The BBC and many other MSM outlets have recently seized on recent measurements concerning ice loss in Antarctica
with the headings “Antarctica’s ice melt has accelerated” and “Antarctica loses three trillion tonnes of ice in 25 years”.
The actual loss over 25 years is 2.7 billion tonnes.
The BBC quote changing losses over time:
In West Antarctica, which is dominated by those marine-terminating glaciers, the assessed losses have climbed from 53 billion to 159 billion tonnes per year over the full period from 1992 to 2017.
On the Antarctic Peninsula, the finger of land that points up to South America, the losses have risen from seven billion to 33 billion tonnes annually. This is largely, say scientists, because the floating ice platforms sitting in front of some glaciers have collapsed, allowing the ice behind to flow faster.
East Antarctica, the greater part of the continent, is the only region to have shown some growth. Much of this region essentially sits out of the ocean and collects its snows over time and is not subject to the same melting forces seen elsewhere. But the gains are likely quite small, running at about five billion tonnes per year.
suggesting losses of 60 billion tonnes rising to 192 billion tonnes while Science News suggest the losses were even greater:
Before 2012, the continent shed ice at a rate of 76 billion tons each year on average, but from 2012 to 2017, the rate increased to 219 billion tons annually.
Although the amount of ice loss sounds scary when quoted in trillion of tonnes, this has been over 25 years and translates to a sea level rise in that time of 7.6 mm ie 0.3mm per year. Globally sea level is rising at a rate of around 1.8mm a year (tide gauges) or 3mm per year (satellite measurements).
However the calculation to arrive at a total of 7.6 mm of sea level rise is more than a little disingenuous and ignores a key factor.
This figure can easily be derived.
The volume of water released is equivalent to mass as 1 tonne of water = 1 m3 of water. However ice in ice sheets has a density of around 0.9167 therefore the mass of ice must be multiplied by 1.090869 to find the equivalent volume of water.
Sea level rise (mm) = (Volume of water (km3)/Surface area of oceans (km2))*10^3
In practice 2.7 trillion tonnes of ice should produce a global rise in sea level of 8.2 mm.
However the loss of ice from Antarctica does not just result from calving from ice shelves and glaciers or the melting of the base of ice shelves or subglacial ice by warmer ocean water, or more likely geothermal heat from the western rift, but also sublimation. Figures on sublimation are hard to obtain but it is suggested that 10 – 15% of the annual precipitation is lost this way
This means that the mass of ice leading to sea level rise must be reduced by a certain proportion.
Annual precipitation over Antarctica is 166 mm per year, which would generate 2.13E+12 tonnes of ice per year if precipitation is water equivalent or 2.32E+11 tonnes of ice per year if precipitation is depth of snowfall. 12.5% loss of this would reduce the contribution to the oceans by 2.56E+11 tonnes per year or 2.79E+10 tonnes per year. However this is too generalised calculation and more reasonable figures are suggested by a NASA study published in Journal of Glaciology in October 2015
suggested that the Antarctic ice sheets added an average of 112 billion tons of ice per year from 1992-2001, and 82 billion tons per year from 2003-2008. 12.5% of this suggest sublimation losses of 1.40E+10
tonnes per year up to 2002 and 1.03E+10 tonnes per year thereafter.
Surface measurements of sublimation show a very complex spatial picture, depending on topography, weather and surface conditions but suggest figures in the order of 0.1 to 0.19 cm. This would translate to losses of 2.03E+10 tonnes per year.
Taking these figures into account the apparent contribution of Antarctica to global sea level rise over the measured period would appear to be as follows:
|Year||Ice Loss (tonnes)||Sublimation (tonnes)||Contribution to oceans (tonnes)||Global Sea Level Rise (mm)||Cumulative (mm)|
However the key factor that is forgotten in all this headlining is that Antarctica is not just releasing water from its margins to the oceans, it is also receiving and storing water from the oceans in the form of snow and ice, which will have the effect of reducing sea level. To fully understand the impact of Antarctica on global sea levels both elements of the hydrological cycle, abstraction of water stored as ice leading to sea level fall and release of water from the ice leading to sea level rise must be accounted for.
The overall impact on sea level is therefore:
|Year||Ice gain (tonnes)||Ice Loss (tonnes)||Sublimation (tonnes)||Global Sea Level Fall (mm)||Global Sea Level Rise (mm)||Nett Sea Level Change (mm)||Cumulative|
So only in 2017 did Antarctica make any positive contribution to Global Sea level rise, assuming that all the models and measurements and analysis used to provide these data are correct.
As so often the headlines and the MSM take on the information is significantly wrong and is clearly intended to bolster the faith of those who fervently believe in AGW.