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Climate change ‘helped to end monsoon 4,000 years ago’

February 27, 2014 in Archaeology, Climate, Drought, History, Monsoon, Palaeoclimatology

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A well and bathing platforms from Harappa, brought low when the monsoon weakened Image: Obed Suhail via Wikimedia Commons

A well and bathing platforms from Harappa, brought low when the monsoon weakened
Image: Obed Suhail via Wikimedia Commons

By Tim Radford

Drought appears to have played a significant part in the collapse of a vibrant community in south-west Asia several thousand years ago, British researchers say – with lessons for us today.

LONDON, 27 February – Climate change can seriously damage a civilisation. An “abrupt weakening” of the summer monsoon in north-west India accompanied the decline of the great cities of the Indus valley more than 4,000 years ago, according to new research by British scientists.

They analysed the oxygen isotopes in snail shells preserved in ancient lake sediments to build up a picture of rainfall patterns in prehistory, and found the first direct evidence that sustained drought contributed to the collapse of a great Bronze Age civilisation, they report in the journal Geology.

The Indus or Harappan civilisation – after Harappa, one of the five great ancient settlements of what is now Pakistan and western India – was marked by the world’s first “megacities”, concentrations of population in built-up areas that covered more than 80 hectares.

“They engaged in elaborate crafts, extensive local trade and long-ranging trade with regions as far away as the modern-day Middle East,” said Cameron Petrie of the University of Cambridge. “But by the mid-second millennium BC, all the great urban centres had dramatically reduced in size or been abandoned.”

The finding links the decline of the Indus civilisation to what now seems a much greater scale event: the failure of Early Bronze Age civilisation in Greece and Crete, the weakening of the Old Kingdom in Egypt, and the crumbling of the Akkadian Empire in Mesopotamia.

Common factor

Researchers last year used pollen grain sediments in an old lake bed in Cyprus to build up a picture of sustained environmental decline that accompanied the collapse of the civilisations linked with Mycenae in Greece and Knossos in Crete. In all cases, there must have been a number of factors at work, but common to them all was a pattern of drought.

Without water crops fail, populations fall and concentrations of people must disperse. Archaeological evidence in the north-west Indian sub-continent has told a story of dispersal. Palaeontological evidence from an old lake bed has confirmed the picture of a changing climate.

“We think we now have a really strong indication that a major climate change event occurred in the area where a large number of Indus settlements were situated,” said David Hodell, an earth scientist at Cambridge, and one of the authors.

“Taken with other evidence from Meghalaya in north-east India, Oman and the Arabian Sea, our results provide strong evidence for a widespread weakening of the Indian summer monsoon across large parts of India 4,100 years ago.”

The authors collected shells of the water snail Melanoides tuberculata from the sediments on an ancient lake, Kotla Dahar in Haryana, India. The changes in oxygen isotope ratios over a period of thousands of years told the researchers a story of a deep lake that became a shallower one, as evaporation accelerated and water supplies dwindled, and then all but disappeared, with an abrupt weakening of the monsoons that lasted around 200 years.

Syrian parallel?

Oxygen occurs in two isotopes: water molecules containing the lighter variant evaporate at a predictably faster rate than the heavier version. In a drought, the ratios of the heavier version increase, and this increase is preserved in the calcium carbonate of the snail shells, which can in turn be dated by radiocarbon measurements.

Archaeological evidence suggests that around the time of the 200-year drought, streets that had once been well-maintained started to fill with rubbish, craftsmanship seemed to become less sophisticated, and the locations of settlements changed.

“It is essential to understand the link between human settlement, water resources and landscape in antiquity,” said Dr Petrie. “We hope that this will hold lessons for us as we seek to find means of dealing with climate change in our own and future generations.”

Simultaneously, a French academic has argued that extended drought may have played a role in the crisis in Syria right now. Francesca de Chatel of Radboud University in the Netherlands writes in Middle Eastern Studies that in her opinion the bloodshed and turmoil in Syria is the culmination of 50 years of sustained mismanagement of water and land resources, capped by a severe drought during the years 2006-2010.

She argues “It was not the drought per se, but rather the government’s failure to respond to the ensuing humanitarian crisis that formed one of the triggers of the uprising, feeding a discontent that had long been simmering in rural areas.” – Climate News Network

Tree roots ‘are natural thermostat’

February 18, 2014 in Carbon Dioxide, Forests, Mountains, Palaeoclimatology, Soil

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In sight of the Carpathians: Mountain forests can cool - and warm - the Earth Image: Horia Varlan from Bucharest, Romania, via Wikimedia Commons

In sight of the Carpathians: Mountain forests can cool – and warm – the Earth
Image: Horia Varlan from Bucharest, Romania, via Wikimedia Commons

By Tim Radford

Trees can influence the climate in unexpected ways, and British researchers say their roots are an important way of helping rocks to weather and drawing carbon dioxide from the atmosphere.

LONDON, 18 February – Trees have become a source of continuous surprise. Only weeks after researchers demonstrated that old forest giants actually accumulate more carbon than younger, fast-growing trees, British scientists have discovered that the great arbiters of long-term global temperatures may not be the leaves of an oak, a pine or a eucalypt, but the roots.

The argument, put by a team from Oxford and Sheffield Universities in the journal Geophysical Research Letters, begins with temperature. Warmer climates mean more vigorous tree growth and more leaf litter, and more organic content in the soil. So the tree’s roots grow more vigorously, say Christopher Doughty of Oxford and colleagues.

They get into the bedrock, and break up it up into its constituent minerals. Once that happens, the rock starts to weather, combining with carbon dioxide. This weathering draws carbon dioxide out of the atmosphere, and in the process cools the planet down a little. So mountain ecosystems – mountain forests are usually wet, and on conspicuous layers of rock – are in effect part of the global thermostat, preventing catastrophic overheating.

The tree is more than just a sink for carbon, it is an agency for chemical weathering that removes carbon from the air and locks it up in carbonate rock.

That mountain weathering and forest growth are part of the climate system has never been in much doubt: the questions have always been about how big a forest’s role might be, and how to calculate its contribution.

Keeping climate stable

US scientists recently studied the rainy slopes of New Zealand’s Southern Alps to begin to put a value on mountain ecosystem processes. Dr Doughty and his colleagues measured tree roots at varying altitudes in the tropical rain forests of Peru, from the Amazon lowlands to 3,000 metres of altitude in the higher Andes.

They measured the growth to 30 cms below the surface every three months and did so for a period of years. They recorded the thickness of the soil’s organic layer, and they matched their observations with local temperatures, and began to calculate the rate at which tree roots might turn Andean granite into soil.

Then they scaled up the process, and extended it through long periods of time. Their conclusion: that forests served to moderate temperatures in a much hotter world 65 million years ago.

“This is a simple process driven by tree root growth and the decomposition of organic material. Yet it may contribute to the Earth’s long-term climate stability. It seems to act like a thermostat, drawing more carbon dioxide out of the atmosphere when it is warm and less when it is cooler”, Dr Doughty said.

If forests cool the Earth, however, they might also warm it up. A team from Yale University in the US has reported in Geophysical Research Letters that forest fires might have had an even greater impact on global warming during the Pliocene epoch about three million years ago than carbon dioxide.

Rapid rise expected

Nadine Unger, an atmospheric chemist, and a colleague have calculated that the release of volatile organic compounds, ozone and other products from blazing trees could have altered the planet’s radiation balance, by dumping enough aerosols into the atmosphere to outperform carbon dioxide as a planet-warmer.

In fact, the Pliocene was at least 2°C or 3°C warmer than the pre-industrial world. The Pliocene is of intense interest to climate scientists: they expect planetary temperatures to return to Pliocene levels before the end of the century, precisely because humans have cleared and burned the forests, and pumped colossal quantities of carbon dioxide into the atmosphere. The greater puzzle is why a rainy, forested and conspicuously human-free world should have been so much warmer.

“This discovery is important for better understanding climate change through Earth’s history, and has enormous implications for the impacts of deforestation and the role of forests in climate protection strategies”, Dr Unger said.

All this scholarship is concerned with the natural machinery of ancient climate change, and the Yale research was based on powerful computer simulations of long-vanished conditions that could not be replicated in a laboratory.

Meanwhile, ironically, forest scientists have had a chance to test the levels of volatile organic discharges from blazing forests because freakish weather conditions in Norway have seen unexpected wild fires in tracts of mountain forest. December was one of Norway’s warmest winter months ever. In one blaze, 430 residents were forced to evacuate. – Climate News Network

Tree find confirms Italian alpine melt

December 16, 2013 in Europe, Glaciers, Palaeoclimatology

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The Italian Alps are warming at twice the global rate, and their glaciers are in retreat Image: Ekharlamov via Wikimedia Commons

The Italian Alps are warming at twice the global rate, and their glaciers are in retreat
Image: Ekharlamov via Wikimedia Commons

By Tim Radford

Evidence from high in the Italian Alps confirms that they are warming at twice the global rate, with the region’s glaciers in retreat everywhere.

LONDON, 16 December – It was only a single, withered conifer needle, but it told a dramatic story of climate change. Glaciologists found it in a set of ice cores drilled through a glacier on top of Mount Ortles, in the Italian Alps.

It lay about 80 metres below the glacial surface, encased in solid ice, and carbon dating confirmed that it had blown from the branches of Larix decidua, the European larch, 2,600 years earlier.

It was found about 30 kilometres from a far more dramatic exposure: the body of Ötzi the Iceman, a mummified Bronze Age corpse revealed  by a melting glacier in 1991.

Both finds deliver the same uncompromising message: for at least 5,000 years – because Ötzi perished around that time – the Italian Alps had continued to stay frozen throughout the year.

And now they are melting. Or, to put it the scientific way, in the words of Paolo Gabrielli, of Ohio State University, who led the project: “Our first results indicate that the current atmospheric warming at high elevation in the Alps is outside the normal cold range held for millennia. This is consistent with the rapid, ongoing shrinking of glaciers at high elevation in this area.”

The problem for all climate scientists – and for glaciologists in particular – is that direct measurements are relatively recent: the oldest thermometer readings date back little more than three centuries, and consistent world coverage began only in the 20th century.

Since climates undergo natural cycles of change on a scale of centuries, measurements over a short period are not, in themselves, of much use. Glaciers, in particular, are a problem: retreat or advance would have been  imperceptible to the small populations likely ever to have observed them.

More evidence likely

Visual records – paintings dating from the early 19th century, in most cases – indicate that today’s glaciers are in retreat, but Romantic Age painters weren’t particularly interested in climate or precision topography, so the evidence from paintings is limited.

But direct measurement of surviving ice really can tell a story, and Gabrielli’s team produced a fragment of this narrative in San Francisco at a meeting of the American Geophysical Union.

The Alps are warming at twice the global rate, and the glaciers are everywhere in retreat. Alto dell’Ortles is the highest glacier in the eastern Alps, at 3,900 metres: its ice is likely to hold much more evidence of climate change and human impact.

As they drilled into the glacier, the research scientists from six nations found that the first 30 metre layer was composed of grainy compacted snow that had partly melted. Below that was nothing but solid, enduring ice all the way down to frozen bedrock.

They could be sure that nothing had changed in this permanent layer of ice for at least 2,600 years, because it had preserved a larch needle from a tree that must have grown at least 2,000 years after Ötzi had perished in the same complex of Alpine glaciers.

“The leaf supports the idea that prehistoric ice is still present at the highest elevations of the region,” Gabrielli said. – Climate News Network

Earth ‘may be doubly sensitive’ to CO2

December 11, 2013 in Arctic, Climate Sensitivity, IPCC, Ocean acidification, Palaeoclimatology

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Bad news for bears - snd for us: Geological proof as well as models show rising CO2 is melting polar ice Image:Alastair Rae via Wikimedia Commons

Bad news for bears – snd for us: Geological evidence as well as models prove rising CO2 is melting polar ice
Image: Alastair Rae via Wikimedia Commons

By Alex Kirby

The sensitivity of the Earth system to a doubling of atmospheric carbon dioxide may be twice as great as scientists had thought, new climate records from the distant past suggest.

LONDON, 11 December – You may think the prospect of climate change is alarming, a call to action to slow down our emissions of carbon dioxide and other greenhouse gases.

You’re almost certainly right. But some scientists are now suggesting you should be much more concerned than you are, because they think we may be seriously underestimating the problem.

The Geological Society of London (GSL) says the sensitivity of the Earth’s climate to CO2 could be double earlier estimates.

The Society has published an addition to a report by a GSL working party in 2010, which was entitled Climate change: Evidence from the Geological Record.
The addition says many climate models typically look at short term, rapid factors when calculating the Earth’s climate sensitivity, which is defined as the average global temperature increase brought about by a doubling of CO2  in the atmosphere.

Scientists agree that a doubling of atmospheric CO2 levels could result in temperature increases of between 1.5 and 4.5°C, caused by rapid changes such as snow and ice melt, and the behaviour of clouds and water vapour.

But what the GSL now says is that geological evidence from palaeoclimatology (studies of past climate change) suggests that if longer-term factors are taken into account, such as the decay of large ice sheets, the Earth’s sensitivity to a doubling of CO2 could itself be double that predicted by most climate models.

CO2′s significance

Dr Colin Summerhayes, who led the statement’s working group, says: “The climate sensitivity suggested by modern climate models may be fine for the short term, but does not encompass the full range of change expected in the long term…”

But he cautions that there are really two “sensitivities” involved: “Climate sensitivity is what happens in the short term in response to a doubling of CO2. But the Earth system sensitivity is what happens in the longer time frame as ice sheets slowly melt, and as sea level slowly rises.

“…The IPCC focuses on… the climate sensitivity – what will happen in the next 100 years. Earth system sensitivity tells you what happens in the next couple of hundred years after that.”

The GSL’s addition also reports new data showing that temperature and CO2 levels recorded in Antarctic ice cores increase at the same time. This, says Summerhayes, “makes the role of CO2 in changing Ice Age climate highly significant.”

Atmospheric carbon levels are currently just below 400 parts per million (ppm) – a figure last seen  between 5.3 and 2.6 million years ago. Global temperatures were then 2-3°C higher than today, and sea levels were several metres higher, due to partial melting of the Antarctic ice sheet.

If the current rate of increase (2 ppm per year) continues, CO2 levels could reach 600 ppm by the end of this century; levels which, says Summerhayes, “have not been seen for 24 million years”.

Models match palaeoclimate

The new GSL statement outlines evidence that a relatively modest rise in atmospheric CO2 levels and temperature leads to significant sea level rise, with oceans more acidic and less oxygenated. Previous such events caused marine crises and extinctions, with the Earth system taking around 100,000 years to recover.

Dr Summerhayes said: “We now have even more confidence from the geological record that the only plausible explanation for current warming is the unprecedented exponential rise in CO2 and other greenhouse gases.

“Recent compilations of past climate data, along with astronomical calculations, show that changes in the Earth’s orbit and axis cooled the world over the past 10,000 years. This cooling would normally be expected to continue for at least another 1,000 years.

“And yet Arctic palaeoclimate records show that the period 1950-2000 was the warmest 50 year interval for 2,000 years. We should be cool, but we’re not.”

He told Climate News Network: “The main implication from my perspective is that the geological record tells us that increasing CO2 increases temperature, melts ice, and raises sea level. This we know independently of any fancy numerical model run by climate scientists.

“However, those climate scientists’ models happen to come up with about the same answer as we get from the geological record, which suggests that the modellers  are likely to be on the right track.” – Climate News Network