Australasian Science: Australia's authority on science since 1938

Climate Records Reveal North–South Divide

climate

It’s little wonder that the two regions have different climate histories: the Southern Hemisphere is a vast oceanic region that is influenced by ocean circulation features such as El Niño, while the Northern Hemisphere is dominated by most of the Earth’s continental masses.

By Joëlle Gergis

The first comprehensive reconstruction of the Southern Hemisphere’s temperature over the past millennium reveals that Northern Hemisphere warm and cool periods were not global. But what about late 20th century warming?

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Long before humans were keeping official weather records, the natural world was tattooing the passing of time, year after year, for centuries. Cycles of good and poor seasons influence the size of a tree’s annual growth ring, while river runoff and changes in sea temperatures affect the thickness of coral skeletons growing in tropical waters.

In cooler locations, the chemical signatures of wet and dry years are layered into cave drip deposits as rain seeps through the porous limestone from the ground above. Ice cores taken from polar regions contain bands of accumulated snow that respond to changes in moisture availability as atmospheric winds shift in response to the seasonal expansion and contraction of sea-ice and high altitude glaciers.

In 1998, Prof Michael Mann of Pennsylvania State University published a seminal temperature reconstruction that used these kinds of natural climate records to estimate temperature variations in the Northern Hemisphere since 1400 AD. Extensions of that study appeared in the Intergovernmental Panel on Climate Change’s (IPCC) third assessment report in 2001 along with four other reconstructions supporting the same conclusion. The graph was termed the “hockey stick” to describe the pattern this showed – a relatively flat period of temperature variations until the start of the 20th century corresponding to the hockey stick’s...

The full text of this article can be purchased from Informit.