Figure above: Thirty-year mean temperatures for the seven PAGES 2k continental-scale regions arranged vertically from north to south. Colours indicate the relative temperature. The most prominent feature of nearly all of the regional temperature reconstructions is the long-term cooling, which ended late in the19th century. North America includes a shorter tree-ring-based and a longer pollen-based reconstruction. Modified from: PAGES 2k Consortium, 2013, Continental-scale temperature variability during the past two millennia ( Nature Geoscience, DOI:10.1038/NGEO1797).
by Steven Phipps
A comprehensive new scientific study has revealed fresh insights into the climate of the past 2,000 years, providing further evidence that the 20th century warming was not a natural phenomenon.
Increasing temperatures after 1900 reversed a previous long-term cooling trend. While the so-called Mediaeval Warm Period and Little Ice Age are found to have had no global expression, the warming during the 20th Century occurred simultaneously in all regions except Antarctica.
The study, published online today in the journal Nature Geoscience, was produced by the PAGES (Past Global Changes) 2k Consortium. Formed seven years ago, this consortium brings together 78 experts from 24 different countries.
By combining records from natural archives such as tree rings, coral reefs and ice cores, the scientists have been able to reconstruct past temperature changes at the continental scale. This reveals new behaviour that was not apparent in previous reconstructions, which have focused on changes at the hemispheric or global scale.
Prior to the 20th century, the most coherent feature is a slow, long-term cooling trend. This was driven by a combination of natural factors, including volcanic eruptions, changes in solar output and changes in the Earth's orbit.
The regional nature of these new reconstructions also reveals that temperature fluctuations from one decade or from one century to the next are not uniform. In particular, there is no evidence for a globally-synchronous Mediaeval Warm Period or Little Ice Age.
While the Northern Hemisphere regions are warmer-than-average during the period from 830 to 1100, Australasia and South America do not experience warmer temperatures until several centuries later.
Likewise, although temperatures in all the regions are generally cooler-than-average from around 1580 to 1880, they do not experience any simultaneous cold period.
After 1900, however, the long-term cooling trend reverses. A coherent warming is apparent in all continents except for Antarctica. The 20th century is the warmest, or nearly the warmest, century in these regions.
Furthermore, the average temperature for the period 1971-2000 is likely to have been higher than at any point during the previous 1400 years. Natural factors alone cannot explain this warming, suggesting that increasing greenhouse gases are responsible.
This new scientific effort featured a strong Australian contribution, with six of the 78 authors coming from the University of New South Wales, the University of Melbourne, the Australian Antarctic Division and the Antarctic Climate & Ecosystems Cooperative Research Centre.
The temperature reconstruction for Australasia shows that our climate has exhibited similar variations to other regions.
Warm conditions persist from the 11th to the 16th centuries, followed by cool conditions that last right through to the early 20th century. However, a rapid warming then occurs, with the period 1971-2000 being hotter than any other time during the previous millennium.
The past has revealed its secrets. Our climate is changing, and it is likely that we are responsible.
Following best scientific practice, all the data collected as part of this study has been made freely available. These new reconstructions will provide a valuable resource for future research, allowing us to evaluate and refine the climate models that are used to project future climate change.