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Climate Sensitivity Estimates and Corrections

You need to be careful in inferring climate sensitivity from observations.

Two climate sensitivity stories this week – both related to how careful you need to be before you can infer constraints from observational data. (You can brush up on the background and definitions here). Both cases – a “Brief Comment Arising” in Nature (that I led) and a new paper from Proistosescu and Huybers (2017) – examine basic assumptions underlying previously published estimates of climate sensitivity and find them wanting.

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  1. C. Proistosescu, and P.J. Huybers, "Slow climate mode reconciles historical and model-based estimates of climate sensitivity", Science Advances, vol. 3, pp. e1602821, 2017.

Something Harde to believe…

Filed under: — gavin @ 25 February 2017

A commenter brings news of an obviously wrong paper that has just appeared in Global and Planetary Change. The paper purports to be a radical revision of our understanding of the carbon cycle by Hermann Harde. The key conclusions are (and reality in green):

  • The average residence time of CO2 in the atmosphere is found to be 4 years.

    [The residence time for an individual molecule is not the same as the perturbation response time of the carbon cycle which has timescales of decades to thousands of years.]

  • The anthropogenic fraction of CO2 in the atmosphere is only 4.3%.

    [Actually, it’s 30%.]

  • Human emissions only contribute 15% to the CO2 increase over the Industrial Era.

    [It’s all of it.]

Since these points contradict multiple independent sources of evidence, I can, without hesitation, predict that there are fundament flaws in this paper that will raise serious questions about the quality of the peer-review that this paper went through. Oddly, this paper is labeled as an “Invited Research Article” and so maybe some questions might be asked of the editor responsible too.

Notwithstanding our last post on the difficulty in getting comments published, this paper is crying out for one.

But this kind of thing has been done before, does not require any great sophistication or computer modeling to rebut, and has come up so many times before (Salby (also here), Beck, Segalstad, Jaworowski etc.), that perhaps a crowd-sourced rebuttal would be useful.

So, we’ll set up an page for this (a site for collaborative LaTeX projects), and anyone who wants to contribute should put the gist of their point in the comments and we’ll send the link so you can add it to the draft. Maybe the citizen scientists among you can pull together a rebuttal faster than the professionals?


  1. H. Harde, "Scrutinizing the carbon cycle and CO2 residence time in the atmosphere", Global and Planetary Change, vol. 152, pp. 19-26, 2017.

Why correlations of CO2 and Temperature over ice age cycles don’t define climate sensitivity

Filed under: — gavin @ 24 September 2016

We’ve all seen how well temperature proxies and CO2 concentrations are correlated in the Antarctic ice cores – this has been known since the early 1990’s and has featured in many high-profile discussions of climate change.

EPICA Dome C ice core greenhouse gas and isotope records.

The temperature proxies are water isotope ratios that can be used to estimate Antarctic temperatures and, via a scaling, the global values. The CO2 and CH4 concentration changes can be converted to radiative forcing in W/m2 based on standard formulas. These two timeseries can be correlated and the regression (in ºC/(W/m2)) has the units of climate sensitivity – but what does it represent?

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The Volcano Gambit

Anyone reading pundits and politicians pontificating profusely about climate or environmental science will, at some point, have come across the “volcano gambit”. During the discussion they will make a claim that volcanoes (or even a single volcano) produce many times more pollutant emissions than human activities. Often the factor is extremely precise to help give an illusion of science-iness and, remarkably, almost any pollutant can be referenced. This “volcano gambit” is an infallible sign that indicates the author is clueless about climate science, but few are aware of its long and interesting history…

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How do trees change the climate?

Filed under: — group @ 27 October 2014 - (Deutsch)

Guest commentary from Abby Swann (U. Washington)

This past month, an op-ed by Nadine Unger appeared in the New York Times with the headline “To save the climate, don’t plant trees”.  The author’s main argument is that UN programs to address climate change by planting trees or preserving existing forests are “high risk” and a “bad bet”. [Ed. There is more background on the op-ed here]

However, I don’t think that these conclusions are supported by the science.  The author connects unrelated issues about trees, conflates what we know about trees from different latitudes, and fails to convey the main point: tropical trees keep climate cool locally, help keep rainfall rates high, and have innumerable non-climate benefits including maintaining habitat and supporting biodiversity.

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How much methane came out of that hole in Siberia?

Filed under: — david @ 13 August 2014

Siberia has explosion holes in it that smell like methane, and there are newly found bubbles of methane in the Arctic Ocean. As a result, journalists are contacting me assuming that the Arctic Methane Apocalypse has begun. However, as a climate scientist I remain much more concerned about the fossil fuel industry than I am about Arctic methane. Short answer: It would take about 20,000,000 such eruptions within a few years to generate the standard Arctic Methane Apocalypse that people have been talking about. Here’s where that statement comes from:
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Arctic and American Methane in Context

Filed under: — david @ 24 November 2013

Lots of interesting methane papers this week. In Nature Geoscience, Shakhova et al (2013) have published a substantial new study of the methane cycle on the Siberian continental margin of the Arctic Ocean. This paper will get a lot of attention, because it follows by a few months a paper from last summer, Whiteman et al (2013), which claimed a strong (and expensive) potential impact from Arctic methane on near-term climate evolution. That economic modeling study was based on an Arctic methane release scenario proposed in an earlier paper by Shakhova (2010). In PNAS, Miller et al (2013) find that the United States may be emitting 50-70% more methane than we thought. So where does this leave us?

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  1. N. Shakhova, I. Semiletov, I. Leifer, V. Sergienko, A. Salyuk, D. Kosmach, D. Chernykh, C. Stubbs, D. Nicolsky, V. Tumskoy, and . Gustafsson, "Ebullition and storm-induced methane release from the East Siberian Arctic Shelf", Nature Geoscience, vol. 7, pp. 64-70, 2013.
  2. G. Whiteman, C. Hope, and P. Wadhams, "Vast costs of Arctic change", Nature, vol. 499, pp. 401-403, 2013.
  3. N.E. Shakhova, V.A. Alekseev, and I.P. Semiletov, "Predicted methane emission on the East Siberian shelf", Doklady Earth Sciences, vol. 430, pp. 190-193, 2010.
  4. S.M. Miller, S.C. Wofsy, A.M. Michalak, E.A. Kort, A.E. Andrews, S.C. Biraud, E.J. Dlugokencky, J. Eluszkiewicz, M.L. Fischer, G. Janssens-Maenhout, B.R. Miller, J.B. Miller, S.A. Montzka, T. Nehrkorn, and C. Sweeney, "Anthropogenic emissions of methane in the United States", Proceedings of the National Academy of Sciences, vol. 110, pp. 20018-20022, 2013.

The new IPCC climate report

The time has come: the new IPCC report is here! After several years of work by over 800 scientists from around the world, and after days of extensive discussion at the IPCC plenary meeting in Stockholm, the Summary for Policymakers was formally adopted at 5 o’clock this morning. Congratulations to all the colleagues who were there and worked night shifts. The full text of the report will be available online beginning of next week. Realclimate summarizes the key findings and shows the most interesting graphs.

Update 29 Sept: Full (un-copyedited) report available here.

Global warming

It is now considered even more certain (> 95%) that human influence has been the dominant cause of the observed warming since the mid-20th century. Natural internal variability and natural external forcings (eg the sun) have contributed virtually nothing to the warming since 1950 – the share of these factors was narrowed down by IPCC to ± 0.1 degrees. The measured temperature evolution is shown in the following graph.

Figure 1 The measured global temperature curve from several data sets. Top: annual values. ​​Bottom: averaged values ​​over a decade.
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El Nino’s effect on CO2 causes confusion about CO2’s role for climate change

Filed under: — rasmus @ 11 September 2012

Are the rising atmospheric CO2-levels a result of oceans warming up? And does that mean that CO2 has little role in the global warming? Moreover, are the rising levels of CO2 at all related to human activity?

These are claims made in a fresh publication by Humlum et al. (2012). However, when seeing them in the context of their analysis, they seem to be on par with the misguided notion that the rain from clouds cannot come from the oceans because the clouds are intermittent and highly variable whereas the oceans are just there all the time. I think that the analysis presented in Humlum et al. (2012) is weak on four important accounts: the analysis, the physics, reviewing past literature, and logic.

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  1. O. Humlum, K. Stordahl, and J. Solheim, "The phase relation between atmospheric carbon dioxide and global temperature", Global and Planetary Change, vol. 100, pp. 51-69, 2013.

Unlocking the secrets to ending an Ice Age

Filed under: — group @ 28 April 2012

Guest Commentary by Chris Colose, SUNY Albany

It has long been known that characteristics of the Earth’s orbit (its eccentricity, the degree to which it is tilted, and its “wobble”) are slightly altered on timescales of tens to hundreds of thousands of years. Such variations, collectively known as Milankovitch cycles, conspire to pace the timing of glacial-to-interglacial variations.

Despite the immense explanatory power that this hypothesis has provided, some big questions still remain. For one, the relative roles of eccentricity, obliquity, and precession in controlling glacial onsets/terminations are still debated. While the local, seasonal climate forcing by the Milankovitch cycles is large (of the order 30 W/m2), the net forcing provided by Milankovitch is close to zero in the global mean, requiring other radiative terms (like albedo or greenhouse gas anomalies) to force global-mean temperature change.

The last deglaciation occurred as a long process between peak glacial conditions (from ~26-20,000 years ago) to the Holocene (~10,000 years ago). Explaining this evolution is not trivial. Variations in the orbit cause opposite changes in the intensity of solar radiation during the summer between the Northern and Southern hemisphere, yet ice age terminations seem synchronous between hemispheres. This could be explained by the role of the greenhouse gas CO2, which varies in abundance in the atmosphere in sync with the glacial cycles and thus acts as a “globaliser” of glacial cycles, as it is well-mixed throughout the atmosphere. However, if CO2 plays this role it is surprising that climatic proxies indicate that Antarctica seems to have warmed prior to the Northern Hemisphere, yet glacial cycles follow in phase with Northern insolation (“INcoming SOLar radiATION”) patterns, raising questions as to what communication mechanism links the hemispheres.

There have been multiple hypotheses to explain this apparent paradox. One is that the length of the austral summer co-varies with boreal summer intensity, such that local insolation forcings could result in synchronous deglaciations in each hemisphere (Huybers and Denton, 2008). A related idea is that austral spring insolation co-varies with summer duration, and could have forced sea ice retreat in the Southern Ocean and greenhouse gas feedbacks (e.g., Stott et al., 2007).

Based on transient climate model simulations of glacial-interglacial transitions (rather than “snapshots” of different modeled climate states), Ganopolski and Roche (2009) proposed that in addition to CO2, changes in ocean heat transport provide a critical link between northern and southern hemispheres, able to explain the apparent lag of CO2 behind Antarctic temperature. Recently, an elaborate data analysis published in Nature by Shakun et al., 2012 (pdf) has provided strong support for these model predictions. Shakun et al. attempt to interrogate the spatial and temporal patterns associated with the last deglaciation; in doing so, they analyze global-scale patterns (not just records from Antarctica). This is a formidable task, given the need to synchronize many marine, terrestrial, and ice core records.
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  1. P. Huybers, and G. Denton, "Antarctic temperature at orbital timescales controlled by local summer duration", Nature Geoscience, vol. 1, pp. 787-792, 2008.
  2. L. Stott, A. Timmermann, and R. Thunell, "Southern Hemisphere and Deep-Sea Warming Led Deglacial Atmospheric CO2 Rise and Tropical Warming", Science, vol. 318, pp. 435-438, 2007.
  3. A. Ganopolski, and D.M. Roche, "On the nature of lead–lag relationships during glacial–interglacial climate transitions", Quaternary Science Reviews, vol. 28, pp. 3361-3378, 2009.
  4. J.D. Shakun, P.U. Clark, F. He, S.A. Marcott, A.C. Mix, Z. Liu, B. Otto-Bliesner, A. Schmittner, and E. Bard, "Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation", Nature, vol. 484, pp. 49-54, 2012.