Doug Hoffman: New Ice Surveys Finds Slower Ice-sheet Melting
A new “comprehensive” report about the melting of Antarctic and Greenland ice sheets is being touted by climate alarmists as “grim news” but in fact says no such thing. This latest estimate, published this week in Science, combines data from many sources including 20 years of satellite data and 32 years of ice-sheet simulations to arrive at a mixed conclusion. It estimates that, between 1992 and 2011, the Antarctic and Greenland ice sheets lost 1350 ± 1010 and 2700 ± 930 Gt of ice, respectively. That is equivalent to an increase in global mean sea level of 11.2 ± 3.8 mm, less than 1/2 an inch. Moreover, while some areas were loosing ice mass others were gaining mass from snowfall. The East Antarctic Ice Sheet (EAIS), which occupies over 75% of Antarctica, experienced mass gains during the final years of the study.
The intent of this new report is quite clear, come up with an unassailable new estimate for ice loss that can be included in the next IPCC global warming report. In “A Reconciled Estimate of Ice-Sheet Mass Balance,” Andrew Shepherd et al. begin with some historical perspective. “Analysis of the geological record suggests that past climatic changes have precipitated sustained ice-sheet contributions, in excess of 10 mm year−1 over millennial time periods, and the prospect of such changes in the future are of greatest concern,” they state, accurately stating that there has been significant variation in ice-sheet loss and ocean levels before the specter of anthropogenic global warming raised its climate destroying head.
Naturally, the authors could not resist bowing to the dominant consensus driven, group think pap: “Even the modest rises in ocean temperature that are predicted over the coming century could trigger substantial ice-sheet mass loss through enhanced melting of ice shelves and outlet glaciers. However, these processes were not incorporated into the ice-sheet models that informed the current global climate projections.” Their data may not be reconciled but the authors are certainly demonstrating that they are go-along types of climate scientists. Exactly how they performed this extensive and exhaustive study is stated early on in the Science paper:
In this assessment, we use 19 years of satellite radar altimeter (RA) data, 5 years of satellite laser altimeter (LA) data, 19 years of satellite radar interferometer data, 8 years of satellite gravimetry data, 32 years of surface mass balance (SMB) model simulations, and estimates from several glacial isostatic adjustment models, to produce a reconciled estimate of ice-sheet mass balance. The satellite data sets were developed by using independent methods and, in the case of the LA, gravimeter, and SMB data sets, through contributions from numerous research groups. To enable a direct comparison, we reprocessed the geodetic data sets with use of common time intervals and common definitions of the East Antarctic, West Antarctic, Antarctic Peninsula, and Greenland ice-sheet (EAIS, WAIS, APIS, and GrIS, respectively) boundaries (16). The maximum temporal extent of the satellite data sets spans the period 1992 to 2011, and results from all geodetic techniques are available between January 2003 and December 2008. Unless stated otherwise, all results are presented with 1-sigma uncertainty estimates.
There are a plethora of abbreviations here but the main thing to bear in mind is that the two big repositories of glacial ice are the Greenland ice-sheet (GrIS) and Antarctica (EAIS, WAIS, APIS). Drawing on all the satellite sources listed above the international team of 47 experts led by Shepherd munged the datasets together—data collected from almost 30 previous ice-sheet studies, including 20 years of data from 10 different satellite missions, adding a large dollop of computer model simulation results. What was found was that “over the course of our 19-year survey, the average rates of mass balance of the AIS and the GrIS were –71 ± 53 and –152 ± 49 Gt year−1, respectively.” The illustration below summarizes the findings.
It should be noted that the scale of the plots is chosen to highlight the variation in the loss rates and do not show the amount of loss with respect to the entire mass of ice present. The left axis denotes cumulative changes in the mass of the EAIS, WAIS, and APIS (top) and GrIS and AIS and the combined change of the AIS and GrIS (bottom). These values are computed from a reconciliation of measurements acquired by satellite RA, the IOM, satellite gravimetry, and satellite laser altimetry (LA). Also shown, labeled on the right axis, is the equivalent global sea-level contribution, calculated assuming that 360 Gt of ice corresponds to 1 mm of sea-level rise.
“Temporal variations in the availability of the various satellite data sets means that the reconciled mass balance is weighted toward different techniques during certain periods,” the authors state. It should also be noted that the response in Greenland is significantly different from the response of the much larger Antarctic ice-sheet. Here is a summary of the Antarctic findings.
The pattern of WAIS imbalance is dominated by mass losses (Amundsen Sea sector) and gains (Kamb Ice Stream) of dynamical origin. Although close to balance during the 1990s, there have been significant mass losses from the APIS since then because of glacier acceleration in the wake of ice-shelf collapse and calving-front retreat. The APIS now accounts for around 25% of all mass losses from Antarctic regions that are in a state of negative mass balance, despite occupying just 4% of the continental area. In contrast, the EAIS, which occupies over 75% of Antarctica, was in approximate balance throughout the 1990s. Although the EAIS has experienced mass gains during the final years of our survey, our reconciled data set is too short to determine whether they were caused by natural fluctuations that are a common feature of Antarctic ice-core records or long-term increases in precipitation that are a common feature of global and regional climate model projections.
In a news article in Nature, Riccardo Riva, a geoscientist at Delft University of Technology in the Netherlands, commented on the results of Shepherd and colleagues’ study. The good news, according to Riva, is that Antarctica is not losing ice as rapidly as suggested by many recent studies. Moreover, snowfall in east Antarctica seems to be compensating for some—if not quite all—of the melting elsewhere in Antarctica.
The figures above show estimated anomalies in cumulative ice-sheet firn mass (A), and mass (B and C), derived from the RACMO regional climate model, satellite RA, and GRACE satellite gravimetry, respectively. Results were computed over the period July 2009 to July 2010 relative to July 2008 to July 2009, a period of high snowfall in East Antarctica (“anomalously high” according to Shepherd et al.).
Also shown are time series data, as resolved by these data sets and three additional climate models [ERA-Interim (ERAI), CFSR, and MERRA], as the average anomaly over four drainage basins of Dronning Maud Land in East Antarctica (shaded areas in the map below).
Consider what the term “reconciliation” means—an attempt to bring multiple datasets into agreement though they represent very different ways of gathering data and yield noticeably different results. The image below, taken from the report, shows a comparison of mass balance estimates of the GrIS, APIS, EAIS, WAIS, AIS, and the AIS plus GrIS, derived from the four independent geodetic techniques of RA (cyan), IOM (red), LA (green), and gravimetry (blue) over the period 2003 to 2008. Also shown is the reconciled result (gray).
“To produce a reconciled ice-sheet mass balance estimate, we computed the average rate of mass change derived from each of the geodetic techniques within the various regions of interest and over the time periods for which geodetic mass rates were derived,” the authors state. As can bee seen from the illustration, some of the methods disagreed significantly, with one method (IOM) constantly overestimating ice loss and another (RA) returning lower loss rates. One method in particular, gravimetry from NASA’s GRACE satellite, falls consistently in the middle, fairly well representing the report’s reconciled result. But averaging a bunch of different data does not remove the errors or biases in those data. Another new paper takes a look at the GRACE data and finds some interesting results.
In Nature article, “Lower satellite-gravimetry estimates of Antarctic sea-level contribution,” Matt A. King et al. examine sources of bias in GRACE ice mass loss estimates for the ice-sheets of Antarctica. “We resolve 26 independent drainage basins and find that Antarctic mass loss, and its acceleration, is concentrated in basins along the Amundsen Sea coast. Outside this region, we find that West Antarctica is nearly in balance and that East Antarctica is gaining substantial mass,” King and colleagues conclude. Why they claim the GRACE based estimates are in error had to do with the slow rebound of Earth’s crust when the weight of ice is removed—change that takes a significant amount of time to take place and is extremely hard to estimate. They explain:
Studies based on data from the GRACE mission offer the prospect of comprehensively measuring regional- to continental-scale mass changes. However, estimates of secular ice-mass change may only be obtained indirectly from these data after the removal of mass change signals due to other processes, and in particular glacial isostatic adjustment (GIA)—the response of the solid Earth to ice and ocean loading during the Late Quaternary period. Accurate modelling of GIA requires accurate knowledge of both the spatiotemporal evolution of the ice sheets, especially after the Last Glacial Maximum, and the Earth’s response to changing surface loads. A general lack of data from Antarctica with which to constrain GIA model parameters has been a key limitation in arriving at a well-constrained estimate of Antarctic mass balance from GRACE observations. However, a variety of recent observations now consistently indicate a significantly smaller ice loss from West Antarctica since the Last Glacial Maximum compared to most previous model reconstructions. These observations have not been incorporated into GIA models previously applied to GRACE data. As a consequence, past studies have overestimated the contemporary GIA signal, resulting in a systematic overestimate of ice-mass loss inferred from GRACE observations and bringing into question the conclusion that consensus has been reached with independent mass-balance approaches.
Emphasis has been added to the last sentence in the quote above because it has direct impact on the supposedly reconciled results reported by Shepherdet al.. King et al. adopted a modified version of a GIA model known as W12a, claiming it provides a much more accurate picture of mass change on the southernmost continent. Their results are captured in the illustration below.
The large upper map, a, denotes best estimate of ice-mass change using the modified W12a GIA model, with basins numbered. The two smaller maps, band c, show basin-specific lower and upper bounds on ice-mass change, respectively, reflecting the potential systematic error in the basin estimates. The systematic errors include contributions from GIA uncertainty, destriping and internal leakage. King et al. claim “leakage effects sum towards zero over multiple basins.”
Both of the papers discussed here concentrate primarily on Antarctica, and rightfully so, since it contains most of Earth’s glacial ice. For Greenland things are a bit murkier. “Assessments of GrIS mass balance require more careful consideration than was possible here, because the surrounding mountain glaciers and ice caps are included in some, but not all, of our geodetic surveys and because the ice-sheet domains varied in area by 2%,” explain Shepherd et al..
It is unclear how these trends, such as ice loss from Greenland, will evolve, says Ian Joughin, one of the paper’s co-authors and a satellite expert at the University of Washington in Seattle: “It really remains unclear whether such losses will decline, whether they’ll level off or they’ll accelerate further.” This should be viewed in light of recent data that show Greenland underwent a similar episode of ice loss in the 1930s.
Indeed, Shepherd et al. admit that their work is biased on too short a time span to draw any meaningful long-term conclusions. “We have shown that assessments of mass imbalance based on short geodetic records should be treated with care, because fluctuations in SMB can be large over short time periods,” they admit, hinting at the study’s fundamental problem. Recent ice-core data reveal that the Antarctic Peninsula area undergoes bouts of rapid warming periodically.
It is striking that supporters of calamitous climate change always base their projections on the last three decades or so—a period that was, in fact, a time of increasing warmth. In this case, they found melting ice around the globe, just not as much as often claimed and certainly not justification for projections into the future for 50 or 100 years. Go back 150 years and people were not worried about retreating glaciers but advancing ones.