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Published: Tuesday 03 September, 2013

isabel marant wedge boots isabel marant wedge boots An ocean switch for global isabel marant wedge boots cooling

The average temperature of the Earth can change dramatically over just a few decades. One of the most striking variations of this sort happened about 12,000 years ago, when the planet was emerging from the last ice age. The gradual warming across the globe was interrupted in the North Atlantic by a sudden return to iceage conditions an episode called the Younger Dryas event.

Now, evidence in Nature1suggests that changes in ocean circulation were responsible. The findings hint that environmental changes that alter circulation in the North Atlantic a possible corollary of global warming caused by greenhouse gases can have a rapid and profound impact on climate.

The last ice age began to thaw about 15,000 years ago. Five thousand years later the great ice sheets that had covered much of North America and northern Europe had retreated towards the North isabel marant wedge boots Pole, and the Earth was about as warm as it is today. But this escape from the deep freeze was rudely interrupted by the Younger Dryas event, which began about 12,900 years ago.

Changes in the shape, tilt and wobble of the Earths orbit around the Sun are thought to have brought about the ice ages. But these orbital changes happen slowly over thousands of years whereas some climate records suggest that the Younger Dryas event was in full swing in under a century. Understanding the causes of such rapid climate change is crucial for calculating the possible ramifications of the humaninduced greenhouse effect.

Greenhouse gases come from natural sources too. Might some change in the processes responsible for this natural greenhouse effect have triggered the Younger Dryas? This is one possibility; another is that the cooling was due to a change in ocean circulation.

The deep waters of the worlds oceans circulate, carrying warm water from the tropics to the poles, and cold, dense water back towards the tropics. This is driven by the fact that water is denser when it is colder and saltier. Warm water from the tropical Atlantic, for example, sinks as it flows north and cools, and as it becomes saltier by the formation of saltfree sea ice. This process is therefore called thermohaline literally heatsalt circulation.

If thermohaline circulation were to shut down, the oceans would cease to bring heat from the tropics to the poles, and the North Atlantic region would become much colder. Is this what happened during the Younger Dryas event as the ice sheets melted, injecting fresh water into the North Atlantic, rendering its waters less dense and so less inclined to sink?

Carsten Rhlemann from the University of Bremen, Germany, and colleagues have analysed ocean sediments in the western tropical North Atlantic Ocean for evidence of the temperature of the surface waters at the time that the organic matter fell to the seabed. The researchers find that isabel marant wedge boots these waters were relatively warm when the North Atlantic was cold during the Younger Dryas and also during an earlier cold episode.

This suggests that the thermohaline circulation had shut down, so that the tropical oceans were retaining their heat. If greenhouse gases were responsible, such cooling would show up everywhere on the planet more or less at the same time.

Could such changes happen again, for example if the deep ocean currents were to shift in a warmer world? Perhaps the words of Wallace Broecker, the US scientist who first proposed the circulation explanation in the 1980s, provide the best answer. I published a full account of my theory as a popularized article in 1987. Unbeknownst to me, the editors added the question Could it happen again? At the time, this statement greatly annoyed me because I had carefully avoided any mention of the future in the article itself. But now in retrospect, perhaps I should forgive them. isabel marant wedge boots