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.