Category Archives: Trade Winds

Atlantic depths may hold key to heat hiatus

A jellyfish floats just above the seafloor of the deep Atlantic Image: NOAA/OAR/OER via Wikimedia Commons
A jellyfish floats just above the seafloor of the deep Atlantic Ocean
Image: NOAA/OAR/OER via Wikimedia Commons

By Tim Radford

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

Hotter extremes belie warming ‘pause’

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California's Death Valley is among the world's hottest places - but many are steadily getting hotter still Image: Pandat at de.wikipedia via Wikimedia Commons

California’s Death Valley is among the world’s hottest places – but many are steadily getting hotter still
Image: Pandat at de.wikipedia via Wikimedia Commons

By Tim Radford

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

Trade winds draw ‘missing’ warmth to deep ocean

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The trade winds make their mark on Hawaii - and they may explain where the "missing" warmth has gone Image: Richard B. Mieremet, Senior Advisor, NOAA OSDIA

The trade winds make their mark on Hawaii – and they may explain where the “missing” warmth has gone
Image: Richard B. Mieremet, Senior Advisor, NOAA OSDIA

By Tim Radford

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