Antarctica blows hot and cold – for now

Antarctica blows hot and cold – for now

Natural variability may at the moment be affecting Antarctica’s temperature more than human activity, but climate change will still be a major influence.

LONDON, 19 April, 2015 – German scientists have identified a pattern of natural change in Antarctica. The discovery appears to suggest that human influence may not be a cause of apparent warming in the seas around the great, frozen continent. Instead, natural variability might have a role.

The finding doesn’t undermine the thesis that human burning of fossil fuels is enriching the atmosphere with greenhouse gases and triggering climate change: it might however account for the so-called pause in the rate of warming this century.

The verdict remains inconclusive. But new evidence published in the journal Climate Dynamics introduces a new layer of complexity in the understanding of the planetary climate system.

In brief, the rate of warming in the northern hemisphere is considerable, and highest in the Arctic Circle. Overall, the southern hemisphere remains colder, and evidence from the Antarctic has been ambiguous, although there have been widely reported fears of potentially dramatic change in West Antarctica. Rapid melting in the region would constitute a “climate tipping point” which would have consequences across the entire planet.

Too simple

But Josef Ludescher of the Institute for Theoretical Physics at the Justus Liebig University of Giessen and colleagues made an analysis of the full range of air temperature records from the southern continent and decided that the simple picture of a consistently cold and hostile world possibly modified by human activity was too simple: instead they found a propensity for “large and enduring natural excursions from the mean.”

Put simply, sometimes the winds blow colder, sometimes not quite so cold, but the coldest spells last for decades, followed by extended periods that might be measurably warmer overall.

The finding – it is based on sophisticated mathematical analysis and like all such hypotheses invites both confirmation and demolition by other climate science researchers – has a consequence: it means that the warming now observed in Antarctica may not be because of human influence. Or it may be that any human influence is so far swamped by a larger cyclic shift between two natural states.

“So far it seemed that there were hardly any major natural temperature fluctuations in Antarctica, so almost every rise in temperature was attributed to human influence,” said Armin Bunde of the Justus Liebig University, another member of the team.

“Global warming as a result of our greenhouse emissions from burning fossil fuel is a fact. However the human influence on the warming of West Antarctica is much smaller than previously thought. The warming of East Antarctica up to now can even be explained by natural variability alone.”

“At the end of this natural cold spell temperatures will rise even more fiercely – globally, but also in Antarctica”

But the study – if backed by other findings – could explain another climate puzzle. Although global warming increased rapidly during the last three decades of the last century, the rate of warming has slowed. All but one of the hottest years ever recorded have been in this century, and 2014 broke all records, but the rate at which the temperatures have risen has slowed.

There have been many potential explanations for this apparent slowdown. And perhaps the proposed Antarctic cycle has a role in that too.

“Our estimates show that we are currently facing a natural cooling period – while temperatures rise slowly but inexorably, due to our heating up the atmosphere by emitting greenhouse gases,” said another of the authors, Hans Joachim Schellnhuber, of the Potsdam Institute for Climate Impact Research.

“At the end of this natural cold spell temperatures will rise even more fiercely – globally, but also in Antarctica, which therefore is in danger of tipping.” – Climate News Network

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Heat is on to slow down faster rise in temperatures

Heat is on to slow down faster rise in temperatures

New research warns that emissions will make drought conditions even more extreme as our climate moves into a period of rapid change.

LONDON, 12 March, 2015 – Analysis of temperature records and reconstructions of past climates indicates that the pace of global warming is about to accelerate.

Although the much-debated “pause” in warming during the 21st century is still under debate, climate scientist now warn that the Earth is about to enter a period of change that will be faster than anything in the last thousand years.

Steven Smith, an integrated modelling and energy
scientist, at the US government’s Pacific Northwest National Laboratory, and colleagues decided to take a look at the short history of temperature records and the somewhat longer “proxy” reconstructions of past climates to look for patterns of the past that might be a guide to the future.

Baseline rates

They then matched the past and examined the future using computer model simulations. Climate periods were considered in 40 year blocks, and were compared to establish a baseline for natural rates of change.

The scientists report in Nature Climate Change that rises now in North America and many parts of the world are greater than the natural range for any rate of change.

And when they tested future emissions scenarios, they confirmed that global warming will pick up speed in the next 40 years in all cases − even in those projections in which the world reduced its greenhouse gas emissions. And if the world doesn’t reduce these emissions, the rate of change in warming will remain high for the rest of the century.

“In these climate model simulations, the world is just now starting to enter a new place, where rates of temperature change are consistently larger than historical values over 40-year time spans,” Dr Smith says. “We need to better understand what the effects of this will be, and how to prepare for them.”

The research is based on simulation, and seems inconsistent with the story of the 21st century, which is that, after a relatively rapid decadal rise in global average temperatures between 1970 and 2000, the rate of rise seemed to slow.

Although almost all the years of the new century so far have been warmer than any in the 20th century, and although 2014 was the warmest year on record so far, the notches on the thermometer each year have been smaller.

But as researchers have repeatedly warned, the real rise may be masked by some kind of natural variation. At least one group in 2014 found that the patterns of extremes of heat seem to be accelerating, even if the averages are not.

“The finding is critical to understanding
what the world will be like
as the climate continues to change”

And now Rong Fu, professor of geological sciences at the University of Texas at Austin, US, has looked at a study by research scientists William Lau, of the University of Maryland, and Kyu-Myong Kim, of the NASA Goddard Space Flight Centre, and seen signs of an intensified pattern of extreme droughts in Australia, the southwest and central US, and southern Amazonia.

The Proceedings of the National Academy of Sciences has published both the original research and the commentary  by Professor Fu.

At the heart of the issue is the impact of increased emissions of carbon dioxide on the pattern of wind circulation that overall dictates the climate of each hemisphere.

This pattern is sometimes called the Hadley Circulation, named after the 18th-century English lawyer and amateur meteorologist, George Hadley, who first identified the mechanism behind the all-important Trade Winds that carried sailing ships across the Atlantic.

It can change with global temperatures. And as the winds change – and the prevailing Trade Winds move away from the tropics – they take the rainfall with them.

Ominous consequences

The guess has been that Hadley Circulation varies naturally. And the PNAS study suggests that it is likely to intensify in a warmer world, with ominous consequences for some already naturally dry regions.

That both Australia and the American southwest are already feeling the heat is not news. But the significance of the research lies in more detailed understanding of why even more is on the cards in future.

“This is the first study that suggests a possible intensification of droughts in the tropic-subtropical margins in warmer climate,” Professor Fu says. “The finding is critical to understanding what the world will be like as the climate continues to change.

“Will the Hadley Circulation continue to expand? Could the intensification of droughts over the tropics be a new norm? These are questions that need to be answered.” – Climate News Network

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Two oceans may explain global warming pause

Two oceans may explain global warming pause

Temperatures may be rising more slowly than expected because of two natural oceanic cycles − the latest refutation of the global warming “pause”.

LONDON, 1 March, 2015 − US scientists have suggested yet another explanation for the so-called pause in global warming. They think it might all be down to the juxtaposition of two independent natural climate cycles – each with periods of half a century or more – one of which is blowing cold, and the other not very hot.

Between them, the phenomena known to meteorologists as the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation could account for the seeming slowdown in predicted temperature rises.

Any pause or hiatus in global warming is only apparent: in fact, 14 of the warmest years on record have happened in the last 15 years and 2014 was scored separately, by the World Meteorological Organisation, the US National Oceanic and Atmospheric Administration, and the US space agency Nasa,  as the warmest on record.

But overall, the palpable increases in average temperatures per decade recorded in the last 30 years of the 20th century have not been maintained, and climate scientists and meteorologists have been trying to work out why.

The latest proposal is from Byron Steinman, a geologist at the University of Minnesota Duluth, and Michael Mann and Sonya Miller of Pennsylvania State University.

Multiple theories

Professor Mann is the scientist who, much to the fury of people who deny climate change, first formulated the famous “hockey-stick graph” which highlights the magnitude of change that threatens to overtake global climate as greenhouse gas levels rise because of human activity.

They report in Science that the northern hemisphere is warming more slowly, not because of the Atlantic oscillation, which has been relatively flat, but because of a second, different but still natural downward trend in the Pacific cycle.

This is not the only explanation on the table. In the past two years Climate News Network has reported that climate scientists certainly expected a slowdown, but just not right now; or that planetary measurements might be incomplete or misleading; or that even though average levels were down, this masked a series of hotter extremes.

The oceans have certainly been under suspicion. One group has already identified the cooling Pacific as a damper on global warming. Another has suggested that in fact the “missing heat” is collecting in the Atlantic depths.

Yet another has questioned the role of the trade winds, while still another has pointed to an upswing in volcanic activity that could have delivered a fine smear of sunblock aerosols to the atmosphere.

“The North Atlantic and North Pacific Oceans appear to be drivers of substantial natural… climate variability on timescales of decades”

Any or all of these could have some role in the big picture. The climate would vary anyway, and the question in every case is: how much would any or all natural variation affect the overall path of change because of increasing carbon dioxide levels in the atmosphere?

The latest study is based on sophisticated climate models that match the predicted impact of the great ocean-atmosphere cycles with the pattern of climate shifts recorded in the past.

“We know that it is important to distinguish between human-caused and natural climate variability so we can assess the impact of human-caused climate change, including drought and weather extremes,” Professor Mann said.

“The North Atlantic and North Pacific Oceans appear to be drivers of substantial natural, internal climate variability on timescales of decades.” – Climate News Network

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Atlantic depths may hold key to heat hiatus

Atlantic depths may hold key to heat hiatus

Researchers analysing millions of oceanographic measurements believe they may finally have got to the bottom of the conundrum about why there is a slowdown in global warming despite greenhouse gas emissions rising.

LONDON, 22 August, 2014 − For years, researchers have puzzled over the temperature rises that haven’t happened – but scientists in China and the US believe they have cracked the mystery of the missing heat.

While calculations indicate that global average temperatures should be rising predictably, the planetary thermometers tell a different story.

But now Xianyao Chen, an oceanographer at the Ocean University of China in Qingdao, and Ka-Kit Tung, an atmospheric scientist at the University of Washington in Seattle, report in Science journal that they think they know where the notional extra heat has gone. It is at the bottom of the Atlantic Ocean.

And this time their conclusion isn’t based only on mathematical models and computer simulations. In their research − funded by the US National Science Foundation and the National Natural Science Foundation of China – they analysed millions of measurements of temperature and salinity taken by oceanographic instruments since 1970, and tracked the pathways that the heat must have taken since the beginning of the 21st century.

High temperatures

But first, a restatement of the conundrum. For more than a century, climate scientists have known that higher levels of carbon dioxide in the atmosphere mean higher atmospheric temperatures. For more than 30 years, every investigation has confirmed this link. And for the last 30 years of the 20th century, as greenhouse gas emissions increased, so did average temperatures.

This rise has continued, with 13 of the 14 warmest years ever recorded all falling in the 21st century, but the rate of increase unexpectedly slowed.

Researchers had expected that there would be some sort of heat hiatus, but not during the first years of the century, and they have been scratching their heads and examining the data again.

Some think that the measurements may be incomplete, or that natural cycles, such as the Pacific cooling event called La Niña, may be at play. Some have suggested that the pattern of trade winds may have a role in taking the warmth into the deep ocean, and some have suspected all along that the heat could be found far below the oceanic surface.

In the same week as the publication in Science, Reto  Knutti, a climate physicist at the federal technology institute ETH Zurich, and his colleague, Markus Huber, reported in Nature Geoscience that the apparent slowdown could be attributed to a cocktail of causes: a longer period of weaker solar irradiance – the sun has its own cycles of intensity − and to the cycle of El Niño and La Niña weather phenomena in the Pacific, and also to incompletely measured data.

“Many of the earlier papers had focused on
symptoms at the surface of the Earth”

But the Science report authors think they have an in-depth solution. “Every week, there’s a new explanation of the hiatus,” said Ka-Kit Tung. “Many of the earlier papers had focused on symptoms at the surface of the Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find an underlying cause.”

The oceans cover 70% of the planet, and are capable of storing 90% of the planet’s heat content. So the two Science report authors argue that a sudden shift in ocean salinity that corresponded with the slowdown of global warming could have triggered the movement of the heat to much deeper waters.

Saltier water is denser, sinks faster, and takes surface heat with it. As the two scientists see it, the depths of the North and South Atlantic have absorbed more heat in the last 14 years than the rest of the global ocean system put together.

This does not mean that global warming is not a problem: heat in the deep oceans is likely to come back to the surface, and to the atmosphere, sooner or later.

Natural cycle

The changes in the Atlantic ocean circulation system are part of a natural cycle that seems to date back many centuries. The surprise discovery by Chen and Tung is that the heat is tucked away in the Atlantic and Southern Oceans, rather than the Pacific − the suspected hiding place until now.

The argument is a complex one, and the latest research probably hasn’t settled the matter.

“All these analyses of ocean heat content are interpreting small changes in ocean temperature, and it will need to be picked over and repeated by others before being fully accepted,” said Professor Andrew Watson, head of the Marine and Atmospheric Science group at the University of Exeter, UK.

And Piers Forster, professor of climate change at the University of Leeds in the UK, said: “Most importantly, this paper is another nail in the coffin of the idea that the hiatus is evidence that our projections of long-term climate change need revising down.

“Variability in the ocean will not affect long-term climate trends, but may mean we have a period of accelerated warming to look forward to.” – Climate News Network

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Hotter extremes belie warming ‘pause’

Hotter extremes belie warming 'pause'

FOR IMMEDIATE RELEASE
Increasing and more frequent extremes of heat affecting wider regions, scientists say, are evidence that it is misleading to claim that climate change has paused.

LONDON, 1 March – If global warming has paused, nobody told the thermometer. Although global average temperature rises have not kept pace with greenhouse gas emissions in the last decade, the mercury has been higher than ever for longer than ever over increasingly larger areas of land, according to a new study in Nature Climate Change.

Sonia Seneviratne from the Swiss Federal Institute of Technology in Zurich and colleagues in Australia and Canada chose not to look at averages but at extremes of temperature. The scientists examined daytime extremes from 1979 onwards, and compared the temperatures of any particular day anywhere to an average of daily temperatures between 1979 and 2012, to identify the hottest 10%. Any region might normally expect 36.5 hottest days in a year; that is, hotter than the average.

Then they looked more closely at temperatures from 1997 to 2012. Regions that experienced 10, 30 or 50 extremely hot days above this average saw the greatest upward trends in extreme hot days over time – and over area. That is, not only were people experiencing greater heat extremes, but they were experiencing them over more days and over more extended regions.

And this consistent upward trend persisted right through the so-called “hiatus” of 1998 to 2012. The year 1998, at the time the hottest ever, coincided with a major El Niño event, the peak of a natural cycle of warmth and cooling in the Pacific.

Extreme extremes change most

Thereafter, although 13 of the 14 warmest-ever years have occurred this century, the rate of increase in warming as a global average has fallen. Climate sceptics used the trend to argue that global warming was an illusion, or part of a natural cycle. Dr Seneviratne and her colleagues do not see it that way.

“It quickly became clear the so-called ‘hiatus’ in global average temperatures did not stop the rise in the number, intensity and area of extremely hot days,” said Lisa Alexander of Australia’s Centre of Excellence for Climate System Science.

“Our research has found a steep upward tendency in the temperatures and number of extremely hot days over land and the area they impact, despite the complete absence of a strong El Niño.”

And her colleague Markus Donat added: “There has been no pause in the increase of warmest daily extremes over the land and the most extreme of the extreme conditions are showing the largest change.

“Another interesting aspect of our research was that those regions that normally saw 50 or more excessive hot days in a year saw the greatest increases in land area impact and the frequency of hot days. In short, the hottest extremes got hotter and the events happened more often.”

‘Illusory’ pause

However, perhaps because the world is mostly ocean, and the extremes have been measured over land, the average, year-on-year rises in temperatures have been lower in the last decade than in previous decades. There have been a number of inconclusive explanations for this phenomenon.

Cyclic changes in trade winds are one explanation; another is that the heat is there, but has been stored in the deep ocean, where measurements are not systematically taken. It’s there somewhere, waiting to be found.

And US scientists argue in the latest issue of the journal Science that the oceans may have an even bigger influence on climate than anybody foresaw, and that persistent cool conditions in the tropical Pacific may be behind what they call the “pause in global warming since 2000.”

But the latest Nature Climate Change paper puts the case that this pause or hiatus is illusory with – for a scientific paper – unusual clarity. “Based on existing observational evidence,” the authors say, “we highlight that the term pause, as applied to the recent evolution of global annual mean temperatures, is ill-chosen and even misleading in the context of climate change.

“Indeed, an apparently static global mean temperature can mask large trends in temperatures at both regional and seasonal scales.” – Climate News Network

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Trade winds draw ‘missing’ warmth to deep ocean

Trade winds draw 'missing' warmth to deep ocean

FOR IMMEDIATE RELEASE
Contrary to some reports, global warming hasn’t stopped or slowed at all, new research suggests. The trade winds have simply carried the heat into the Pacific Ocean – temporarily.

LONDON, 10 February – Australian and US scientists think they know where a lot of global warming has been concentrated: it has been tucked away below the surface waters of the western Pacific Ocean. And the agency that took the heat out of the atmosphere and transferred it into a liquid form could have been the equatorial trade winds.

Matthew England from the Australian Centre of Excellence for Climate System Science and colleagues report in Nature Climate Change that a dramatic acceleration in the winds has drawn heat from the atmosphere and transferred it to the ocean: cooler waters have risen to the surface to mask the transaction.

Climate sceptics – and some climate scientists – talk about a slowdown, or a pause, or a hiatus in global warming. In fact, temperatures have gone on rising and 13 of the 14 warmest years ever recorded have all occurred since 2000. But the rate of rise in global average temperatures since 2000 has not been as fast as the rate during the 1980s and 1990s.

Since greenhouse gas levels have continued to rise, and since scientists are sure of their atmospheric physics, then there was some “missing heat” to be accounted for.

Researchers have variously suggested that a puzzling increase in deep ocean temperatures could be one explanation or that perhaps the unevenness of temperature measurements around the planet might be another. But both suggestions were hypotheses: nobody had an answer that could be tested by any kind of experiment.

Slower rate

Professor England and colleagues worked with observed winds, surface air temperatures, and a set of ocean climate models to calculate what may have happened.

The global warming story has always been one of fits and starts: a warming that ought to have been observed 70 years ago stalled between 1940 and 1970, and when it resumed, did so in fits and starts. The overall trend continued upward, but the rate of rise slowed noticeably during the last decade.

Ocean circulation loops are driven by winds, and speed up as the winds intensify. Cool waters well up, warm waters descend. And intensify is just what the trade winds have done. They began strengthening during the 1990s, a process which continues today. Once the researchers added the trade winds to their calculations, the global average temperatures looked very like the observations during the hiatus.

They also found that four-fifths of the surface temperature cooling occurred after 2000, which confirmed that wind acceleration was the key contributor.

“Scientists have long suspected that extra ocean heat uptake has slowed the rise of global average temperatures, but the mechanism behind the hiatus remained unclear”, said Professor England.

Rapid rise on the way

“But the heat uptake is by no means permanent: when the trade wind strength returns to normal – as it inevitably will – our research suggests heat will quickly accumulate in the atmosphere. So global temperatures look set to rise rapidly out of the hiatus, returning to the levels projected within as little as a decade.”

The same mechanism could explain the slowdown between 1940 and 1970. In 1938, the British scientist G S Callendar argued that rising carbon dioxide levels should mean global warming but the evidence proved elusive, perhaps because the trade winds accelerated during those decades.

Richard Allan, professor of climate science at the University of Reading in the UK, said the current slowdown was only a temporary reprieve.

“Measurements from satellites and ocean buoys show that the planet is absorbing more heat than it is radiating out to space and the heat is building up in the oceans.

“This new research suggests that when the trade winds weaken again, the planet can expect rapid warming of the surface to resume, as greenhouse gas concentrations continue to rise.” – Climate News Network

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