(RENO) – Using more than 500 temperature records, obtained from tree rings, pollen, corals, lake and marine sediments, glacier ice, and historical documents, a team of 80 international scientists has revealed that the 30-year period from 1971-2000 was statistically warmer than any other 30-year period in the last 1,400 years.
Hydrogeologists and atmospheric scientists from Nevada’s Desert Research Institute (DRI), led by Joe McConnell Ph.D., performed a precise dating of one of the West Antarctic Ice Sheet (WAIS) Divide ice cores used in the study.
The stable isotope record (d18O) derived from the WAIS ice core contained information on past temperatures in this remote polar region. Together with an ice core record that was recently obtained from James Ross Island, on the Antarctica Peninsula, the two datasets are the first to give insight into climate variability over the last 2,000 years in West Antarctica.
DRI postdoctoral fellow Michael Sigl, a member of the team that analysed the WAIS ice core in DRI’s unique ultra-trace ice core analytical laboratory, said DRI’s team “built a robust, continental scale record of past climate variability by synchronizing stable isotope time series using records of sulphate that was deposited over Antarctica by explosive volcanism.”
Sigl explained that ice cores contain seasonally resolved aerosol records from biomass burning, sea salt and dust deposition that allow scientists to successfully determine the age of the ice samples, similar to counting annual growth layers in trees.
Forty-two volcanic events from predominantly tropical volcanoes were identified in the various ice cores and used as time markers to place the temperature reconstructions from all over Antarctica on a common and precise timescale.
The study was coordinated by the international Past Global Changes (PAGES) organization, which receives funding mainly from the Swiss and US National Science Foundations. Previous attempts to reconstruct temperature changes focused on hemispheric or global-scale averages, which are important, but overlook the pronounced regional-scale differences that occur along with global changes, according to the authors.
"A key aspect of the (PAGES) consortium effort was to engage regional experts who are intimately familiar with the evidence for past climate changes within their regions," said Darrell Kaufman, professor at the Northern Arizona University in Flagstaff and leading co-author of the article. "Several mathematical procedures were applied to reconstruct the continental temperature time series and they were compared to assess the extent to which the main conclusions of the study stood up to the different analytical approaches."
Long-term cooling trend reversed
The most coherent feature in nearly all the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century and the authors’ suggests was likely caused by a combination of factors such as an overall increase in volcanic activity, a decrease in solar irradiance, changes in land cover, and slow changes in earth’s orbit.
The study found that the warming during the last century has reversed this long-term cooling trend, with cold temperatures only remaining in the Antarctic region. An analysis of the average temperatures over 30-year periods indicates that interval from 1971-2000 was probably warmer than any other 30-year period in the last 1,400 years, the authors’ report.
Cooler 30-year periods between the years 830 and 1910 AD were particularly pronounced during weak solar activity and strong tropical volcanic eruptions. Both phenomena often occurred simultaneously and led to a drop in the average temperature during five distinct 30- to 90-year intervals between 1251 and 1820. Warming in the twentieth century was on average twice as large in the northern continents as it was in the Southern Hemisphere. During the past 2000 years, some regions experienced warmer 30-year intervals than during the late 20th century. For example, in Europe the years between 21 and 80 AD were likely warmer than the period 1971-2000.
The study also reports that evolution of temperature change across all the continents was noticeably more similar within the hemispheres than between the Northern and Southern Hemisphere.
"Distinctive periods, such as the Medieval Warm Period or the Little Ice Age stand out, but do not show a globally uniform pattern," said professor Heinz Wanner of the University of Bern and one of the original architects of the PAGES 2k Network.
By around 1,500 AD temperatures did indeed fall below the long-term mean across the globe.
However, in the Arctic, Europe and Asia this temperature drop occurred several decades earlier than in North America and the Southern Hemisphere. These new findings, Wanner believes, “will certainly stimulate vibrant discussions within the research community.”
For more information about the PAGES project, other resources, and additional contact information please visit: http://www.pages-igbp.org/workinggroups/2k-network
To view the Nature GeoScience paper please visit: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1797.html
“Continental-scale temperature variability during the past two millennia”
Nature Geoscience (2013) doi:10.1038/ngeo1797
Received 09 December 2012, Accepted 11 March 2013, Published online 21 April 2013
PAGES was established in 1991 to facilitate international research into understanding climatic and environmental dynamics by studying the past. The program receives funding mainly from the Swiss and US national science foundations. In 2006, ambitious scientists in the PAGES network decided to organize an initiative to reconstruct the climate of the last 2000 years in unprecedented quality.
The PAGES 2k Consortium, in coordination with NOAA Paleoclimatology, has tabulated all of the data used for the continental-scale reconstructions in a uniform and user-friendly format. This data product will be useful in future studies, including as a benchmark for comparisons with climate-model simulations that attempt to account for both anthropogenic and natural factors in projections of future climate.
Additional information about the study will be available on the PAGES website at - http://www.pages.unibe.ch/
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