[1] 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/yr 2 for Greenland and 14.5 ± 2 Gt/yr 2 for Antarctica, for a combined total of 36.3 ± 2 Gt/yr 2 . This acceleration is 3 times larger than for mountain glaciers and ice caps (12 ± 6 Gt/yr 2 ). If this trend continues, ice sheets will be the dominant contributor to sea level rise in the 21st century.

3. Results [10] Both ice sheets exhibit large inter‐annual variations in SMB (Figure 1). Percentage‐wise, these variations are comparable, but the absolute values are 3–4 times larger in Antarctica compared to Greenland due to the larger total SMB in Antarctica. In Greenland, SMB values have decreased by 12.9 ± 1 Gt/yr2 since 1992 due to a steady increase in surface runoff, whereas precipitation has not changed at a detectable level. In Antarctica, we observe a 5.5 ± 2 Gt/yr2 decrease in SMB since 1992, which is consistent with studies indicating no significant increase in SMB over the past 50 years [Monaghan et al., 2006]. Figure 1 Open in figure viewer PowerPoint Monthly surface mass balance, SMB (open circle), and yearly ice discharge compensated for grounding line retreat, D* (solid triangle), for (a) Greenland and (b) Antarctic Ice Sheets between 1992 and 2009 over a grounded area of respectively, 1.7 million km2 and 12.427 million km2, with error bars in gigaton per year (1012 kg/yr or trillion tons per year). The acceleration rate in SMB and D*, in Gt/yr2, is determined from a linear fit of the data (dotted lines). [11] In contrast, ice discharge, D*, exhibits smooth variations during the time period, and a steady increase with time, except in 2005 when two large glaciers accelerated simultaneously in East Greenland. The acceleration rate in ice discharge in Greenland is 9.0 ± 1 Gt/yr2 for 1992–2009. In Antarctica, the acceleration is also 9.0 ± 1 Gt/yr2 for the same time period. [12] We compare the MBM and GRACE results for the same area, i.e., the grounded extent of ice sheets excluding GIC, on a monthly time scale but with a 13‐month smoothing applied to the data, for the common time period 2002.9 to 2009.5. In Greenland, the agreement in M(t) demonstrated at seasonal and annual timescales [van den Broeke et al., 2009] is extended here to dM/dt and d2M/dt2 (Figure 2a). The mass losses estimated from MBM and GRACE are within ±20 Gt/yr, or within their respective errors of ±51 Gt/yr and ±33 Gt/yr. The acceleration in mass loss is 19.3 ± 4 Gt/yr2 for MBM and 17.0 ± 8 Gt/yr2 for GRACE. The GRACE‐derived acceleration is independent of the GIA reconstruction, a constant signal during the observational period. Figure 2 Open in figure viewer PowerPoint Total ice sheet mass balance, dM/dt, between 1992 and 2009 for (a) Greenland; (b) Antarctica; and c) the sum of Greenland and Antarctica, in Gt/yr from the Mass Budget Method (MBM) (solid black circle) and GRACE time‐variable gravity (solid red triangle), with associated error bars. The acceleration rate in ice sheet mass balance, in gigatons per year squared, is determined from a linear fit of MBM over 18 yr (black line) and GRACE over 8 yr (red line). [13] In Antarctica, we find an excellent agreement between the two techniques (Figure 2b). The dM/dt values differ by ±50 Gt/yr, or within the error bar of ±150 Gt/yr for MBM and ±75 Gt/yr for GRACE. In 2006, the MBM mass loss was approximately 200 ± 150 Gt/yr (regression line), which is comparable to Greenland's 250 ± 40 Gt/yr, and equivalent to 0.6 ± 0.4 mm/yr sea level rise. The total contribution from both ice sheets amounted to 1.3 ± 0.4 mm/yr sea level rise. [14] The temporal variability in Antarctic SMB introduces a large modulation of the GRACE signal with a 3.6‐year periodicity according to the signal autocorrelation. Taking this periodic signal into account, we retrieve an acceleration in mass loss from the GRACE data of 13.2 ± 10 Gt/yr2 (Figure 2b). For the same time period, the acceleration in mass loss from the MBM data is 15.1 ± 12 Gt/yr2. Both estimates have a large uncertainty because of the short period of observation and the large temporal variability in SMB. As for Greenland, the GRACE‐derived acceleration is independent of the GIA correction, a larger residual uncertainty in Antarctica than in Greenland. [15] The excellent agreement of the GRACE and MBM records over the last 8 years validates the 18‐year MBM record. The results also indicate that an observation period of 8 years is probably not sufficient for these methods to separate the long‐term trend in ice sheet acceleration from temporal variations in SMB, especially in Antarctica. When we use the extended time period 1992–2009, the significance of the trend improves considerably. The MBM record indicates an acceleration in mass loss of 21.9 ± 1 Gt/yr2 for Greenland and 14.5 ± 2 Gt/yr2 for Antarctica. The lower uncertainty reflects the reduced influence of temporal variations in SMB for the longer record. The uncertainty in acceleration is thus reduced to 5% for Greenland and 10% for Antarctica. When the mass changes from both ice sheets are combined together (Figure 2c), the data reveal an increase in ice sheet mass loss of 36.3 ± 2 Gt/yr2.

4. Discussion [16] Using techniques other than GRACE and MBM, the mass loss of mountain glaciers and ice caps (GIC), including the GIC surrounding Greenland and Antarctica, has been estimated at 402 ± 95 Gt/yr in 2006, with an acceleration of 11.8 ± 6 Gt/yr2 over the last few decades [Kaser et al., 2006; Meier et al., 2007]. Our GRACE estimates and associated errors account for the leakage from the Greenland and Antarctica GIC, and, as discussed earlier, this leakage is small. The MBM estimates completely exclude the GIC. In year 2006, the total ice sheet loss was 475 ± 158 Gt/yr (regression line in Figure 2c), which is comparable or greater than the 402 ± 95 Gt/yr estimate for the GIC. More important, the acceleration in ice sheet loss of 36.3 ± 2 Gt/yr2 is three times larger than that for the GIC. If this trend continues, ice sheets will become the dominant contribution to sea level rise in the next decades, well in advance of model forecasts [Meehl et al., 2007]. [17] It is important to examine whether the acceleration in mass loss may continue. In Greenland, the increase in runoff, which contributes more than half the total loss, is likely to persist in a warming climate [Hanna et al., 2008], and continue to exhibit large inter‐annual variations. For ice dynamics, the GRACE data and the interferometric ice motion record indicate that the mass loss has decreased in southeast Greenland since 2005, yet still maintains above its level in 1996, but has increased in the northwest Greenland since 2006 [Khan et al., 2010]. Collectively, these observations reveal an ice sheet still in transition to a regime of higher loss. [18] In Antarctica, Pine Island Glacier accelerated exponentially over the last 30 years: 0.8% in the 1980s, 2.4% in the 1990s, 6% in 2006 and 16% in 2007–2008 [Rignot, 2008], and quadrupled its thinning rate in 1992–2008 [Wingham et al., 2009]. Simple model projections predict a tripling in glacier speed once the grounding line retreats to a deeper and smoother bed [Thomas et al., 2004]. Dynamic losses are therefore likely to persist and spread farther inland in this critical sector. A small positive increase in Antarctic SMB could offset these coastal losses, but this effect has not yet been observed. [19] If the acceleration in ice sheet loss of 36.3 ± 2 Gt/yr2 continues for the next decades, the cumulative ice sheet loss would raise global sea level by 15 ± 2 cm in year 2050 compared to 2009/2010. The GIC would contribute a sea level rise of 8 ± 4 cm, and thermal expansion of the ocean would add another 9 ± 3 cm based on the average of scenarios A1B, A2 and B1 [Meehl et al., 2007], for a total rise of 32 ± 5 cm. At the current rate of acceleration in ice sheet loss, starting at 500 Gt/yr in 2008 and increasing at 36.5 Gt/yr2, the contribution of ice sheets alone scales up to 56 cm by 2100. While this value may not be used as a projection given the considerable uncertainty in future acceleration of ice sheet mass loss, it provides one indication of the potential contribution of ice sheets to sea level in the coming century if the present trends continue.

5. Conclusions [20] This study reconciles two totally independent methods for estimating ice sheet mass balance, in Greenland and Antarctica, for the first time: the MBM method comparing influx and outflux of ice, and the GRACE method based on time‐variable gravity data. The two records agree in terms of mass, M(t), mass change, dM(t)/dt, and acceleration in mass change, d2M/dt2. The results illustrate the major impact of monthly‐to‐annual variations in SMB on ice sheet mass balance. Using the two‐decade long MBM observation record, we determine that ice sheet loss is accelerating by 36.3 ± 2 Gt/yr2, or 3 times larger than from mountain glaciers and ice caps (GIC). The magnitude of the acceleration suggests that ice sheets will be the dominant contributors to sea level rise in forthcoming decades, and will likely exceed the IPCC projections for the contribution of ice sheets to sea level rise in the 21st century [Meehl et al., 2007].

Acknowledgments [21] This work was performed at the Earth System Science Department of Physical Sciences, University of California Irvine and at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryospheric Science Program. NCAR is funded by the National Science Foundation. This work was financially supported by Utrecht University and the Netherlands Polar Programme. [22] E. Calais thanks two anonymous reviewers.