Himalayan glaciers: the big picture is a montage.

U nusual miscarriages of science (1, 2) recently rocked climate change science and glaciology. An infamous paragraph, uncharacteristic of the rest of the contribution of Working Group II to the Intergovernmental Panel on Climate Change Fourth Assessment, claimed that Himalayan glaciers would disappear by 2035 (1). In such a monumental report, errors can be expected. However, this error, explicated in ref. 3, shredded the reputation of a large and usually rigorous international virtual institution. The gaffe by the Intergovernmental Panel on Climate Change helped to trigger a global political retreat from climate change negotiations, and it may prove to have been one of the more consequential scientific missteps in human history. An equally incorrect claim, on a different timescale, was that large Himalayan glaciers may be responding today to climate shifts 6,000– 15,000 y ago (2). However, both mistakes (1, 2) and some solid scientific reporting on Himalayan glacier dynamics (4–10) highlight large gaps in the observational record. In PNAS, Fujita and Nuimura (11) competently reduced the knowledge gap. Fujita and Nuimura (11) have shown a globally relevant point in reference to the Himalayas. Climate change is heterogeneous, oscillatory, and trending; consequently, glacier responses are heterogeneous, oscillatory, and trending as well. The measured mass balances of three glaciers in the study by Fujita and Nuimura (11) were between −0.5 and −0.8 m/y water-equivalent thinning. The smallest of the three, AX010 (AX), was thinning by about −0.8 m/y. A simple rule (12) relating glacier volume to area suggests that AX’s present mean thickness is ∼24 m; at recent thinning rates, it would indeed disappear in the next few decades, perhaps as soon as 2035. Our rough estimate does not consider the rate of rise of the equilibrium line altitude (ELA; where mass gain by snowfall is balanced exactly by loss caused by melting) calculated by Fujita and Nuimura (11). If this ELA rise rate is maintained, the mass balance will become still more negative, and the glacier will disappear even earlier. Two other benchmark glaciers are larger and thicker. Fujita and Nuimura (11) found that Yala Glacier (YL) also has a rising ELA, but it may take several decades to ascend above and thus doom the glacier. The larger glacier, Rikha Samba (RS), has a less negative balance, approximately −0.5 m/y, and in 2035, it will be only slightly reduced in area and thickness. Many larger Himalayan glaciers are also thinning by up to 1 m/y (7–9). With mean thicknesses up to a few hundred meters, the largest glaciers are unlikely to disappear this century, even if thinning accelerates. However, massive glaciers such as Khumbu (Nepal), Gangotri (India), and Siachen (India/Pakistan) will retreat rapidly if supraglacial lakes form and set the