Posted by: JohnnyRook | January 4, 2009

Alpine Glaciers in Bolivia Shrink, Contributing to Sea Level Rise

Two new articles accepted for publication by the American Geophysical Union, but not yet published, shed more light on alpine glacier shrinkage and on glacier contribution to sea level rise. The first of the articles examines the retreat of glaciers in Bolivia’s Cordillera Real.

Figure 1. Map of glaciers in the Cordillera Real. The studied area is shown in the red square. Meteorological stations are shown on the map.

Glacier decline between 1963 and 2006 in the Cordillera Real, Bolivia

<a href=”“>As in other areas of the world, the Himalayas, Chile, Switzerland, etc. research shows a decline in the mass balance (accumulation-ablation) of the Bolivian glaciers.

As the research team lead by Alvaro Soruco of IRD-LGGE, BP 96, 38402 Saint Martin d’Hères, France. writes:

The volume changes of 21 glaciers in the Cordillera Real have been determined between 1963 and 2006 using photogrammetric measurements. These data form the longest series of mass balances obtained with such accuracy in the tropical Andes. Our analysis reveals that temporal mass balance fluctuations are similar, revealing a common response to climate over the entire studied region. The mass of these glaciers has clearly been decreasing since 1975 without any significant acceleration of this trend over recent years. We have found a clear relationship between the average mass balance of these glaciers as a function of exposure and altitude. From this relationship, the ice volume loss of 376 glaciers has been assessed in this region. The results show that these glaciers lost 43 % of their volume between 1963 and 2006, essentially over the 1975-2006 period and 48% of their surface area between 1975 and 2006.

… mass balance variability and precipitation variability differ by almost an order of magnitude. A change in precipitation of less than 70 mm cannot directly explain changes in mass balances larger than 500 mm. Considering that the precipitation lapse rate is small [Sicart et al., 2007], it would be tempting to conclude that the influence of precipitation on mass balance is low. However, as shown in previous studies [Wagnon et al., 2001; Sicart, 2002], the feedback on albedo induced by solid precipitation is more significant in explaining the large variability of annual glacier mass balance than the annual variability of accumulation itself. Indeed, although precipitation is weak, a thin snow cover of high albedo is sufficient to stop an intense melting.

In any case, note that the interannual mass balance variability, shown for Zongo Glacier greatly exceeds the decadal variability. For instance, the mass loss of Zongo over the two years 1997-1999 corresponds to 60% of the 1997-2006 loss (Figure 2a) and reveals that annual mass balances are strongly affected by ENSO events [Wagnon et al., 2001].

In addition, our measurements relative to the sample of 21 glaciers over the 1963-2006 period show that the area changes are poorly related to mass balance over each period (R²<0.15) and over the whole period (R²=0.08). Detailed measurements of area/length changes on the Zongo glacier [Vuille et al., 2008] lead to the same conclusion. Indeed, the strong retreat observed over the 1992-2006 period compared to the 1963-2006 average does not reflect the mass balance evolution. This confirms that area or length changes are poor indicators for climate change analysis at a decadal time-scale.

[See the discussion in the second article below for an possible explanation of what does drive area or length changes]

Centred mass balance analysis over four periods between 1963 and 2006 shows that temporal mass balance fluctuations are similar, revealing a common response to climate over the entire studied region, except for the smallest glaciers which can be strongly affected by local conditions.

The cumulative mass balances of these glaciers do not show any acceleration of the trend. The strong retreat of the Zongo snout observed over the 1992-2006 period does not reflect the mass balance changes of this glacier. Although our dataset does not allow us to clearly link the mass balance variations with precipitation and temperature variations, mass balance variations observed at a decadal scale are roughly in agreement with temperature and precipitation changes.

The differences of the cumulative mass balance trends observed over the 1963-2006 period can be explained to a large extent by the exposure and altitude of each glacier. The highest glaciers and glaciers with exposures between east and south experienced less negative mass balances. Using the strong relationship found between mass balance, exposure and altitude, the ice volume loss of 376 glaciers assessed in this region over the period 1963-2006, corresponds to 43% of their volume. In addition, our surface area measurements of these 376 glaciers indicate an overall shrinkage in glacier area of 48% between 1975 and 2006. In the future, these results will be used to assess the impact of glaciers shrinkage on water runoff, especially for La Paz city.

Huayna Potosí in the Central Bolivian Cordillera

The second AGU article lead by David B. Bahr of the Department of Physics and Computational Science, Regis University, Denver, CO. looks at the accumulation area of glaciers and icecaps relative to mass volume.

Sea-level rise from glaciers and ice caps: a lower bound

One of the most easily measured dimensions of a glacier, the accumulation area, is linked to future changes in glacier volume and consequent changes in sea level. Currently observed accumulation areas are too small, forcing glaciers to lose 27% of their volume to attain equilibrium with current climate. As a result, at least 184 ± 33 mm of sea-level rise are necessitated by mass wastage of the world’s mountain glaciers and ice caps even if the climate does not continue to warm. If the climate continues to warm along current trends, a minimum of 373 ± 21 mm of sea-level rise over the next 100 years is expected from glaciers and ice caps. When compared to recent estimates from all other sources, melt water from glaciers must be considered as a particularly important fraction of the total sea-level rise expected this century.

In other words, where accumulation areas are too small, one can predict by what percentage glaciers will shrink in the future as well as make additional predictions regarding the contribution of glacier melt to sea level rise. The authors are thus able to deduce how much glacial melt will contribute to sea level rise even if the climate warms no further as well as how much it will rise if it continues to warm at current rates.

On average, a glacier in equilibrium will accumulate snow on its upper reaches and ablate snow and ice at its lower elevations. The [accumulation area ratio] AAR is the ratio of the accumulation area to the area of the entire glacier, with values for healthy glaciers ranging from approximately 0.4 to 0.8 (Meier and Post, 1962). Variations in the equilibrium AAR are caused by differences in glacier shapes and mass balance profiles, (x), which give the net accumulation minus ablation of snow and ice at any position x. The balance profile is a direct consequence of climate (precipitation and temperature) which can vary both regionally and locally due to orographic and other meteorological factors.

If we assign each glacier an equilibrium AAR0 that indicates its value when the glacier’s net balance is zero, then an AAR < AAR0 indicates that the accumulation area has shrunk, the glacier is overextended, and the glacier must retreat to reach a new equilibrium with current climate. This hypothesized retreat involves the implicit but reasonable assumption that changes in the balance regime happen quickly relative to changes in the area of the glacier. Typical e-folding response times for glacier flow range from 10’s to 1000’s of years depending on the glacier’s size (Bahr et al., 1998; Pfeffer et al., 1998; Jóhannesson et al., 1989), while a glacier’s climate can change every year. Therefore, as the climate warms, a glacier’s current AAR will not represent an equilibrium value. To reach an equilibrium with the current climate, the glacier will have to slowly change size until AAR = AAR0. Note that in this case, the AAR is an observed medium term average, calculated over a period long enough to eliminate interannual variability but significantly shorter than the time scale of adjustment to equilibrium.

Our estimate places no bounds on time and only indicates the final outcome after all glaciers reach equilibrium. Therefore, it is possible that the additional ~80 mm represents sea-level rise that will occur after the 100 year time scale of previous estimates. However, with an e-folding response time that averages on decadal to century time scales for most glaciers, the bulk of the 184 mm of predicted rise is expected within this century.

Long term mass balance data from 86 mountain glaciers and ice caps from around the world shows that the equilibrium AAR0 differs for each glacier but averages 0.57 ± 0.01 (see Supplementary Data). The same data indicate that the average AAR from 1997-2006 is only 0.44 ± 0.02, suggesting that glaciers and ice caps around the world are out of equilibrium, as expected. The ratio AAR / AAR0 = αr gives a measure of the extent to which each glacier is out of equilibrium (Dyurgerov et al., submitted), and in this case 1 – αr = 0.23 or approximately 23% out of equilibrium on average. The following analysis converts each glacier’s αr to a change in glacier volume. By summing over all glaciers this gives an estimate of sea-level rise.

If a similar reduction of AARs occurs over the next 30 to 40 years, then we can reasonably expect the average AAR to drop linearly from roughly 0.54 in 1961 to 0.44 in 2007 to 0.31 by 2050. This is a conservative estimate – observations indicate a faster than linear decrease in global ice mass balance over the last 40 years (Kaser et al., 2006). Although the actual decrease in AAR may be faster than linear, this conservative estimate represents a 30% decrease from the current value. As a rough approximation, we can assume that the AAR of every glacier decreases by the same percentage, giving an estimate of the fractional volume change pv for each glacier. In that case, the minimal sea-level rise from glaciers and ice caps will more than double to 373 ± 21 mm over the next 100 years.

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Responses

  1. Just wanted you to know that I’m reading and appreciating your mission and dedication. You and B are both in my thoughts. All the best.

    • Thank you so much, Karen. It’s so nice to hear from you! I hope that you had wonderful holidays and are doing well. Becci and I wish you all the best for 2009.


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