades but over centuries are expected to have a range of impacts that differ by region. These include, e.g., human water supplies, effects on dry-season wheat and maize agriculture in certain regions of rain-fed farming such as in Africa, increased fire frequency, ecosystem change, and desertification.
Anthropogenic carbon dioxide will cause irrevocable sea level rise....thermal expansion alone can be expected to be associated with substantial irreversible commitments to future changes in the geography of the Earth because many coastal and island features would ultimately become submerged. (Note: recall that the model did not take into account sea level rise due to melting of glaciers or the ice sheets of Antarctica and/or Greenland - such melting adds many meters to sea level rise; also recall that 1 meter equals a little over 3 feet.)
Finally, the paper ends with a discussion of policy implications:
It is sometimes imagined that slow processes such as climate change pose small risks, on the basis of the assumption that a choice can always be made to quickly reduce emissions and thereby reverse any harm within a few years or decades. We have shown that this assumption is incorrect for carbon dioxide emissions, because of the longevity of the atmospheric CO2 perturbation and ocean warming. Irreversible climate changes due to carbon dioxide emissions have already taken place, and future carbon dioxide emissions would imply further irreversible effects on the planet, with attendant long legacies for choices made by contemporary society. Discount rates used in some estimates of economic trade-offs assume that more efficient climate mitigation can occur in a future richer world, but neglect the irreversibility shown here. Similarly, understanding of irreversibility reveals limitations in trading of greenhouse gases on the basis of 100-year estimated climate changes (global warming potentials, GWPs), because this metric neglects carbon dioxide’s unique long-term effects. In this paper, we have quantified how societal decisions regarding carbon dioxide concentrations that have already occurred or could occur in the coming century imply irreversible dangers relating to climate change for some illustrative populations and regions. These and other dangers pose substantial challenges to humanity and nature, with a magnitude that is directly linked to the peak level of carbon dioxide reached.
Sobering? Yes. Depressing? Yes. A call for immediate action? Yes!!!

Assessing dangerous climate change through an update of the Intergovernmental Panel on Climate Change (IPCC) ‘‘reasons for concern’’. J.B. Smith, et. al. Proceedings of the National Academy of Science. 106:4133. 2008.

Wed, 22 Apr 2009 08:43:48 -0400

Abstract: Article 2 of the United Nations Framework Convention on Climate Change [United Nations (1992) http://unfccc.int/resource/docs/convkp/conveng.pdf. Accessed February 9, 2009] commits signatory nations to stabilizing greenhouse gas concentrations in the atmosphere at a level that “would prevent dangerous anthropogenic interference (DAI) with the climate system.” In an effort to provide some insight into impacts of climate change that might be considered DAI, authors of the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) identified 5 “reasons for concern” (RFCs). Relationships between various impacts reflected in each RFC and increases in global mean temperature (GMT) were portrayed in what has come to be called the “burning embers diagram.” In presenting the “embers” in the TAR, IPCC authors did not assess whether any single RFC was more important than any other; nor did they conclude what level of impacts or what atmospheric concentrations of greenhouse gases would constitute DAI, a value judgment that would be policy prescriptive. Here, we describe revisions of the sensitivities of the RFCs to increases in GMT and a more thorough understanding of the concept of vulnerability that has evolved over the past 8 years. This is based on our expert judgment about new findings in the growing literature since the publication of the TAR in 2001, including literature that was assessed in the IPCC Fourth Assessment Report (AR4), as well as additional research published since AR4. Compared with results reported in the TAR, smaller increases in GMT are now estimated to lead to significant or substantial consequences in the framework of the 5 “reasons for concern.”

This is an open access article and the full report is on the PNAS website here.

Working Group comments: This paper updates for policy makers the impact of increasing global temperature on various environmental and economic “reasons for concern” (RFC - see below for the detailed definition of each RFC). These areas of concern and the probability for increasing detrimental effects are shown in this figure (the so called “burning embers” figure) in which the color change from white to yellow to red indicates worsening consequences for each RFC plotted against future temperature change (up to 5 degrees Celsius). The authors make the point of not specifically identifying what constitutes “dangerous” changes, leaving that up to the policy makers. However, they clearly indicate that as the color changes from yellow to red we are going to be going from bad to worse. Here’s what each RFC means and some of the authors conclusions: Risk to Unique and Threatened Systems - “addresses the potential for increased damage to or irreversible loss of unique and threatened systems, such as coral reefs, tropical glaciers, endangered species....small island states and indigenous communities”. Example: “new and stronger evidence” since the 2001 IPCC report indicate the likelihood that 20-30% of known plant and animal species risk extinction if global temperatures exceed 1.5 to 2.5 degrees Celsius. Risk of Extreme Weather Events - “tracks increases in extreme events with substantial consequences for societies and natural systems. Examples include increases in the frequency, intensity, or consequences of heat waves, floods, droughts, wildfires, or tropical cyclones”. Example: with just a 1 degree Celsius rise in global temperature there will increases in droughts, heat waves and floods that will adversely impact humans, wildlife and ecosystems. Distribution of impacts - “some regions, countries, and populations face greater harm from climate change....and some may benefit.” Example: specific populations, particularly the elderly and poor, and some regions more than others, will be more adversely impacted by increasing global temperature. Aggregate Damages - “covers comprehensive measures of impacts....such as monetary damages, lives affected, or lives lost.” Example: “it is likely that there will be higher damages for larger magnitudes of increased global mean temperature, and the net costs of impacts of warming are projected to increase over time....climate change over the next century is likely to adversely affect hundreds of millions of people through increased coastal flooding after a further 2 degree Celsius warming from 1990 levels, reduction in water supplies (0.4 to 1.7 billion people) affected with less than a 1 degree Celsius warming from 1990 levels and increased health impacts (that are already being observed).” Risk of Large-Scale Discontinuities - “represents the likelihood that...tipping points would occur, any of which would be accompanied by very large impacts.” Examples cited include deglaciation (partial or complete) of the West Antarctica or Greenland ice sheets. The authors state that “for a global average temperature increase of 1-4 degrees Celsius” there is a “medium confidence” that “at least partial deglaciation of the Greenland ice sheet, and possibly the West Antarctica ice sheet, would occur over a period ranging from centuries to millennia” resulting in 4-6 meters of sea level rise. Complete deglaciation of the Greenland ice sheet would raise sea levels by 7 meters and would be irreversible. The authors conclude that the IPCC 2007 report underestimated sea level rise that will be observed in coming centuries.

If you comprehend the summary above, then the authors statement that “since 2000, the trajectory of global emissions is above the highest SRES scenario” is sobering and scary. In other words, greenhouse gas emissions, and therefore global warming, is exceeding the worst case scenario modeled in the 2007 IPCC report (the A1F1 model which predicts mean global temperature increase of 2.4 to 6.4 degrees Celsius by 2100) and we may very likely shoot beyond the red zone depicted in the burning embers figure in the next 90 years.

Probabilistic Forecast for 21st Century Climate Based on Uncertainties in Emissions (without Policy) and Climate Parameters. A.P. Sokolov, et al. Massachusetts Institute of Technology. January 2009.

Sat, 18 Apr 2009 21:01:35 -0400

Abstract: The MIT Integrated Global System Model is used to make probabilistic projections of climate change from 1861 to 2100. Since the model’s first projections were published in 2003 substantial improvements have been made to the model and improved estimates of the probability distributions of uncertain input parameters have become available. The new projections are considerably warmer than the 2003 projections, e.g., the median surface warming in 2091 to 2100 is 5.1oC compared to 2.4oC in the earlier study. Many changes contribute to the stronger warming; among the more important ones are taking into account the cooling in the second half of the 20th century due to volcanic eruptions for input parameter estimation and a more sophisticated method for projecting GDP growth which eliminated many low emission scenarios. However, if recently published data, suggesting stronger 20th century ocean warming, are used to determine the input climate parameters, the median projected warning at the end of the 21st century is only 4.1oC. Nevertheless all our simulations have a very small probability of warming less than 2.4oC, the lower bound of the IPCC AR4 projected likely range for the A1FI scenario, which has forcing very similar to our median projection. The probability distribution for the surface warming produced by our analysis is more symmetric than the distribution the IPCC due to a different feedback between the climate and the carbon cycle, resulting from a different treatment of the carbon-nitrogen interaction in the terrestrial ecosystem.

The full report is published online at the MIT Joint Program on the Science and Policy of Global Change website here.

Working Group comments: This report models the predicted temperature change at the end of the century if the status quo is maintained. AR4 refers to the IPCC 2007 report and A1FI refers to the worst case IPCC modeled scenarios for future climate change. Their conclusions are shown in this graph. The red outer lines represent the 5% and 95% percentiles and the middle red line represents the median. The blue lines similarly represent modeling from a 2003 report by the same group at MIT. The new data shows that temperature change by the end of the century will be between about 3.5 and 7.4 degrees warmer; with a mean of about 5 degrees. That’s degrees in Celsius folks. In Fahrenheit that translates to a temperature change of between 6.3 and 13.3 degrees hotter than it is now.