If Earth’s climate continues to warm, then the volume of present-day ice sheets will decrease. Melting of the current Greenland ice sheet would result in a sea-level rise of about 6.5 meters; melting of the West Antarctic ice sheet would result in a sea-level rise of about 8 meters (table 1). The West Antarctic ice sheet is especially vulnerable, because much of it is grounded below sea level. Small changes in global sea level or a rise in ocean temperatures could cause a breakup of the two buttressing ice shelves (Ronne/Filchner and Ross). The resulting surge of the West Antarctic ice sheet would lead to a rapid rise in global sea level. Reduction of the West Antarctic and Greenland ice sheets similar to past reductions would cause sea level to rise 10 or more meters. A sea-level rise of 10 meters would flood about 25 percent of the U.S. population, with the major impact being mostly on the people and infrastructures in the Gulf and East Coast States (fig. 3). Researchers at the U.S. Geological Survey and elsewhere are investigating the magnitude and timing of sea-level changes during previous interglacial intervals. Better documentation and understanding of these past changes will improve our ability to estimate the potential for future large-scale changes in sea level.

Climate scientist James Hansen and his colleague Makiko Sato have released a new discussion paper with updated data on ice sheet mass loss from Greenland and Antarctica, with implications for possible multi-metre sea level rise this century. It makes for some interesting reading - there is a link to Hansen's website and the paper at the end. The thesis that Hansen has put forward for several years is that Ice Sheet collapse is a non-linear process: that with the inclusion of amplifying climate feedbacks it is likely to follow an exponential rate of acceleration - a doubling rate. It might be a 10 year doubling time, or less. This will lead to extensive sea level rise, perhaps in the order of 5 metres this century. But accurate data measurements of ice mass loss via laboriously estimating mass input and output has only been available since the early 1990s, and accurate satellite measurements (Gravimetry) via the GRACE satellites since 2000. What these measurements show is that ice mass loss from both Greenland and Antarctica are accelerating, but the data for the time period is still too short to determine whether ice sheet mass loss will follow a somewhat linear path, or an exponential path doubling every 10 years or shorter time period.

Mass balance values for the observation period 2014/15 have been reported from more than 130 glaciers worldwide. The mass balance statistics (Table 1) are calculated based on all reported values and on available data from the 40 reference glaciers with continued observation series of more than 30 years (Table 2). In addition, preliminary mass balance values are given for 2015/16 for some glaciers.

The average mass balance of the glaciers with available long-term observation series around the world continues to be negative, with tentative figures indicating a further thickness reduction of 1.1 meters water equivalent (m w.e.) during the hydrological year 2015. The new data continues the global trend in strong ice loss over the past few decades and brings the cumulative average thickness loss of the reference glaciers since 1980 at almost 20 m w.e. (see Figures 1 and 2). All so far reported mass balance values, given in Table 3, are tentative.

Current conditions: contribution from melting glaciers Global sea level is currently rising as a result of both ocean thermal expansion and glacier melt, with each accounting for about half of the observed sea level rise, and each caused by recent increases in global mean temperature. For the period 1961-2003, the observed sea level rise due to thermal expansion was 0.42 millimeters per year and 0.69 millimeters per year due to total glacier melt (small glaciers, ice caps, ice sheets) (IPCC 2007). Between 1993 and 2003, the contribution to sea level rise increased for both sources to 1.60 millimeters per year and 1.19 millimeters per year respectively (IPCC 2007).

Surface melting doesn't contribute to sea level rise because the water percolates back into the snow and refreezes. But it does reduce the reflectivity of the ice, known as albedo, with consequences for how much sunlight the Arctic region absorbs, and how much ice stays frozen. Together, with soot which also decreases reflectivity, the ice albedo was pushed below a certain threshold in 1889 and 2012, making it vulnerable to rapid ice loss, say the authors. Lead author Kaitlin Keegan explains such big surface melting events won't be out of place by 2100: "With both the frequency of forest fires and warmer temperatures predicted to increase with climate change, widespread melt events are likely to happen much more frequently in the future."

The Greenland ice sheet may be more vulnerable to climate change than previously thought, scientists reported in a Nature Geoscience paper on Sunday. The team of US scientists discovered deep channels extending horizontally below the surface of the Greenland sheet, which mean large parts of the glaciers lie on land that's below sea level. Just like in West Antarctic glaciers, warm water coming into contact with the edge of the glacier forms vast pools under the ice sheet, melting it from the bottom up, the paper explains.