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How to make sense of ‘alarming’ sea level forecasts

Sea level rise is one of the biggest worries of climate change. This image is from the Witness King Tides project, which aims to visualise sea level rise using large tides and storm surges. Witness King Tides/Flickr, CC BY-NC-SA

How to make sense of ‘alarming’ sea level forecasts

Sea level rise is one of the biggest worries of climate change. This image is from the Witness King Tides project, which aims to visualise sea level rise using large tides and storm surges. Witness King Tides/Flickr, CC BY-NC-SA

You may have read recent reports about huge changes in sea level, inspired by new research from James Hansen, NASA’s former Chief Climate Scientist, at Columbia University. Sea level rise represents one of the most worrying aspects of global warming, potentially displacing millions of people along coasts, low river valleys, deltas and islands.

The Intergovernmental Panel on Climate Change, the UN’s scientific climate body, forecasts rises of approximately 40 to 60 cm by 2100. But other studies have found much greater rises are likely.

Hansen and 16 co-authors found that with warming of 2C sea levels could rise by several metres. Hansen’s study was published in the open-access journal Atmospheric Chemistry and Physics Discussion, and has not as yet been peer-reviewed. It received much media coverage for its “alarmist” findings.

So how should we make sense of these dire forecasts?

What we’re pretty sure about

According to the IPCC sea level rise has accelerated from 0.05 cm each year during 1700-1900 to 0.32 cm each year during 1993-2010. Over the next century the IPCC expects an average rise of 0.2 to 0.8 cm each year.

Observed and projected sea level rise. IPCC AR5
Sea level rise has accelerated. IPCC AR5

The collapse of the West Antarctic ice sheet would add several tens of centimetres to the total.

The IPCC report adds that “it is very likely that there will be a significant increase in the occurrence of future sea level extremes” and “it is virtually certain that global mean sea level rise will continue for many centuries beyond 2100, with the amount of rise dependent on future emissions”.

Looking to the past

The IPCC estimates stand in sharp contrast to projections made by some climate scientists, in particular James Hansen who pointed out in 2007 and in his and his colleagues’ latest study of the effects of ocean warming on the ice sheets.

The IPCC reports did not take into account rates of dynamic ice sheet breakdown, despite satellite gravity measurements reported in the peer-reviewed literature by other scientists.

In Greenland, ice loss reached around 280 gigatonnes of ice each year during 2003-2013, whereas in Antarctica the loss reached around 180 gigatonnes of ice each year during the same period. Both ice sheets appear to be undrgoing accelerated rates of ice melt, as shown in the diagrams.

Melting of the Greenland ice sheet recorded by satellites. GRACE
Melting of the West Antarctic ice sheet recorded by satellites. GRACE

Hansen and his 16 colleagues reach their conclusion by looking at both the present and the past. During the Eemian interglacial, a period between ice ages around 130,000–115,000 years ago, average global temperatures were around 1C warmer than temperatures before the industrial revolution – that is, similar to today’s temperatures. In Greenland temperatures were about 8C warmer (the rise in polar temperatures is generally higher than the rise in tropical and subtropical temperatures, due to the ice-water albedo contrast effect). This led to sea level rise of around 6-7 metres, to a large extent due to melting of the Antarctic ice sheet.

The study points out that during the Eemian contact between the warming ocean and ice sheets led to abrupt disintegration of the ice, raising sea levels by several metres over period of 50-200 years, an extreme rate exceeding current IPCC estimates. The concern is that similar high rates of warming and of sea level rise may pertain in future.

For these reasons Hansen’s group considers sea level could reach several meters toward the end of the century.

These authors state: “We conclude that 2C global warming above the pre-industrial level, which would spur more ice shelf melt, is highly dangerous. Earth’s energy imbalance, which must be eliminated to stabilize climate, provides a crucial metric.”

Earth with a sea level rise of six meters. NASA

Criticisms of the study

Extensive criticism of this conclusion followed. Kevin Trenberth, of the US National Centre for Atmospheric Research, stated “there are way too many assumptions and extrapolations for anything here to be taken seriously other than to promote further studies.”

Greg Holland, also from the US National Centre for Atmospheric Research, stated: “There is no doubt that the sea level rise, within the IPCC, is a very conservative number, so the truth lies somewhere between IPCC and [James Hansen].”

Michael Mann stated Hansen’s estimates are prone to a very large “extrapolation error”.

Media comments range from positive to derogatory. However, few comments respond in detail to the comprehensive analysis by the authors of Hansen’s 2015 paper.

Could it be worse?

The consequences of advanced ice melt include the increased discharge of icebergs from a disintegrating ice sheet, as occurred in the past during stadial phases of interglacial periods. Stadials are sharp cooling phases following peak temperatures, caused by the discharge of cold melt water into the ocean. Such discharges constitute a negative feedback, namely cooling.

Past stadial phases, in the wake of peak temperatures, included the Younger dryas (12,900 – 11,700 years-ago) and melting of the Laurentian ice sheet 8,500 years-ago.

A stadial freeze, predicted due to a collapse of the North Atlantic Thermohaline Current would follow in the wake of large-scale melting and discharge of large parts of the Greenland ice sheet. With further rise in atmospheric CO2 this would constitute a transient stage in global warming.

Warming of 2-4C implies a rise in sea level by several to many metres. Future sea level rise, once it reaches equilibrium with temperature rise of about 2C above pre-industrial temperature, could reach levels on the scale of the Pliocene (pre-2.6 million years ago) around 25+/-12 metres. Temperature rise of 4C higher than pre-industrial would be consistent with peak Miocene (about 16 million years ago) equilibrium sea levels of about 40 meters.

We don’t know how long it would take for seas to rise that high with rising temperatures. However the extreme rise rate of atmospheric greenhouse gas concentrations, higher than 2 ppm CO2 per year, if continues, threatens an accelerating rate of sea level rise.

If so, it follows human civilisation has now begun to preside over a major change to the map of planet Earth.