Tuesday, March 22, 2016

Ice Melt, Sea Level Rise and Superstorms: The Threat of Irreparable Harm - by Jim Hansen

A massive boulder on a coastal ridge in North Eleuthera, the Bahamas. A new research paper claims it was likely moved there by powerful storms during the last warm period of Earth history, 120,000 years ago, and warns that such stormy conditions could recur because of human emissions of greenhouse gases. (Credit: Charles Ommanney/The Washington Post, via Getty Images) Click to Enlarge.
A more immediate threat is the likelihood of shutting down the oceans overturning circulations in the North Atlantic and Southern oceans.  That’s where superstorms come in. Let me explain.

We use global climate modeling, paleoclimate data – that’s Earth’s ancient climate history – and modern observations of the ocean and ice sheets to study effects of ice melt on Greenland [2] and the Antarctic ice shelves (tongues of ice extending from Antarctica into the Southern Ocean).[3]

Greenland and Antarctica are beginning to melt because of global warming.  So far it is just a tiny, tiny fraction of the ice sheets that has melted.  However, this fresh meltwater spilling out onto the North Atlantic and into the Southern Ocean already is having important effects. [F16]

We conclude that light freshwater added to upper layers of the ocean is already beginning to shut down North Atlantic Deep Water formation and Antarctic Bottom Water formation.  This will have enormous consequences in future decades, if full shutdown is allowed to occur.
The surface manifestation of slowdown of the deep circulations is cooling in the North Atlantic southeast of Greenland and in the Southern Ocean.  These coolings are prominent in our model by the middle of the 21st century.  However, on multiple grounds, we conclude that the real world responds faster to freshwater than the models do.

First, let’s note that North Atlantic cooling, if the overturning circulation shuts down entirely, will have large effects.  The tropics continue to warm as CO2 increases.  If Greenland freshwater shuts down deepwater formation and cools the North Atlantic several degrees, the increased horizontal temperature gradient will drive superstorms stronger than any in modern times.  All hell would break loose in the North Atlantic and neighboring lands.

[5 1st boulder] Such a situation occurred in the last interglacial period, 118 thousand years ago. The tropics were about 1°C warmer than today because Earth’s spin axis was tilted less than today.  Ocean core data show that deepwater formation shut down, the North Atlantic cooled, and there is evidence of powerful superstorms at about that time, powerful enough for giant waves to toss 1000 ton megaboulders [6] onto the shore in the Bahamas.  Some scientists think these boulders may have been tossed by a tsunami, but we present multiple lines of evidence that the boulders and other geologic features are best explained as the result of superstorms.

An important point is that if we let ice melt from Greenland become large enough to fully shut down the AMOC, the Atlantic overturning circulation, it will be permanent as far as the public is concerned.  It takes several centuries for AMOC () to get moving again.

However, superstorms will not be the most important consequence of global warming, if it continues to grow.  The most important effect will be sea level rise.  Here too, the most complete analysis must account for paleoclimate data, which shows that ice sheets, when they disintegrate, can go quickly, non-linearly, yielding multi-meter sea level rise in a century, even when the climate forcing is weaker than the human-made climate forcing.
More important, for sea level, is what is happening around Antarctica.  Sinking of heavy, salty cold water near the Antarctic coast normally forms Antarctic Bottom Water, thus also bringing relatively warm water to the surface, where it releases heat to the atmosphere and space.

Now, as fresh meltwater from Antarctic ice shelves increases, it tends to put a cold low density lid on the Southern Ocean.  This reduces exchange with the surface, so the heat stays in the ocean, raising the temperature of ocean water at the depth of ice shelves, an amplifying feedback.

In a global perspective, cold freshwater lenses around Antarctica increase the planet’s energy imbalance.  The added energy goes into the ocean where it is available to melt ice shelves.

These feedbacks support our conclusion that melt in response to strong forcing will be nonlinear. These feedbacks, with meltwater driving subsurface warming, also help us understand and gain a consistent picture of rapid nonlinear climate oscillations in the paleoclimate record.

[8] Paleoclimate data makes clear that when ice sheets melt, they can go fast.  However, we do not know the characteristic time for the nonlinear ice sheet response to growing climate forcing.  Eventually ice sheet models may give us an answer, but for now our best guide is observations.

Unfortunately records of growing annual mass loss by the ice sheets are short.  The Greenland data can be fit as well by 10-year or 20-year doubling times, but already Greenland is losing several hundred cubic kilometers of ice per year.  Feedbacks for Greenland, with its surface melt, are different than for Antarctica, but there are several amplifying feedbacks.  Greenland response to global warming will be nonlinear, but likely with a different characteristic doubling time.

[9] Antarctic mass loss is smaller. Most melting so far is ice shelves, which does not show up in gravity satellite measurements of mass change.  However, as ice shelves disappear, the discharge of non-floating ice will accelerate.

If ice sheet mass loss has a 10-year doubling time, meter-scale sea level rise would be reached in about 50 years, and multi-meter sea level rise a decade later. 20-year doubling time would require about 100 years.

The data records are too short.  But if we wait until the real world reveals itself clearly, it may be too late to avoid sea level rise of several meters and loss of all coastal cities.  I doubt that we have passed a point of no return, but frankly we are not certain of that.
The relevance is that I believe we are already witnessing the beginning of this cooling southeast of Greenland and cooling around Antarctica in response to freshwater from ice melt.

Read more at Ice Melt, Sea Level Rise and Superstorms:  The Threat of Irreparable Harm - by Jim Hansen

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