James Hansen worries that “we may be approaching a point of no return, a situation in which our children inherit a climate system undergoing changes that are out of their control, changes that will cause them irreparable harm”. He’s looked at the models, at current observations, and at what happened during the Eemian interglacial 118,000 years ago, and he doesn’t like what he sees.

During the Eemian, when global average temperatures were about 1°C more than now, sea level was about 3-4 metres higher than now for a considerable time. Then about 118,000 years ago, towards the end of the interglacial, it peaked at 6-9 meters, including a rise of 2-3 metres within several decades. A similar sea level rise of several metres now would see the inundation of many of the world’s major cities.

Also there were huge storms at mid-latitudes in the North Atlantic which would make Superstorm Sandy look mild. Hansen thinks that climate change may be entering a phase where similar events could occur this century.

As background, in my post Scoping long-term sea level rise I cited IPCC5 WGI Chapter 13 as giving the following breakup of the contributing factors to SLR:

Thermal expansion – 1.1mm

Glaciers except Greenland and Antarctica – 0.76mm

Greenland – 0.33mm

Antarctica – 0.27mm

Land water storage – 0.38mm

Total – 2.8mm

Observed SLR – 3.2mm

Residual – 0.4mm

Concerning the residual, more recent research indicates that thermal expansion may have been underestimated.

Greenland and Antarctica combined only contribute 0.6mm pa to sea level rise, a minor contribution. Yet we know that sooner or later ice sheet decay is going to become the main driver. Hansen thinks that may be starting to happen now, with cool fresh meltwater spilling into the ocean from Greenland and Antarctica, disrupting the huge ocean currents, the Atlantic Meridional Overturning Circulation (AMOC) and the Southern Ocean Meridional Overturning Circulation (SMOC).

At this point I should mention that Hansen’s paper has 18 very distinguished joint authors. This site has an abbreviated version of the ‘Ice Melt Paper’, plus links to the full version, an explanatory video and three newspaper articles. Elsewhere there’s a transcript of the video, a post at Open Mind with a long discussion thread, and a post by Graham Readfearn.

Hansen et al show a map of the January-February 2016 global temperature pattern, which is very similar to this NASA map for March via Open Mind, which I’ve used as it highlights the important features more clearly:

The important features are the cool patch in the North Atlantic, the warming immediately below and near the east coast of the USA (that water was 3°C warmer than normal during Hurricane Sandy) and the ocean cooling near Antarctica, which possibly has even greater potential for sea level rise.

Hansen et al looked at the models, which they say are lethargic compared to real world observations and assume more deep water mixing than actually takes place, so they looked to paleoclimate evidence to see what might happen. While the Eemian is not an analogue for future climate change, it has clues which contribute to what may actually happen in the the pattern of ice sheet decay.

Their conclusions are best summarised in the abstract of the main paper:

The modeling, paleoclimate evidence, and on-going observations together imply that 2°C global warming above the pre-industrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) non-linearly growing sea level rise, reaching several meters over a timescale of 50–150 years.

These predictions differ fundamentally from existing climate change assessments, for example they suggest “markedly reduced warming or even cooling in Europe”.

This image shows the global temperature pattern predicted in 2055-2060 with one metre of ice melt:

The overall pattern looks remarkably like what we’ve been getting recently. Noteworthy is that the Arctic is still seen as warming, contra the discussion at Open Mind.

The basic physical mechanism is that cool water just above freezing point pools near the surface, as it is less dense than saltwater, so it doesn’t sink. However, global warming continues, with the excess heat taken up elsewhere, not least in the ocean. The paper goes into feedback mechanisms whereby ice sheet melting is speeded up.

They think ice sheet melting may on average double every 10, 20 or 40 years, and have developed scenarios to illustrate what may happen. The following graph models the global temperature rise with 1 metre and 3.8 metres of melt water:

If the Atlantic Meridional Overturning Circulation (AMOC) stops, Hansen says All Hell Will Break Loose with huge storms at mid-latitudes and sea level could take off to the extent that we may have several metres this century.

It would take centuries for AMOC to restart, whereas SMOC could crank up again after decades.

Hansen and company are not certain they are right so they discuss observations and modelling studies needed to refute or clarify their assertions. Hansen thinks we haven’t reached a point of no return, but the future looks complicated, non-linear, uncertain and with considerable risk. The possibilities are alarming. Hansen says “we have a global emergency. Fossil fuel CO2 emissions should be reduced as rapidly as practical.”

Out of interest, this graph from the paper shows trends in sea level rise:

This graph shows the typical climate response time of most climate models:

The graph shows the percentage response if a quantity of CO2 is dumped into the atmosphere. About half the eventual response comes in the first 20 years, but only 60% in the first hundred. Hansen et al think that 75% may be more accurate.

However, the main problem is that most models used by IPCC simply exclude ice melt, and most models don’t deal with added freshwater in a way that reflects the real world. Hansen and his colleagues have had a go at rectifying these deficiencies.