Tech & Science Global sea level rise rate speeding up, 25 years of satellite data confirms

05:30  13 february  2018
05:30  13 february  2018 Source:   ABC News

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The study found that early satellite data had exaggerated the rate of sea level rise in the 1990s, masking the recent acceleration. A U.N. panel of climate scientists said in 2014 that sea levels could rise by up to about a meter by 2100.

A sea level rise is an increase in global mean sea level as a result of an increase in the volume of water in the world’s oceans. Sea level rise is usually attributed to global climate change by thermal expansion of the water in the oceans and by melting of ice sheets and glaciers on land.

Satellite data indicates sea levels will rise around 61 cm between now and 2100.© Pexel: Sebastian Voortman/ABC News Satellite data indicates sea levels will rise around 61 cm between now and 2100. The rate of global sea level rise is accelerating as ice sheets in Antarctica and Greenland melt, an analysis of the first 25 years of satellite data confirms.

The study, by US scientists, has calculated the rate of global mean sea level rise is not just going up at a steady rate of 3mm a year, but has been increasing by an additional 0.08mm a year, every year since 1993.

If the rate of change continues at this pace, global mean sea levels will rise 61 centimetres between now and 2100, they report today in the journal Proceedings of the National Academy of Sciences.

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The rise in global sea levels has accelerated since the 1990s amid rising temperatures, according to a new study. The annual rate of sea level rise has increased by 50 per cent to 3.3 millimetres each year in 2014 from 2.2 millimetres each year in 1993.

However, the annual rate of rise over the past 20 years has been 0.13 inches (3.2 millimeters) a year , roughly twice the average speed of the preceding 80 years . The rise in sea levels is linked to three primary factors, all induced by this ongoing global climate change

"That's basically double the amount you would get if you only had 3 mm a year with no acceleration," said the study's lead author Steven Nerem of the University of Colorado.

But that figure, which is broadly in line with climate modelling, is likely to be a conservative estimate of global mean sea level rise in the future, said Professor Nerem.

"When you try to extrapolate numbers like this you're assuming sea level change and acceleration are going to be the same as they've been over the past 25 years.

"But that's probably not going to be the case.

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This latest NOAA data shows unchanging linear trends in the rate of sea level rise worldwide with many of these records including 100 year and longer measurement duration periods. The UN IPCC AR5 WG1 report claims that

Another study that month found “ sea level rising 60% faster than projected.” Here is the plot of the NASA temperature data : “Since 1975, global average surface air temperature has increased at a rate of 0.17 deg.C/decade,” Tamino notes.

"We're seeing changes in Greenland and Antarctica that are almost certainly going to be bigger than that in the future," he said.

Putting a number on sea level rise

Global warming drives sea level rises in two ways: by melting land-based ice sheets, and heating up ocean water causing it to expand.

Sea levels have been recorded by a series of four satellites, starting with the 1992 launch of the TOPEX/Poseidon satellite, in addition to long-term data captured by tidal gauges.

The Jason-3 satellite is the most recent in a series of four satellites to measure sea levels since 1993.© Supplied: NASA/ABC News The Jason-3 satellite is the most recent in a series of four satellites to measure sea levels since 1993. Professor Nerem said analysis of tidal gauge records and decadal changes in satellite data in the past had indicated global mean sea level rise was accelerating, but it had been hard to pin down a number.

"We always felt that there was an acceleration, but it's very small and it's difficult to detect," he said.

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The study, published by Nature Climate Change, used refined satellite estimates to show global sea level rise has increased from 2.2 millimetres each year in 1993, to 3.3 millimetres each year in 2014.

Near- global ocean temperature data sets made available in recent years allow a direct calculation of thermal expansion. It is believed that on average, over the period from 1961 to 2003, thermal expansion contributed about one-quarter of the observed sea level rise

To arrive at their number, Professor Nerem and colleagues adjusted the satellite data for short-term factors such as the El Niño/La Niña climate patterns, as well as the 1991 eruption of Mount Pinatubo, which caused sea levels to drop just before the launch of the TOPEX satellite.

They also cross-referenced tide gauge and satellite data to correct anomalies in the TOPEX satellite record proposed in an earlier research co-authored by John Church of the Climate Change Research Centre at the University of New South Wales (UNSW).

"This is the first satellite-based estimate of an acceleration number," Professor Nerem said.

"The number is useful because you can take the rate of sea level change and the acceleration, and extrapolate it in the future and see how it agrees with climate models."

The figure calculated by Professor Nerem's study is similar to those predicted by the Intergovernmental Panel on Climate Charge (IPCC) under its upper 8.5 scenario, which assumes increasing greenhouse gas emissions.

UNSW's Professor Church is the co-convening lead author of chapters on sea level rise in the most recent IPCC assessment report.

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The annual rate of sea level rise increased to 3.3 millimeters (0.13 inch) in 2014 – a rate of 33 centimeters (13 inches) if kept unchanged for a Fire In The Sky: Major Correction To Satellite Data Shows 140% Faster Atmospheric Heating Since 1998. Air Temps Rising Faster Than Surface Temps.

He said the length of the new study and the correction of the satellite data was important.

"This is very solid confirmation … there is an acceleration and it's the right magnitude to be consistent with IPCC, Professor Church said.

Ice sheet melting is a real concern

Professor Nerem's team also looked at data from the Gravity Recovery and Climate Experiment (GRACE) satellite, which monitored changes in Earth's gravitational field, to determine where the figure was coming from.

They found the bulk of the acceleration was caused by the melting of the Greenland and Antarctic ice sheets, which contributed 0.02mm and 0.03mm a year, every year, to the overall acceleration rate.

Professor Church said the Antarctic was contributing more to acceleration than previous estimates.

"Ice sheets have huge amounts of equivalent sea level stored in them," Professor Church said.

"I think that's a real concern. The ice sheets are contributing measurably to this acceleration," he said.

Aerial image of rivers and meltwater lakes on the Greenland ice sheet.© Supplied: Maria-Jose Vinas/ABC News Aerial image of rivers and meltwater lakes on the Greenland ice sheet. Professor Nerem said the next step was to continue looking at the satellite data to get a longer-term picture.

"This was a first detection in the satellite altimeter record, so we just barely have enough time series to feel comfortable publishing an estimate of acceleration," Professor Nerem said.

"In addition, we'll certainly be watching our data to see if there are any rapid changes in the ice sheets that might be detected."

One of the important tools that will enable them to do that will be the launch of a new GRACE satellite in April.

"That allows us to directly observe the ice sheets," he said.

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