OP article wrote:[...]
So the talk of sea level rise should not be in centuries, it should be decades or perhaps even single years. For 10,000 years, during all of human civilisation sea level remained stable leading us to believe that coastlines remained roughly in the same place. A century ago the sea began to rise one millimetre a year, 20 years ago it had reached two millimetres and this century it has risen to 3 millimetres.
[...]
That's not entirely a correct description, although it is true that there has on average been a sea level rise of millimeters per year, centimeters per decade. Here's first a graph of the past few thousand years, then more recent history where global warming has accelerated the rate of sea level rise:
Ice melting naturally requires an amount of energy transferred into the ice sufficient to melt it, and there are multiple mechanisms by which such can be provided in a given period of time.
One mechanism of energy transfer is solar radiation received directly.
For example, consider if solar radiation absorbed changes by up to X watts per square meter: X joules per second per square meter, which is (31.5 * X) MJ per year. Since the mass of a cubic meter of ice is about 900 kg, with ice having
its heat of fusion of 3.3E5 J/kg, a cubic meter of ice takes 300+ MJ to melt.
As a result, such a change in solar radiation absorbed is a sufficient rate of energy transfer to directly melt up to 0.1 * X meter thickness of ice per year.
As a random example, if part of an ice sheet is 2 kilometers or 2000 meters thick, then X watts per square meter on its top surface is nominally sufficient to melt about all of it in 20000 / X years or a greater period of time. In other words, the preceding theoretically provides enough energy to accomplish that when summed up over such a period of time.
Total average solar irradiance in the high latitude regions tends to be on the order of 100 watts per square meter (varying with location), although much is reflected rather than absorbed by the ice, but it is the net change that would be more relevant here. Such could reach up to multiple percent from global warming.
The preceding was essentially talking about radiative heat transfer. Of course, it is not the only form of heat transfer possible. For example, if a part of an ice sheet does break up and fall into the ocean, then that segment can conduct heat from warmer surrounding water, and the rate of energy transfer into pieces of ice may be much beyond that from sunlight on its surface.
To now get into talking about the real world directly, one can look at the Greenland ice sheet.
The volume of ice in the Greenland ice sheet is
about 3 million cubic kilometers, with about 3 million trillion kilograms mass. Thickness varies, being up to several kilometers in some parts, less thick elsewhere.
A 2006 study concluded that the rate of ice loss in Greenland accelerated compared to 2004, reaching 240 cubic kilometers per year.
That would correspond to a loss rate approaching 0.01% per year. Nominally, it would be approaching 0.1% per decade or 1% melting per century, except extrapolation over too long of a timeframe is not fully valid since the rate can and does change over time.
Here's a 2006 news article on such:
BBC wrote:Greenland melt 'speeding up'
The meltdown of Greenland's ice sheet is speeding up, satellite measurements show.
[...]
Estimated monthly changes in the mass of Greenland's ice sheet suggest it is melting at a rate of about 239 cubic kilometres (57.3 cubic miles) per year.
This figure is about three times higher than an earlier estimate of the mass loss from Greenland made using the first two years of Grace measurements.
[...]
Dr Chen and colleagues partly attribute this to increased melting in the past one-and-a-half years and partly to better processing of the data.
"Acceleration of mass loss over Greenland, if confirmed, would be consistent with proposed increased global warming in recent years," the authors wrote in Science.
This would amount to a contribution to global sea level rise from Greenland of about half a millimetre (0.02 inches) each year [with total current sea level rise being multiple millimeters per year with other sources such as melting Antarctic ice].
From here.
"
- PeZook
