Alister Doyle and Simon Johnson Reuters Yahoo News 28 Sep 13;
STOCKHOLM (Reuters) - Leading climate scientists said on Friday they were more convinced than ever that humans are the main culprits for global warming, and predicted the impact from greenhouse gas emissions could linger for centuries.
The Intergovernmental Panel on Climate Change (IPCC) said in a report that a hiatus in warming this century, when temperatures have risen more slowly despite growing emissions, was a natural variation that would not last.
It said the Earth was set for more heatwaves, floods, droughts and rising sea levels from melting ice sheets that could swamp coasts and low-lying islands as greenhouse gases built up in the atmosphere.
The study, meant to guide governments in shifting towards greener energies, said it was "extremely likely", with a probability of at least 95 percent, that human activities were the dominant cause of warming since the mid-20th century.
That was an increase from "very likely", or 90 percent, in the last report in 2007 and "likely", 66 percent, in 2001.
U.N. Secretary-General Ban Ki-moon said the study was a call for governments, many of which have been focused on spurring weak economies rather than fighting climate change, to work to reach a planned U.N. accord in 2015 to combat global warming.
"The heat is on. Now we must act," he said of the report agreed in Stockholm after a week of talks between scientists and delegates from more than 110 nations.
U.S. Secretary of State John Kerry said the report was a wake-up call. "Those who deny the science or choose excuses over action are playing with fire," he said, referring to skeptics who question the need for urgent action.
They have become emboldened by the fact that temperatures rose more slowly over the last 15 years despite increasing greenhouse gas emissions, especially in emerging nations led by China. Almost all climate models failed to predict the slowing.
"LOOKING FOR THE CURE"
European Climate Commissioner Connie Hedegaard said it was time to treat the Earth's health. "If your doctor was 95 percent sure you had a serious disease, you would immediately start looking for the cure," she said.
Compiled from the work of hundreds of scientists, the report faces extra scrutiny this year after its 2007 edition included an error that exaggerated the rate of melting of Himalayan glaciers. An outside review later found that the mistake did not affect its main conclusions.
The IPCC said some effects of warming would last far beyond current lifetimes.
Sea levels could rise by 3 meters (9 feet, 10 inches) under some scenarios by 2300 as ice melted and heat made water in the deep oceans expand, it said. About 15 to 40 percent of emitted carbon dioxide would stay in the atmosphere for more than 1,000 years.
"As a result of our past, present and expected future emissions of carbon dioxide, we are committed to climate change and effects will persist for many centuries even if emissions of carbon dioxide stop," said Thomas Stocker, co-chair of the talks.
The IPCC said humanity had emitted about 530 billion tons of carbon, more than half the 1 trillion ton budget it estimated as a maximum to keep warming to manageable limits. Annual emissions are now almost 10 billion tons and rising.
Explaining a recent slower pace of warming, the report said the past 15-year period was skewed by the fact that 1998 was an extremely warm year with an El Nino event - a warming of the ocean surface - in the Pacific.
It said warming had slowed "in roughly equal measure" because of random variations in the climate and the impact of factors such as volcanic eruptions, when ash dims sunshine, and a cyclical decline in the sun's output.
Rajendra Pachauri, chair of the IPCC, told Reuters the reduction in warming would have to last far longer - "three or four decades" - to be a sign of a new trend.
And the report predicted that the reduction in warming would not last, saying temperatures from 2016-35 were likely to be 0.3-0.7 degree Celsius (0.5 to 1.3 Fahrenheit) warmer than in 1986-2005.
Still, the report said the climate was slightly less sensitive than estimated to warming from carbon dioxide.
A doubling of carbon in the atmosphere would raise temperatures by between 1.5 and 4.5 degrees Celsius (2.7 to 8.1F), it said, below the 2-4.5 (3.6-8.1F) range in the 2007 report. The new range is identical to the ranges in IPCC studies before 2007.
The report said temperatures were likely to rise by between 0.3 and 4.8 degrees Celsius (0.5 to 8.6 Fahrenheit) by the late 21st century. The low end of the range would only be achieved if governments sharply cut greenhouse gas emissions.
And it said world sea levels could rise by between 26 and 82 cm (10 to 32 inches) by the late 21st century, in a threat to coastal cities from Shanghai to San Francisco.
That range is above the 18-59 cm estimated in 2007, which did not take full account of Antarctica and Greenland.
Bjorn Lomborg, author of "The Skeptical Environmentalist" said "the IPCC's moderate projections clearly contradict alarmist rhetoric" of higher temperature and sea level rises by some activists.
(Additional reporting by Nina Chestney in London, Barbara Lewis in Brussels, Valerie Volcovici in Washington; editing by Alistair Scrutton and Mark Trevelyan)
IPCC climate report: six things we've learned
The IPCC's long-awaited fifth assessment report says it is 'extremely likely' that humanity is to blame for global warming. What else can we learn from the report?
Adam Vaughan theguardian.com 27 Sep 13;
• Scientists are more certain than ever that humanity is to blame for rising temperatures. The head of the United Nations' World Meteorological Organisation, Michel Jarraud, said "it is extremely likely that changes in our climate system in the past half century are due to human influence." The report says: "Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system."
• Concentrations of CO2 and other greenhouse gases in the atmosphere have increased to levels that are unprecedented in at least 800,000 years. The burning of fossil fuels is the main reason behind a 40% increase in CO2 concentrations since the industrial revolution.
• We're likely to surpass rises of 2C by 2100, the threshold of warming that governments have pledged to hold temperatures to, and beyond which dangerous consequences including drought, floods and storms are expected. "What is very clear is we are not" on the path to keeping temperatures below 2C, said Thomas Stocker, one of the co-chairs of today's report. Global temperatures are likely to rise by 0.3C to 4.8C by the end of the century, the report said.
• Sea level rises are coming. "Global mean sea level will continue to rise during the 21st century," says today's report, by a further 26-82cm by 2100, but Stocker said "there is no consensus in the scientific community over very high sea level rises".
• Scientists said claims that the rate of temperature rises in the last 15 years has slowed did not affect the big picture, and temperatures are going up in the long-term. Climate trends "should not be calculated for periods of less than 30 years," said Stocker.
• The amount of carbon the world can burn without heading for dangerous levels of warming is far less than the amount of fossil fuels left in the ground. "The IPCC carbon budget to stay below 2C is 800-880 gigatonnes of carbon (GTC). 531 GTC had already been emitted by 2011. So we have 350 GTC left, which is much less than the carbon stored in fossil fuel reserves," notes our correspondent Fiona Harvey.
IPCC climate report: the digested read
The fifth assessment report from the IPCC looks at everything from oceans and sea ice to carbon budgets and geoengineering
Damian Carrington and John Vidal theguardian.com 27 Sep 13;
Global change
The global climate has already changed in many ways that are unprecedented in the past hundreds or thousands of years, the world's scientists and governments concluded in the new report from the Intergovernmental Panel on Climate Change. These changes have affected every region of the globe, on land and at sea. Continued carbon emissions will drive further heatwaves, sea level rise, melting ice and extreme weather. The changes will last for centuries and limiting the effects would require "substantial and sustained" cuts in carbon dioxide, the scientists report.
Temperatures
Scientists are now at least 66% certain that the last three decades are the warmest in 1400 years, with global temperature having risen by 0.9C in the last century. However, more than 90% of the heat trapped by greenhouse gases is being stored in the oceans.
By mid-century, scientists predict a further rise of 1.4-2.6C if carbon emissions continue to rise as they are today. If emissions were halted almost immediately and significant carbon was extracted from the atmosphere, the rise by mid-century would be 0.4-1.6C.
The scientists predict the average temperature between 2080 and 2100 will be 2.6-4.8C higher than today if emissions are unchecked. They are 90% certain that heatwaves will be more frequent and longer.
In the oceans, the strongest surface warming is expected in tropical and sub-tropical regions, up to 2C by 2100 and posing a grave threat to coral reefs which sustain much sealife. Scientists conclude that a collapse of the Gulf Stream that warms western Europe, as dramatised in the film The Day After Tomorrow, is very unlikely this century but cannot be ruled out afterwards.
Oceans
Global sea level has already risen by 20cm in the last century and scientists are now 90% certain that the rate of the rise will increase. The tide line is rising as warming glaciers and ice sheets pour hundreds of billions of tonnes of water into the oceans each year, but an equally big factor is the warming – and therefore expansion – of the seawater itself.
The new projections for the average sea level in the period 2080-2100 are greater than in the 2007 report, ranging from 45-82cm higher than now if nothing is done to curb emissions to 26-55cm if carbon emissions are halted and reversed. In the former case, sea level could have risen by a 98cm by the end of the century, seriously threatening cities from Shanghai to New York and meaning hurricanes and cyclones inflict far worse damage when they hit shorelines.
Sea level projections have been controversial because exactly how fast glaciers and ice sheets will slip into the sea is not well known. A collapse in ice sheets is therefore not included in the estimates and could add tens of centimetres more to the rise. Because the big Greenland and Antarctic ice sheets are slow to melt, scientists predict melting and sea level rise will continue for centuries. If a temperature rise of between 1C and 4C is sustained, the vast Greenland ice sheet will completely melt adding 7m to sea level, scientists predict, but over the course of a millennium.
The acidity of the ocean is also increasing, due the large amounts of carbon dioxide it is absorbing, and this will continue. This will harm shell-forming sealife but scientists are still determining to what extent.
Ice
The impact of warming is crystal clear in the faster rates of melting in virtually all the world's glaciers and the huge ice sheets in Greenland and Antarctica. The ice sheets have been shedding at least five times more water in the 2000s than in the 1990s, the scientists report. Northern hemisphere snow cover has fallen by 11% a decade since 1967 and the temperature of the seasonally frozen ground, or permafrost, has increased by 2-3C in Russia and Alaska.
Arctic sea ice has been melting by 9-14% a decade since 1979, while sea ice around Antarctica has been increasing by 1-2%, probably due to current changes.
Scientists are 90% sure that Arctic sea ice, snow cover and glaciers will continue to shrink. The scientists say a "nearly ice-free" Arctic ocean in September is at least 66% likely before 2050. By 2100 between 35% and 85% of the remaining world glacier volume will have vanished if emissions are not cut. Permafrost is also 99% likely to shrink further.
Extremes
It is 90% certain that the number of warm days and nights has increased globally and heatwaves have become more frequent, lasting longer in Europe, Asia and Australia. Droughts have also become more frequent and intense in the Mediterranean and west African regions.
The number of heavy rainfall events over land has increased in more regions than it has decreased. It is virtually certain that the frequency and intensity of the strongest tropical cyclones in the North Atlantic has increased since the 1970s.
The scientists concluded it is 99% certain that the frequency of warm days and warm nights increases in the next decades, while that of cold days and cold nights to decrease. The frequency and intensity of extreme downpours is very likely to increase in many populous regions.
Pause
The last decade has been the warmest on record but although CO2 concentrations in the atmosphere have continued to accelerate, surface air temperatures have only marginally increased in the past 15 years, leading some to suggest global warming has stopped. The IPCC scientists reject this, reporting that while the warming trend is robust over decades, there is "substantial" variability within decades. They conclude: "Trends based on short records ... do not in general reflect long-term climate trends."
They add that the heat being trapped by global warming in 2011 was 43% more than the estimate for 2005 in their last report and that over 90% of all the heat added enters the oceans.
Carbon budget
Scientists calculate that nearly half of all the carbon dioxide that can be safely emitted without raising temperatures above a dangerous 2C has already been emitted. This, says the IPCC, means governments must act quickly to have a reasonable change of avoiding 2C. It is also very likely that more than 20% of emitted CO2 will remain in the atmosphere longer than 1,000 years after man-made missions have stopped. According to the IPCC, a large fraction of climate change is thus "irreversible on a human time-scale", except if man-made CO2 emissions are sucked out of the atmosphere over a long period.
Geoengineering
The scientists report that "geoengineering" the climate by reducing the amount of sunlight being absorbed by earth or by extracting and storing carbon dioxide and other climate-changing emissions is theoretically possible. But, the IPCC warns, there is insufficient knowledge to assess how effective such methods, such as pumping sunscreen chemical into the stratosphere, would be and warns of "side effects and long-term consequences on a global scale."
Abrupt change
It is "very likely" that the so-called Gulf Stream, which ferries warm water to western Europe, will weaken over the 21st century. But it is "very unlikely" to collapse or undergo a major transition this century. Further warming will lead to significant methane emissions from permafrost over the next century, equivalent to 50 to 250 billion tonnes of CO2. But the IPCC scientists do not assess the possibility of catastrophic releases this century.
Uncertainties
In terms of data, information is still limited in some locations and especially from before 1950s. There is also limited data from oceans below 700m.
Theoretical uncertainties are how pollution affects cloud formation and the planet's overall climate "sensitivity", ie how much it responds to extra CO2 in the atmosphere. The new report slightly reduces the minimum climate sensitivity but at the report's launch event, co-chair Thomas Stocker said that change, if realised would slow the impacts of climate change by just a few years.
There is uncertainty about the contribution of human activity to changes in tropical cyclones and droughts.
Other explanations of warming
The scientists state: "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." The report rules out any significant contribution from changing solar cycles, volcanoes and cosmic rays.
