Last time CO2 was this high, the world was underwater? No actually
Ice sheets didn't melt 3 million years B.C., say boffins
OK, so levels of atmospheric CO2 are rising through 0.0004 (or 400 parts per million) at the moment. Disaster, right? The last time the world saw carbon levels like this, some three million years ago, the mighty ice sheets of Greenland and the Antarctic had melted from the heat and the seas were 35 metres higher than they are today. Anybody who doesn't live up a mountain will soon find themselves underwater. Aaargh!
Not so much, according to new research.
The idea that the seas were 35 metres higher 3 million years back comes mainly from scientists examining ancient high-tide marks found along coastal cliffs and scarps - particularly some often-used ones on the US eastern seaboard. By determining the ages of the rocks and marks, scientists have come to the conclusion that the seas were much, much higher then - and thus, that the Greenland ice and large parts of the Antarctic ice as well must have been melted at the time.
According to a crew of top boffins led by Professor David Rowley of Chicago uni, the problem with this is that over these sorts of timescales, areas of the Earth's crust rise and fall as much as the sea does. And nobody thus far has taken account of that - it has just been assumed that the rocky coasts have remained fixed with respect to the centre of the Earth, which means that the studies thus far have been - basically - wrong.
"No prediction of ancient ice volumes can ever again ignore the Earth's interior dynamics,” says Rowley.
The prof and his colleagues' new investigation has sought to reconstruct the behaviour of the crust along the oft-studied scarp running up the US coast from Florida to Vermont. And it turns out that over the past three million years, interactions in the Earth's mantle have lifted the entire coastline and the ancient tidemarks with it - giving a false impression of much higher sea levels.
Using the corrected, much lower sea levels, Rowley's team say that in fact the world's ice sheets didn't melt nearly as much back in the old days of 400+ ppm CO2 as people think. According to a statement highlighting the new research:
Until now, many research groups have studied this shoreline and concluded that during a warm period three million years ago, the Greenland, West Antarctic and a fraction of East Antarctic ice sheets collapsed, raising the sea level at least 35 metres. But the new findings by Rowley and his team suggest that these ice sheets, particularly the East Antarctic Ice Sheet (the world’s largest), were probably more stable.
The new paper has just been published in Science.
"It is the kind of study that changes how people think about our past climate and what our future holds," comments Rowley, bluntly. ®