James Hansen, Pushker Kharecha, Makiko Sato, Valerie Masson-Delmotte, Frank Ackerman, David J. Beerling, Paul J. Hearty, Ove Hoegh-Guldberg, Shi-Ling Hsu, Camille Parmesan, Johan Rockstrom, Eelco J. Rohling, Jeffrey Sachs, Pete Smith, Konrad Steffen, Lise Van Susteren, Karina von Schuckmann, James C. Zachos are variously climate scientists and economists.
Thus Dr James Hansen is an eminent US climate scientists, the former head of NASA’s Goddard Institute for Space Studies and an adjunct professor at 101–Nobel-Laureate Columbia University (see: http://en.wikipedia.org/wiki/James_Hansen ).
Jeffrey Sachs is an eminent US economist, professor of economics at 101–Nobel-Laureate Columbia University and Director of The Earth Institute at Columbia University (see: http://en.wikipedia.org/wiki/Jeffrey_Sachs ).
James Hansen, Pushker Kharecha, Makiko Sato, Valerie Masson-Delmotte, Frank Ackerman, David J. Beerling, Paul J. Hearty, Ove Hoegh-Guldberg, Shi-Ling Hsu, Camille Parmesan, Johan Rockstrom, Eelco J. Rohling, Jeffrey Sachs, Pete Smith, Konrad Steffen, Lise Van Susteren, Karina von Schuckmann, James C. Zachos on the terminal carbon budget (2013): “We assess climate impacts of global warming using ongoing observations and paleoclimate data. We use Earth’s measured energy imbalance, paleoclimate data, and simple representations of the global carbon cycle and temperature to define emission reductions needed to stabilize climate and avoid potentially disastrous impacts on today’s young people, future generations, and nature. A cumulative industrial-era limit of ~500 GtC fossil fuel emissions [370 Gt C by 2013 i.e. 130 GtC or 477 Gt CO2 left] and 100 GtC storage in the biosphere and soil would keep climate close to the Holocene range to which humanity and other species are adapted. Cumulative emissions of ~1000 GtC, sometimes associated with 2°C global warming, would spur “slow” feedbacks and eventual warming of 3–4°C with disastrous consequences. Rapid emissions reduction is required to restore Earth’s energy balance and avoid ocean heat uptake that would practically guarantee irreversible effects. Continuation of high fossil fuel emissions, given current knowledge of the consequences, would be an act of extraordinary witting intergenerational injustice. Responsible policymaking requires a rising price on carbon emissions that would preclude emissions from most remaining coal and unconventional fossil fuels and phase down emissions from conventional fossil fuels… McKibben [255] uses results of M2009 [Meinshausen et al] to infer allowable fossil fuel emissions up to 2050 if there is to be an 80% chance that maximum warming in the 21st century will not exceed 2°C above the pre-industrial level. M2009 conclude that staying under this 2°C limit with 80% probability requires that 2000–2049 emissions must be limited to 656 GtCO2 (179 GtC) for 2007–2049. McKibben [255] used this M2009 result to determine a remaining carbon budget (at a time not specified exactly) of 565 GtCO2 (154 GtC) if warming is to stay under 2°C. Let us update this analysis to the present: fossil fuel emissions in 2007–2012 were 51 GtC [5], so, assuming no net emissions from land use in these few years, the M2009 study implies that the remaining budget at the beginning of 2013 was 128 GtC. Thus, coincidentally, the McKibben [255] approach via M2009 yields almost exactly the same remaining carbon budget (128 GtC) as our analysis (130 GtC). However, our budget is that required to limit warming to about 1°C (there is a temporary maximum during this century at about 1.1–1.2°C, Fig. 9), while McKibben [255] is allowing global warming to reach 2°C, which we have concluded would be a disaster scenario! This apparently vast difference arises from three major factors…” [1].
[1]. James Hansen, Pushker Kharecha, Makiko Sato, Valerie Masson-Delmotte, Frank Ackerman, David J. Beerling, Paul J. Hearty, Ove Hoegh-Guldberg, Shi-Ling Hsu, Camille Parmesan, Johan Rockstrom, Eelco J. Rohling, Jeffrey Sachs, Pete Smith, Konrad Steffen, Lise Van Susteren, Karina von Schuckmann, James C. Zachos, “Assessing “dangerous climate change”: required reduction of carbon emissions to protect young people, future generations and Nature”, PLOS One, 8 (12), 3 December 2013: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0081648 .
[Editorial note: In a major re-assessment, World Bank analysts Robert Goodland and Jeff Anfang have estimated that annual greenhouse gas pollution totals 63.8 Gt CO2-e as compared to the FAO estimate of 41.8 Gt CO2-e (see Robert Goodland and Jeff Anfang. “Livestock and climate change. What if the key actors in climate change are … cows, pigs and chickens?”, World Watch, November/December 2009: http://www.worldwatch.org/files/pdf/Livestock%20and%20Climate%20Change.pdf ) i.e. the World has 477 Gt CO2/ (63.8 Gt CO2 per year) = 7.5 years left to zero emissions ( or 477 Gt CO2/ (41.8 Gt CO2 per year) = 11.4 years left to zero emissions on the basis of the FAO estimate. Using the Goodland and Anfang estimate and a GWP of methane relative to CO2 of 105, Dr Gideon Polya has estimated 5.3 years left relative to December 2012 based on a 600 Gt CO2 terminal budget (see Gideon Polya, " Doha climate change inaction. Only 5 years left to act", MWC News, 9 December 2012: http://mwcnews.net/focus/analysis/23373-gideonpolya-climate-change.html ) with this now being 4.3 years left relative to December 2013].