Two new studies aim to quantify limits on the amount of greenhouse emissions necessary to avoid dangerous global warming
David Biello, Scientific American 29 Apr 09;
To avoid catastrophic climate change, the world will need to emit less than one trillion metric tons of carbon between now and 2050, according to two new papers published in Nature today. In other words, there is only room in the atmosphere to burn or vent less than one quarter of known oil, natural gas and coal reserves.
Atmospheric concentrations of carbon dioxide have reached 386 parts per million—and rising, because every year, human activity spews more than 30 billion metric tons of CO2. So far, that's led to warming of roughly 0.8 degree Celsius (1.4 degrees Fahrenheit). The question is: How much more can we safely emit? The two new papers attempt an answer.
"There is a simple and predictable relationship between the total amount of carbon injected into the atmosphere and peak projected warming," says physicist Myles Allen of the University of Oxford in England and lead author of one of the studies. "Releasing a trillion [metric] tons of carbon into the atmosphere may cause a most likely peak warming of 2 degrees C [3.6 degrees F], which many identify as the danger point." An average temperature rise of 2 degrees C or lower has been adopted by the European Union and other countries—110 in all—as a goal for any treaty to control climate change, and has been identified by scientists, including the authors of the U.N. Intergovernmental Panel on Climate Change report, as a point at which most climate changes become damaging.
But CO2—and the carbon at its molecular core—is not the only greenhouse gas. Others—ranging from methane to hydrofluorocarbons (HFCs)—"could contribute as much as 10 to 40 percent of the warming induced by CO2 alone," says climatologist Malte Meinshausen of the Potsdam Institute for Climate Impact Research (PIK) in Germany, lead author of the second study appearing in Nature. That drops the overall budget for atmospheric emissions to roughly 750 billion metric tons of carbon between the years 2000 and 2050. "To limit the risk to a one-in-four chance [of 2 degree C warming], then total CO2 in the first half of the 21st century has to be kept below [one trillion] metric tons."
Put another way: humanity can only afford to burn and vent less than one quarter of known oil, natural gas and coal reserves. Already, between 2000 and 2006, the world emitted roughly 234 billion metric tons of CO2—and roughly one third of the total trillion metric ton "budget" has already been spent to date. "We can burn less than a quarter of known economically recoverable fossil fuel reserves between now and 2050," says co-author and climatologist William Hare, also of the PIK. "Not much at all of coal reserves can be burnt and still keep warming below the 2 degree [C] limit."
Meeting that target will require global emissions to peak and begin to decline before 2020—unless countries desire drastic cuts later in the century. "If you burn a [metric] ton of carbon today then you can't burn it tomorrow, you've got a finite stock," says co-author and physicist David Frame of Oxford. "How would you apportion that finite stock of carbon?" That is a decision the world's governments will have to make in coming months and years.
Global greenhouse gas emissions will need to be at least 50 percent below 1990 levels by 2050—that means cuts by industrialized countries such as the U.S. of more than 90 percent—and on a path to zero emissions. A market for carbon—charging emitters for the ability to pollute—could help. A $50 price for a metric ton of CO2 by 2020 might do the trick based on economic modeling, Frame says, allowing emissions to peak some 25 percent above 1990 levels. "After that the carbon price would need to progressively increase to push carbon emissions down globally to where you would stay within this budget," he adds. "The 80 percent reduction [by 2050 pledge] from the U.S. is a good start but it's not enough to limit warming in order to meet [the] 2 degree [C increase] with high confidence."
Of course, 2 degrees C warming—a further 1.2 degrees C from the current level of heating—will in and of itself have a host of unpleasant effects, such as ongoing sea-level rise that will swamp coastlines and island states. Since 1750 humanity has added 520 billion metric tons of greenhouse gases to the atmosphere and we're on pace to add that much again within 40 years—a scenario that is likely to result in catastrophic climate change.
"The longer we let [emissions] rise, the harder and costlier reductions become," Allen notes.
'Safe' climate means 'no to coal'
Richard Black, BBC News 29 Apr 09;
About three-quarters of the world's fossil fuel reserves must be left unused if society is to avoid dangerous climate change, scientists warn.
More than 100 nations support the goal of keeping temperature rise below 2C.
But the scientists say that without major curbs on fossil fuel use, 2C will probably be reached by 2050.
Writing in Nature, they say politicians should focus on limiting humanity's total output of CO2 rather than setting a "safe" level for annual emissions.
The UN climate process focuses on stabilising annual emissions at a level that would avoid major climate impacts.
But this group of scientists says that the cumulative total provides a better measure of the likely temperature rise, and may present an easier target for policymakers.
"To avoid dangerous climate change, we will have to limit the total amount of carbon we inject into the atmosphere, not just the emission rate in any given year," said Myles Allen from the physics department at Oxford University.
"Climate policy needs an exit strategy; as well as reducing carbon emissions now, we need a plan for phasing out net emissions entirely."
Forty years
The UN climate convention, agreed at the 1992 Rio Earth Summit, commits countries to avoiding "dangerous" climate change, without defining what that is.
The EU proposed some years ago that restricting the rise to 2C from pre-industrial times was a reasonable threshold, and it has since been adopted by many other countries, although some - particularly small islands - argue that even 2C would result in dangerous impacts.
Temperatures have already risen by about 0.7C during the industrial age.
Dr Allen's analysis suggests that if humanity's CO2 emissions total more than about one trillion tonnes of carbon, the 2C threshold is likely to be breached; and that could come within a lifetime.
"It took us 250 years to burn the first half trillion," he said, "and on current projections we'll burn the next half trillion in less than 40 years."
Inherent uncertainties in the modelling mean the temperature rise from the trillion tonnes could be between 1.3C and 3.9C, Dr Allen's team calculates, although the most likely value would be 2C.
Oil change
The "trillion tonnes" analysis is one of two studies published in Nature by a pool of researchers that includes the Oxford group and scientists from the Potsdam Institute for Climate Change Impact Research in Germany.
The second study, led by Potsdam's Malte Mainshausen, attempted to work backwards from the 2C goal, to find out what achieving it might mean in practice.
It suggests that the G8 target of halving global emissions by 2050 (from 1990 levels) would leave a significant risk of breaching the 2C figure.
"Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming," said Dr Mainshausen.
"If we continue burning fossil fuels as we do, we will have exhausted the carbon budget in merely 20 years, and global warming will go well beyond 2C."
If policymakers decided they were happy to accept a 25% chance of exceeding 2C by 2050, he said, they must also accept that this meant cutting emissions by more than 50%.
That would mean only burning about a quarter of the carbon in the world's known, economically-recoverable fossil fuel reserves. This is likely to consist mainly of oil and natural gas, leaving coal as a redundant fuel unless its emissions could be captured and stored.
Both analyses support the view of the Stern Review and the Intergovernmental Panel on Climate Change (IPCC) in suggesting that making reductions earlier would be easier and cheaper than delaying.
But according to Potsdam's Bill Hare, a co-author on the second paper, some key governments appear to favour pledging milder cuts in the near term in return for more drastic ones in decades to come.
"We have a number of countries - the US, Japan, Brazil - saying 'we will emit higher through to 2020 and then go down faster'," he said.
"That might be true geophysically, but we cannot find any economic model where emissions can fall in the range that this work shows would be necessary - around 6% per year."
Major intervention
Myles Allen's group has made the argument before that focussing on humanity's entire carbon dioxide output makes more scientific and political sense than aiming to define a particular "safe" level of emissions, or to plot a pathway assigning various ceilings to various years.
Some greenhouse gases, such as methane, have a definable lifetime in the atmosphere, meaning that stabilising emissions makes sense; but, said Dr Allen, CO2 "doesn't behave like that".
"There are multiple levers acting on its concentration and it does tend to accumulate; also models have to represent the possibility of some feedback between rising temperatures and emissions, such as parts of the land turning from carbon sinks into sources, for example."
The Nature papers emerge in a week that has seen the inaugural meeting of President Obama's Major Economies Forum on Energy and Climate, a new version of a body created under President Bush that brings together 17 of the world's highest-emitting countries for discussion and dialogue.
During the opening segment, Secretary of State Hillary Clinton re-affirmed the administration's aim of cutting US emissions by 80% from 1990 levels by 2050 - a target espoused by some other developed countries.
But according to Malte Meinshausen's analysis, even this reduction may not be enough to keep the average global temperature rise within 2C, assuming less developed nations made less stringent cuts in order to aid their development.
"If the US does 80%, that equates to about 60% globally, and that offers only a modest chance of meeting the 2C target," he said.
Last week saw the publication of data showing that industrialised countries' collective emissions rose by about 1% during 2007.
To meet climate goal, cut fossil fuels use: study
Yahoo News 29 Apr 09;
PARIS (AFP) – Meeting a widely-supported goal to tackle global warming means that humanity will be able to burn less than a quarter of the proven reserves of fossil fuels by 2050, a study released on Wednesday said.
The paper, published by the British journal Nature, implies only a revolution in energy use can achieve the aim of limiting warming to less than two degrees Celsius (3.8 degrees Fahrenheit) above pre-industrial levels.
To achieve the objective -- embraced by the European Union (EU) and many scientists -- means that only 1,000 billion tonnes of carbon dioxide (CO2) can be emitted between 2000 and 2050, it said.
By comparison, the world has emitted a third of that amount in just nine years.
"If we continue burning fossil fuels as we do, we will have exhausted the carbon budget in merely 20 years, and global warming will go well beyond two degrees," said the study's lead author, Malte Meinshausen of the Potsdam Institute for Climate Impact Research in Berlin.
Fossil fuels -- coal, gas and oil -- provide the backbone of the world's energy supplies.
But they are also the main source of heat-trapping carbon gases blamed for warming Earth's atmosphere and driving changes to weather patterns.
Boosting efficient use of these fuels so that they are no longer dangerous or switching to cleaner alternatives carries an economic cost, and this is the biggest stumbling block in efforts to defuse the threat.
Meinshausen said the change should not be delayed and cautioned that even with a 2 C (3.8 F) warming, there would still be unprecedented risk.
"Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming," he said in a press release.
"We shouldn't forget that a 2 C [3.8 F] global mean warming would take us far beyond the natural temperature variations that life on Earth has experienced since we humans have been around."
"To keep warming below 2 C [3.8 F], we cannot burn and emit the CO2 from more than a quarter of the economically recoverable fossil fuels up to 2050, and in the end, only a small fraction of all known fossil-fuel reserves," said co-author Bill Hare, also of the Potsdam Institute.
The study said world emissions of greenhouse gases have to be cut by more than 50 percent by 2050 levels compared to 1990 levels if the risk of busting the 2 C (3.6 F) ceiling is to be limited to 25 percent. In addition, reductions would have to be made from 2020.
The Group of Eight (G8) countries have pledged an emissions reduction of at least 50 percent by 2050.
But they have not identified a benchmark year against which this should be measured nor set an intermediate date by which emissions cuts should start.
Negotiations are underway under the UN's Framework Convention on Climate Change (UNFCCC) for agreeing on emissions cuts beyond 2012, when current pledges under the Kyoto Protocol expire. The UNFCCC wants to wrap up a deal in a conference in Copenhagen in December.
The world has already warmed by about 0.8 C (1.44 F) since the start of the Industrial Revolution, and another 0.5 C (O.9 F) or so is considered inevitable over coming decades given past greenhouse gas emissions.
To keep warming low, deeper pollution cuts needed
Seth Borenstein, Associated Press 29 Apr 09;
WASHINGTON – If the world is going to limit global warming to just a few degrees, it has to slash carbon dioxide pollution much more than now being discussed, two new science studies say.
Carbon dioxide from the burning of fossil fuels — coal, oil and natural gas — is the chief cause of global warming.
The studies found there's a limit to how much manmade carbon dioxide can be added to the air before warming exceeds an increase of 2 degrees Celsius or 3.6 degrees Fahrenheit — the level that many governments have set as a goal. World average temperatures going higher than that may be dangerous, some scientists say.
To keep under that danger level, the world has to spew less than 1.1 trillion tons of carbon dioxide in the first half of the 21st Century, according to studies published in Thursday's edition of the journal Nature.
In the first nine years of the century, the world has already emitted one-third of that amount and is on pace to hit that trillion ton limit in just 20 years, said climate researcher Malte Meinshausen of Germany's Potsdam Institute for Climate Impact Research and lead author of one of the studies.
Even if the world ducks under that emissions limit, there is still a 25 percent chance of temperatures exceeding the dangerous mark, he said.
President Barack Obama said he wants to cut U.S. emissions of carbon dioxide by 80 percent. That is a "good start but it's not enough to limit warming," said Bill Hare, a study co-author who is also at the Potsdam Institute.
Assuming that other countries cut their per-person emission levels to match the United States, the United States has to cut its overall pollution by 90 to 95 percent to keep the world from exceeding the 1.1 trillion ton mark, Hare said.
Cutting emissions means not burning as much fossil fuels, leaving about three quarters of the known reserves in the ground, the study authors said.
"Not much at all of coal reserves can be burnt and still keep below" the 3.6 degrees of warming, Hare said.
World emissions must start dropping by 2015, otherwise cuts will have to be too draconian, Meinshausen said.
The studies, which used computer models, take a different approach than other research on figuring out how much carbon dioxide in the air is too much. Instead of the proportion of carbon dioxide in the air at any given time, they looked at the total amount spewed out over many decades to arrive at a tipping point of 1.1 trillion tons.
Stephen Schneider of Stanford University who paints a worst case scenario for global warming in a commentary in the journal, said the studies make it seem like scientists know where there's a solid danger line for emissions, when they don't. The papers acknowledge there is a 25 percent chance the limit should be lower. Schneider said that's a pretty big risk when the consequences of being wrong are severe.
"If you had a 25 percent chance that walking into a room would give you serious flu, would you?" Schneider asked.
Climate countdown: Half a trillion tonnes of carbon left to burn
To avoid dangerous climate change of 2C, the world can only burn another half a trillion tonnes of carbon, climate change experts warn
David Adam, guardian.co.uk 29 Apr 09;
The world has already burned half the fossil fuels necessary to bring about a catastrophic 2C rise in average global temperature, scientists revealed today.
The experts say about half a trillion tonnes of carbon have been consumed since the industrial revolution. To prevent a 2C rise, they say, the total burnt must be kept to below a trillion tonnes. On current rates, that figure will be reached in 40 years.
Myles Allen, a climate expert at Oxford University who led the new study, said: "Mother Nature doesn't care about dates. To avoid dangerous climate change we will have to limit the total amount of carbon we inject into the atmosphere, not just the emission rate in any given year."
The scientists say their research could simplify political attempts to tackle global warming, which encompass a range of targets and timetables. Such proposals usually set future limits on the amount of carbon dioxide allowed to build up in the atmosphere, such as 450 parts per million (ppm), or as future emission rates, such as the UK's pledge to slash emissions 80% by 2050.
The new study effectively re-frames such targets as an available budget - to avoid dangerous climate change of 2C the world can only burn another half a trillion tonnes of carbon.
Writing in today's Nature, Allen and colleagues say a trillion tonnes of carbon burnt would be likely to produce a warming of between 1.6C and 2.6C, with a "most likely" 2C rise.
Chris Huntingford of the NERC Centre for Ecology and Hydrology said: "Research often reveals new complexities, but this analysis could actually simplify matters for policy makers. The relationship between total emissions and future warming can be inferred largely from quantities we can observe, and is remarkably insensitive to the timing of future emissions."
The key implication of the research, the scientists say, is that access to fossil fuels must somehow be rationed and eventually turned off, if the 2C target is to be met. "If country A burns it then country B can't," said Bill Hare, a climate expert with the Potsdam Institute in Germany. "It's like a draining tank."
The research also highlights that continued high rates of fossil fuel use in the next decade will demand extraordinary cuts in emissions in future decades to hit the 2C target. Allen said: "If you use too much [carbon] this year, it doesn't mean the planet will come to an end. It means you have to work even harder the next year."
A separate study, also published today in Nature, led by Malte Meinshausen at the Potsdam Institute, use a similar approach and sets a different carbon budget. They say the world can only emit 190bn tonnes of carbon between now and 2050 if it aims for a 2C rise. Emissions over 310bn tonnes in that time lead to a 50% chance of going over 2C.
The new research does not say anything about the likelihood of reaching the 2C target. They simply change the way progress towards the target is measured.
In an accompanying commentary article, the scientists behind both studies say: "These results are not incompatible with current proposals for near-term emission targets -- the small size of the cumulative emission budgets to 2050 reinforces the need for global CO2 emissions to peak around or before 2020 so that emission pathways remain technologically and economically feasible."
They add: "Having taken 250 years to burn the first half trillion tonnes of carbon we look set, on current trends, to burn the next half trillion in less than 40. No one could credibly suggest that we should carry on with business as usual to the 2040s and then somehow suddenly stop using fossil fuels, switch to 100% carbon capture or just shut down the world economy overnight."
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