The trouble with predicting climate change

When you hear predictions about climate change, scientists often refer to simple models of gradual transformation. But new evidence suggests that climate change is likely to be abrupt, with the temperature of the planet rising quickly over only a few decades.

University of Bristol Earth scientist Paul Valdes has spent his career researching how to predict natural climate change. This week in Nature Geoscience, he argues that we need to understand climate change as a series of "tipping points" rather than gradual, incremental shifts. His evidence for this? Four times in recent geologic history when Earth's climate has changed dramatically in just a few decades.

Valdes gives a quick overview of four abrupt, unexplained climate changes in his paper:

The Palaeocene–Eocene Thermal Maximum.
A rapid warming event about 55.8 Myr ago started with warm climate conditions with a smaller difference between temperatures at the Equator and the high latitudes. Complex climate models do not adequately simulate the warm climate before the abrupt change set in.

The desertification of northern Africa.
Between about 9,000 and 5,500 yr ago, the region that is now the Sahara was much wetter and supported a steppetype vegetation. The transition to the current desert state occurred in decades to centuries. Complex climate models fail to simulate the vegetated state, and can not therefore capture this event of rapid change.

Collapse of the Atlantic meridional overturning circulation.
During the glacial period between about 120,000 and 12,000 yr ago, the meriodional overturning circulation in the Atlantic Ocean collapsed during six Heinrich events [periods of rapid cooling], most probably in response to fresh water entering the North Atlantic. Complex climate models simulate such a shut-down - but only in response to a freshwater injection as much as ten times the magnitudes estimated for the past.

Dansgaard–Oeschger rapid warming events.
Between Heinrich events, 25 incidences of rapid warming, by up to 8 °C within a few decades in Greenland, are consistently recorded in the ice cores. We don't even fully understand the mechanisms for such changes and simulating the final one of these events required an injection of fresh water into the ocean that was large and many thousand years longer than is thought realistic.

You'll notice that the common thread uniting these events is their rapidity, and the mystery underlying why they happened so fast. The Sahara went from a relatively wet area, to the dry sands of today, in just a few decades. And what about those Dansgaard-Oeschger rapid warming events? We know they happened, and they happened fast. But how? And why?

Valdes is concerned that our current ideas about climate change, especially those in a recent report from the Intergovernmental Panel on Climate Change, don't deal with these "tipping point" events where the climate changes very dramatically. Humans could be causing such a tipping point, or we could be about to go through a natural one. Either way, we need to be prepared for the possibility that the regions of the planet we once considered habitable may lose that status — and it could happen in the next fifty years.

And yes, this means that the environmental science in The Day After Tomorrow is still completely wrong.

Read the full scientific paper via Nature Geoscience

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