Global Temperatures, Volcanic Eruptions, and Trees that Didn’t Bark

As noted earlier, our main conclusions are insensitive to the precise details of the forcing estimates used, the volcanic scaling assumptions made, and the precise assumed climate sensitivity. They were also insensitive to the details of the biological tree growth model over a reasonable range of model assumptions. The conclusion that tree-ring temperature reconstructions might suffer from age model errors due to missing rings is bound to be controversial. A few points are worth making here. First of all, our conclusion is quite specific to temperature-sensitive trees at treeline, and it does not imply more general problems in the larger discipline of dendrochronology. Secondly, the conclusion at this stage simply a hypothesis, a hypothesis that can account for these key enigmatic features in the actual tree-ring hemisphere temperature reconstruction: the attenuation, and the increasing (back in time) delay and temporal smearing of the cooling response to past volcanic forcing. Were an equally successful and more parsimonious hypothesis to be provided for these observations, I would be the first to concede and defer to this alternative explanation.

One argument against the specific conclusion of missing growth rings is that trees are carefully cross-dated when forming regional chronologies, and this precludes the possibility of chronological errors. That, however, assumes that there are at least some trees within a particular region that will not suffer a missing ring during the years where our model predicts it. Yet our prediction is that all trees within a region of synoptic or lesser scale where growing season temperatures lie below the growth threshold will experience a missing ring. Thus, cross-dating within that region, regardless of how careful, cannot resolve the lost chronological information. It is my hope that dendroclimatologists will reassess raw chronologies more carefully and critically assess the extent to which the predicted features might indeed be present in the underlying tree-ring data. Again, this paper presents a hypothesis for explaining some enigmatic features of existing tree-ring temperature reconstructions. It is hardly the last word on the matter.

Finally it is worth discussing the potential wider implication of these findings. Climate scientists use the past response of the climate to natural factors like volcanoes to better understand how sensitive Earth’s climate might be to the human impact of increasing greenhouse gas concentrations, e.g. to estimate the equilibrium sensitivity of the climate to CO2 doubling i.e. the warming expected for an increase in radiative forcing equivalent to doubling of CO2 concentrations. Hegerl et al (2006) for example used comparisons during the pre-industrial of EBM simulations and proxy temperature reconstructions based entirely or partially on tree-ring data to estimate the equilibrium 2xCO2 climate sensitivity, arguing for a substantially lower 5%-95% range of 1.5–6.2C than found in several previous studies. The primary radiative forcing during the pre-industrial period, however, is that provided by volcanic forcing. Our findings therefore suggest that such studies, because of the underestimate of the response to volcanic forcing in the underlying data, may well have underestimated the true climate sensitivity.

It will be interesting to see if accounting for the potential biases identified in this study leads to an upward revision in the estimated sensitivity range. Our study, in this regard, once again only puts forward a hypothesis. It will be up to other researchers, in further work, to assess the validity and potential implications of this hypothesis.

Page 3 of 3 | Previous page

56 comments on this post.
  1. Ron Broberg:

    From the paper:

    We employed variants of the VaganovShashkin tree-growth model:
    G(t)=g_e(t)min[g_T(t);g_W(t)]
    g_E(t), g_T(t) and g_W(t) represent solar, thermal and moisture-dependent factors determining growth respectively.

    Two naive questions:

    1) How is the solar dependent factor analyzed? Wouldn’t g_E be dependent on cloud parameters?

    2) Would a carbon-dioxide factor g_C(t) (derived perhaps from stomata studies) be a useful inclusion? Is it a factor in the ‘divergence’?

  2. Aaron Lewis:

    What is your estimate as to how much the “accounting” may have to be revised? Are we talking a few minor revisions of interest to experts in the field or significant revisions that affect public policy, risk management, planning, and engineering?

  3. James Staples:

    I’m wondering several things (And, Yes, I will read your Nature GeoSci Item, and no doubt find the answers – though you can still take this, from a RealClimate Fan Constructive Editorially Criticising This Item POV, as just that):
    Where, exactly, where did the AD 1258 Eruptions Occur, and what other evidence is there for there apparent severity?
    Could it be that the Eruptions in question occured during, say, a certain season – during, say, a Monsoon of great fierceness – that diverted a disproportionate amount of material onto the sampling sites?
    Is there any evidence thatthese AD1258 Eruptions may have been, say, low in Sulfate Emissions – and, thus, low in generation of upper stratospheric aerosols?
    Love.

  4. Jim Gill:

    Only tangentially related to this article but….
    Do you know about, or can you refer me to someone who may know about, the climate effects of the other Tambora-scale volcanic eruption (VEI=7) of the last millenium — from Changbaishan (Baitoushan) on the China-NKorea border (42oN latitude) sometime between 960-1025 AD? The magma had an unusual chemical composition (=comendite) and its eruption is calculated to have released 100 km3 of particulates into the stratosphere, ~1800 megatons of H2O, 45 megatons each of Cl and F, but only 2 megatons of S. Seems odd for something of this magnitude to have little climate consequence. I have studied aspects of this eruption for some years.

  5. Jim Eager:

    “42oN latitude”

    That’s a key factor. Surely you can understand why.

  6. Jaros Prlwytzkofsky:

    comments?
    [edit]

    [Response:Sure. Don’t waste our time.–Jim]