Posts Tagged ‘PETM’

Past and Future Climate Change

Friday, December 18th, 2009

If we fail to mitigate the extent of climate change, then we will have to adapt to a world that will look very different. How different? We can look into the past to see what may now lie ahead.

Levels of atmospheric carbon dioxide continue to rise at an alarming rate

Levels of atmospheric carbon dioxide continue to rise at an alarming rate

Over the past half billion years, the planet has alternated between two climate modes: cold and dry, or hot and humid. The switch from one stae to the other is correlated with atmospheric carbon dioxide levels approaching 500 ppm. We should at present still be in the middle of a cold, dry glacial period, but we have shaken ourselves out of that by increasing atmospheric carbon dioxide to sufficient levels that the planet is warming – the ice caps are melting, glaciers are receding, permafrost is losing its permanence, winters are shorter, ocean levels are rising, ocean water is starting to acidify. At this extraordinary rate of change, we will relatively soon shift to the hot and humid mode of planet climate.

When did this last happen? 55 million years ago, atmospheric carbon dioxide levels rose to about 1000 parts per million. The polar seas warmed by 8-10 degrees centigrade, and sea levels were high. The pH of oceans dropped, resulting in mass extinctions of plankton dependent on calcium for their skeletons. On land, immense habitat changes forced the migration and dispersal of mammals, and the diversification of deer, horses and primates – including ourt own ancestors. It was a time of ecological upheaval.

The event occurred at the boundary of the Paleocene and Eocene Geological Epochs, so it is known as the Paleocene Eocene Thermal Maximum, or PETM. We dont know what caused the warming, though methane, trapped frozen as clathrates in polar sediments, may have become free as the temperature rose, and accelerated the warming. It took about 20,000 years for the warming to reach its maximum, and then it lasted, in a stable fashion, for 100,000 years. Then gradually the carbon dioxide levels dropped, sea and global temperatures dropped, and the planet shifted to its dryer, colder mode once again.

Clathrate: Methane (green) trapped in cage of frozen water molecules (red)

Clathrate: Methane (green) trapped in cage of frozen water molecules (red)

So what’s different this time? All the same events appear to be occurring. The difference is the rate of change. What took 20,000 years then may now occur in a century or less. Communities and species have a chance to evolve to tolerate new conditions when there is time, but decades or even a century is far too short for evolutionary change. The planet will survive this – it has seen worse – but we will see major habitat and ecosystem changes in the decades ahead, rising seas and major storms will force many millions of humans to become migrants, and life for all of us will become more difficult.

None of this needed to happen. Although we still have the opportunity to mitigate the harshest of the stresses we need action, now. The Copenhagen Conference on Climate Change was a lost opportunity. Without concerted global action by all of us around this little planet, the climate change events of 55 million years ago will soon return, and we’ll have to try to persist on a hot and humid planet.