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Responses to volcanoes in tree rings and models

Filed under: — gavin @ 29 November 2012

Houston, we have a problem.

Admittedly, not a huge problem and not one that most people, or even most climatologists, are particularly fascinated by, but one which threads together many topics (climate models, tree rings, paleo-climate) which have been highlighted here in the past. The problem is that we have good evidence in the ice core records for very large tropical eruptions over the last 1000 years – in particular the eruptions in 1258/1259, 1452 and 1809 to 1815 – but for which many paleo-reconstructions barely show a blip in temperature. Models, in attempting to simulate this period, show varied but generally larger (and sometimes much larger) responses. The differences are significant enough to have prompted a few people to try and look into why this mismatch is occurring.

Whenever there is a mismatch between model and observation, there are, roughly speaking, at least three (non-exclusive) possibilities: the model is wrong, the observational data are wrong or the comparison is not like-with-like. There have been many examples of resolved mismatches in each category so all possibilities need to be looked at.

As described in a previous post earlier this year, Mann et al., 2012 (pdf), postulated that for extreme volcanoes, the cooling would be sufficient to saturate the growth response, and that some trees might `skip´ a ring for that year leading to a slight slippage in tree-ring dating, a potential smearing of the composite chronologies, and a further underestimate of the cooling in tree-ring based large-scale reconstructions.

This hypothesis has now been challenged by a group of authors in a comment (Anchukaitis et al.) (pdf, SI, code), who focus on the appropriateness of the tree ring growth model and the spatial pattern the volcanic climate responses. The Mann et al. response (pdf, SI) presents some further modeling and 19th century observational data in support of the original hypothesis.

Of course, there are still two other possibilities to consider. First, the models may have an excessive response. This could be due to either models responding excessively to the correct forcing, or could be related to an excessive forcing itself. There are indeed some important uncertainties in estimating the history of volcanic forcing – which involves inferring a stratospheric aerosol load (and effective radius of the particles and their distribution) from a network of sulphate peaks in ice cores in Greenland and Antarctica. For example, the forcing for the big eruption around 1453 differs by a factor of 2 in the inferred forcing (-12 W/m2 and -5.4 W/m2) in the two estimates proposed for the recent model-intercomparison (Schmidt et al., 2012). Note too that the details of how aerosols are implemented in any specific model can also make a difference to the forcing, and there are many (as yet untested) assumptions built into the forcing reconstructions.

It is also conceivable that climate models overreact to volcanic forcing – however, excellent matches to the Pinatubo response in temperature, radiative anomalies, water vapour and dynamic responses, where we know the volcanic aerosol load well, make that tricky to support (Hansen et al, 2007) (pdf, SI). (As an aside, the suggestion in this paper that the response to Krakatoa (1883) was underpredicted by the historical SST fields was partially vindicated by the results from HadSST3 which showed substantially more cooling).

The third possibility is that some tree-ring reconstructions can’t be easily compared to simple temperature averages from the models. As both the original paper and the comment suggests, there are important effects from memory from previous years in ring widths and, potentially, increases in diffuse light post-eruption promoting growth spurts. This needs to be assessed using more sophisticated forward models for tree ring growth applied to the models’ output – a feature in both the Mann et al, and Anchukaitis et al. approaches. More work is likely needed on this, and using the wider variety of model experiments coming out of CMIP5/PMIP3.

There is clearly potential for these competing hypotheses to get sorted out. Information from newly-digitised old instrumental records in the early 19th Century such as shipping records for the East India Company (Brohan et al, 2012), doesn’t support the largest modelled responses to Tambora (1815), but does suggest a response larger and more defined than that seen in some reconstructions. However, other 19th Century temperature compilations such Berkeley Earth show larger responses to Tambora – though there are spatial sampling issues there as well. There is also the potential for non-tree ring based reconstructions to provide independent confirmation of the magnitude of the response.

So while neither of the latest comments and responses provide a definitive answer to the principal problem, there is certainly lots of scope for extended and (hopefully) productive discussions.


  1. M.E. Mann, J.D. Fuentes, and S. Rutherford, "Underestimation of volcanic cooling in tree-ring-based reconstructions of hemispheric temperatures", Nature Geoscience, vol. 5, pp. 202-205, 2012.
  2. K.J. Anchukaitis, P. Breitenmoser, K.R. Briffa, A. Buchwal, U. Büntgen, E.R. Cook, R.D. D'Arrigo, J. Esper, M.N. Evans, D. Frank, H. Grudd, B.E. Gunnarson, M.K. Hughes, A.V. Kirdyanov, C. Körner, P.J. Krusic, B. Luckman, T.M. Melvin, M.W. Salzer, A.V. Shashkin, C. Timmreck, E.A. Vaganov, and R.J.S. Wilson, "Tree rings and volcanic cooling", Nature Geoscience, vol. 5, pp. 836-837, 2012.
  3. M.E. Mann, J.D. Fuentes, and S. Rutherford, "Reply to 'Tree rings and volcanic cooling'", Nature Geoscience, vol. 5, pp. 837-838, 2012.
  4. G.A. Schmidt, J.H. Jungclaus, C.M. Ammann, E. Bard, P. Braconnot, T.J. Crowley, G. Delaygue, F. Joos, N.A. Krivova, R. Muscheler, B.L. Otto-Bliesner, J. Pongratz, D.T. Shindell, S.K. Solanki, F. Steinhilber, and L.E.A. Vieira, "Climate forcing reconstructions for use in PMIP simulations of the Last Millennium (v1.1)", Geoscientific Model Development, vol. 5, pp. 185-191, 2012.
  5. J. Hansen, M. Sato, R. Ruedy, P. Kharecha, A. Lacis, R. Miller, L. Nazarenko, K. Lo, G.A. Schmidt, G. Russell, I. Aleinov, S. Bauer, E. Baum, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Cohen, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, J. Jonas, M. Kelley, N.Y. Kiang, D. Koch, G. Labow, J. Lerner, S. Menon, T. Novakov, V. Oinas, J. Perlwitz, J. Perlwitz, D. Rind, A. Romanou, R. Schmunk, D. Shindell, P. Stone, S. Sun, D. Streets, N. Tausnev, D. Thresher, N. Unger, M. Yao, and S. Zhang, "Climate simulations for 1880–2003 with GISS modelE", Climate Dynamics, vol. 29, pp. 661-696, 2007.
  6. P. Brohan, R. Allan, E. Freeman, D. Wheeler, C. Wilkinson, and F. Williamson, "Constraining the temperature history of the past millennium using early instrumental observations", Climate of the Past, vol. 8, pp. 1551-1563, 2012.

Don’t estimate acceleration by fitting a quadratic…

Filed under: — stefan @ 20 November 2012

… if your data do not look like a quadratic!

This is a post about global sea-level rise, but I put that message up front so that you’ve got it even if you don’t read any further.

The reputable climate-statistics blogger Tamino, who is a professional statistician in real life and has published a couple of posts on this topic, puts it bluntly:

Fitting a quadratic to test for change in the rate of sea-level rise is a fool’s errand.

I’d like to explain why, with the help of a simple example. Imagine your rate of sea-level rise changes over 100 years in the following way:
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Stronger regional differences due to large-scale atmospheric flow.

A new paper by Deser et al. (2012) (free access) is likely to have repercussions on discussions of local climate change adaptation. I think it caught some people by surprise, even if the results perhaps should not be so surprising. The range of possible local and regional climate outcomes may turn out to be larger than expected for regions such as North America and Europe.

Deser et al. imply that information about the future regional climate is more blurred than previously anticipated because of large-scale atmospheric flow responsible for variations in regional climates. They found that regional temperatures and precipitation for the next 50 years may be less predictable due to the chaotic nature of the large-scale atmospheric flow. This has implications for climate change downscaling and climate change adaptation, and suggests a need to anticipate a wider range of situations in climate risk analyses.

Although it has long been recognised that large-scale circulation regimes affect seasonal, inter-annual climate, and decadal variations, the expectations have been that anthropogenic climate changes will dominate on time scales longer than 50 years. For instance, an influential analysis by Hawking & Sutton (2009) (link to figures) has suggested that internal climate variability account for only about 20% of the variance over the British isles on a 50-year time scale.
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  1. C. Deser, R. Knutti, S. Solomon, and A.S. Phillips, "Communication of the role of natural variability in future North American climate", Nature Climate Change, vol. 2, pp. 775-779, 2012.
  2. E. Hawkins, and R. Sutton, "The Potential to Narrow Uncertainty in Regional Climate Predictions", Bulletin of the American Meteorological Society, vol. 90, pp. 1095-1108, 2009.

ClimateDialogue: Exploring different views on climate change

Filed under: — group @ 15 November 2012

This is a guest posting from some Dutch colleagues on a new online experiment in fostering dialogue on climate change. Bart Verheggen has asked us to host this quick introduction. We are interested to hear if you think this is a good idea.

Guest Commentary by Bart Strengers (PBL) offers a platform for discussions between invited climate scientists on important climate topics that have been subject to scientific and public debate. The goal of the platform is to explore the full range of views currently held by scientists by inviting experts with different views on the topic of discussion. We encourage the invited scientists to formulate their own personal scientific views; they are not asked to act as representatives for any particular group in the climate debate.

Obviously, there are many excellent blogs that facilitate discussions between climate experts, but as the climate debate is highly polarized and politicized, blog discussions between experts with opposing views are rare.


The discovery, early 2010, of a number of errors in the Fourth IPCC Assessment Report on climate impacts (Working Group II), led to a review of the processes and procedures of the IPCC by the InterAcademy Council (IAC). The IAC-report triggered a debate in the Dutch Parliament about the reliability of climate science in general. Based on the IAC recommendation that ‘the full range of views’ should be covered in the IPCC reports, Parliament asked the Dutch government ‘to also involve climate skeptics in future studies on climate change’.

In response, the Ministry of Infrastructure and the Environment announced a number of projects that are aimed to increase this involvement. is one of these projects.

We are starting ClimateDialogue with a discussion on the causes of the decline of Arctic Sea Ice, and the question to what extent this decline can be explained by global warming. Also, the projected timing of the first year that the Arctic will be ice free will be discussed. With respect to the latter, in its Fourth Assessment Report in 2007, IPCC anticipated that (near) ice free conditions might occur by the end of this century. Since then, several studies have indicated this could be between 2030-2050, or even earlier.

We invited three experts to take part in the discussion: Judith Curry, chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology; Walt Meier, research scientist at the National Snow & Ice Data Center (NSIDC) in Boulder, Colorado; and Ron Lindsay, Senior Principal Physicist at the Polar Science Center of the University of Washington in Seattle.

Future topics that will be discussed include: climate sensitivity, sea level rise, urban heat island-effects, the value of comprehensive climate models, ocean heat storage, and the warming trend over the past few decades.

Our format

Each discussion will be kicked off by a short introduction written by the editorial staff, followed by a guest blog by two or more invited scientists. The scientists will start the discussion by responding to each other’s arguments. It is not the goal of ClimateDialogue to reach a consensus, but to stimulate the discussion and to make clear what the discussants agree or disagree on and why. 
To round off the discussion on a particular topic, the ClimateDialogue editor will write a summary, describing the areas of agreement and disagreement between the discussants. The participants will be asked to approve this final article, the discussion between the experts on that topic will then be closed and the editorial board will open a new discussion on a different topic.

The public (including other climate scientists) are also free to comment, but for practical reasons these comments will be shown separately.

The project organization consists of an editorial staff of three people and an advisory board of seven people, all of whom are based in the Netherlands. The editorial staff is concerned with the day-to-day operation of researching topics, finding participants for the discussion and moderating the discussions between the experts. The main task of the advisory board is to guard the neutrality of the platform and to advise the editorial staff about its activities

The project leader is Rob van Dorland of the Royal Netherlands Meteorological Institute (KNMI), a senior scientist and climate advisor in the Climate Services section and is often active at the interface between science and society. The second member is Bart Strengers. He is a climate policy analyst and modeler in the IMAGE-project at the PBL Netherlands Environmental Assessment Agency (PBL) and has been involved in the discussion with climate skeptics for many years. The third member is Marcel Crok, an investigative science writer, who published a critical book (in Dutch) about the climate debate.

We welcome comments here and are happy to answer any questions regarding this project. You can also send an email to info [at] climatedialogue [dot] org.

Weighing change in Antarctica

Filed under: — group @ 13 November 2012

Guest commentary by Matt King, Michael Bentley and Pippa Whitehouse

Determining whether polar ice sheets are shrinking or growing, and what their contribution is to changes in sea level, has motivated polar scientists for decades. Genuine progress began in the early 1990s when satellite observations started to provide (nearly) spatially comprehensive sets of observations. Three very different, and hence complementary, approaches are now employed, although each has a particular limitation:

  • Satellite altimetry: measurements of ice sheet volume changes from laser or radar altimeters (e.g. IceSat) can be converted to mass changes through correction of spatially- and temporally-varying surface density together with spatial extrapolation to unsampled regions. The main limitation lies in the models used to correct for surface density changes.
  • Input-minus-output: calculating the difference between the mass of snow accumulated and that of the ice (and meltwater) being discharged gives the mass imbalance. The snow accumulation is normally estimated from numerical models and the discharge is computed using the multiple of measured velocity at the edge of the ice sheet with its measured or inferred ice thickness and density. Thus, uncertainty in accumulation models and sub-glacial topography at the grounding line propagate into mass balance uncertainties.
  • Satellite gravimetry: changes in Earth’s gravity field can be measured from satellite (e.g. from Gravity Recovery and Climate Experiment, GRACE) and used to determine changes in ice mass but only after accounting for mass-change effects that are not due to ice mass redistribution – in particular the glacial isostatic adjustment (GIA).

Our recently published Nature paper (King et al, 2012), used GRACE gravity data to infer the ice mass trends as in previous work, but with an updated estimate of the GIA correction.
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  1. M.A. King, R.J. Bingham, P. Moore, P.L. Whitehouse, M.J. Bentley, and G.A. Milne, "Lower satellite-gravimetry estimates of Antarctic sea-level contribution", Nature, vol. 491, pp. 586-589, 2012.

Trying to shoot the messenger

Filed under: — gavin @ 7 November 2012

Does this sound familiar? A quantitative prediction is inconvenient for some heavily invested folks. Legitimate questions about methodology morph quickly into accusations that the researchers have put their thumb on the scale and that they are simply making their awkward predictions to feather their own nest. Others loudly proclaim that the methodology could never work and imply that anyone who knows anything knows that -it’s simply common sense! Audit sites spring up to re-process the raw data and produce predictions more to the liking of their audience. People who have actually championed the methods being used, and so really should know better, indulge in some obvious wish-casting (i.e. forecasting what you would like to be true, despite the absence of any evidence to support it).

Contrarian attacks on climate science, right?
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Short term trends: Another proxy fight

Filed under: — gavin @ 1 November 2012

One might assume that people would be happy that the latest version of the Hadley Centre and CRU combined temperature index is now being updated on a monthly basis. The improvements over the previous version in terms of coverage and error estimates is substantial. One might think that these advances – albeit incremental – would at least get mentioned in a story that used the new data set. Of course, one would not be taking into account the monumental capacity for some journalists and the outlets they work for to make up stories whenever it suits them. But all of the kerfuffle over the Mail story and the endless discussions over short and long term temperature trends hides what people are actually arguing about – what is likely to happen in the future, rather than what has happened in the past.

The fundamental point I will try and make here is that, given a noisy temperature record, many different statements can be true at the same time, but very few of them are informative about future trends. Thus vehemence of arguments about the past trends is in large part an unacknowledged proxy argument about the future.

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Unforced Variations: Nov 2012

Filed under: — group @ 1 November 2012

I can’t think what people might want to talk about this month…