Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2We can draw on geodetic and gravimetricmeasurements of multidecadal mass balanceto reinforce our understanding of calving rates.To illustrate, Larsen et al. (2007) estimatedthe mass balance in southeastern Alaska andadjacent British Columbia as –16.7 ± 4.4 Gt a –1 .Earlier, Arendt et al. (2002) measured glaciersacross Alaska by laser altimetry and estimatedan acceleration in mass loss for the entire statefrom 52 ± 15 Gt a –1 (mid-1950s to mid-1990s)to 96 ± 35 Gt a –1 (mid-1990s to 2001). These aresignificantly greater losses than the equivalentdirect estimates, and much of the discrepancymust be due to under-representation of calvingin the latter. This under-representation iscompounded by a lack of basic information.The extent, and even the total terminus length,of glacier ice involved in calving is not known,although a substantial amount of information isavailable in scattered sources.Global mass-balance estimates suffer fromuncertainty in total glacierized area, and therate of shrinkage of that area is not known accuratelyenough to be accounted for. A furtherproblem is delineating the ice sheets so as toavoid double-counting or omitting peripheralice bodies.Measured glaciers are a shifting population.Their total number fluctuates, and the list ofmeasured glaciers changes continually. Thecommonest record length is 1 year; only about50 are longer than 20 years. These difficultiescan be addressed by assuming that eachsingle annual measurement is a random sample.However, the temporal variance of such a shortsample is difficult to estimate satisfactorily,especially in the presence of a trend.On any one glacier, a small number of pointmeasurements must represent the entire glacier.It is usually reasonable to assume that the massbalance depends only on the surface elevation,increasing from net loss at the bottom to netgain above the equilibrium line altitude. Atypical uncertainty for elevation-band averagesof mass balance is ± 200 kilograms per squaremeter per year (kg m –2 a –1 ), but measurementsat different elevations are highly correlated,meaning that whole-glacier measurements haveintrinsic uncertainty comparable with that ofelevation-band averages.At the global scale, the number of measuredglaciers is small by comparison with the totalnumber of glaciers. However the mass balanceof any one glacier is a good guide to the balanceof nearby glaciers. At this scale, the distance towhich single-glacier measurements yield usefulinformation is of the order of 600 km. Glacierizedregions with few or no measured glacierswithin this distance obviously pose a problem.If there are no nearby measurements at all, wecan do no better in a statistical sense than to setthe regional average equal to the global average,attaching to it a suitably large uncertainty.3.3.3 Historical and RecentBalance RatesTo extend the short time series of measuredmass balance, Oerlemans et al. (2007) havetried to calibrate records of terminus fluctuations(i.e., of glacier length) against thedirect measurements by a scaling procedure.This allowed them to interpret the terminusfluctuations back to the mid-19th century inmass-balance units. Figure 2.7 shows modeledmass loss since the middle of the 19th century,at which time mass balance was near to zerofor perhaps a few decades. Before then, massbalance had been positive for probably a fewcenturies. This is the signature of the LittleIce Age, for which there is abundant evidencein other forms. The balance implied by theOerlemans et al. (2007) reconstruction is a netloss of about 110 to 150 Gt a –1 on average overthe past 150 years. This has led to a cumulativerise of sea level by 50–60 mm.It is not possible to detect mass-balanceacceleration with confidence over this timespan, but we do see such an acceleration overthe shorter period of direct measurements(Fig. 2.7). This signature matches well withthe signature seen in records of global averagesurface-air temperature (Trenberth et al., 2007).Temperature remained constant or decreasedslightly from the 1940s to the 1970s and hasbeen increasing since. In fact, mass balance alsoresponds to forcing on even shorter time scales.For example, there is a detectable small-glacierresponse to large volcanic eruptions. In short,small glaciers have been evolving as we wouldexpect them to when subjected to a small butgrowing increase in radiative forcing.52