BY PAIGE BROWN
Determining patterns of how local and global climate conditions vary from decade to decade and century to century is paramount to differentiating between natural climate variability, or patterns of climate that vary regularly from hot to cold periods every few decades for example, and human-driven climate forcing in the 20th century. When instrumental records of temperature collected from meteorological stations and satellite observations are inadequate for determining longer scale patterns of climate variability around the world, researchers such as Kristine DeLong, assistant professor of geography at LSU, must turn to records buried in geological samples.
The skeletons of these boulder-size corals, like tree rings, provide a datable history of environmental conditions that can date back hundreds to thousands of years ago.
“The corals can speak to us and tell us what it was like in the past,” DeLong said. “I tell my students that the coral is like a Rosetta stone – the coral records the history of environmental factors such as temperature in its skeleton, we just need to understand how to read the skeleton.”
DeLong and collaborators’ research, published in June 2012 as an advance online publication in the prestigious journal Nature Climate Change, confirms that, contrary to previous assumptions, climate in the Southern Hemisphere does not work the same as climate in its northern counterpart. Temperature records extracted from the corals of New Caledonia match up beautifully with a newly discovered pattern of Pacific climate variability known as the South Pacific Decadal Oscillation, or SPDO. Patterns of climate that vary on 10-20 year timescales associated with the SPDO are known to impact rainfall anomalies in the western tropical Pacific region. DeLong and collaborators found that ocean temperature in the southwest tropical Pacific does not vary with changes in solar irradiance and sunspot activity, whereas a dip in sunspot activity coincides with a cold period known as the Little Ice Age in the Northern Hemisphere. DeLong suspects that patterns in ocean currents may play a larger role in controlling climate variations in the South Pacific. “The South Pacific Ocean takes up nearly a quarter of the Earth,” DeLong said. “We basically didn’t know how climate over a quarter of the Earth changed going back more than 50 years ago, which is the limit of instrumental records for that part of world. We don’t know all the answers yet, but we are making strides in getting there by studying geological records such as corals.” DeLong and collaborators’ research also reveals that the pattern of climate variation in the southwest tropical Pacific changed significantly after 1893, a date coinciding with the onset of a recent global warming trend.
DeLong and her collaborators found that during the period of global warming in the 20th century, climate oscillations in the South Pacific have been speeding up.
“Patterns climate are changing faster,” DeLong said. “Before the global warming trend, multidecadal variability in the southwest tropical Pacific existed on longer time scales than it does now. It is neat that we were able to detect these changes in climate oscillations. It’s one of those things that is really hard to pin down with just instrumental records.”
As New Caledonia was not modernly settled by Europeans until late 1800s, the fact that DeLong and collaborators recorded local temperatures matching up with the global warming trend reveals a globally connected climate.
“It drives home how we are all connected through the atmosphere, whether we like it or not,” DeLong said. “The atmosphere is inherently a noisy system, but there are certain things on long term scales that we can predict. Over the long term if you change the temperature of the ocean, it affects the atmosphere.”
DeLong and collaborator’s study is the first to use records from two different coral cores and is unique in the use of extremely precise methodologies to extract temperature data and absolute age of coral samples accurate down to the year. With this improved chronological accuracy, climate modelers can use the new record of southwest tropical Pacific climate to improve simulations of future climate conditions based on both natural climate variability and human induced changes.
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For more information about the original research article, click http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1583.html.