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General · 20th July 2015
Ray Grigg
During the geological epoch known as the Pliocene, about 5 to 3 million years ago, atmospheric carbon dioxide levels were about 400 parts per million — the same as they are today. Among the major effects, our planet was about 3°C – 5°C warmer and our ocean levels about 30 metres higher. This makes the Pliocene a useful model if scientists are to anticipate the eventual effects of our comparable carbon dioxide concentrations.

But the model is particularly useful for another reason. The suspected cause of the initial Pliocene warming was a sudden surge in atmospheric carbon dioxide levels, probably from volcanic activity in carbonized fossil deposits. A process that took place over 20,000 to 30,000 years in the Pliocene, however, has been compressed by our burning of fossil fuels to a mere 300 years.

We know from analyzing air bubbles in ice core samples from Antarctica that present CO2 levels are the highest in 800,000 years. But it's possible to infer from diverse geological evidence, including plant and animal material in sediment and rock, that our levels are the highest since the peak of the Pliocene about 4 million years ago.

Notably, our present target of keeping the increase in global average temperature to 2°C means a ceiling of 450 ppm of atmospheric carbon dioxide, 25 ppm higher than at the peak of the Pliocene.

Studies of the Pliocene mean we can be reasonably certain about the eventual climate effects of carbon dioxide at these levels. When combined with the massive heat transfers from altered ocean currents, this seems to explain why the interior of large continents became dry savannahs and the poles warmed considerably more than the tropics.

But the immediate effects of 400 ppm CO2 is uncertain. “Climate lag” and “thermal inertia” are complex processes not yet fully understood. Basic physics dictates that our planet will warm. But exact projections are complicated by the present rate of rise — about 100 times faster than the Pliocene. True to predictions, however, the Arctic is currently heating about 4 times faster than the tropics, so summer sea ice will be gone within a few decades.

Since this ice is already afloat in the Arctic Ocean, melting will not affect ocean levels. But it will accelerate warming. Antarctica, however, is another matter. The West Antarctic ice sheet is already showing signs of instability as altered ocean currents melt its base. The collapse of this massive reserve of ice, when combined with melting from Greenland’s ice sheet, should send global oceans to the predicted rise of 30 metres.

How quickly? We don't know. In geological terms, with atmospheric carbon dioxide at 400 ppm, the 30 metre rise is likely inevitable. But we live in a human time scale. The warming trend of the early Pliocene was a least twice as long as the entire history of our civilization. Given the rapid rate of our carbon emissions, just how quickly our oceans will rise is conjectural.

Of course, we don't need 30 metres to cause havoc. Storms and tidal surges are already causing coastal flooding. And a metre or two would be sufficient to inundate many cities and communities, a prospect we should consider when contemplating our carbon emissions.