Deep Water

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.

13 thoughts on “Deep Water”

  1. Matt Ridley is worth a look re this (and other apocalyptic threats).

    “…So, should we worry or not about the warming climate? It is far too binary a question. The lesson of failed past predictions of ecological apocalypse is not that nothing was happening but that the middle-ground possibilities were too frequently excluded from consideration. In the climate debate, we hear a lot from those who think disaster is inexorable if not inevitable, and a lot from those who think it is all a hoax. We hardly ever allow the moderate “lukewarmers” a voice: those who suspect that the net positive feedbacks from water vapor in the atmosphere are low, so that we face only 1 to 2 degrees Celsius of warming this century; that the Greenland ice sheet may melt but no faster than its current rate of less than 1 percent per century; that net increases in rainfall (and carbon dioxide concentration) may improve agricultural productivity; that ecosystems have survived sudden temperature lurches before; and that adaptation to gradual change may be both cheaper and less ecologically damaging than a rapid and brutal decision to give up fossil fuels cold turkey…”

  2. Yeah, it looks like Hearty was wrong about this and did not account for the effects of glacial isostatic adjustment. But 9 metres of sea level rise implies disintegration of the marine-based ice sheet margins of East Antarctica.

  3. One problem with using interglacials as models for the response of sea level to temperature increases is that the geographical shape of the temperature increase is very different from what one would expect from greenhouse warming: the initial Milankovich warming takes place at the ice sheet’s edges and is amplified by albedo feedback. This means relatively more of the warming takes place at high latitudes, where we would expect most of the melting impact on continental ice sheets to be.

    What this means is that one risks overestimating the sensitivity of sea level to global temperature changes that are lacking this high-latitude excess, such as those caused by greenhouse warming. Better is to go back even farther in time, to the Pliocene, when there was a warmer greenhouse climate. But of course that is harder.

    1. Mustakissa – The sea level response from the previous interglacials, as an analogue, is more likely to be an underestimate. The complete opposite of what you suggest.

      Firstly, the warming (insolation) was focused predominately on the Northern Hemisphere, due to the change in the Earth’s tilt and orbital characteristics. Despite this, the West Antarctic ice sheet repeatedly collapsed during the previous interglacials, and was the main contributor to sea level rise, not the Greenland ice sheet, as one would expect.

      Secondly, the change in radiative forcing due to Milankovitch is small compared to the human-driven change in modern-day forcing brought about by increased greenhouse gases. And much of this forcing has yet to be felt because of the strong, but short-lived, role of reflective industrial pollution particles (aerosols), and also the sluggish response of full ocean warming.

      And thirdly, although fraught with large uncertainty, the idea that the previous interglacials were as warm as, or warmer than, today is highly contestable. If you delve into the scientific literature, there are some that indicate the Eemian interglacial, for instance, was probably as warm as the Early 20th century or even earlier.

      This would make much more sense because we know that during the Pliocene, with atmospheric CO2 concentrations similar to today (around 400ppm), that global sea level was around 20-25 metres higher than modern-day. This is seriously at odds with the interglacials, if one is to accept that global temperatures were similar during these intervals. Even if we take into consideration the length of the intervals (only around 10,000 years for the interglacials) there still is a major disparity. This becomes less problematic with the realization that the interglacials were cooler than the Pliocene.

      There is a paper, awaiting publication, that addresses the coupling of global temperature with sea level stretching back many millions of years, but given that the West Antarctic ice sheet disintegrated repeatedly during the interglacials – from the small Milankovitch nudge, we should expect that much, and more, over the coming centuries.

      1. Dappledwater,

        it’s complicated. The precession of the Earth axis together with the long axis of the Earth orbital ellipse produce summer warming in the North. Most of the feedback going into this is albedo feedback from the melting of the continental ice sheets in the North. Antarctica doesn’t have a lot of albedo feedback to give by comparison.

        The change in tilt of the Earth’s axis OTOH affects North and South likewise. But irrespective of that, both initial forcing patterns are weighted toward high latitudes (N and S) compared to greenhouse forcing, which is spatially almost homogeneous. Feedbacks (e.g., water vapour) will tend to follow this pattern.

        And yes, I agree that it is worrisome how strongly WAIS responds to small changes in forcing.

  4. There was an interesting aspect raised recently that the actual amounts of sea level change if the icecaps melt may be surprising. For eg, if Greenland lets go, the sea level around Greenland, Baffin Land and even Scotland would drop, as the gravitational mass of the icecap disappears allowing water that is currently drawn toward it to move away. Likewise, Antarctica current sea level is much higher than it would be if its huge mass of ice melted, to the extent that NZ’s sea level may not change much. Sadly, the regions likely to suffer rises most are the equatorial countries, often those most vulnerable, whose current coastlines are lower that the global average due the volumes of sea water drawn toward the icecaps, quite apart from the amount locked in the ice. It’s a poetic justice (sort of) that one area likely to be disproportionately affected will be Washington DC.

    1. Given the trend of the previous interglacials, i.e. West Antarctic ice sheet disintegration, and the Earth’s wobble this is likely to induce, the east coast of North America will probably experience sea level rises around 40% higher than the global average. And it’s sinking too anyway – a consequence of being levered up by the giant Laurentide ice sheet, and still now subsiding due to the ice sheet’s complete disintegration.

      I don’t agree this is poetic justice though, because those who caused this millennial-scale melt will be long-dead by time the serious consequences begin to be felt.

      1. I’m hoping that we’ll see sufficient melting in the near term for realisation to dawn and for opprobrium to be heaped upon them. Not necktie parties perhaps, although our descendants a few generations from now (if any) won’t regard the 20th Century with much favour. To be honest, I’m as guilty as the Koch Bros.

        Not that the money-grubbers will care, the catastrophic melting of the Arctic Ocean is just regarded as a splendid opportunity to make more money, without the merest whiff of concern about the causes and consequences.

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