STOCKHOLM — The world’s top climate scientists on Friday formally embraced an upper limit on greenhouse gases for the first time, establishing a target level at which humanity must stop spewing them into the atmosphere or face irreversible climatic changes. They warned that the target is likely to be exceeded in a matter of decades unless steps are taken soon to reduce emissions. 
 
 Unveiling the latest United Nations assessment of climate science, the experts cited a litany of changes that were already under way, warned that they were likely to accelerate and expressed virtual certainty that human activity is the main cause. “Climate change is the greatest challenge of our time,” said Thomas F. Stocker, co-chairman of the Intergovernmental Panel on Climate Change, the United Nations-sponsored group of scientists that produced the report. “In short, it threatens our planet, our only home.” 
 
 The panel, in issuing its most definitive assessment yet of the risks of human-caused warming, hoped to give impetus to international negotiations toward a new climate treaty, which have languished in recent years in a swamp of technical and political disputes. The group made clear that time was not on the planet’s side if emissions continued unchecked. 
 
 “Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes,” the report said. “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” 
 
 The new report is a 36-page summary for world leaders of a 900-page report that is to be released next week on the physical science of climate change. That will be followed by additional reports in 2014 on the most likely impacts and on possible steps to limit the damage. A draft of the summary leaked last month, and the final version did not change greatly, though it was edited for clarity. 
 
 Going well beyond its four previous analyses of the emissions problem, the panel endorsed a “carbon budget” for humanity — a limit on the amount of the primary greenhouse gas, carbon dioxide, that can be produced by industrial activities and the clearing of forests. No more than one trillion metric tons of carbon could be burned and the resulting gases released into the atmosphere, the panel found, if planetary warming is to be kept below 3.6 degrees Fahrenheit (2 degrees Celsius) above the level of preindustrial times. That temperature is a target above which scientists believe the most dangerous effects of climate change would begin to occur. 
 
 Just over a half-trillion tons have already been burned since the beginning of the Industrial Revolution, and at the rate energy consumption is growing, the trillionth ton will be burned sometime around 2040, according to calculations by Myles R. Allen, a scientist at the University of Oxford and one of the authors of the new report. More than three trillion tons of carbon are still left in the ground as fossil fuels. 
 
 Once the trillion-ton budget is exhausted, companies that wanted to keep burning fossil fuels would have to come up with ways to capture carbon dioxide and store it underground. In the United States, the Obama administration is moving forward with rules that would essentially require such technology, which is likely to be costly, for any future coal-burning power plants; the president’s Republican opponents have accused him of waging a “war on coal.” 
 
 The Intergovernmental Panel on Climate Change is a worldwide committee of hundreds of scientists that issues major reports every five or six years, advising governments on the latest knowledge on climate change. 
 
 The group has now issued five major reports since 1990, each of them finding greater certainty that the world is warming and greater likelihood that human activity is the chief cause. The new report finds a 95 to 100 percent chance that most of the warming of recent decades is human-caused, up from the 90 to 100 percent chance cited in the last report, in 2007. 
 
 But the new document also acknowledges that climate science still contains uncertainties, including the likely magnitude of the warming for a given level of emissions, the rate at which the ocean will rise, and the likelihood that plants and animals will be driven to extinction. The scientists emphasized, however, that those uncertainties cut in both directions and the only way to limit the risk is to limit emissions. 
 
 Climate-skeptic organizations assailed the new report as alarmist even before it was published. 
 
 The Heartland Institute, a Chicago organization, issued a document last week saying that any additional global warming would likely be limited to a few tenths of a degree and that this “would not represent a climate crisis.” 
 
 One issue much cited by the climate doubters is the slowdown in global warming that has occurred over the past 15 years. The report acknowledged that it was not fully understood, but said such pauses had occurred in the past and the natural variability of climate was a likely explanation. ||||| Photo Advertisement Continue reading the main story 
 
 Nations have so dragged their feet in battling climate change that the situation has grown critical and the risk of severe economic disruption is rising, according to a draft United Nations report. Another 15 years of failure to limit carbon emissions could make the problem virtually impossible to solve with current technologies, experts found. 
 
 
 
 
 
 
 
 
 
 A delay would most likely force future generations to develop the ability to suck greenhouse gases out of the atmosphere and store them underground to preserve the livability of the planet, the report found. But it is not clear whether such technologies will ever exist at the necessary scale, and even if they do, the approach would probably be wildly expensive compared with taking steps now to slow emissions. 
 
 The report said that governments of the world were still spending far more money to subsidize fossil fuels than to accelerate the shift to cleaner energy, thus encouraging continued investment in projects like coal-burning power plants that pose a long-term climate risk. 
 
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 While the spread of technologies like solar power and wind farms might give the impression of progress, the report said, such developments are being overtaken by rising emissions from fossil fuels over the past decade, especially in fast-growing countries like China. And one of the most important sources of low-carbon energy, nuclear power, is actually declining over time as a percentage of the global energy mix, the report said. 
 
 Unless far greater efforts are made to reduce emissions, “the fundamental drivers of emissions growth are expected to persist despite major improvements in energy supply” and in the efficiency with which energy is used, the report said. 
 
 The new warnings come in a draft report from the Intergovernmental Panel on Climate Change, a United Nations panel of climate experts that won the Nobel Peace Prize in 2007 for its efforts to analyze and communicate the risks of climate change. The report is not final, but a draft dated Dec. 17 was leaked this week and was first reported by Reuters. The New York Times obtained a copy independently. 
 
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 Business leaders will tackle many of the problems raised in the draft next week, at the World Economic Forum in Davos, Switzerland, where a day will be devoted to addressing the rising economic costs of climate change — and the costs to businesses and governments of solving the problem. 
 
 Within the business community, “there is an awakening of increasing economic risk — a recognition that operating conditions are changing and we need to respond,” said Dominic Waughray, head of environmental initiatives for the forum. “There has been a failure of government to address these solutions. If there is an alliance of companies that can bite off pieces of the puzzle, it might help.” 
 
 In the dry language of a technical committee, the draft outlines an increasingly dire situation. 
 
 Even as the early effects of climate change are starting to be felt around the world, the panel concluded that efforts are lagging not only in reducing emissions, but also in adapting to the climatic changes that have become inevitable. 
 
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 It is true, the report found, that the political willingness to tackle climate change is growing in many countries and new policies are spreading, but the report said these were essentially being outrun by the rapid growth of fossil fuels. 
 
 While emissions appear to have fallen in recent years in some of the wealthiest countries, that is somewhat of an illusion, the report found. The growth of international trade means many of the goods consumed in wealthy countries are now made abroad — so that those countries have, in effect, outsourced their greenhouse gas emissions to countries like China. 
 
 Emissions in the United States rose slightly in 2013, but are still about 10 percent below their 2005 levels, largely because of the newfound abundance of natural gas, which produces less greenhouse gases than burning coal. 
 
 The Kyoto Protocol, an international treaty meant to limit emissions, has “not been as successful as intended,” the report found. That is partly because some important countries like the United States refused to ratify it or later withdrew, but also because of flaws within the treaty itself, the report found. The treaty exempted developing countries from taking strong action, for instance, a decision that many experts now say was a mistake. 
 
 Efforts are underway to negotiate a new international treaty to replace Kyoto, but it is not even supposed to take effect until 2020, and it is unclear whether countries will agree on ambitious goals to limit emissions. It is equally unclear how much political support a new treaty will gain in China and the United States, the world’s largest emitters. 
 
 The Obama administration is pushing for a deal, but any treaty would have to be ratified by the Senate; many Republicans and some coal-state Democrats are wary, fearing economic damage to the country. 
 
 The new report suggests, however, that the real question is whether to take some economic pain now, or more later. 
 
 Nations have agreed to try to limit the warming of the planet to 3.6 degrees Fahrenheit above preindustrial levels. Even though it will be exceedingly difficult to meet, this target would still mean vast ecological and economic damage, experts have found. But the hope is that these would come on slowly enough to be somewhat manageable; having no target would be to risk catastrophic disruption, the thinking goes. 
 
 As scientists can best figure, the target requires that atmospheric concentrations of carbon dioxide, the main greenhouse gas, stay below 500 parts per million. The level recently surpassed 400, and at present growth rates will surpass 500 within a few decades. 
 
 If countries permit continued high emissions growth until 2030, the draft report found, the target will most likely be impossible to meet, at least without a hugely expensive crash program to rebuild the energy system, and even that might not work. 
 
 If emissions do overshoot the target, the report found, future generations would probably have to develop ways to pull greenhouse gases out of the air. It is fairly clear this will be technically possible. It could be achieved, for instance, by growing bioenergy crops that take up carbon dioxide, burning the resulting fuel and then injecting the emissions into underground formations. But such efforts would compete with food production, already under strain. 
 
 The leaked draft is the third and final segment of a major report that the climate change panel is completing in stages. The first segment, published in September, found a 95 percent or greater likelihood that humans are the main cause of climate change. The second, due out in March, is expected to warn about the likely consequences of climate change, including risks to the food supply. 
 
 The third, focusing on policies to limit the damage, is due for publication in Berlin in April. ||||| The world’s leading climate scientists have warned there is only a dozen years for global warming to be kept to a maximum of 1.5C, beyond which even half a degree will significantly worsen the risks of drought, floods, extreme heat and poverty for hundreds of millions of people. 
 
 The authors of the landmark report by the UN Intergovernmental Panel on Climate Change (IPCC) released on Monday say urgent and unprecedented changes are needed to reach the target, which they say is affordable and feasible although it lies at the most ambitious end of the Paris agreement pledge to keep temperatures between 1.5C and 2C. 
 
 The half-degree difference could also prevent corals from being completely eradicated and ease pressure on the Arctic, according to the 1.5C study, which was launched after approval at a final plenary of all 195 countries in Incheon in South Korea that saw delegates hugging one another, with some in tears. 
 
 Quick guide What difference would restricting warming to 1.5C make? Show Hide A key finding of the new IPCC report is the dramatic difference that restricting warming to 1.5C above pre industrial levels would have on the global environment. The scientists found: • By 2100, global sea level rise would be 10cm lower with global warming of 1.5C compared with 2C. • Extreme heatwaves will be experienced by 14% of the world's population at least once every five years at 1.5C. But that figure rises to more than a third of the planet if temperatures rise to 2C • Arctic sea ice would remain during most summers if warming is kept to 1.5C. But at 2C, ice free summers are 10 times more likely, leading to greater habitat losses for polar bears, whales, seals and sea birds. 
 
 • If warming is kept to 1.5C, coral reefs will still decline by 70-90% but if temperatures rise to 2C virtually all of the world's reefs would be lost 
 
 
 
 We must reduce greenhouse gas emissions to net zero or face more floods Read more 
 
 “It’s a line in the sand and what it says to our species is that this is the moment and we must act now,” said Debra Roberts, a co-chair of the working group on impacts. “This is the largest clarion bell from the science community and I hope it mobilises people and dents the mood of complacency.” 
 
 Policymakers commissioned the report at the Paris climate talks in 2016, but since then the gap between science and politics has widened. Donald Trump has promised to withdraw the US – the world’s biggest source of historical emissions – from the accord. The first round of Brazil’s presidential election on Sunday put Jair Bolsonaro into a strong position to carry out his threat to do the same and also open the Amazon rainforest to agribusiness. 
 
 The world is currently 1C warmer than preindustrial levels. Following devastating hurricanes in the US, record droughts in Cape Town and forest fires in the Arctic, the IPCC makes clear that climate change is already happening, upgraded its risk warning from previous reports, and warned that every fraction of additional warming would worsen the impact. 
 
 Scientists who reviewed the 6,000 works referenced in the report, said the change caused by just half a degree came as a revelation. “We can see there is a difference and it’s substantial,” Roberts said. 
 
 At 1.5C the proportion of the global population exposed to water stress could be 50% lower than at 2C, it notes. Food scarcity would be less of a problem and hundreds of millions fewer people, particularly in poor countries, would be at risk of climate-related poverty. 
 
 Facebook Twitter Pinterest Attendees take a photo before the opening of the 48th session of the IPCC in Incheon. Photograph: Jung Yeon-Je/AFP/Getty Images 
 
 At 2C extremely hot days, such as those experienced in the northern hemisphere this summer, would become more severe and common, increasing heat-related deaths and causing more forest fires. 
 
 But the greatest difference would be to nature. Insects, which are vital for pollination of crops, and plants are almost twice as likely to lose half their habitat at 2C compared with 1.5C. Corals would be 99% lost at the higher of the two temperatures, but more than 10% have a chance of surviving if the lower target is reached. 
 
 Sea-level rise would affect 10 million more people by 2100 if the half-degree extra warming brought a forecast 10cm additional pressure on coastlines. The number affected would increase substantially in the following centuries due to locked-in ice melt. 
 
 Oceans are already suffering from elevated acidity and lower levels of oxygen as a result of climate change. One model shows marine fisheries would lose 3m tonnes at 2C, twice the decline at 1.5C. 
 
 Sea ice-free summers in the Arctic, which is warming two to three times faster than the world average, would come once every 100 years at 1.5C, but every 10 years with half a degree more of global warming. 
 
 The final tick box is political will Jim Skea 
 
 Time and carbon budgets are running out. By mid-century, a shift to the lower goal would require a supercharged roll-back of emissions sources that have built up over the past 250 years. 
 
 The IPCC maps out four pathways to achieve 1.5C, with different combinations of land use and technological change. Reforestation is essential to all of them as are shifts to electric transport systems and greater adoption of carbon capture technology. 
 
 Carbon pollution would have to be cut by 45% by 2030 – compared with a 20% cut under the 2C pathway – and come down to zero by 2050, compared with 2075 for 2C. This would require carbon prices that are three to four times higher than for a 2C target. But the costs of doing nothing would be far higher. 
 
 “We have presented governments with pretty hard choices. We have pointed out the enormous benefits of keeping to 1.5C, and also the unprecedented shift in energy systems and transport that would be needed to achieve that,” said Jim Skea, a co-chair of the working group on mitigation. “We show it can be done within laws of physics and chemistry. Then the final tick box is political will. We cannot answer that. Only our audience can – and that is the governments that receive it.” 
 
 He said the main finding of his group was the need for urgency. Although unexpectedly good progress has been made in the adoption of renewable energy, deforestation for agriculture was turning a natural carbon sink into a source of emissions. Carbon capture and storage projects, which are essential for reducing emissions in the concrete and waste disposal industries, have also ground to a halt. 
 
 Reversing these trends is essential if the world has any chance of reaching 1.5C without relying on the untried technology of solar radiation modification and other forms of geo-engineering, which could have negative consequences. 
 
 Facebook Twitter Pinterest A nearly ice-free Northwest Passage in the Arctic in August 2016. Photograph: VIIRS/Suomi NPP/Nasa 
 
 In the run-up to the final week of negotiations, there were fears the text of the report would be watered down by the US, Saudi Arabia and other oil-rich countries that are reluctant to consider more ambitious cuts. The authors said nothing of substance was cut from a text. 
 
 Bob Ward, of the Grantham Research Institute on Climate Change, said the final document was “incredibly conservative” because it did not mention the likely rise in climate-driven refugees or the danger of tipping points that could push the world on to an irreversible path of extreme warming. 
 
 The report will be presented to governments at the UN climate conference in Poland at the end of this year. But analysts say there is much work to be done, with even pro-Paris deal nations involved in fossil fuel extraction that runs against the spirit of their commitments. Britain is pushing ahead with gas fracking, Norway with oil exploration in the Arctic, and the German government wants to tear down Hambach forest to dig for coal. 
 
 At the current level of commitments, the world is on course for a disastrous 3C of warming. The report authors are refusing to accept defeat, believing the increasingly visible damage caused by climate change will shift opinion their way. 
 
 “I hope this can change the world,” said Jiang Kejun of China’s semi-governmental Energy Research Institute, who is one of the authors. “Two years ago, even I didn’t believe 1.5C was possible but when I look at the options I have confidence it can be done. I want to use this report to do something big in China.” 
 
 The IPCC global warming report spares politicians the worst details | Bob Ward Read more 
 
 The timing was good, he said, because the Chinese government was drawing up a long-term plan for 2050 and there was more awareness among the population about the problem of rising temperatures. “People in Beijing have never experienced so many hot days as this summer. It’s made them talk more about climate change.” 
 
 Regardless of the US and Brazil, he said, China, Europe and major cities could push ahead. “We can set an example and show what can be done. This is more about technology than politics.” 
 
 James Hansen, the former Nasa scientist who helped raised the alarm about climate change, said both 1.5C and 2C would take humanity into uncharted and dangerous territory because they were both well above the Holocene-era range in which human civilisation developed. But he said there was a huge difference between the two: “1.5C gives young people and the next generation a fighting chance of getting back to the Holocene or close to it. That is probably necessary if we want to keep shorelines where they are and preserve our coastal cities.” 
 
 Johan Rockström, a co-author of the recent Hothouse Earth report, said scientists never previously discussed 1.5C, which was initially seen as a political concession to small island states. But he said opinion had shifted in the past few years along with growing evidence of climate instability and the approach of tipping points that might push the world off a course that could be controlled by emissions reductions. 
 
 “Climate change is occurring earlier and more rapidly than expected. Even at the current level of 1C warming, it is painful,” he told the Guardian. “This report is really important. It has a scientific robustness that shows 1.5C is not just a political concession. There is a growing recognition that 2C is dangerous.”
– The UN's climate change panel for the first time charted the upper limit of acceptable global carbon emissions today—and we're already halfway to it. Based on an internationally agreed upon goal of keeping temperatures within 3.6 degrees Fahrenheit of their pre-industrial levels, humanity can only allow gas from 1 trillion tons of burned carbon to enter the atmosphere, the Intergovernmental Panel on Climate Change said in its report, according to the New York Times. We're on pace to hit that ceiling in 2040. The much-anticipated report was released this morning following a furious night of editing, but the substance of it hasn't changed much since a draft was released in August. The report also presents what, according to the BBC, is considered the most comprehensive statement on how climate change works, concluding with 95% certainty that human activity "has been the dominant cause" of the warming trend. That, the AP points out, is as certain as scientists are that cigarettes kill you, and more certain than they are that vitamins improve your health.