At last we know why the monstrous ice sheets that once ground across the continents vanished when they did. Stephen Battersby reports

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22 May 2010 | NewScientist | 33

Milankovitch cycles

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COVER STORY

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Immense ice sheets have grown and shrunk many times JA

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The cave code

34 | NewScientist | 22 May 2010

Timeline of the ice agesStalagmites from China have helped establish the precise timing of events as ice ages end

Ice age termination

Intensity of sunshine in July at 65° north, based on combined effect of orbital cycles

Obliquity (tilt of Earth’s axis)

Strength of Asian monsoon, based on uranium-thorium isotope ratios in stalagmites from different caves

Carbon dioxide levels, based on ice cores (parts per million)

Relative sea level, based on oxygen isotope ratios in deep-sea sediments

350 300 250 200

Age (thousands of years ago)

150 100 50 0

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22 May 2010 | NewScientist | 35

EccentricityPrecessionObliquity

Earth’s orbit varies from being nearly circular to more elliptical in a cycle lasting 95,000 or 125,000 years. This means the planet gets more sunshine during part of the year and less during the rest

SUN

Orbit

The Earth’s axis wobbles, completing a circle every 22,000 years. This changes which hemisphere is tilted towards the sun at the point in Earth’s orbit when it is closest to or farthest away from the sun

The tilt of the Earth’s axis swings from 22 to 24 degrees and back in a cycle lasting 41,000 years. Since the tilt causes the seasons, a greater tilt makes summers hotter and winters colder

The Milankovitch cyclesPeriodic changes in Earth’s orbit alter the amount of sunshine different parts of the planet receive

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PrecessionObliquity

The Earth’s axis wo22,000 years. Thistowards the sun atclosest to or farthe

The tilt of the Earth’s axis swings from 22 to 24 degreesand back in a cycle lasting 41,000 years. Since the tilt causes the seasons, a greater tilt makes summers hotter and winters colder

The Milankovitch cyclesPeriodic changes in Earth’s orbit alter the amount of sunshine different parts of t

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” Bats stole our guide rope. The cave has many branches, and we lost our way”

Stalagmites hold clues to ancient climate changeH

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36 | NewScientist | 22 May 2010

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There is no doubt that carbon dioxide is a major player in the coming and going of the ice ages. When the planet starts warming as ice ages end, atmospheric CO2 levels start rising, amplifying the effects of orbital changes. What is less clear is where the extra CO2 comes from.

CO2 is less soluble in warm water, so the warming of oceans would lead to its release. The change in solubility cannot fully explain the rises in CO2 at the ends of ice ages, though. It appears there are several sources, perhaps including a rather surprising one: coral. The formation of corals’ carbonate skeletons releases CO2, and a rise in sea level will lead to a burst of reef building, as existing reefs grow upwards and as corals colonise shallow waters where

land has been submerged. So according to the “coral reef hypothesis”, up to half of the rise in CO2 during ice age terminations might be due to coral growth.

This idea was proposed more than a decade ago, but has won little support because the rise in CO2 was thought to precede the rise in sea level. “Since we conclude that they do shift together, the coral reef mechanism could be involved,” says Larry Edwards of the University of Minnesota in Minneapolis (see main story). “In our scenario, the possible mechanisms (including the coral reef mechanism) fit together in a plausible sequence, which starts with a simple trigger of insolation rise initiating the demise of the ice sheets.”

BLAME THE CORALS Changing pace

Stephen Battersby is a freelance writer based in London

The growth of coral reefs releases carbon dioxide

LAURENTIDE ICE SHEET

SCANDINAVIAN ICE SHEET

Fresh water pours into the Atlantic and shuts down the overturning circulation

Because less heat is carried north, the southern oceans warm and release carbon dioxide

The entire planet warms, leading to further melting

and CO2 release, triggering the end of the ice age

CO2

Increased summer sunshine leads to melting of ice sheet

How the ice ages endA little extra sunshine can trigger a complex series of feedbacks