20 30J u ly 2 0 0 2 J a n . 2 0 0 3 J u ly 2 0 0 3 J a n . 2 0 0 4 J u ly 2 0 0 4 J a n . 2 0 0 5 J u ly 2 0 0 5 J a n . 2 0 0 6 J u ly 2 0 0 6 J a n . 2 0 0 7 J u ly 2 0 0 7 J a n . is 5.9 cm, compared with 10.3 cm for soil water and 13.8 cm for TWS. These relationships are consistent with those reported in previous studies of soil-water/groundwater covariability 5,14,22 . The soil-water time series reflects rainfall anomalies during the period (discussed below) and exhibits no significant trend. On the other hand, TWS and groundwater decline steadily from 2003 onwards. We calculate the rate of depletion of groundwater to be 4.0 6 1.0 cm yr 21 . Assuming 2 a specific yield of 0.12, the regional mean rate of water As best can be determined from the coarse GRACE observations, maximum rates of groundwater depletion are centred on Haryana. Groundwater levels also appear to be declining quickly in western Uttar Pradesh, to the east of Haryana. If there is groundwater depletion in Pakistan, to the northwest, it seems to be much less severe.Although six years is a short period from which to assess a longterm trend with confidence, two pieces of evidence support our conclusion that severe groundwater depletion is occurring as a result of human consumption rather than natural variability. First, the Indian Ministry of Water Resources reports that groundwater withdrawals exceed recharge in the three states we studied 2 . Irrigation accounts for about 95% of the consumption 2 ; about 28% of the area is irrigated 23 . Second, there was no shortage of rainfall in the region to cause a natural decline in water storage. GLDAS modelled soil-water fields integrate the effects of precipitation, solar radiation, air temperature and other meteorological factors that directly or indirectly influence groundwater storage 4 . The trend in simulated soil-water storage during the period of study was 0.4 cm yr 21 . This supports the notion that groundwater declines were not caused by natural climate variability. It also confirms that the computed groundwater trend is not a mathematical artefact caused by the subtraction of a large positive soil-water trend from the GRACE-derived TWS trend.We conclude that withdrawals for irrigation and other uses are depleting the groundwater reserves of Rajasthan, Punjab and Haryana at a rate of 4.0 6 1.0 cm yr 21 equivalent height of water, or 17.7 6 4.5 km 3 yr 21 . The Indian Ministry of Water Resources reports that the difference between annual available recharge and annual withdrawals in the region is a 13.2 km 3 yr 21 deficit 2 . Our result implies that the portion of irrigated water that replenishes the aquifers is less and/or the rate of withdrawal is more than the Indian government has estimated. Apparently, most of the groundwater withdrawn subsequently is lost from the region as a result of increases in run-off and/or evapotranspiration. Between August 2002 and October 2008, the region lost 109 km 3 of groundwater, which is double the capacity of India's largest reservoir, the Upper Waingang...

Warming and Melting Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172 ) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly—in some cases, not even agreeing about whether there is a net loss or a net gain—making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183 ) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise.

Ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques, over different time periods, and at various levels of spatial detail. Considerable disparity remains between these estimates due to the inherent uncertainties of each method, the lack of detailed comparison between independent estimates, and the effect of temporal modulations in ice sheet surface mass balance. Here, we present a consistent record of mass balance for the Greenland and Antarctic ice sheets over the past two decades, validated by the comparison of two independent techniques over the last 8 years: one differencing perimeter loss from net accumulation, and one using a dense time series of time‐variable gravity. We find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss. In 2006, the Greenland and Antarctic ice sheets experienced a combined mass loss of 475 ± 158 Gt/yr, equivalent to 1.3 ± 0.4 mm/yr sea level rise. Notably, the acceleration in ice sheet loss over the last 18 years was 21.9 ± 1 Gt/yr2 for Greenland and 14.5 ± 2 Gt/yr2 for Antarctica, for a combined total of 36.3 ± 2 Gt/yr2. This acceleration is 3 times larger than for mountain glaciers and ice caps (12 ± 6 Gt/yr2). If this trend continues, ice sheets will be the dominant contributor to sea level rise in the 21st century.

GRACE and Movement Together Recent measurements of the rate of mass loss from the Greenland ice sheet vary approximately by a factor of three. Resolving these discrepancies is essential for determining the current mass balance of the ice sheet and to project sea level rise in the future. Van den Broeke et al. (p. 984 ) obtained consistent estimates from two independent methods, one based on observations of ice movement combined with model calculations and the other on remote gravity measurements made by the GRACE (Gravity Recovery and Climate Experiment) satellites. The combination of these approaches also resolves the separate contributions of surface processes and of ice dynamics, the two major routes of ice mass loss.

[1] We review the sea-level and energy budgets together from 1961, using recent and updated estimates of all terms. From 1972 to 2008, the observed sea-level rise (1.8 ± 0.2 mm yr −1 from tide gauges alone and 2.1 ± 0.2 mm yr −1 from a combination of tide gauges and altimeter observations) agrees well with the sum of contributions (1.8 ± 0.4 mm yr −1 ) in magnitude and with both having similar increases in the rate of rise during the period. The largest contributions come from ocean thermal expansion (0.8 mm yr −1 ) and the melting of glaciers and ice caps (0.7 mm yr −1 ), with Greenland and Antarctica contributing about 0.4 mm yr −1 . The cryospheric contributions increase through the period (particularly in the 1990s) but the thermosteric contribution increases less rapidly. We include an improved estimate of aquifer depletion (0.3 mm yr −1 ), partially offsetting the retention of water in dams and giving a total terrestrial storage contribution of −0.1 mm yr −1 . Ocean warming (90% of the total of the Earth's energy increase) continues through to the end of the record, in agreement with continued greenhouse gas forcing. The aerosol forcing, inferred as a residual in the atmospheric energy balance, is estimated as −0.8 ± 0.4 W m −2 for the 1980s and early 1990s. It increases in the late 1990s, as is required for consistency with little surface warming over the last decade. This increase is likely at least partially related to substantial increases in aerosol emissions from developing nations and moderate volcanic activity. Citation: Church, J. A.,

[1] Eleven monthly GRACE gravity field solutions are now available for analyses. We show those fields can be used to recover monthly changes in water storage, both on land and in the ocean, to accuracies of 1.5 cm of water thickness when smoothed over 1000 km. The amplitude of the annually varying signal can be determined to 1.0 cm. Results are 30% better for a 1500 km smoothing radius, and 40% worse for a 750 km radius. We estimate the annually varying component of water storage for three large drainage basins (the Mississippi, the Amazon, and a region draining into the Bay of Bengal), to accuracies of 1.0-1.5 cm.

We use monthly measurements of time‐variable gravity from the GRACE (Gravity Recovery and Climate Experiment) satellite gravity mission to determine the ice mass‐loss for the Greenland and Antarctic Ice Sheets during the period between April 2002 and February 2009. We find that during this time period the mass loss of the ice sheets is not a constant, but accelerating with time, i.e., that the GRACE observations are better represented by a quadratic trend than by a linear one, implying that the ice sheets contribution to sea level becomes larger with time. In Greenland, the mass loss increased from 137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009, i.e., an acceleration of −30 ± 11 Gt/yr2 in 2002–2009. In Antarctica the mass loss increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006–2009, i.e., an acceleration of −26 ± 14 Gt/yr2 in 2002–2009. The observed acceleration in ice sheet mass loss helps reconcile GRACE ice mass estimates obtained for different time periods.

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