The rising and falling of the seas

The figure to the left shows the ordering of Earth’s geological periods, going back about four and half billion years.  Homo erectus, our ancestors, showed up during the Pleistocene epoch, which lasted about 2.6 million years, as evidenced by the geological record.  During the Pleistocene the climate underwent a series of dramatic swings, producing alternate periods of glaciation and warming periods that led to major retreats of the glaciers.  Homo sapiens—the first humans, you might say—showed up about 200,000 years ago. The little boxes denoting the different epochs in the figure are not to scale.  If they were, the Pleistocene would be a very narrow box, and the Holocene would be just a line. We humans are very recent entrants onto the stage of Earth’s evolution. 

The Holocene represents the major retreat of the last wave of glaciation.  The glaciers covered Canada, dipped down into what is now the Midwest, covered New York and all of New England.  It was only 11,700 years ago that the most recent major glaciers retreated.  The climate since then has been relatively stable, with no periods of dramatic change.  It is this stability that enabled humans to expand from the tropical regions toward the poles, especially in the northern hemisphere.  In their retreats, those massive glaciers gouged out the Great Lakes, and their movements ground rocks to a fine powder that, along with sand, covered land surfaces and formed the base for seeds to germinate and form the vegetation that eventually covered much of the land.

During this time Earth’s climate was determined in large measure by the amount of carbon dioxide, CO₂,  in the atmosphere. Climatologists and geologists have developed powerful means to measure CO₂ levels in the past several million years, and have also been able to reliably estimate atmospheric temperatures over this same period. The two are strongly correlated.  On the basis of this and much corroborating evidence, we can be quite sure that an increase in atmospheric CO₂ level produces warming, which in turn results in melting of glacial ice and an increase in sea level.

The table above should bring home the fact that we’re such newcomers to the planet!  Humans have no real experience with major climate change.  The Holocene is regarded as a period of quite stable climate as compared with earlier epochs, long before humans made their debut.  The geological record and ocean sediments, provide evidence that there were periods in Earth’s history that were truly overpowering, that changed nearly everything.  The Russian scientist, Mikhail Budyko, was one of the pioneers of studies on global climate.  He produced a simple physical model of equilibrium in which the incoming solar radiation absorbed by the Earth's system is balanced by the energy re-radiated to space as thermal energy. The results of his calculations were startling.  In 1972, he calculated that a 50% increase in atmospheric CO₂ would melt all the polar ice, whereas reducing the CO₂ level by half could lead to a complete glaciation of the Earth.  Budyko was not the first to advance these ideas.  For example, the Swedish scientist, Svante Arrhenius, had come to similar conclusions much earlier.  But Budyko had a quantitative model.   He predicted that if ice sheets advanced far enough out of the polar regions, a reinforcement could occur whereby the increased reflectiveness of the ice led to more cooling and the formation of more ice, until the entire Earth was covered in ice.

His prediction regarding loss of the polar ice is being borne out; as the CO2 the arctic ice is diminishing year by year, and is likely to be completely melted in the summer months in 20 to 30 years. But what about that other prediction? Was there ever a time when the CO₂ level was so low that the planet was covered with ice? It’s hard to imagine, but evidence that Earth was once completely glaciated, the so-called Snowball Earth hypothesis, has been accumulating.  The American climatologist Joseph Kirschvink published a paper in 1992 in which he argued that the presence of banded iron formations in certain geological deposits is consistent with such a global glacial episode. It happened very long ago, 650 million years back, in the Proterozoic Eon (see the figure). But once such a frozen landscape is formed, what could break the planet out of such a frozen state?  Several mechanisms might have been at work.  One candidate is that CO₂ began to accumulate from volcanic outgassing. There could have been plenty of that going on during those long-ago times.

I recently ran across a paper in Science, entitled: “Rapid sea level rise in the aftermath of a Neoproterozoic snowball Earth”.  It was my introduction to the idea of “Snowball Earth”.  Naturally, I had to read it. If Earth’s surface were covered with ice, what was the sea level under all that ice? The gist of the paper is that, with so much water being tied up as ice, the level of the liquid ocean underneath it had to be much lower than it is today.  Scientists can only guess, but they estimate that the water level would have been from about 0.7 to 1 mile lower than today.  So when conditions suddenly changed, and the glaciers began melting at a furious rate, sea levels rose, scientists estimate, about 100 times faster than at present—on the order of a foot a year.

We humans seem to have been programmed by evolution to think short term.  That characteristic is coming to haunt us in dealing with global climate change today.  We have trouble focusing on events likely to ensue in the future, even if they’re only a century away.  We should be thinking about the possibility that glacial melting could come more rapidly than scientists now estimate. There’s a lot we could be doing to head off some of projected global warming, but we seem content to put it off.  It may be tempting to buy a pricey home or condo near the water, but future rates of sea level rise could make that an even poorer investment than it now seems to be.