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Steve:          Welcome to the Scientific American podcast Science Talk, posted on December 8th, 2011.  I'm Steve Mirsky.  One of the most worrisome consequences of global warming is that sea levels could rise substantially this century and beyond.  Scientists look back into the geological record to see how carbon dioxide levels in the atmosphere correlated to sea level and track a few basic measurements of the atmosphere in current times to predict how much sea level might rise in the future.  Eelco Rohling, an ocean and climate scientist at the University of Southampton in England, has studied the paleoclimate record going back 50 million years.  On December 6th, at the annual meeting of the American Geophysical Union in San Francisco, Rohling presented calculations indicating that sea level worldwide could rise far more than scientists had previously thought.  He also sat down with Scientific American senior editor, Mark Fischetti to explain how the basic measurements of carbon dioxide in parts per million in the air and the added heat in the atmosphere that the carbon dioxide creates called Forcing leads to water, water everywhere.

Fischetti:          We hear about several different sorts of key numbers, and that's part per million of carbon dioxide or the global forcing, which is in watts per square meter, and give us the basic numbers and which ones matter most?

Rohling:          Okay, so I think the first number, really, to take into account is the preindustrial carbon dioxide value, which is round about 280 parts per million in the atmosphere.  Now that is, you could say, that's the zero level from which we started to increase.  We're currently very, very close to 400, so we have increased it a lot, and we have increased it by 120 ppms. And to put that in context during the last ice age, we were at a 180, so we were 100 ppm lower than in the preindustrial state.  So we've already increased CO2 by more than from the last glacial maximum to the present interglacial.

Fischetti:          Okay. So that's the parts per million. So now let's equate that to watts per meter squared and you can explain what the forcing really means.

Rohling:          So, the watts per meter square with CO2, you have to convert.  So if we have the first doubling of CO2, so we go from 280 to 560 ppms—bearing in mind we're already on 400—but if we go to 560 that equates to roughly four watts per meter square of radiative forcing on the planet.  That's four watts per meter square, so you can do the division on the 400 that we're on at the moment,  so we're not on a doubling now.  So currently we have experienced something between two and three watts per meter square already since the preindustrial times and there's been a bit of global dimming to take into account and the net forcing we are experiencing at the moment, relative to preindustrial is 1.6.

Fischetti:          Let me ask you just about the global dimming, because that's something that I think people don't understand, and it's an interesting factor.  What causes the global dimming and how does that affect the numbers?

Rohling:          So global dimming is essentially when we were combusting especially, sort of, coal and, you know, dirty combustion of fuels, we pump out aerosol particles.  So that you know, these are little combustible particles in soot etcetera, into the atmosphere and these particles act as nucleation for clouds and the clouds are reflective.  So, the clouds reflect a little bit more of the incoming solar radiation and as a result, the global dimming has done exactly what it says—it's slightly reduced the incoming solar radiation and slightly offset the radiative forcing from the, say, CO2 increase and that has reduced the value down from two to three watts per meter square from the greenhouse gas forcing down to about 1.6 net forcing.

Fischetti:          Great. And so the irony then is that as we burn cleaner sources of fuel, we don't help keep carbon dioxide's effects down, is that right?

Rohling:          That's correct. I mean, what's going to happen now is that we are, you know, getting much cleaner in the way that we combust things and as a result of global dimming is going to reduce, and that's going to give us a little bit more warming. But of course, you know, there are enormous benefits from cleaner burning, so I'm not advocating that you should stop that. (laughs)

Fischetti:          No I don't think we would advocate that either. (laughs)   So okay, so then in this session we've been doing here at AGU, this all, kind of, comes down to sea-level rise and what can we expect.  So if you could then bring all these together—parts per millions, the global forcing and sea-level rise—based on the paleoclimate record, which is, kind of, the really more a recent data that the new view is built on.

Rohling:          Yeah, so what we see is that for a current level of forcing, so 1.6 watts per meter square net forcing, if we look in the relationship that we now recognize between sea-level change and climate forcing, we're are, more or less, looking at in the equilibrium state, natural equilibriumstate, where the planet would like to be that is similar to where we were 3.5 million years ago and that's where we're looking at sea level, you know, at least 15 meters, maybe 25 meters above the present. So, we're talking about this period 3.5 million years ago, this is the middle Pleistocene, and that's where the CO2 concentrations were round about 400 ppm; and if we want to look at CO2 concentrations considerably higher than that, we're going to go much deeper in time, and then we're really going into periods where sea level was even higher.  We're looking at an ice-free planet, for example, 55 million years ago and then sea level would be 60 to 70 meters above the present. But that's a really scary number that will take an enormously long period to achieve. But the 15 to 20 meters is where the planet, at the moment for the current climate forcing in the atmosphere today, where the climate would like to be, if you give it enough time.

Fischetti:          And the related question then is okay, 15 to 25 meters higher—how long would it take?  I know that's the, sort of, the impossible question, but maybe there's a rate per year, decade or something that that might equate to?

Rohling:          So, the big thing to point out there is these are very long-term processes, but what we're doing now is we're warming the planet up so fast, like really pulling an elastic band really quickly. So, what we're doing is we're creating a disequilibrium, and that could lead to very fast steps in the sea-level adjustment, in the ice volume adjustment.  We can get some measure of how fast this could go by looking at the last interglacial 125 thousand years ago.  At that time the sea level had risen to about four or six meters above the present due to natural circumstances, but we've recently reconstructed the rate at which this happened, and that was rates of a meter or more per century at which this happened.  So I think it's very realistic, if we want to look at the adjustment to that big disequilibrium then that we have generated, to look at those sort of rates of change that we will eventually achieve; and maybe not this century, we'll be working our way up to that, but certainly in the next century, we need to think about that as the rate of sea-level rise.

Steve:          Go to http://www.ScientificAmerican.com for more of Mark Fischetti's coverage of the activities at the AGU meeting as well as reporting from David Biello, our man at the U.N. Climate Change negotiations in Durban, South Africa. And follow us on Twitter, where you'll get a tweet every time a new article hits our Web site.  Our Twitter name is @sciam (S-C-I-A-M).  For Science Talk, I'm Steve Mirsky.  Thanks for clicking on us.