Next year, in December, delegates from more than 190 nations will gather in Paris to take another shot at completing a new global treaty on climate change. This will be the 21st Conference of the Parties under United Nations auspices since the first summit meeting in Rio de Janeiro in 1992.  For the most part, these meetings have been exercises in futility, producing just one treaty — in Kyoto in 1997 — that asked little of the big developing countries and was never ratified by the United States Senate. But if the Intergovernmental Panel on Climate Change’s most recent report is to be taken seriously, as it should be, the Paris meeting may well be the world’s last, best chance to get a grip on a problem that, absent urgent action over the next decade, could spin out of control.  The I.P.C.C., composed of thousands of the world’s leading climate scientists, has issued three reports in the last seven months, each the product of up to six years of research. The first simply confirmed what has been known since Rio: global warming is caused largely by the burning of fossil fuels by humans and, to a lesser extent, by deforestation. The second, released in Japan three weeks ago, said that profound effects were already being felt around the world, including mounting damage to coral reefs, shrinking glaciers and more persistent droughts, and warned of worse to come — rising seas, species loss and dwindling agricultural yields.  The third report, released last week, may be the most ominous of the three. Despite investments in energy efficiency and cleaner energy sources in the United States, in Europe and in developing countries like China, annual emissions of greenhouse gases have risen almost twice as fast in the first decade of this century as they did in the last decades of the 20th century. This places in serious jeopardy the emissions target agreed upon in Rio to limit warming to no more than 2 degrees Celsius (3.6 degrees Fahrenheit) above the preindustrial level. Beyond that increase, the world could face truly alarming consequences.  Avoiding that fate will require a reduction of between 40 percent and 70 percent in greenhouse gases by midcentury, which means embarking on a revolution in the way we produce and consume energy.  That’s daunting enough, but here’s the key finding: The world has only about 15 years left in which to begin to bend the emissions curve downward. Otherwise, the costs of last-minute fixes will be overwhelming. “We cannot afford to lose another decade,” says Ottmar Edenhofer, a German economist and co-chairman of the committee that wrote the report. “If we lose another decade, it becomes extremely costly to achieve climate stabilization.”  The report does not tell governments what to do — presumably, that’s for them to decide in Paris — but it lists approaches, mostly familiar, some technologically advanced. The most obvious, and probably the most difficult to negotiate, is to put a global price on carbon, either through a system of tradable permits like that adopted by Europe (and rejected by the United States Senate) or through a carbon tax of some sort, thus driving investments to cleaner fuels.  A more plausible pathway is to get each country to adopt binding emission reduction targets and then allow them to choose how to get there — ramping up nuclear energy, phasing out coal-fired plants in favor of cleaner natural gas (though natural gas itself would have to someday give way to low-carbon alternatives), and vastly increasing renewable sources like wind and solar, which still supply only a small fraction of the world’s energy (less than 5 percent for wind and solar combined in the United States). All this will require a huge shift in investment, both private and public, from fossil fuels.  Governments have an enormous amount of work to do in devising emission reduction strategies by next year. As always, American leadership will be required, meaning leadership from the top. Confronted with a hostile Congress, President Obama has commendably moved on his own to reduce emissions through regulations, first with cars and now with coal-fired power plants. And he has done so without a great deal of public support. However compelling the science, global warming has not generated the kind of public anxiety and bottom-up demand for change that helped win the big fights for cleaner air and water in the late 1960s and early 1970s. This makes his job harder but no less urgent.
There are years, not decades, left to start reducing greenhouse gas emissions, and American leadership is urgently needed.