Scientists have long warned that failure to halt or significantly slow greenhouse gas emissions would do irreparable harm to the Earth, society and civilization. Crucial thresholds for change are ever nearer — because time has passed and those deadlines are more proximate and because the pace of change is accelerating. The Intergovernmental Panel on Climate Change (IPCC), in the most authoritative assessment of climate change, has released its latest report and the conclusions are grim: Keeping global warming below a critical limit demands “rapid, far-reaching and unprecedented changes in all aspects of society.” Immediate action is required, but history gives us little reason to be optimistic.

The IPCC is a scientific body under the United Nations, set up in 1988 to provide authoritative assessments of climate change. It shared the 2007 Nobel Peace Prize with environmental activist and former U.S. Vice President Al Gore. Membership is open to all members of the World Meteorological Program and the U.N. Environmental Program. It does not conduct its own analysis but instead assesses published literature.

After the 2015 Paris climate agreement, the IPCC was tasked with providing a special report to detail impacts of climate change of 1.5 degrees Celsius, what will be required to prevent further warming, and mitigation and adaptation options to deal with those impacts. Nearly 100 authors from 40 countries, along with 133 contributing authors, joined the effort: A 34-page “summary for policymakers” that cited more than 6,000 articles and studies was presented Monday at the 48th Session of the IPCC, which was held in Incheon, South Korea.

While IPCC reports are cautious and conservative by nature, the conclusions of this report are alarming. It notes that global temperatures have already risen by about 1 degree — in other words, we are already two-thirds of the way to the 1.5 C threshold set as a target by the Paris accord. If the world continues its current trajectory, that level will be reached in 2030.

The impact of this climb is already evident: rising sea levels, more extreme weather, Arctic melt, to name but three. Worse, it concludes that even if governments honored their pledges as part of the Paris accord, global temperatures would rise about 3 degrees on average by the end of the century.

Scientists once believed that 1.5 C was a good target and 2 C was “the limit.” This is no longer the case. Now 2 degrees is intolerable for some parts of the world, and as Jennifer Morgan, the executive director of Greenpeace International, explained, “1.5 degrees is the new 2 degrees.”

It is hypothetically possible to hold the line at 1.5 degrees: The end of all greenhouse gas emissions this week would do the trick. But that is virtually impossible. Experts believe that global net emissions of carbon dioxide need to fall 45 percent from 2010 levels by 2030 and reach “net zero” by mid-century if the 1.5 degree target is to be hit. It is “possible within the laws of chemistry and physics,” reports one IPCC expert, but that would demand a virtual transformation of modern societies, forcing change in energy policy, urban policy, construction and transportation. The IPCC concludes that “there is no documented historic precedent” for such a shift.

Currently, global carbon dioxide emissions total 40 billion tons a year. To reach the 1.5 degree target, there must be reductions in emissions in the next decade of more than 1 billion tons per year — a sum that exceeds the emissions of virtually all emitting countries. The IPCC reckons that by the midpoint of the century, there must be a phaseout of the use of coal.

Difficult as such a development would be, the withdrawal of the United States — the world’s second-largest greenhouse gas emitter — from the Paris agreement, the U.S. government’s dismissal of human contributions to climate change and its disregard for regulations to cut such emissions makes that goal even more unlikely. The prospect of 2 billion more people in the world and the resulting demands on the ecosystem paints a bleaker picture still.

Reaching “net zero” is not a technical problem. A solution demands social, economic and political considerations that are well beyond the world’s current capacity.

Japan knows well the impact of climate change: This summer the entire country was walloped by extreme weather and more will likely follow. Japan has made sustainable environmental policies a priority, pushing the Kyoto Protocol in the early 1990s and putting those concerns at the heart of its diplomacy. It must continue to do so, for example, working with like-minded nations to ensure that the December meeting of the Paris climate agreement musters a consensus on how to meet the ever-intensifying challenge of climate change.

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