Global warming hiatus explained and it's not good news

You may have heard that global warming has 'paused' but it's only one part of a bigger picture and the search for understanding has equipped climate scientists with better tools than ever.

"It is frustrating," says climate scientist Michael Mann from his office at Penn State University in the United States.

"There certainly has not been a hiatus in global warming — global warming hasn't stopped, even though you still hear those contrarian talking points," he says.

Professor Mann, the director of the university's Earth System Science Centre, is famous for his 'hockey stick' graph that reconstructed 1,000 years of global temperatures showing a dramatic spike towards the end of the 20th century.

The 'pause', also known as the 'slow down' or the 'hiatus', refers to the average rate of warming across the whole planet's surface in the last 15 years or so. The latest major report (pdf) from the United Nations Intergovernmental Panel on Climate Change said the rate of warming between 1998 and 2012 had been about 0.05°C per decade.

This rate, the report said, was "smaller than the rate calculated since 1951" which was 0.12°C per decade.

"The occurrence of the hiatus in global mean surface temperature trend during the past 15 years raises the two related questions of what has caused it and whether climate models are able to reproduce it," the report said (pdf).

This was proof enough for some commentators that computer models of the climate were wrong and that the risks of global warming may have been overblown.

Businessman and former ABC chairman Maurice Newman, the Prime Minister's top business advisor, has written that "temperatures have gone nowhere for 18 years".

Dr Scott Power, senior principal research scientist at the Bureau of Meteorology, was an author of the IPCC report.

"If you look at just these 15-year periods of globally averaged surface air temperature, then they fluctuate quite a bit," he says.

"We expect these fluctuations. What we know is happening is that the planet is warming in response to human increase in greenhouse gases very largely but there are fluctuations because of natural processes."

Power said that 1998, the start of the 'pause', was a particularly hot year due to the natural El Niño climate pattern that has a warming influence on worldwide temperatures.

Power said that if you choose a 15-year period starting in 1996 instead of 1998 then the rate of warming almost triples to 0.14°C per decade.

"Globally average surface temperature is just one measure of changes in the Earth's climate system," he says.

During the 15-year 'hiatus' period, studies of other aspects of the climate system have continued to show warming as expected.

The world's oceans have continued to gain heat, a recent study has found. And, in late March, a study in the prestigious journalScience found Antarctica's ice sheets were melting at an accelerating rate.

Many scientists have pointed to the recent extended periods of ocean cycles that are in phases that tend to have a cooling effect on temperatures at the surface of the planet.

The sun has also been in a state of unusually low activity, which can also have a cooling influence.

Volcanic eruptions generally cool the planet's surface. A study in the journal Nature Geoscience found that some climate models did not properly account for the higher levels of volcanic activity in the early 20th century. This meant that some models had overestimated the amount of atmospheric warming during the so-called slow-down.

Two studies led by scientists at the University of New South Wales found an increase in the strength of trade winds around the equator in the Pacific Ocean had pushed warmer surface waters deeper, having an overall cooling effect on global surface temperatures.

Dr Shayne McGregor, of the UNSW Climate Change Research Centre and an author of both the studies, said: "We have a certain amount of energy coming in at the top of the atmosphere and a certain amount of energy going out. With global warming we are changing that balance so the system is retaining heat.

"About 97 per cent of all the heat capacity of the Earth is in the ocean — that's where all the energy gets stored."

He said the increase in the trade winds was temporary. When they returned to normal, the rise in global temperatures would "come back faster".

A separate study by Dr Power found computer models were not simulating the changes in trade winds.

Professor Mann is a co-author of research published in February in Science that also found a role for the oceans in temporarily slowing the rate of global warming.

He said: "Our research reinforces work that tied the slow-down in surface warming — what I call the faux pause because it's not a real pause — with increased heat burial below the surface of the tropical Pacific Ocean.

"The problem is that it's temporary. We will fairly soon see the climate warm even faster than the models predicted."

An analysis by US government scientists published in Nature Climate Change suggested if temporary natural fluctuations were ignored then world was probably now warming at a rate of about 0.2°C per decade — higher than the IPCC's longer-term average. By 2020 this rate was expected to rise to 0.25°C per decade.

One group of scientists working at Australia's ARC Centre of Excellence for Climate System Science wrote last year that the term 'pause' was "ill-chosen and even misleading" in the context of climate change.

In the journal Nature Climate Change, the scientists showed during the 'hiatus' there had been an increase in the number of extreme hot days being experienced around the world.

"This is particularly relevant for climate change impacts, as changes in the warmest temperature extremes over land are of the most relevance to human health, agriculture, ecosystems and infrastructure," they wrote.

Click here to read the rest from Graham Readfearn at ABC Environment

Posted by Guest Author on Thursday, 9 April, 2015

Creative Commons License The Skeptical Science website by Skeptical Science is licensed under a Creative Commons Attribution 3.0 Unported License.