Climate Denial Crock of the Week

Above, my video from a year ago on accelerating mass loss in West Antarctica.  Below, evidence that that melt is showing up in sea level rise.

Washington Post:

The new research published Monday in the journal Nature Climate Change shows that the rate of sea-level rise appears to have accelerated over the past 15 years, a period in which scientists elsewhere documented a surprisingly rapidly retreat of some of Earth’s great ice masses, from Greenland to West Antarctica.

The findings appear to contradict earlier studies suggesting that the rate of sea-level rise had actually slowed slightly in recent years.

Australian scientists detected the increase in a study that analyzed decades of records from tidal gauges around the world, together with satellite data that show changes in water levels as well as subtle shifts in land formations.

Using these more precise measurements, the researchers discovered that scientists had slightly overstated sea-level rise that occurred in the 1990s, and underestimated the rate of increase since 1999, said Christopher Watson, a University of Tasmania geodesist who co-authored the study along with colleagues from the university and from Australia’s national science agency, the Commonwealth Scientific and Industrial Research Organization.

Science:

In the past, researchers have used tide gauges to keep tabs on the performance of satellite altimeters, which use radar to measure the height of the sea surface. The comparison allowed them to sniff out and cope with any issues that cropped up with the satellite sensors. Tide gauges themselves are not immune to problems, however; the land on which they rest can shift during earthquakes, or subside because of groundwater withdrawal or sediment settling. These processes can produce apparent changes in sea level that have nothing to do with the oceans.

So Watson’s team tried to correct for the rise and fall of tide gauge sites by using nearby global positioning satellite (GPS) stations, which measure land motions. If no GPS stations were present, they used computer models to estimate known changes, such as how some regions continue to rebound from the last glaciation, when heavy ice sheets caused land to sink.

The newly recalibrated numbers show that the earliest part of the satellite record, collected between 1993 and 1999 by the first altimetry mission, known as TOPEX/Poseidon, appears to have overstated sea level rise. That’s probably because a sensor deteriorated, ultimately forcing engineers to turn on a backup instrument. When combined with data from subsequent satellite missions, those inflated TOPEX/Poseidon numbers gave the appearance that sea level rise was decelerating, even as the global climate warmed.

Also contributing to the apparent slowdown was a hiccup caused by natural climate variation, says John Church, a climate scientist at CSIRO in Hobart, Australia, and a co-author of the new study. Around 2011, “there was a major dip in sea level associated with major flooding events in Australia and elsewhere,” he says. Intense rainfall transferred water from the oceans to the continents, temporarily overriding the long-term sea level trend.

The corrected record now shows that sea level rose 2.6 millimeters to 2.9 millimeters per year since 1993, compared to prior estimates of 3.2 millimeters per year. Despite the slower rates, the study found that sea level rise accelerated by an additional 0.04 millimeters per year, although the acceleration is not statistically significant. Watson says he expects that trend to grow stronger as researchers collect more data.

The acceleration falls in line with predictions from the Intergovernmental Panel on Climate Change (IPCC), Watson notes. “We’re tracking at that upper bound” of the IPCC’s business-as-usual scenario for greenhouse gas emissions, he says, which could bring up to one meter of sea level rise by 2100.

Where’s the acceleration coming from?  Antarctica is waking up…

John Abraham in The Guardian:

The new paper provides an improvement to the resolution of GRACE.(satellite mass measurements) They find that the western part of Antarctica is losing mass at 121 billion tons (gigatons) a year. This rate has increased recently. In particular, in one region (the Amundsen Sea coast, the ice loss has doubled in the past six years). In the east, there is a small mass gain (approximately 30 gigatons a year). This mass gain partially offsets what is happening in the west but there is still a large loss of water to the sea each year.

I asked the lead author about this study and he told me,

It is very important that we continue long term monitoring of how mass changes in ice sheets. For West Antarctica in particular this is important because of how it is thought to be more unstable, where the feedbacks can cause more and more ice loss from the land over time. These strong regional accelerations that we see are very robustly measured and imply that Antarctica may become a major contributor to sea level rise in the near future. This increase in the mass loss rate, in ten years, accelerations like that show that things are beginning to change on human time scales.

The conclusions of the authors fits well with a study that I published with Ted Scambos late last year. In that paper, we discussed the ice loss from Antarctica. Our particular focus was on the rising ocean levels. What we showed by reviewing the literature was that as Antarctica’s mass decreases, changes to the gravity field will actually cause sea level rise to be greater than expected in the Northern Hemisphere.