Recent Sea Level

A her a century of polar exploration, the past decade of satellite measurements has painted an altogether new picture of how Earth's ice sheets are changing. As global temperatures have risen, so have rates of snowfall, ice melting. and glacier flow. Although the balance between these opposing processes has varied considerably on a regional scale, data show that Antarctica and Greenland are each losing mass overall. Our best estimate of their combined imbalance is about ray megatons (Ct) of ice per year. enough to raise sea level by 0.35 mm per year. lids is only a modest contribution to the present rate of sea level rise of 3.0 mm per year. However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers. which has accelerated over the past decade. In both continents, there are suspected triggers for the accelerated ice discharge surface and ocean warming, respective, over the course of the amt century, these processes could rapidly counteract the snowfall gains predicted by present coupled climate models. Antarctica and Greenland hold enough ice to raise global sea levels by some 70 m (2), and, according to the geological record (2), collapses of Earth's former ice sheets have caused in-creases of up to so m in less than 500 years. Such a rise, were it to occur today, would have tremendous societal implications (3). Even a much more gradual rise would have great impact. Accordingly, one goal of sociological survey (e.g., (4.511 is to determine the contemporary sea level contribution due to Antarctica and Greenland. For much of the moth century, however, the size of these ice sheets hindered at-tempts to constrain their mass trends, because estimating whole ice sheet mass change could be done only by combining sparse local sue. keys. with consequent uncertainty. For example. a 1992 review (6) concluded that the available sociological measurements allowed Antarctica to be anything from a Goo Ct/year sink to a Sui Gt/year source of ocean mass Boo Ct of ice equals 1.4 mm equivalent sea level (ESL)). accounting for nearly all of the both•century sea level trend of r.8 mm/year (s) or, in the other direction, leaving a mass shortfall of some moo Gather. Even the ZOOS Intergovernmental Panel on Climate Change (IPECAC) report (s) preferred models to observations in estimating Antarctic and Greenland sea level contributions. However, in the past decade, our knowledge of the contemporary mass imbalances of Antarctica and Greenland has been trans-formed by the launch of a series of satellite. based sensors. Since 1998, there have been at least 54 satellite-based estimates (7-20) of the mass imbalance of Earth's ice sheets (Table 1). At face value. their range of some -366 to 53 Gt/year, or 1.0 to -0.15 mm/year sea level rise equivalent, explains much of the ecstatic component of century sea level rise (x.5 mm/year in (as)), but we argue that the contribution is smaller and the problem of closing the century sea level budget remains. Equally, the new observations provide a picture of considerable regional variability and, in particular, the long•predicted OAT snowfall-driven growth [e.g., (so, 2.2)] is being offset by large mass losses from particular ice-stream and glacier flows (e.g.,  23)1. There is, more-over, evidence in Greenland and Antarctica of recent accelerations in these flows (12, 24, as). It is apparent that the late both and early ant-century ice sheets at least are dominated by regional behaviors that are not captured in the models on which the Intergovernmental Panel on Climate Change (IPECAC) predictions have depended, and there is renewed speculation (26, ) or accelerated scan level rise from the ice sheets under a constant rate of climate • warming. Methods and Their Sensitivity to Accumulation Rate The Massachusetts method [e.g., (9, 12)compares the mass gain due to snowfall with mass losses due to sublimation, melt water runoff, and ice that flows into the ocean. It has been given new impetus by the capability of inter thermometric synthetic aperture radar (In STAR) to determine ice surface velocity. This has improved earlier estimates of the ice flux to the ocean (5) and pm. vised a capability to identify accelerations of ice flow. The method is hampered by a lack of accuse-rate accumulation and ice thickness data. For Antarctica, where surface melting is negligible, accumulation may be determined by spatially averaging the history of accumulation recorded in ice cores, or from meteorological forecast models. Estimates of the temporally averaged accumulation or 'mean" accumulation range. respectively, from 1752 to 1924 Gt/year (t) and from 1475 to 2331 Gt/year (28). The meteorological data are acknowledged to be of inferior