Warming over the 20th century produced the hottest global average temperatures in 1400 years, a major scientific research project has found.
In a paper published in the journal Nature Geoscience on 21 April, an international team of scientists reconstructed temperatures over the past 2000 years using proxy records such as tree-ring measurements, pollen sampling, coral reefs, ice cores and historical records.
The study found that the global warming that began in the late-19th century reversed a long cooling trend across the planet that lasted well over 1000 years.
One of the authors of the paper, Dr Steven Phipps from the University of NSW's ARC Centre of Excellence for Climate System Science, 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 research shows that in just a century the Earth has reversed 1400 years of cooling."
When the reconstructed temperatures are averaged across the planet, the best estimate of past temperatures finds 1971–2000 was warmer than any other time in nearly 1400 years.
But some of the seven different regions studied may have experienced similar or warmer temperatures at other times.
The paper says the findings do not consider uncertainty associated with the temperature estimates, and the reconstruction for each area covered different periods within the 2000 years, depending on the availability of data.
In Australia and Asia, the reconstructed temperature was highest during 1971–2000 than at any other period over the studied timelines.
The paper says the millennium-long cooling trend – and earlier warmer temperatures – were primarily driven by natural cycles in the Earth's orbit and some volcano and solar activity. But natural influences do not account for the dramatic global temperature rises of the 20th century, the report says.
Rising greenhouse gas emissions are attributed to human activity, mainly from the burning of fossil fuels and, while the recent warming is a global trend, other temperature anomaly periods – such as the Mediaeval Warm Period and the Little Ice Age – occurred only regionally.
Professor Jonathan Overpeck, visiting fellow of the Victorian Centre for Climate Change Adaptation Research, said it confirmed that recent global and continental-scale warming was "very unusual" in recent Earth history and was driven mostly by human emissions of greenhouse gases.