One of the clearest and most worrying signs that something odd is happening to Earth’s climate is that sea levels are rising steadily. Records of long-term tidal gauges show that sea levels were 17-20 cm higher at the start of the twenty-first century than 100 years before, correlating with a globally averaged rise in temperature of 0.75 degrees. Recent and very precise satellite measurements taken over the past few decades have confirmed this trend. How much of a concern is this change?
Such a rate and magnitude of sea level rise is certainly not unprecedented in Earth’s past. For instance, at the end of the last ice age, around ten thousand years ago, the oceans rose by as much as 120 m over a few thousand years – and up to 1 m every 20 years for a sustained 400 year period – engulfing the previously dry land that connected mainland Australia to Tasmania and New Guinea. Looking deeper in time, global climate was an average of 2 to 3 degrees warmer than at present some 3 million years ago, and sea levels were 35 ±18 m above the shoreline of today.
The implications of these past changes in sea level for future rises are worrying, but uncertain. Our best models predict that the globe will heat up by anywhere between 1.8 to 6.4 degrees Celsius over the next century. The United Nations Intergovernmental Panel on Climate Change (IPCC), in their 2007 review, somewhat reassuringly indicated that the probable amount of sea level rise by the year 2100 will be 18 to 59 cm. Although a rise at the upper end of this range would be catastrophic for low-lying coastal and island communities and ecosystems, many societies will, at a cost, be able to adapt. However, the most recent science suggests that these IPCC projections could turn out to be gross underestimates.
The IPCC projections of sea level rise are based largely on the slow, steady and inexorable thermal expansion of the oceans (as water heats, its volume increases) with some additional contributions from the melting of mountain glaciers (almost all of which are expected to be gone by mid century). They do not include “rapid and dynamical changes in ice flow”, because the IPCC was too uncertain about how likely or influential these changes might be. This means that the 18 to 59 cm projection is really a best-case scenario, given that it assumes the great icecaps of Greenland and West Antarctica remain largely intact.
Yet the rate of ice loss from these two polar realms, as identified by satellite measurements of the change in gravity of the ice masses, has more than doubled over the last decade. It now represents a loss of up to 150 cubic kilometres of ice per year, from each region. Similarly, the rate of sea level rise has doubled over the last 10 years, and now exceeds the upper-end predictions made by the IPCC only a few years before. The suspicion of many scientists is that this acceleration is being driven by self-reinforcing feedbacks. This includes the loss of reflectivity (“albedo”) caused by sunlight striking dark exposed rock rather than white ice, the lubricating effect of melt water penetrating to the bottom of glaciers, and many other “dynamical changes”.
If both Greenland and West Antarctica shed the entirety of their ice burden, global sea levels would rise by 12 to 14 m. Although these icecaps would not disintegrate within a century, the loss of even a third of their mass – quite plausible if the rate of polar ice loss continues to double each decade – would force up the oceans by at least 4 m, with disastrous socioeconomic and environmental consequences. Further, the inertia accumulated by the slowly heating, yet physically vast oceans, means that should large-scale polar melting begin, it will almost certainly be impossible to halt.
The likelihood of such a frightening scenario unfolding is currently quite uncertain. Perhaps in 10 to 20 years, when science has accumulated considerably more information on ice flow dynamics, and measured another two decades of ice loss, we will be more confident about the true risk of catastrophic sea level rise.
The question is, can we afford to wait until we are sure?