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Science Media Centre: Reconstruction of 2000 years of global temperatures – experts respond


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A group of international scientists, including New Zealanders, have for the first time reconstructed temperatures for seven continental-scale regions for the past 2,000 years.

Screen Shot 2013-04-22 at 9.33.27 AMThe detailed global climate record reveals that the period from the 1970s until 2000 was the warmest in 1,400 years.

“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,” said Dr Steven Phipps, an author of the paper from the University of New South Wales’ ARC Centre of Excellence for Climate System Science.

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

Below New Zealand and Australian experts comment.

[*Continental-scale temperature variability during the past two millennia, Kaufman et al., Nature Geoscience]

Climate scientist, Dr. Andrew M. Lorrey, National Institute of Water and Atmospheric Research and contributor to the project comments:

“New Zealand’s specific contribution to the paper was from a series of tree ring chronologies that were generated over many decades by researchers based here and abroad.

“This PAGES2k paper represents a ‘work in progress’ that shows how temperature for Australasia, and other continental-scale regions of the globe, has varied over the last 2000 years.
The study clearly shows a good deal about natural variability and how the Australasian domain compares to the rest of the world.

“There were times in the past when Australasia, including New Zealand, was clearly not in synch with other regions. There are also unique signatures in the past that show Australasian temperatures were different from all other regions. This points to the fact that palaeoclimate data from New Zealand are valuable, and they can help us understand past climate changes at both a local and global scale.

“We have a unique perspective to offer to in a project like this because we have such a diverse array of natural archives in New Zealand.

“Project contributors are presently using tree rings, glacial deposits, lake sediments and stalagmite records from caves to reconstruct New Zealand’s past terrestrial climate.

“By looking at these archives, we hope to reveal what precipitation, temperature and atmospheric circulation were like beyond the instrumental record. We hope to learn more about significant events like severe droughts and periods of storminess in the pre-written era and why they occurred, which can help put current events into a long-term perspective.”

Dr Jim Salinger, climate scientist, 2012 Lorrey Lokey Visiting Professor, Stanford University comments:

“The paper by the PAGES consortium examining temperature variability for representative regions of a quarter of the surface of the earth is very significant. It is also consistent with New Zealand past climate work. The PAGES research is the first truly global examination of temperature fluctuations, as against previous ones for the Northern Hemisphere only.

“The next noteworthy finding is that no multidecadal synchronous global warm period would qualify for a Medieval Warm Period (MWP), firstly postulated by the pioneer of UK climate change studies Hubert Lamb. This study finally sets this hypothesis aside.

“PAGES show that from 500 to 1900 CE the global cooling rate was 0.1 to 0.3°C/1000 years culminating in generally cooler period from the 17^th to 19^th century. New Zealand is fortunate in that it is rich in paleoclimatic data including pollen caught in peat bogs, speleothems (stalagmites and stalactites), tree rings and glacier deposits, the latter indicating cooler periods of climate.

“These indicate a record consistent with the PAGES research. Warmer periods of New Zealand climate around 1200, in the 1300s and 1500s CE. Cooler periods of climate are indicated in the 1600s, 1720 CE and 1845 – 1880 CE. The recent cooler periods in the last millennium locally culminating from 1600 tob 1900 CE are in concert with those seen in the PAGES study.

“Common to New Zealand evidence and PAGES is that the 1971-2000 period was the warmest in the last millennium and beyond, and the average 20th century land temperature was 0.4°C higher than that averaged over the preceding five centuries. Research from New Zealand and PAGES shows that the end of the 20th century has been unusually warm in the record of the last millennia.”

Dr James Renwick, Associate Professor of Physical Geography, School of Geography, Environment and Earth Sciences, Victoria of University, comments:

“This careful analysis of a comprehensive range of proxies covering the past two thousand years of climate history demonstrates yet again that the modern era exhibits a very different climate to what has gone before. For all the natural variations seen over past millennia, the rapid changes we are seeing now, induced by human emissions of greenhouse gases, are another story.

“The rapid rate of change, and just the magnitude of change, are obviously very out of the ordinary. That thirty years of warming at the end of the 20th century could more than reverse 1400 years of cooling is extremely striking. What it implies about the magnitude of change we are likely to see in the future is concerning, to say the least.”

Comments gathered from Australian scientists by our colleagues at the AusSMC:

Dr Helen McGregor is an AINSE Research Fellow in the School of Earth and Environmental Sciences at the University of Wollongong. Dr McGregor also co-leads the Ocean 2K sub-branch of the PAGES 2k Network, which is separate from the groups contributing to the article above – she had no input into the analysis, interpretation, or drafting of the Nature Geoscience paper. She comments:

“This paper synthesises data of past climate change, where the data comes from corals, ice cores, tree rings, lake and marine sediments, historical records, cave deposits, and pollen climate archives. What makes this paper unique is that it looks at regional signals of climate change, rather than producing one global compilation. Knowing how individual regions differ in their climate trends is important as it will improve our ability to plan for and manage future climate change. A second strength of the paper is that combines the climate archive data using a range of different methods. This makes their key finding, that for all regions the cooling from 600-1900AD is reversed by recent warming, a robust result.

“That individual decades in the regional compilations are warmer (or cooler) than the 1971-2000AD interval is unsurprising. These decadal variations represent a combination of climate variability and the inherent noise in records making up the compilation. There is always natural climate variability and what we are seeing today is the human-induced climate change superimposed over the top. For most regions, on balance, the 1971-2000AD period is unusually warm and I would expect to see the decades to come being even warmer.”

Professor Jonathan Overpeck is Professor of Geosciences and Atmospheric Sciences at the University of Arizona, USA. He is currently on sabbatical at the University of Melbourne, where he is a Visiting Fellow of the Victorian Centre for Climate Change Adaptation Research, as well as a Visiting Scientist with the Australian Research Council’s Centre of Excellence For Climate System Science. He comments:

“The work is most exciting in that it provides confirmation that recent global – and continental scale – warming is indeed very unusual in recent Earth history, and that this recent warming is driven mostly by human emissions of greenhouse gases. The work confirms that there was no period of global warmth similar to that of the last 60 years in the preceding ca. 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.

“The new paper also shows the change that volcanic eruptions and changes in solar irradiance can imprint on the global climate system, but once again that these natural causes of climate variability cannot explain the most recent period of global warming.

“Although the paleotemperature record from Australia presented in the paper is shorter than available for some of the other continents, it does highlight how the change in Australia mirrors the more global patters of temperature variation over the last 1000 years. This result is not surprising, and fits well with the wide range of other evidence that global climate change – i.e., global warming – is having a significant impact on the climate of Australia.

“The evidence is thus overwhelming that Australia will continue to warm and also experience more periods of unusual dryness unless global warming is stopped.”

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