Guest essay by Eric Worrall

A study published in AAAS “Science Advances” has attempted to resurrect discredited claims of a dangerously high climate sensitivity, by suggesting that climate sensitivity increases as the world warms.

Climate change may be escalating so fast it could be ‘game over’, scientists warn New research suggests the Earth’s climate could be more sensitive to greenhouse gases than thought, raising the spectre of an ‘apocalyptic side of bad’ temperature rise of more than 7C within a lifetime. It is a vision of a future so apocalyptic that it is hard to even imagine. But, if leading scientists writing in one of the most respected academic journals are right, planet Earth could be on course for global warming of more than seven degrees Celsius within a lifetime. And that, according to one of the world’s most renowned climatologists, could be “game over” – particularly given the imminent presence of climate change denier Donald Trump in the White House. … In a paper in the journal Science Advances, they said the actual range could be between 4.78C to 7.36C by 2100, based on one set of calculations. …

Read more: http://www.independent.co.uk/news/science/climate-change-game-over-global-warming-climate-sensitivity-seven-degrees-a7407881.html

The abstract of the study;

Nonlinear climate sensitivity and its implications for future greenhouse warming Global mean surface temperatures are rising in response to anthropogenic greenhouse gas emissions. The magnitude of this warming at equilibrium for a given radiative forcing—referred to as specific equilibrium climate sensitivity (S)—is still subject to uncertainties. We estimate global mean temperature variations and S using a 784,000-year-long field reconstruction of sea surface temperatures and a transient paleoclimate model simulation. Our results reveal that S is strongly dependent on the climate background state, with significantly larger values attained during warm phases. Using the Representative Concentration Pathway 8.5 for future greenhouse radiative forcing, we find that the range of paleo-based estimates of Earth’s future warming by 2100 CE overlaps with the upper range of climate simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Furthermore, we find that within the 21st century, global mean temperatures will very likely exceed maximum levels reconstructed for the last 784,000 years. On the basis of temperature data from eight glacial cycles, our results provide an independent validation of the magnitude of current CMIP5 warming projections.

Read more: http://advances.sciencemag.org/content/2/11/e1501923

The money quote from the study full text;

… This paleodata-based TCRP is now applied to the RCP8.5 forcing scenario until year 2100 CE, following the equation above for deriving an estimated global mean SAT response. The anthropogenic forcing results in a global mean SAT anomaly of 5.86 K by year 2100 with respect to PI values. The uncertainties in S and the ocean’s heat uptake efficiency as discussed above result in a likely range of 4.78 to 7.36 K for the global mean SAT anomaly. Comparing our paleo-based estimate of future warming to the multimodel ensemble mean projections of the CMIP5 (52), we found that our projection results in a slightly higher global mean SAT anomaly. The current Intergovernmental Panel on Climate Change (IPCC)/CMIP5 projection under the RCP8.5 scenario results in a global mean SAT increase of 4.84 K for the year 2100 (with respect to PI values). The corresponding multimodel ensemble values range from 3.42 to 6.40 K. …

Read more: Same as above

My question – if a warmer world is more sensitive to CO2, why was the Cretaceous, which ended 66 million years ago, only 4C warmer than today, despite a CO2 level more than 4x higher than today’s CO2 level? If we apply the conclusions of the paper, temperatures in the warm Cretaceous should have been 14c higher than today’s temperatures – an absurdity which should excuse this paper from further serious consideration.

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