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Climate sensitivity uncertainties leading to more concern

Posted on 6 November 2018 by greenman3610

This is a re-post from Yale Climate Connections

This month’s Yale Climate Connections “This is not cool” video provides valuable context for public understanding of ECS – equilibrium climate sensitivity.

The video opens with Patrick Brown of the Carnegie Institute for Science, Stanford University, explaining that ECS is the measure of the amount of global warming expected from a doubling of the carbon dioxide levels from pre-industrial times. Penn State’s Michael Mann says that doubling could come as early as mid-century, and he says a 3-degree C increase in warming is generally considered to be in the mid-range of estimates.

But Mann points to lots of uncertainty over that 3-degree figure and says some project the increase may be “as little as” two degrees C and others lean toward an increase of 4.5 to 5 degrees C.

Pointing to research he conducted with scientist Ken Caldeira of Stanford, Brown says that the models best simulating the recent past “tend to produce more warming” than those low- or mid-range estimates. He says their research “cuts off the probability of these low estimates of warming” and instead indicates higher-end estimates “appear to be more likely.”

“Uncertainty is not our friend” in this case, Mann says, as the climate system may be “even more sensitive than we thought.”

Scientist Andrew Dessler of Texas A&M University agrees with Mann’s point that “the evidence may indeed be pointing to the problem being worse than we had anticipated, not better.”

Dessler said his “best guess” currently, based on the evidence he’s seen, calls for an increase of 3 to 4 degrees C from a doubling of CO2 concentrations over pre-industrial levels.

Author and activist Bill McKibben says a critical unknown involves not solely the sensitivity of the climate per se, but of civilizations generally – political, economic, and psychological systems. He points to the recent tragedies involving Puerto Rico as a result of Hurricane Maria as illustrative.

“The idea that climate sensitivity from observations is a lot lower than the models, that the models are ‘running hot'” and showing more warming and not less … “that idea is headed for the junkyard,” Dessler concludes.

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Comments 1 to 5:

  1. The evidence points towards medium to high climate sensitivity, and didn't the recent research paper finding increased ocean heat content also point to high climate sensitivity?

    Yet the sceptical lobby are still claiming otherwise, and claiming no warming since 1998 (despite recent record temperatures since 2015 staring them in the face). It's beyond human comprehension.

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  2. I can't get my head around this one.  Is climate sensitivity determined by comparing the observed rise in temperature with the observed rise in Carbon dioxide.  Should we be using the temperature rise above what it would have been if only the Milankovitch cycle has been in play???  Puzzled.

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    Moderator Response:

    [JH] The final paragraph of the OP contains the phrase "climate sensitivity". If you click on it you will see the oficial IPCC/WMO definition of climate sensitivity.  

  3. William @2, this is my understanding, although Im not a climate scientist.My understanding is climate scientists do look at paleo climate data to estimate climate sensitivity,comparing C02 and temperature and subtracting the effects of the milankovitch cycle, but I would guess the trouble is you also have past volcanic activity, sometimes at scale, and the energy output of the sun has changed on long term time scales. There are also limits in the fossil data they use.

    Some of this stuff is not known with great reliability, so this might explain the quite wide range of climate sensitivities, even those from studies based largely on paleo climate data.

    Although such data still suggests 3 degrees is the most likely sensivity, possibly more but not less. Personally I would put my money on the paleo climate data rather than other ways of estimating sensitivity.

    Climate sensitivity can also be estimated with modelling and more recent observations. The following articles describes different ways used to calculate sensitivity: Explainer: How scientists estimate ‘climate sensitivity’

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  4. And the problem with estimating climate sensitivity using the modern temperature observations since the 1980's - 2108, and subtracting the milankovitch cycle is the modern warming period is its just too short to be 100% certain, some feedbacks haven't fully developed, aerosols distort the record and effects are not 100% certain. But again it still points towards 3 degrees.

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  5. Some very eminent climate scientists have examined ECS and are agreed that it is around 3°C, possibly higher. They note that CO2 concentrations in the atmosphere are around 406 ppm and continue to rise at an accelerating rate. On this data they conclude that pre-industrial CO2 concentration (~280 ppm) is likely to be double and average global temperature rise 3°C above the pre-industrial average, possibly by 2050.

    This is wrong because it does not take into account the significant rise in other, more potent greenhouse gasses. As I have pointed out, anthropogenic methane emissions now stand at 1,860 ppb compared to pre-industrial concentration of ~700 ppb - and methane is x35 more powerful than CO2. I have also drawn attention to continued emission of man-made halogen gasses which are up to x5,000 more powerful than CO2.

    Although electrification of transport and stationary machines has made a slow start, net production and use of fossil-fuelled vehicles continues to rise. The result is production of increased emission of greenhouse gases such as Nitrous Oxides. Here again I have drawn attention to the need to rapidly reverse this trend – an outcome which could eventuate over the next decade.

    Coal combustion for electricity generation is a major source of CO2 emissions. Its use is not in decline and proposals for significant expansion of its use, if implemented, could see rapid increase in its concentration in the atmosphere. Without an international agreement akin to the Montreal Protocol, with severe penalties for its breach and strict policing, reduction in it’s use to zero by 2050 – required to avoid warming of 2°C – may not be achieved.

    The above shows that rather than using likely rise in concentration of CO2 as an indication of future average global temperature rise, we should be calculating and using actual and expected increase in Carbon Dioxide equivalent (CO2eq) as a more accurate indicator of future temperature increase. Why do leading climate scientists ignore this and insist on using CO2 concentration only – ignoring the effect of all other greenhouse gas emissions when estimating future temperature rise?

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