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Planetary Watch: Does Climate Change Go Slow Before Going Fast?

by Bill Chameides | September 29th, 2008
posted by Erica Rowell (Editor)

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The Earth’s climate has changed abruptly from warm periods to cold periods. Is it possible to predict when the next shift will occur?

One of the greatest worries about the current global warming is that it may trigger a rapid shift in the climate — a move to a radically different one, perhaps even an ice age. How do we know if an abrupt change is approaching? A new study suggests that it will be difficult to tell — climate change may go slow before it goes fast.

Over the Earth’s 4.5 billion-year history, our climate has seen major fluctuations, often with rapid transitions from one state to another. Many scientists believe that the Earth has experienced periods of extreme cold and hot periods.

During extreme cold periods, referred to as “Snowball Earth,” much of the Earth’s surface, if not all, was covered by ice. Temperatures dipped well below freezing. During extreme hot periods, referred to as “Hothouse Earth,” there was no ice and temperatures climbed well above 100 degrees Fahrenheit. In contrast, today’s climate regime, referred to as “Icehouse” and covering the last 2 million years, has some permanent ice but not nearly as much as Snowball Earth. This ice periodically grows in extent (we call those ice ages), and shrinks in extent, like the current warm period.

Geologic evidence suggests Snowball Earth occurred three times: about 2 billion, then 700 million, and most recently 600 million years ago. The last Hothouse Earth ended about 50 million years ago. Scientists suspect that large changes in carbon dioxide (CO2) and other greenhouse gases coupled by realignments in the position of the continents played a role in these major climate shifts.

Tipping Points for Previous Climate Shifts

These changes in climate probably did not occur gradually, but in an abrupt shift — like the proverbial straw that broke the camel’s back. The climate was pushed and pushed (for example by a rise or fall in CO2) until a tipping point was reached, setting off a radical change in climatic conditions sometimes in as little as a few decades.

This is worrisome. Will the current increase in CO2 pollution trigger an abrupt and potentially catastrophic climate change? Is it possible for us to predict when a shift is approaching?

A new study by Vasilis Dakos and colleagues at Wageningen University in the Netherlands and the Potsdam Institute for Climate Research in Germany, published in the Proceedings of the National Academy of Sciences, suggests that prediction will not be easy. Their analysis of geologic data suggests that many of the past abrupt shifts in the Earth’s climate have been proceeded by remarkably stable conditions. At first blush this might seem to be counterintuitive, but it is actually characteristic of many systems subject to an external perturbing force.

Consider a very simple example: A ball on a track with a series of peaks and valleys and the ball sitting in one of the valleys (see figure). If you push the ball gently up the hill, it will roll upward, slow, stop, and fall back. As you push harder, the ball goes up higher, stopping further along before rolling back down. Now imagine you push the ball hard enough so that it tops the hill and rolls down the other side. The point where the ball moves the slowest will always be at the hill’s peak — in a sense at its tipping point. (If you have trouble visualizing this, try pushing a baseball up a small hill in your neighborhood.)

Dakos et al. argue that such a ball scenario is analogous to the way our climate moves into a new state. The period of slowest change occurs just before it moves past a tipping point toward a new climate regime.

Abrupt Climate Change Will Be Difficult to Spot Coming, Study Suggests

I find these results scientifically compelling but not especially reassuring. It suggests that being able to get consensus on whether an abrupt climate change is approaching will be quite difficult. If the climate becomes increasingly stable, some will undoubtedly conclude that global warming is no longer an issue. And how could they be proven wrong? On the contrary, Dakos’s results, in my opinion, raise the stakes in the grand greenhouse-gas atmospheric experiment we have undertaken.

The current climate regime, which began at the end of the last ice age about 12,000 years ago, has been remarkably stable. We are now slowly pushing the climate to higher temperatures by emitting CO2 and other greenhouse gas pollution. Most likely, the climate response will be gradual, as it has been over the past 100 years, but more abrupt and radical changes can’t be ruled out. It seems unlikely that we will be able to provide an unambiguous warning that such changes are imminent.

Further Reading

“Snowball Earth,” January 21, 2000, scientificamerican.com – www.atmos.washington.edu/2007Q4/211/Hoffman2000_snowball.pdf

Snowballearth.org

Snowball Freeze-Fry Scenario – www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig5.pdf

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