Among the most iconic image of Al Gore’s “An Inconvenient Truth” was that of coastlines and cities disappearing beneath rising seas.

Sea level rise is certainly one of the more worrisome impacts of climate change, but the film’s disregard of the time scales involved in sea level rise may have led some to think that sea level rise on the order of 20 feet is probable in this century. Scientists cannot completely rule out such rapid sea level rise, but the general sense in the climate science community is that a lower but still worrying degree of sea level rise is more likely.

Estimates of climate change-driven sea level rise are rife with uncertainties. In addition to the indirect, but not insubstantial, impact of land subsidence in some areas, three primary factors contribute to sea level increases: the thermal expansion of water, the rate of ice melt, and dynamic ice sheet movements.

Water, like most materials, generally expands as it gets warmer, at least above 4 degrees C or 39.2 degrees F, below which water molecules actually become less dense as they freeze. Higher global temperatures will warm the upper layers of ocean water in the short term, and both upper and deep ocean water over centuries. Climate scientists can model the rate of heat transfer in ocean waters to estimate the rate of ocean warming and the resulting thermal expansion of ocean water. According to the most recent IPCC report (pdf), this thermal expansion is expected to result in 0.17 to 0.41 meters (0.56 to 1.35 feet) of sea level rise in the highest warming scenario (with between 2.4 and 6.4 degrees C warming by 2100), and 0.13 to 0.32 meters (0.43 to 1.05 feet) in the “mean” warming case (with between 1.7 and 4.4 degrees C warming).

Warming temperatures will also increase the rate of ice melt. Mountain glaciers and land-based ice sheets – for example, much of Greenland and Antarctica – are expected to lose ice mass in a warmer world. (Sea ice losses, like the dramatic ones seen in the Arctic last year, have little net effect on sea level since ice and water displace roughly equal volumes of water.)

Ice melt resulting from warming is a rather slow process because of the high thermal inertia of ice. It takes an enormous amount of energy to melt ice, so while major ice sheets will eventually melt in a warmer world, complete melting would occur only over many centuries and even millennia in the absence of any dynamic ice sheet changes. Non-dynamic ice sheet melting would result in 0.04 to 0.23 meters (0.13 to 0.75 feet) of sea level rise in the high warming scenario and 0.04 to 0.20 meters (0.13 to 0.20 feet) in the mean warming scenario.

Until recently, glaciologists had focused primarily on direct ice melt. However, as Ian Howat of the University of Washington explains:

Over the past few years there has been a major revolution in the way scientists think about ice sheet response to climate change. Previously, it was assumed that the big ice sheets react very slowly to climate, on the order of centuries to millennia. This is because surface melting and precipitation was thought to be the dominant way in which ice sheets gain and lose mass under changes in climate. However, over the past five years we have observed that the flow speed of the ice sheets, and therefore the rate at which the ice flows to ocean, can change dramatically over very short time scales.

Ice sheet flow rates can increase as a result of warming through the formation of moulins, or holes in the ice through which melt-water bores down to the rock below. Water sinking below the ice sheet can help lubricate ice sheets, causing them to flow faster. The melting of sea ice surrounding land-locked glaciers can also speed up flows if that sea ice had served to impede glacial flow.

The distinction between ice sheet melt and dynamic ice sheet movement has often been a confusing one, but it is an important distinction. As Michael Oppenheimer explains,

The issue still seems to perplex many journalists and others because there are two entirely distinct aspects of the sea level rise problem that are emphasized, depending on which scientists are speaking. On the one hand, these ice sheets are large enough to ultimately raise sea level by 7m and about 5m, for Greenland and West Antarctica, respectively. On the other, the recent observations that caused such a stir report a current contribution to the rate of sea level rise not exceeding ~1mm/yr from both ice sheets taken together … The key question is whether the ice sheet contribution could accelerate substantially (e.g., by an order of magnitude) either in this century or subsequently… From the point of view of societal and ecosystem adaptation, the timescale over which ice sheets might disintegrate, which may be on the order of centuries or millennia according to the two extremes posited in the literature, is crucial.

Scientists cannot now accurately quantify the probable effects of dynamic ice sheet flows on sea level rise, but some researchers warn that dynamic ice sheet flows may result in more sea level rise than thermal expansion and ice sheet melt combined. NASA’s James Hansen, outspoken in his concerns over the dangers of dynamic ice sheet flows, has argued that it is “almost inconceivable that ‘business as usual’ climate change will not result in a rise in sea level measured in meters within a century.” He points to his “deep concern about the stability of ice sheets in the face of ‘business as usual’ global warming scenarios.” Hansen suggests that sea level rise on the order of 15 feet this century is not out of the question, given evidence from periods of rapid warming at the end of past glacial periods.

In its recent Fourth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) estimated that sea levels will rise between 0.26 and 0.59 meters (0.85 to 1.94 feet) in the high warming scenario and between 0.21 and 0.48 meters (0.69 to 1.57 feet) in the mean warming scenario by the decade from 2090-2099.

These figures have raised a few eyebrows, as they are slightly lower than the sea level rise projections from the prior IPCC report (in some part because of a switch from 2100 to 2090-2099 as the modeled period). The IPCC’s Third Assessment Report had included a very speculative estimate of sea level rise resulting from dynamic ice sheet flows.

Research findings in the seven years between those third and fourth IPCC reports, however, revealed that dynamic ice sheet flows were potentially far more significant than had previously been thought, and the authors of the most recent report decided not to include an estimate of sea level rise from dynamic ice sheet flows. They noted that:

Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.

James Annan, a climate scientist at the Frontier Research Centre for Global Change in Japan, says the IPCC decision to exclude dynamic ice sheet flows suggests that the majority of climate scientists are not as convinced as Hansen is. He points out that:

If a significant proportion of the IPCC authors agreed, it is surely even more inconceivable that they would have brushed such a critical aspect of climate change under the carpet in the way they did (by presenting figures which excluded any assessment of ice sheet collapse).

It seems clear that even if dynamic ice sheet flows turn out to have a less dire effect than Hansen fears, the latest IPCC figures for sea level rise are likely too conservative. Stefan Rahmstorf and a team of other scientists published an influential paper in Science in 2007 showing that observed sea level rise has been close to double the mean predicted by IPCC models over the past decade, as shown in the chart below.

Chart showing IPCC Third Assessment Report sea level rise projections (mean projections shown by dashed blue line) compared to observations (shown by the solid blue and red lines). Note that IPCC AR4 estimates are similar to those shown for the Third Assessment Report, with a slightly narrower range. Figure from Rahmstorf et al. 2007 via Realclimate.

Eric Steig, a geochemist at the University of Washington in Seattle and Realclimate contributor, suggests that the evidence for accelerated dynamic ice sheet flow and melt rates needs to be to be considered carefully. He reminds us that:

It is also important to remember that the data showing accelerating mass loss in Antarctica and rapid glacier flow in Greenland only reflect a very few years of measurements – the GRACE satellite has only been in operation since 2002, so it provides only a snapshot of Antarctic mass changes. We don’t really know whether these observations reflect the long term trend … On the other hand, none of the new evidence points in the direction of smaller rates of sea level rise in the future, and probably nudge us closer to the upper end of the IPCC predictions.

Sea level rise projections remain an area of substantial uncertainty. With all the attention the issue rightly receives, there likely will be many more important studies on the possibility of accelerated sea level rise resulting from dynamic ice sheet flows.

For the time being, however, and while scientists move to address lingering uncertainties, journalists should resist any temptation to report the most dire consequences as settled science.