Continued high fossil fuel emissions this century are predicted to yield … nonlinearly growing sea level rise, reaching several meters over a timescale of 50–150 years. — Statement from a new scientific study led by Dr James Hansen entitled Ice Melt, Sea Level Rise, and Superstorms.
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This week, Dr James Hansen and colleagues published one hell of a groundbreaking bit of scientific research. It’s a multi-disciplinary study incorporating the work of 19 top climate scientists, glaciologists, paleoclimatologists, and other Earth Systems researchers. Scientists from NASA, GEOMAR, JPL, and other top research agencies including recognized names like Dr Eric Rignot and Dr Makiko Sato all appear on the contributors list.
(Rates of sea level rise since 1900 and associated with a 1.1 C jump in global temperatures have already shown a non-linear progression. Ice Melt, Sea Level Rise, and Superstorms attempts to pin down just how fast glacial melt rates will increase over the coming decades.)
The paper covers three topics related to the rapid accumulation of fossil fuel driven greenhouse gasses in the atmosphere and related rapid warming — Ice Melt, Sea Level Rise, and Superstorms. In other words, the paper looks into what will likely be the initiation of a Heinrich Event during the 21st Century so long as high levels of human greenhouse gas emissions continue.
A Heinrich Event for the 21st Century
For those not familiar with a Heinrich Event — it’s one of those disastrous climate change related incidents that you really don’t want to see emerge. One that drives rapid sea level rise, wrenching climate dislocations, and is likely also a trigger for regional and possibly hemispheric superstorms. Something that’s occurred numerous times in the geological past when the great Greenland and West Antarctic ice sheets warmed enough to disgorge armadas of ice bergs into the North Atlantic and/or Southern Ocean. The kind of thing that scientist Steve Pacala called a Climate Monster in the Closet. And Dr. James Hansen and colleagues’ new study is the first of its kind to scientifically explore the potential occurrence of just such a freak and dangerous event during the 21st Century.
Because the paper covers such a broad range of topics related to Heinrich Events, I’ve decided to write a two-blog post covering it. This post will focus on the ice melt and sea level rise issues. The superstorm-generating aspect of Heinrich Events — which Dr Hansen and colleagues found was capable of producing waves powerful enough to pluck 1,000 ton boulders from the sea floor and deposit them upon hillsides in the Bahamas 130 feet above sea level 115,000 years ago — is something we’ll cover in a second related post over the next few days.
Warm Ocean Waters Attacking Weak Glacial Underbellies
The chief driver of Heinrich Events is spiking rates of glacial melt issuing from the Greenland and West Antarctic ice sheets and related outflow of ice bergs and fresh water into the North Atlantic or the Southern Ocean. Hansen and colleagues’ paper builds on recent work by Eric Rignot and others who’ve found that the contact of warming ocean waters with the submerged sea faces of glacial cliffs and undersides of floating ice shelves is a primary driver for melt and ice berg release during periods of local and global temperature increase.
(Illustration from Ice Melt, Sea Level Rise, and Superstorms shows how ocean stratification acts as an amplifying feedback to glacial melt. Cool, fresh surface waters generated by the initial ice release set up a kind of ocean heat conveyor belt that delivers more and more warm water to the submerged underbellies of the great ice sheets. In Greenland, prograde beds limit the amount of ice that can be released in sudden events. In Antarctica, retrograde beds below sea level set up a situation where the amplifying melt feedback is further enhanced.)
Grounding glaciers and ice shelves are, at first, weakened by slow but ramping melt rates. Eventually, the glaciers and shelves collapse due to the weakening process of melt which leads to a surge of previously buttressed ice sliding out into the oceans. As more fresh melt water expands over the ocean surface, it traps heat into deeper layers of the water column near the submerged glacial faces. So initial melt produces an amplifying feedback that delivers more ocean heat to the ice and, in turn, results in more ice rushing out into the North Atlantic or the Southern Ocean.
Exponential Rates of Glacial Melt and Sea Level Rise
It is this mechanism that Hansen and colleagues fear will come into play over the course of the 21st Century. Their paper identifies a risk that such a mechanism could set up 5, 10, or 20 year melt doubling times for Greenland, West Antarctica or both this Century. A new perspective from some of the world’s top scientists that assumes the risk of non linear melt is high enough to present a major concern. As an example, under a 10 year doubling time, the current approximate 3 mm per year sea level rise would double to 6 mm per year by 2026, 12 mm per year by 2036, 2.4 cm per year by 2046, and nearly 5 cm per year by 2056.
Doubling times in non linear events often don’t fit a pure exponential curve — instead tending to follow a series of spikes and recessions with major transitional events coming at the end of any ‘curve.’ But Hansen’s particular perspective is useful given the fact that current rates of sea level rise do not appear to be following a linear pattern and due to the fact that the mechanism for large, Heinrich Event type glacial melt spikes is becoming more supported in the observational science.
(It’s still early days for Greenland and Antarctic melt. However, current trend lines do point toward a potential for multi-meter sea level rise this Century. Image source: Ice Melt, Sea Level Rise, and Superstorms.)
Early measures of Greenland and Antarctica ice mass loss imply 8-19 year melt doubling times for Greenland and 5-10 year melt doubling times for Antarctica. For reference, if both these ice systems continued to double mass loss on a roughly 10 year basis, total sea level rise by the 2090s would equal 5 meters or 16.4 feet. By contrast, a 5 year doubling time would result in 5 meters of sea level rise by the late 2050s and a 20 year doubling time would result in nearly a meter of sea level rise by the end of this Century and 5 meters worth of sea level rise by 2160.
Hansen notes that these are still early days and it is unlikely that ice sheet response trends have become clear at this stage. However, initial trend lines, though likely to be less accurate, appear to pose some cause for concern. In addition, Hansen points out that rates of sea level rise are less likely to be constrained by ice sheet inertia during periods when global temperatures are rapidly rising. Projected rates of global temperature increase in the range of 1-5 C this Century is on the order 20-100 times faster than during the end of the last ice age — at the upper end covering all of the 10,000 years worth of ice age warming in just one Century. And Hansen notes that this potentially extreme rate of temperature increase poses a much greater risk of rapid glacial destabilization than is indicated by current IPCC glacial melt models.
Hansen’s research also points to the likelihood that rapid glacial melt would temporarily put a break on rates of global atmospheric warming by cooling local ocean surfaces and increasing the rate of heat transfer into middle ocean layers. And it’s this energy flip-flop and related heightened imbalance that provides a pretty severe potential storm set-up as rates of glacial melt ramp up.
Links:
Ice Melt, Sea Level Rise, and Superstorms
Climate Guru James Hansen Warns of Much Worse Than Expected Sea Level Rise
Climate Monsters We Want to Keep in the Closet
Melting in West Antarctica Could Raise Seas By 3 Meters
Hat Tip to DT Lange
Hat Tip to Colorado Bob
Hat Tip to TodaysGuestIs
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