25/08/2020

We Pieced Together The Most Precise Records Of Major Climate Events From Thousands Of Years Ago. Here’s What We Found

The Conversation - Ellen Corrick | John Hellstrom | Russell Drysdale

Markus Rex/Alfred Wegener Institute via AP

Authors


Between 115,000 and 11,700 years ago, the Earth would have been almost unrecognisable. 

 Massive ice-sheets covered northern Europe and northern Asia, and about half of North America, and global sea-levels were as much as 130 meters lower than today.

In this period, known as the “last glacial period”, the climate was much cooler and drier than today. It was punctuated by some of the largest and most rapid climate change events in Earth’s recent geological history.

For a long time, scientists have pondered how closely timed these abrupt climate change events were between Greenland and other regions of the world — far beyond the Arctic.

In our research, published today in Science, we’ve shown abrupt climate changes across the Northern Hemisphere and into the southern mid-latitudes occurred simultaneously, within decades of each other, throughout the last glacial period. We’ve also determined exactly when the abrupt changes occurred, much more precisely than before.

This can help us predict how abrupt climate changes might play out in the future.

A series of abrupt climate changes

Scientists can peer into Earth’s climate history through long ice cylinders, called “ice cores”, drilled from the Greenland ice sheet. Changes in the chemical composition of these ice cores reveal that the surrounding atmospheric temperature repeatedly warmed by 8-16℃, and each time within just a few decades.

An ice core. Ancient ice can reveal what the surrounding climate was once like. AAP Image/ACECRC

Each warming event was followed by a more gradual period of cooling. These abrupt warming and cooling events happened more than 25 times throughout the last glacial period, and are known as Dansgaard-Oeschger events.

They reflect changes in circulation patterns of the Atlantic Ocean.While we have records of climate changes from many regions, the relative timing of these changes between Greenland and across the northern hemisphere into the southern sub-tropics is not well understood.

This has been difficult to resolve because we need very precisely dated records to make exact comparisons in timing. Ice cores provide a wealth of information about Dansgaard-Oeschger events. But while they faithfully reproduce the patterns of past climate, they are difficult to date very precisely.

Crystal time capsules beneath our feet

For our study, we turned to more precisely datable climate records: those from cave stalagmites.

Stalagmites are cave mineral deposits, which build up layer-by-layer on the cave floor. Their growth is fed by water dripping from the cave ceiling, which carries with it a chemical signal of temperature and rainfall conditions above the cave at that time. This signal is trapped in the crystal structure of the growing stalagmite.

Stalagmites can be dated very precisely, by measuring the decay of minute amounts of uranium trapped in them. This key feature enables us to compare the timing of climate events from place to place.

Stalagmites hold chemical signals that reveal what the climate above the cave was like thousands of years ago. Shutterstock

However, long, high-quality stalagmite records are rare. Scientists from around the world have been working for more than 20 years to produce these records. Only now that enough records are available, we are able to make precise comparisons of the timing of Dansgaard-Oeschger events between different regions.

We collated and compared 63 published stalagmite records from caves in Asia, Europe and South America, and we determined the timings of abrupt climate changes in each.

What we found

Our results show that during each Dansgaard-Oeschger event, climate changes felt in Asia, South America and Europe occurred within decades of one another. Being able to determine this level of synchrony is remarkable, given we are looking at events that occurred many tens of thousands of years ago.

This means that as large temperature increases were occurring in Greenland, abrupt changes were also occurring in air temperature and rainfall in Europe, and in the monsoon systems in Asia and South America.

So why is this important? First of all, finding that climate change events occurred in lots of different parts of the world within decades provides clues as to how they started in the first place.

It tells us the changes were likely propagated from the North Atlantic region to these locations through the re-organisation of patterns in atmospheric circulation. And knowing this can help scientists narrow down the underlying triggers, which are still not conclusive.

And our findings mean the precise ages from the stalagmites can be used to better date ice cores, enhancing one of the most important records we have of the last glacial climate.

In the last glacial period, vast ice sheets covered much of the world. Shutterstock

Implications for the future

The abrupt climate changes we studied occurred under very different conditions compared to the climate of today.

While our ancestors lived through the last glacial period, humans are unlikely to experience Dansgaard-Oeschger events for many thousands of years, until the earth has again cooled to glacial temperatures.

However, piecing together the puzzle of how abrupt climate changes took place in the past will help us to understand how abrupt climate changes might occur in the future. For example, our findings will help validate climate models used to predict climate changes.

Showing that profound changes in climate can occur simultaneously across large regions of the Earth highlights just how unstable and interconnected our climate system can be.

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Earth Has Lost 28 Trillion Tonnes Of Ice In Less Than 30 Years

The Guardian - 

‘Stunned’ scientists say there is little doubt global heating is to blame for the loss

Husky dogs wade over sea ice during an expedition in north-western Greenland, where ice loss has been triggered by rising sea levels and atmospheric temperatures. Photograph: Steffen Olsen/Centre for Ocean and Ice /AFP/Getty Images

A total of 28 trillion tonnes of ice have disappeared from the surface of the Earth since 1994.

That is stunning conclusion of UK scientists who have analysed satellite surveys of the planet’s poles, mountains and glaciers to measure how much ice coverage lost because of global heating triggered by rising greenhouse gas emissions.

The scientists – based at Leeds and Edinburgh universities and University College London – describe the level of ice loss as “staggering” and warn that their analysis indicates that sea level rises, triggered by melting glaciers and ice sheets, could reach a metre by the end of the century.

“To put that in context, every centimetre of sea level rise means about a million people will be displaced from their low-lying homelands,” said Professor Andy Shepherd, director of Leeds University’s Centre for Polar Observation and Modelling.

The scientists also warn that the melting of ice in these quantities is now seriously reducing the planet’s ability to reflect solar radiation back into space. White ice is disappearing and the dark sea or soil exposed beneath it is absorbing more and more heat, further increasing the warming of the planet.

In addition, cold fresh water pouring from melting glaciers and ice sheets is causing major disruptions to the biological health of Arctic and Antarctic waters, while loss of glaciers in mountain ranges threatens to wipe out sources of fresh water on which local communities depend.

“In the past researchers have studied individual areas – such as the Antarctic or Greenland – where ice is melting. But this is the first time anyone has looked at all the ice that is disappearing from the entire planet,” said Shepherd. “What we have found has stunned us.”

The level of ice loss revealed by the group matches the worst-case-scenario predictions outlined by the Intergovernmental Panel on Climate Change (IPCC), he added.

The group studied satellite surveys of glaciers in South America, Asia, Canada and other regions; sea ice in the Arctic and Antarctic; ice sheets that cover the ground in Antarctica and Greenland; and ice shelves thatprotrude from the Antarctic mainland into the sea. The study covered the years 1994 to 2017.

A glacier melting in Svalbard archipelago. Photograph: Jan Tove Johansson/Getty Images

The researchers’ conclusion is that all the regions have suffered devastating reductions in ice cover in the past three decades and these losses are continuing.

“To put the losses we’ve already experienced into context, 28 trillion tonnes of ice would cover the entire surface of the UK with a sheet of frozen water that is 100 metres thick,” added group member Tom Slater from Leeds University. “It’s just mind-blowing.”

As to the cause of these staggering losses, the group is adamant: “There can be little doubt that the vast majority of Earth’s ice loss is a direct consequence of climate warming,” they state in their review paper, which is published in the online journal Cryosphere Discussions.

“On average, the planetary surface temperature has risen by 0.85C since 1880, and this signal has been amplified in the polar regions,” they state. Both sea and atmospheric temperatures have risen as a result and the resulting double whammy has triggered the catastrophic ice losses uncovered by the group.

In the case of the melting ice sheet in Antarctica, rising sea temperatures have been the main driver while increasing atmospheric temperatures have been the cause of ice loss from inland glaciers such as those in the Himalayas. In Greenland, ice loss has been triggered by a combination of both sea and atmospheric temperatures increasing.

Meltwater rushes from Boyabreen glacier in Fjaerland, Norway – a process accelerated by global heating. Photograph: Sean Gallup/Getty Images

The team stressed that not all the ice that was lost over that period would have contributed to sea level rises.

“A total of 54% of the lost ice was from sea ice and from ice shelves,” said Leeds University researcher Isobel Lawrence. “These float on water and their melting would not have contributed to sea level rises. The other 46% of meltwater came from glaciers and ice sheets on the ground, and they would have added to sea level rise.”

The group’s results were published 30 years after the first assessment report of the IPCC was published, at the end of August 1990. This outlined, in stark terms, that global warming was real and was being triggered by increasing emissions of greenhouse gases from the burning of fossil fuels.

Despite warnings from scientists, these emissions have continued to rise as global temperatures continued to soar. According to figures released by the Met Office last week, there was a 0.14C increase in global temperatures between the decade 1980-89 and the decade 1990-1999, then a 0.2C increase between each of the following decades.

This rate of increase is expected to rise, possibly to around 0.3C a decade, as carbon emissions continue on their upward trajectory.

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The Observer View On The Climate Catastrophe Facing Earth

The Guardian - 

Thirty years ago we were warned. Now is our last chance to listen

Icebergs off the south-east coast of Greenland, a region that is exhibiting an accelerated rate of ice loss. Photograph: Andrew Bossi/Goddard Space Flight Center/NASA

Thirty years ago this week, the population of Earth was given official notification that it faced a threat of unprecedented magnitude.

Emissions of carbon dioxide and other greenhouse gases, spewed into the atmosphere from factories and vehicles burning fossil fuels, were pinpointed, definitively, as triggers of future climate change.

Melting icecaps, rising sea levels and increasing numbers of extreme weather events would be the norm for the 21st century unless action were taken, warned the authors of the first assessment report of the Intergovernmental Panel on Climate Change (IPCC).

The scientists had been charged by the IPCC, which had been set up two years earlier, with establishing whether climate change was a real prospect and, if it was, to look at the main drivers of that threat. They concluded, in a report released in August 1990, that the menace was real and that coal, gas and oil would be the principal causes of global heating.

Unless controls were imposed on their consumption, temperature rises of 0.3C a decade would be occurring in the 21st century, bringing havoc in their wake.

Three decades later, it is clear that we have recklessly ignored that warning. Fossil fuels still supply 80% of the world’s energy, carbon dioxide levels in the atmosphere continue to rise and global temperatures are still increasing.

According to Met Office statistics, there was a 0.14C increase in global temperatures in the decade that followed publication of the first assessment report. This was then followed by a 0.2C increase in each of the following two decades. The world could easily heat by 3C by the end of the century at this rate, warn scientists.

The impact on the world will, by then, be catastrophic. As the Observer reveals this week, our overheating planet has already lost a staggering 28tn tonnes of ice from its ice sheets and glaciers, triggering sea level rises that are now accelerating at a rate that matches the worst-case scenario predictions of the IPCC.
Such an event would have been impossible in a world with limited, pre-industrial carbon dioxide levels in its atmosphere
Or consider the heatwave that struck Siberia this year, bringing forest fires, permafrost melting and pest invasions in its wake. Such an event would have been impossible in a world with limited, pre-industrial levels of carbon dioxide in its atmosphere.

It would therefore be tempting to blame IPCC scientists for this troubling state of affairs. Should they not have guided us more adroitly through these worrying times? Could they not have acted with more precision? Certainly, the organisation is not without its critics who have claimed, on occasions, that it is overstaffed and sometimes slipshod in its work.

However, we should be careful when apportioning blame for the world’s failure to act over climate change. It is the government members of the IPCC who are at fault for ignoring their own scientists’ warnings.

They have allowed lobbying by the fossil fuel industry to play havoc with attempts to limit carbon emissions, while nations such as Canada, Saudi Arabia and the United States have blocked all attempts to limit global fossil fuel consumption.

By contrast, the IPCC has at least made the world aware of the impending crisis, a task of considerable complexity. Getting scientific experts from 195 nations to agree anything can be likened to the herding of a similar number of bad-tempered cats.

Thanks to the IPCC, we are at least aware of the problem that now faces our world. We know exactly how much fossil fuel we have left to burn if we want to limit global temperature rises to a relatively safe rise of 1.5C.

Individual nations have until next year – at the United Nations climate change conference in November – to announce how they will achieve those reductions in oil, gas and coal burning in order to make that target possible and to halt global heating.

It is an achievable aspiration even at this late date. We still have hope, in other words.

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