Book 2.indb - US Climate Change Science Program

Abrupt Climate Changewater) or, more conveniently at the global scale,Gt a –1 (change of total mass, in gigatons peryear). A change in mass balance is measuredin Gt a –2 , gigatons per year per year: faster andfaster loss or gain.3.3.2 Mass-Balance Measurementsand UncertaintiesMost measurements of the mass balance ofsmall glaciers are obtained in one of twoways. Direct measurements are those inwhich the change in glacier surface elevationis measured directly at a network of pits andstakes. Calving is treated separately. In geodeticmeasurements, the glacier surface elevationis measured at two times with reference tosome fixed external datum. Recent advancesin remote sensing promise to increase thecontribution from geodetic measurements andto improve spatial coverage, but at present theobservational database remains dominated bydirect measurements. The primary source forthese is the World Glacier Monitoring Service(WGMS; Haeberli et al., 2005). Kaser et al.(2006) (see also Lemke et al., 2007; Sec. 4.5)present compilations which build on the WGMSdataset and extend it significantly.In Figure 2.3 (see also Table 2.2), the three spatiallycorrected curves agree rather well, whichmotivated Kaser et al. (2006) to construct theirconsensus estimate of mass balance, denotedMB. The arithmetic-average curve C05a isthe only curve extending before 1961 becausemeasurements are too few at those times forarea-weighting or spatial interpolation to bepracticable. The early measurements suggestweakly that mass balances were negative. After1961, we can see with greater confidence thatmass balance became less negative until theearly 1970s, and that thereafter it has beengrowing more negative.The uncorrected C05a, a simple arithmeticaverage of all the measurements, generallytracks the other curves with fair accuracy. Apparentlyspatial bias, while not negligible, is ofonly moderate significance. However the C05aestimate for 2001–04 is starkly discordant. Thediscordance is due in large part to the Europeanheat wave of 2003 and to under-representationof the high arctic latitudes, where measurementsare few and 2003 balances were onlymoderately negative. It illustrates the extentto which spatial bias can compromise globalestimates. The other curves, C05i, DM05,and O04, each attempt to correct carefully forspatial bias.Mass-balance measurements at the glaciersurface are relatively simple, but difficultiesarise with contributions from other parts ofthe glacier. Internal accumulation is one of themost serious problems. It happens in the lowerpercolation zones of cold glaciers (those whoseinternal temperatures are below freezing)when surface meltwater percolates beneath thecurrent year’s accumulation of snow. Internalaccumulation is impractical to measure andis difficult to model with confidence. It is aplausible conjecture that there are many morecold glaciers than temperate glaciers (in whichmeltwater can be expected to run off ratherthan to refreeze).Table 2.2. Global small-glacier mass balance for differentperiods. Consensus estimates (Kaser et al., 2006), includingsmall glaciers in Greenland and Antarctica, of global averagespecific mass balance (b); global total mass balance (B), equal toA×b where A=785×10 9 m 2 is the areal extent of small glaciers;and the sea level equivalent (SLE), equal to –B/(ρ w AO), whereρ w =1,000 kg m –3 and ocean area AO=362×10 12 m 2 .Period b (kg m –2 a –1 ) B (Gt a –1 ) SLE (mm a –1 )1961–2004 –231±101 –182±78 0.50±0.221961–1990 –173±89 –136±70 0.37±0.191991–2004 –356±121 –280±95 0.77±0.262001–2004 –451±89 –354±70 0.98±0.19The calving of icebergs is a significant source ofuncertainty. Over a sufficiently long averagingperiod, adjacent calving and noncalving glaciersought not to have very different balances, butthe time scale of calving is quite different fromthe annual scale of surface mass balance, and itis difficult to match the two. Tidewater glacierstend to evolve by slow growth (over centuries)alternating with brief (decades-long) episodes ofrapid retreat. Many tidewater glaciers are undergoingsuch retreat at present, but in general theyare under-represented in the list of measuredglaciers. The resulting bias, which is known tobe opposite to the internal-accumulation bias,must be substantial.51