Perhaps now the deniers will cease their attacks on the science of climate change, and the American public will, at last, fully accept that global warming is a danger now and an even graver threat to future generations.

On Monday, the Intergovernmental Panel on Climate Change, a United Nations group that since 1990 has been issuing increasingly grim warnings about the consequences of a warming planet, released its most powerful and sobering assessment so far. Even now, it said, ice caps are melting, droughts and floods are getting worse, coral reefs are dying. And without swift and decisive action to limit greenhouse gas emissions from fossil fuels and other sources, the world will almost surely face centuries of climbing temperatures, rising seas, species loss and dwindling agricultural yields. The damage will be particularly acute in coastal communities and in low-lying poor countries — like Bangladesh — that are least able to protect themselves.

The report’s conclusions mirrored those of a much shorter but no less disturbing report issued two weeks ago by the American Association for the Advancement of Science, the world’s largest scientific society. Like the panel, the association declared that the world is already feeling the effects of global warming, that the ultimate consequences could be catastrophic, and that the window for effective action is swiftly closing.

The intergovernmental panel’s report (a companion report later this month will discuss what governments should do) could carry considerable weight with delegates to next year’s climate change summit meeting in Paris, at which the members of the United Nations will again try, after years of futility, to fashion a new global climate treaty. And together, the two reports could build public support for President Obama’s efforts to use his executive authority to limit greenhouse gases, most recently with a plan issued on Friday to reduce methane emissions from landfills, agricultural operations and oil and gas production and distribution.