Deep Water

By Bryan Walker 18/08/2012

We know that sea level rise is an inevitable consequence of the global warming that our continued burning of fossil fuels is causing. What we don’t know is how much to expect and how soon to expect it. Journalist Daniel Grossman in his Kindle Single Deep Water: As Polar Ice Melts, Scientists Debate How High Our Oceans Will Rise explores the momentous issue by looking at the work of three scientists who study the past history of elevated sea levels to get a better understanding of what is likely ahead for humanity. Grossman writes from a close acquaintance with climate science and his ability to distil the science in readily understandable form for the general reader is outstanding.

Paul Hearty, “talented and cantankerous”, is a geologist who has argued from his studies of inter-glacial periods that if the Earth warms by two degrees the huge glaciers in Greenland and Antarctica could substantially melt in a short space of time. His field work in the Bahamas and Bermuda, which he regards as a relatively stable region geologically, has led him to the conclusion that in the warm interglacial 400,000 years ago (Stage 11) sea level rose by as much as around 19 metres. Paleoclimatologist Maureen Raymo doesn’t share that view but it was Hearty she invited in 2009 to collaborate in field work with her in Western Australia seeking evidence of sea level rise in the Pliocene. Grossman travelled with them as journalist and gives a lively account of the expedition.

Raymo’s respect for Hearty’s field work is considerable, and for some time she felt he had established the case for a higher sea level rise in the Stage 11 interglacial than most credited. Then it occurred to her that the Bahamas and Bermuda may have been involved in vertical lift as the great weight of the Laurentide ice sheet depressed the land on which it sat, causing a bulge in surrounding crust. Grossman uses the analogy of a waterbed to illustrate the effect. When the ice sheet subsequently melts the depressed land rises and the surrounding bulges sink. The Stage 11 interglacial on which Hearty’s work was focused was a long one, allowing, she surmised, more time for the subsidence of Bermuda and the Bahamas than has yet occurred in our own period.  Raymo took her theory to geophysicist Jerry Mitrovica who applied the adjustment factors that he had worked out over the years as he studied unique local circumstances which affect local sea level change relative to global average change. By his calculations the Bahamas and Bermuda had indeed subsided much more during the Stage 11 interglacial than they have today. Some 8.5 metres more in fact, meaning that the sea level rise recorded in Hearty’s findings was close to the 9.4 metres proposed by most other researchers.

The matter is not settled, of course, and Hearty is not persuaded. But it’s a fascinating picture Grossman presents of scientific argument and counter-argument. And if Hearty’s conclusions prove mistaken, he is not mistaken in finding that the features he locates in the field are in fact evidence of a shore line, not the result of storm winds or tsunamis as some have suggested.

Grossman has chronicled a scientific debate which may mean that the sea level rise we can eventually expect, as the global temperature continues to rise, will be more like 9 metres than 19. There’s no comfort for humanity there. Denialists inclined to make something out of the difference of opinion would be well astray. As Mitrovica comments, 9 metres is still plenty of water. And he is troubled by work he has shared on the most recent interglacial, which also reached a temperature only a little warmer than today’s. Yet that appears to have been enough to cause the Greenland and West Antarctic ice sheets to lose most of their ice and sea level to rise by around 7 metres over time, which points to the possibility of a higher sea level rise this century than previous estimates.

Grossman reports that most scientists with whom he has spoken expect a sea level rise of around a metre this century. But there is profound uncertainty about the behaviour of the ice sheets of Greenland and Antarctica. Will these continent-scale glaciers remain languid in their transport of ice to the sea or under the influence of warming will they become more akin to bounding torrents?  He explains very clearly for the lay reader the difference between the gradual surface melting of an ice-cube and the dynamics of glacier movement that can cast into the sea quantities of ice which dwarf the amounts that dribble off from surface melting.

Grossman’s short book is science journalism at its best, informative, accessible, and yes, entertaining. It’s an intriguing first hand glimpse into paleoclimatologists at work, piecing together the results of their research, sometimes conflicting in their conclusions but pointing undeniably towards grave consequences for humanity if we set in train the disintegration of the world’s major ice sheets. There is clearly warning enough, in spite of the uncertainties, that the danger is real and may already be in process. That’s the seriousness underlying the highly readable narrative in which the book is cast.

Note: There’s a useful article in The Yale Forum on Climate Change and the Media discussing TED Books, the digital books publisher of Deep Water, and referring to Grossman’s book in that context.