Sir,
Reply to the comments of J. Oerlemans on “Mass balance of glaciers other than the ice sheets” by Cogley and Adams
Our statements (Reference Cogley and Adams.Cogley and Adams, 1998; CA), challenged by Oerlemans, were not “firm” but tentative. We were careful to qualify our main conclusion at several places. Given the extreme under-sampling of mass balance, it would not be surprising if our conclusion were shown to be in error, but we do not think that Oerlemans has done so.
Oerlemans notes that many of the larger measured glaciers are in Norway. If, following CA, we define “larger” as “larger than 16 km2”, then the nine larger glaciers from Norway, occupying the 16–32 and 32–64 km2 size classes, contribute 68 of the 440 annual balance measurements made on larger glaciers. If they and other Norwegian glaciers are excluded, we find that the Norwegian glaciers do indeed shift our results in the direction argued by Oerlemans. However, these glaciers are not near to “the peak of the frequency distribution of observed sizes (fig. 7b of CA), and CA’s size-corrected estimate of the small-glacier contribution to sea-level rise, 0.058 mm a−1, is revised only to 0.066 mm a−1 when they are excluded.
More seriously, we see no reason why Norwegian glaciers should be given special attention. The fact that many of the larger Norwegian glaciers have positive mass balance is not really relevant. What would be relevant would be a demonstration that Norwegian glaciers are so globally atypical as to make the available sample unrepresentative of the world’s small glaciers. We do not think that this can be done. However, we agree entirely with Oerlemans on the need for regional differentiation. Indeed we took some trouble to evaluate spatial bias, and our paper contains an estimate of its magnitude: about –60 mm a−1, or +0.10 mm a−1 of equivalent sea-level rise.
Our results are not in conflict with those of (Reference Dyurgerov and Meier.Dyurgerov and Meier 1997; DM). The cumulative data of DM shown in Oerlemans’ figure 1 are for practical purposes identical with those shown as annual averages in figure 5a of CA. It follows that the latter are not in conflict with the modelling results of (Reference Zuo and Oerlemans.Zuo and Oerlemans 1997; ZO). In fact, Oerlemans’ figure 1 reveals that the best agreement between ZO and DM (and hence CA) is for ΔT ≃ 0 K. A reasonable interpretation of this agreement is that (i) ZO’s model suggests that small glaciers were in equilibrium during 1865–95, while (ii) the measurements of DM and CA suggest that, when biases are allowed for, small glaciers were close to equilibrium during 1961–90. To assimilate this latter claim, the reader should mentally differentiate the curves in Oerlemans’ figure 1, and should note that ZO’s model assumes a (calibrated) dependence of balance on temperature, while the DM and CA data are complementary in that they demonstrate such a dependence.
CA’s conclusions, restated succinctly, are as follows. Firstly, a naive calculation yields a moderately negative estimate of global average mass balance. Secondly, this estimate must be revised upwards because at least three biases distort the result: (a) neglect of internal accumulation, (b) the spatially uneven distribution of the measured glaciers, and (c) the size bias identified in CA’s figure 7. Oerlemans does not address bias a, the significance of which is emphasized by results reported recently by Reference Bazhev, Rototaeva, Heintzenberg, Stenberg and Pinglot.Bazhev and others (1998) and Reference Rabus and Echelmeyer.Rabus and Echelmeyer (1998); he may not have understood that we had already addressed bias b; and we show above that his comments on bias c do not affect our conclusion. Thirdly, it is not practical to correct all of these biases at once, because they are not additive and are probably correlated. The extent of overlap needs to be determined carefully, which will require a substantial effort. Fourthly, CA’s analysis, when taken as a whole, entails the conclusion that small glaciers were probably close to equilibrium during 1961–90.
Our empirical demonstration of the size bias warrants a practical response in the medium-term disposition of measurement effort, but its physical causes also deserve study. In this regard we accept Oerlemans’ argument that sensitivity to precipitation should be examined as well as sensitivity to temperature. His earlier work (e.g. Reference Oerlemans and Fortuin.Oerlemans and Fortuin, 1992) certainly shows that precipitation plays a role, as might be expected. However the published micrometeorological works cited by Oerlemans do not discuss precipitation and so do not bear on the question, which remains open. There is no global precipitation dataset of a quality comparable to that available for temperature, and large-scale analysis is therefore not straightforward.
Oerlemans claims that “The temperature sensitivity of mass balance as a function of glacier size cannot be determined by comparing a hemispheric mean temperature signal with unevenly distributed mass-balance measurements”. Why not? Standard temperature climatologies are based on measurements which, though far more numerous, are no less unevenly distributed than the mass-balance measurements. Improving the glaciological estimates deserves high priority, but dismissing what is currently available would not be a good first step. The mass-balance measurements, as assembled by CA and DM, constitute the best observational estimates glaciology has to offer for comparison with large-scale measures of climatic change.
