Last 100 years reverse 1400 years of global cooling

Rebecca Scott
Media Officer
University of Melbourne
+613 83440181

Alvin Stone
Media and Manager
Centre of Excellence Climate System Science, UNSW
+612 9385 8953

The first continental-scale reconstruction of temperatures over the past 2000 years has found 20th Century warming was a global event that has produced the hottest global average temperature in 1400 years.

The study by 78 scientists from 24 countries was published and combined Northern and Southern Hemisphere temperature reconstructions has highlighted the unusual nature of the 20th century warming.

The findings are in stark contrast to the Mediaeval Warm Period and Little Ice Age, which the study revealed did not occur simultaneously across the globe.

The international study led by the Past Global Changes (PAGES) network in Switzerland, included Australian scientists from the University of Melbourne, the ARC Centre of Excellence for Climate System Science at the University of New South Wales and the Australian Antarctic Division.

Co-author of the paper from the University of New South Wales’ ARC Centre of Excellence for Climate System Science, Dr Steven Phipps said the striking feature about the sudden rise in 20th Century global average temperature is that it comes after an overall cooling trend that lasted more than a millennium.

"This work has transformed our understanding of temperatures over the past 2000 years," he said

"This research shows that in just a century the Earth has reversed 1400 years of cooling.

"Armed with this information, future researchers will able to better understand the causes of climate variability at a regional and global level and help forecast the changes we can expect as our planet warms."

To reconstruct 2000 years of temperature, the researchers used data from 511 individual proxy records. The majority of these records came from tree ring measurements but also included coral reefs, cave formations, ice cores, lake sediments and some historical documents.

The 2000-year temperature snapshot revealed by the researchers showed a long-term cooling trend before human influences began to become significant. This trend was primarily driven by natural cycles in the Earth’s orbit. At the same time there were also natural fluctuations caused by volcanic eruptions and variations in solar activity.

It is these natural variations, and in particular the changes in the Earth’s orbital cycle, that explain why some of the average global temperatures prior to AD 600 were as warm as today. However, none of these natural influences account for the dramatic global temperature rises of the 20th Century.

Palaeoclimatologist Professor Jonathan Overpeck from the University of Arizona, US who is a Visiting Fellow of the Victorian Centre for Climate Change Adaptation Research at the University of Melbourne and Visiting Scientist with the ARC’s Centre of Excellence For Climate System Science, said the work was very exciting.

"The work has confirmed that there was no period of global warmth similar to that of the last 60 years in the preceding circa 2000 years, and adds to the evidence that the so-called Medieval Warm Period was quite different from the recent period of human-caused warming," he said.

"This paper has provided confirmation that recent global and continental scale warming was very unusual in recent Earth history, and that this recent warming is driven mostly by human emissions of greenhouse gases.

"The evidence in the paper is overwhelming that Australia will continue to warm, and this means the region will likely also experience more periods of unusual dryness unless global warming is stopped."