A paper by Roe & O’Neal (2008) looks at the use of glacier modelling and numerical weather modelling to discriminate between glacier mass balance variations due to natural climate variability or due to global large-scale climate change. Using the Mount Baker glaciers in Washington State, USA, as a basis for the modelling; natural variability was shown to be able to produce 2 to 3 kilometre changes in glacial length over decadal and centennial timescales. As a result, the 1.3 to 2.5 km changes in length on Mount Baker during the Little Ice Age (LIA) can be attributed to this natural variability instead, with no need for global climate change. Instead, they could be the product of yearly weather variation.
The paper notes that all of the processes which contribute to mass accumulation and ablation are ultimately controlled by climate and that large-scale climate changes will often drive glacier variations. However, changing climate doesn’t have to always be the cause of these glacier variations. The authors look at how the statistical distribution of atmospheric variables in a constant climate do not change over time, and how this also means that variability is also an intrinsic part of a constant climate. Historical records and reconstructions of glacial response times show that glaciers are able to reflect this variability. And whilst this variability in glacier response time may vary from years to centuries, the paper asks the key question of whether these past movements are statistically different from the normal response of a glacier to the variability shown to exist in a constant climate.
Whilst the paper used historical climate data from sites situated over Mount Baker in the Cascade Range of western Washington, USA, to show glacial response through the use of a linear model; its intention was not to simulate any one glacier. Rather, the observations were used to show the much wider effects of climate variations in a stationary climate on glacier length.
Figure 1. Glaciers of Mt. Baker (USGS, 1999)
The authors recognise the similar approach taken to that of Reichart et al. (2002), who ultimately drew very similar conclusions about the preindustrial fluctuations of LIA glaciers being explained as within the internal variability of the climate system. The Reichart et al. (2002) paper used a global climate model and a dynamic glacier model for two European glaciers to show this. An important point from this study is that whilst LIA glacier advances did not exceed natural variability, the present retreat since then certainly does. Therefore, whilst climate change isn’t needed to explain LIA variability, it is needed for changes today and in the foreseeable future. Furthermore, whilst studies of this type are useful to change our knowledge of past glacier sensitivity, they do not show the kind of glacial retreat that we can see today as a result of anthropogenic activity.
It is quite common in the literature to find decadal climate variability given as the reason for glacier variability on these timescales. This includes the Pacific Decadal Oscillation (PDO) which is the main orbital forcing factor on sea-surface temperatures in the North Pacific and on climate patterns in the Pacific northwest. What this study and others (Huybers and Roe, 2009) showed, was that there was actually little or no significant interannual memory for the atmospheric variables that control glacier variability and so they therefore don't fully explain the observed variance. Indeed, the authors state that the appearance of decadal variability in time series of the PDO is often artificially exaggerated by the application of a multiple-year running mean through the data.
At one of the monitoring sites used in this study, a test for autoregression was applied to the long term record of climate, and showed that there was no statistically significant interannual relation for either precipitation or melt-season temperature. Put simply this shows that one year had no relation to the next. The paper also criticises the often-used technique of applying a five-year running average through climate data, which may give an artificial view of decadal climatic periods. They also accept that whilst there are chance intervals of high or low conditions over several years, the use of a running average highly exaggerates this, rather than accepting the existence of high year to year variability. To reiterate the point here, there was little to no observed interannual relationship and thus no evidence for interannually linked climate regimes.
As can be seen in the wider glaciological literature as well as for Mount Baker, much of the pre-anthropogenic glacial history has been explained as the result of decadal climate regimes. However, in the findings of this study, it is shown to be the intrinsic inertia or memory found for each glacier and not the climate system which drives previous long term variations. The simple model used in this study showed significant centennial variability, with an amplitude of 2-3 km for glacier length. As the authors state very clearly:
“Thus there is centennial, and even millennial variability in the spectrum, all fundamentally driven by the simple integrative physics of a process with a perhaps-surprisingly short timescale, and forced by simple stochastic year-to year variations in climate.”
Evidently, variability on such a time-scale can be used to reinterpret past glacial histories and this is attempted in the paper. The authors state that from the instrumental observations and the model parameters used to show natural variability:
“...the 1.3- to 2.5-km length fluctuations on Mount Baker attributed to the LIA can be accounted for by the model without recourse to changes in climate.”
What is stated here is that the kilometre-scale length variations of glacier terminus don’t need large changes to temperature and precipitation to trigger them. Instead the natural interannual variability of a constant climate will do this.
What was also noted, was the importance of measuring small-scale climate forcing patterns, to fully understand individual glacier response. The model produced results which suggest that random climatic fluctuations over the past 1000 years may have been enough to produce large changes in glacier length. Indeed, many moraines may be products of much earlier advances and not necessarily synchronous with each other, nor part of a global pattern. The paper notes that the current world-wide retreat, as opposed to the past regional and local variance observed in this study, is a strong indicator of global climate change.
In summary, the suggestion that long-term glacier variability can occur in a constant climate was explored. The authors found that it would be impossible to rule out the effect of variations observed in a stationary climate on past glacier length. The obvious effects on how we understand past glacier changes is looked at for the LIA although not developed further. Overall, the paper offers a very exciting look at how we can see glacier dynamics in many different ways. But whilst it may dispute past climate forcing on glaciers, it reinforces the point that global retreats in glacial length seen today are indisputably as a result of anthropogenic climate change.
Figure 2. The peak of Mt. Baker
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