Friday, May 18, 2012

Climate research has a ring of truth

Adam Morton 
The Age, May 19, 2012   

VISITING the giant kauri trees of Northland, on New Zealand's north-west coast, is like stepping back in time. Ancient conifer pines that over centuries have escaped damage from fire and forestry, the surviving kauris are up to 50 metres tall and five metres across - as wide as a 12-seater mini-bus is long.

Scientists estimate some have survived for two millennia and consider them the southern hemisphere's answer to California's redwoods, the world's biggest trees. Like the redwoods, age has transformed the kauris into a time capsule. Their tree rings - inner markings that reveal growth patterns through centuries - carry precise insights into changes in the world's climate conditions stretching back long before the advent of modern scientific measurement.

Since the 1970s, scientists have been extracting this knowledge using increment borers - giant corkscrews that collect long slivers of timber thin enough to slide inside a drinking straw. A decent sample will include hundreds of tree rings spanning centuries. When scientists get really lucky, they find a preserved dead tree that collapsed into a swamp and fossilised, allowing them to stitch together a picture dating back millennia.

On each data collection trip at least three samples of tree ring information are removed from each pine examined and a minimum 10 trees sampled across a tract of forest. The goal is to get a clear climate signal unaffected by natural variation between individual trees.

''Tree research is all about replication. You are looking at the growth patterns and trying to figure out whether the tree is responding to temperature or rainfall,'' says Dr Joelle Gergis, a palaeoclimatologist at the University of Melbourne's School of Earth Sciences, who spent two years collecting and analysing kauri data. Early last decade, data collected at Northland led to a peer-approved study that named the years over the past four centuries in which the climate had been most influenced by strong El Nino events over the Pacific.

This week the north New Zealand tree ring data was used to help paint a much bigger picture: what scientists say is the most complete climate record available for Australasia covering the past millennium.

The first study of its kind, it drew on 27 biological and geological records, including 12 tree ring data sets, mainly from New Zealand, 13 coral sets from Indonesia, the south-west Pacific and Western Australia and two ice cores from Antarctica. Scientists used the data to produce a long-term temperature record, and found the past five decades were very likely to have been the warmest since 1000AD.

Published in the Journal of Climate and drawing on decades of work by 30 scientists, it will help form the basis for the Australasian palaeoclimate section of the next report by the Intergovernmental Panel on Climate Change, due in 2013-14.

According to Gergis, the study's lead author, rigorous analysis had confirmed the reconstruction was robust. The numbers were crunched in 3000 different ways.

"The take-home message here is that 95 per cent of our 3000 reconstructions show that there are no 50-year periods in the past 1000 years that match or exceed the post-1950 warming," she says.

Co-author and University of Melbourne climate science professor David Karoly says the study for the first time establishes that claims there was a substantial mediaeval warm period hotter than today had no basis in Australasia. The study uses climate proxies - surrogates for the record of observed temperatures that date back to only the early 20th century.

Initially, the data from tree rings and other sites was tested for its ability to reconstruct temperatures between 1921 and 1990. The palaeoclimate records from 50 sites were compared to the actual temperature record for these years. The palaeoclimatic data that did not display a statistically significant temperature signal, but was found to have been more strongly influenced by other climate factors such as rainfall, was excluded. But the data from the 27 sites that remained collectively matched the actual temperature increase with a high correlation coefficient of 0.83, and were considered suitable for use as a proxy for the real thing to reconstruct temperatures over previous centuries.

The results matched what was known about certain historical periods. It was found early European settlers would have suffered through the coldest period of the past millennium in the 1830s and 1840s - the peak of what is known as the global little ice age. In pre-industrial times, the warmest lengthy stretch was found to be between 1238 and 1267, which the study estimates was 0.09 degrees cooler than the mid-to-late 20th century average.

But the warmest decades were found to be the last three examined: the 1970s, 1980s and 1990s. Instrumental temperature records show the first decade of the 21st century was hotter again.

The study is considered a step towards filling a gap in the long-term temperature record. To date, data from the southern hemisphere has been sparse compared with the north, in part because ocean cover is greater and also because scientific measurements on what land there is began later.

In the northern hemisphere, an earlier study had already found the second half of the 20th century was very likely to be the warmest in the past 500 years, and likely to be the warmest for the past 1300 years.

The historic reconstructions are being used as part of an international collaboration known as Past Global Changes, or PAGES. Its backers aim to improve the accuracy of future climate projections by reconstructing the past 2000 years of climate across the world. Those projections come from computer model simulations. These were also used in the PAGES work.

Dr Steven Phipps, a research fellow at the University of New South Wales Climate Change Research Centre, used three climate models to simulate the temperature patterns of the past millennium, at different times factoring in natural drivers of temperature such as volcanic eruptions and the sun and the human influence of greenhouse gas emissions.

Examined side by side, neither the average of the three climate model simulations nor the reconstructed temperature record shows any long-term trend before about 1850. In both, regional temperatures fall in line with known events, such as the 1815 eruption of Mount Tambora in Indonesia. Otherwise they bounce up and down independently. "What this demonstrates is that these variations are likely to be due to natural climate variability, which is a random process," Phipps says.

After 1850, both the model simulation and reconstruction show an unprecedented leap in warming. Phipps carried out three simulations in a bid to explain the warming, variously excluding known natural and human influences. He found the warming could only be produced through the inclusion of greenhouse gas emissions - clear evidence, he says, of human impact on Australasian temperature.

The veracity of the proxy methods used is not universally accepted. Professor Graham Farquhar, a biophysicist at the ANU's Research School of Biology, says the use of surrogates is problematic. For example, he says trees are likely to have grown faster between 1921 and 1990 due to the increased atmospheric concentrations of carbon dioxide, not just the rise in temperature. ''It is obviously very useful to have such data, but I can't see it as being definitive,'' he says.

The authors dismiss this concern. Karoly says that nothing is certain in science, but the results draw from a range of sites and using state-of-the-art statistical methods can be accepted with high confidence: ''It is reinforcing that barrage of scientific information that confirms that the climate is warming and increasing greenhouse gases are the major cause.''

Wednesday, May 16, 2012

Ocean temperature made Queensland floods worse: study

Nicky Phillips 
The Age, May 16, 2012

Abnormally high ocean temperatures off the coast of northern Australia contributed to the extreme rainfall that flooded three-quarters of Queensland over the summer of 2010-11, scientists report.

A Sydney researcher, Jason Evans, ran a series of climate models and found above average sea surface temperatures throughout December 2010 increased the amount of rainfall across the state by 25 per cent on average.

While the study did not look at the cause of ocean warming in the region, a physical oceanographer, Matthew England, said climate change could not be excluded as a possible driver of this extreme rainfall event.

Between December 23 and 28 many places experienced up to 400 millimetres of rain in a few days. "That [means] 100 millimetres of rain was attributable to sea surface temperatures," said Dr Evans, a future fellow at the University of NSW's Climate Change Research Centre.

While the flooding occurred during one of the strongest La Nina events on record it was insufficient to produce the extreme rainfall recorded, he said.

The effect of the high sea surface temperatures coupled with the impact of a La Nina, both of which are associated with above average rainfall over eastern Australia, plus tropical cyclone Tasha, combined to create an extreme weather event, he said.

The resulting floods stretched across 1.3 million square kilometres all the way to Brisbane, caused billions of dollars in damage and killed 35 people.

Matthew England, who was not involved in the study, said ocean temperatures off northern Australia were the highest on record at the time of the Queensland floods.

"While the La Nina event played a big role in this record ocean warmth, so too did the long-term warming trend over the past 50 years," Professor England, the co-director of the UNSW Climate Change Research Centre, said.

To measure the extent high sea surface temperature contributed to the rainfall, Dr Evans used a regional climate model to compare the effect of the 2010 December sea surface temperatures with the sea surface temperatures from previous La Nina events.

"All the simulations produced high rainfall, but only the simulations with the high sea surface temperatures [from December 2010] were able to produce extreme precipitation," said Dr Evans, whose findings are published in the journal Geophysical Research Letters.

Warmer sea surface temperatures increase the amount of moisture transferred from the ocean to the atmosphere.

"If you've got the right winds they carry this moisture to land, and [because] land is hotter than the ocean during the day it will cause convection and rain," Dr Evans said.

While both simulations underestimated the amount of rain that fell in Queensland in December 2010, the model that used the higher sea surface temperatures came closest to the rainfall recorded in the region.

"The model doesn't replicate the observations perfectly but it clearly shows what we saw, which was from Cairns to south-east QLD all [regions] received abnormally high precipitation," he said.

If increases in sea surface temperatures can be attributed to global warming, the probability of La Nina events producing extreme rainfall in the future would also rise, he said.

Last 50 years were Australia's hottest: study

By Matthew Carney 
ABC Online, May 17, 2012
For the first time scientists have provided the most complete climate record of the last millennium and they found that the last 50 years in Australia have been the warmest.
The researchers from Melbourne University used 27 different natural indicators like tree rings and ice cores to come to their conclusion, which will be a part of the next United Nations intergovernmental panel on climate change report.
The findings show that no other period in the last 1,000 years matches the temperature rises Australia and the region has experienced in the last 50 years.
Report co-author Joelle Gergis says the findings are significant.
"It does show that the post-1950 warming is unusual in the Australasian region," she said.
"A lot of these sorts of studies have come out of the northern hemisphere and for the first time we have been able to say 'well we have collected all of our natural records from our region and this is what it shows and the warming is real and it is in the Australian region. It is not in some far away place'," she said.
Dr Gergis says the study used decades of work from 30 scientists who had been collecting natural data to reconstruct temperatures before human records started in 1910.
"So really what these are are climate proxies. They are not direct temperature records but we use them as stand-ins or surrogates for temperature records," she said.
"What we do is compare these natural records with the observed temperature records and then develop a statistical relationship and take that relationship back centuries into the past."
Co-author Professor David Karoly says the strength of the study is that it relied more on direct observations and measurements than climate modelling.
"Nothing is absolutely certain in science, but we say with very high confidence because we have repeated the analysis alone for the uncertainties that the warming in the last 50 years is very unusual and very likely cannot be explained by natural climate variability alone," he said.
Dr Gergis says the scientists have minimised the variability in their model by crunching the data 3,000 different ways.
"What we were able to see is that in 95 per cent of the reconstructions, we actually see that the post-1950 warming observed in the region is unprecedented in the context of the last millennium so it is not dependent on a loss of records back in time or the different combination of record," he said.
The government-funded study will be Australia's contribution to the fifth Intergovernmental Panel on Climate Change report due in 2014.