(NASA satellite shot of Antarctica on October 13 of 2014. Recent scientific papers point toward a vicious cycle of Antarctic glacial melt. Expanding sea ice results from increased cold, fresh water outflows from melting land-anchored glaciers spreading out along the ocean surface and protecting the floating ice. Meanwhile, rapidly warming waters concentrate in a layer beneath the ice to further accelerate melting of the giant glaciers’ bases. Image source: LANCE-MODIS.)
With fewer and fewer logical straws to grasp for plausibly denying an obvious and inexorable warming of the global climate system, climate change deniers have resorted to pointing toward an expanding veil of sea ice near Antarctica as ‘proof positive’ that global warming really isn’t happening.
But recent scientific papers reveal that what may well appear to be a soothing light at the end of an imaginary cooling tunnel is more a freight train of global heat aimed directly at the ice sheets’ weak underbellies. For the last time the cool, fresh waters of an initial Antarctic melt expanded out along the surface, likely temporarily enhancing the range of sea ice as well, below-surface warmth ran beneath the ice and rapidly melted sea-fronting glaciers, leading to a sea level rise of about 14 feet in just one century.
In essence, the expanding skein of ice and fresh water concentrated warmth where it was needed least — at the bases of massive glaciers submerged in hundreds of feet of warming water. The heat melted the glacier from the bottom up, floated the glaciers and then flooding further inland beneath the ice to do still more damage.
And it is the start of just this process that we are witnessing now. How fast it proceeds will be critical to the rate of sea level rise going forward.
As for all that extra sea ice? Well, that’s merely the last gasp of coolness running along the surface waters — sent out by the dying glaciers.
Current Sea Level Rise Unprecedented in 6,000 Years
(Past and future sea level rise as shown in this IPCC AR5 WG1 graphic. Note the steady rate of sea level increase beginning at around 1880 and continuing on through the 21st Century. Also note the recent uptick in observed sea level rise together with end 21st Century projections by the IPCC. It is also worth noting that many still consider the IPCC projections to be a bit too conservative, especially when considering business as usual projections of 3-5 C or greater warming by the end of this century. Image source: IPCC.)
It is likely that we are just in the first stages of such a catastrophic process of ice sheet decline. A process that will last for centuries, but one that is already having a profound impact on the world’s oceans and coastlines.
For a new study, published yesterday in the Proceedings of the National Academy of Sciences, has found that sea level rise over the past century is the fastest it has been since the end of the last ice age — when vast surges of water erupted from the melting glaciers.
The study, which compiled over 1,000 measurements of sea level over the past 35,000 years from sediment samples, found that at no time during the most stable period of the Holocene have seas ever risen so fast as they are now rising. This 6,000 year period saw no increase or decrease in sea level exceeding 15-20 centimeters over 200 year time-frames. But during the 100 years from 1900 to 2000, seas rose by 20 centimeters, more than doubling highest rates of variance during the last 6,000 years.
Increasing Heat Melts Glaciers, Swells Seas
The increases to sea level are a result of added ocean and atmospheric heat. A warming pushed ever-higher by a rapidly expanding heat-trapping gas emission.
Such direct heating of the ocean causes water to thermally expand. The added atmospheric and ocean heat also goes to work melting glaciers at the surface and where the glaciers contact the warming seas. These glaciers, in turn, add great volumes of water to the world’s oceans. The upshot of a 0.6 degree Celsius warming of the atmosphere and near surface world ocean during the 20th Century.
But both this heating and melt were just the start. For the atmospheric warming hit 0.8 C by the early second decade of the 21st Century even as top 700 meter ocean heat content spiked to unexpectedly high levels. Meanwhile, forecast rates for rising seas and temperatures are even more extreme for the coming years and decades.
(Tell-tale 100 km long plumes of sediment carried out from beneath Greenland’s glaciers by floods of melt water as seen in this NASA satellite shot from September of 2014. Surface melt from Greenland tunnels through the ice sheet base. Once there, it flows from beneath the ice sheet and out into the oceans — carrying with it loads of sediment flushed from beneath the glacier. Melt from both the Greenland and West Antarctic ice sheets has greatly increased during recent years. Image source: Earth Observatory.)
Current Rate of Sea Level Rise More than 60% Faster Than 20th Century Mean
If the average rate of sea level rise was 2 mm per year during the 20th Century, the past two decades have witnessed a more than 60% rate of increase over even that unprecedented rate. For current sea level rise measures show a 3.27 mm per year increase.
Most scientists expect an ever more extreme rate of atmospheric warming over the 21st Century to ramp this already rapid rate higher — with annual increases likely to exceed 1 cm before 2100 arrives. Such rates would push end 21st Century sea level rise well into the end ice age range of 1.2 meters every 100 years — with chances for even greater rates of increase going forward.
The IPCC has identified a likely sea level rise in the range of 2-3 feet by the end of this Century (60-100 cm), with many outside analysts identifying a range between 2-9 feet (60-300 cm) as possible given the potential for 3-5 C warming under business as usual fossil fuel emissions (Researchers at the Neils Bohr Institute recently established a range from 2-6 feet but note that sea level rises of 80 cm [2.5 feet] are most likely this century and increases of greater than 6 feet have a probability of less than 5% through 2100).
(Current rate of sea level rise as measured by AVISO. Note the 3.27 mm per year rise that has been ongoing since 1992 with an increasing flux beginning around 2010. Image source: AVISO.)
For context, current global CO2e atmospheric heat forcing is in the range of 481 ppm CO2e. The last time CO2 equivalent heat forcing hit such levels, millions of years ago, oceans were 75-120 feet higher and temperatures were about 3.6 C warmer than they are today.
It is also worth noting that it took 10,000 years for the Earth to warm by about 5 C at the end of the last ice age. Current and expected human greenhouse gas emissions (without a rapid transition to renewable energy sources and zero carbon civilizations) could well achieve a similar level of warming in just 180 to 270 years (1880 to 2150) — a pace more than 30 times faster than what was witnessed then.