PARIS, Feb. 2 — In a grim and powerful assessment of the future of the planet, the leading international network of climate scientists has concluded for the first time that global warming is “unequivocal” and that human activity is the main driver, “very likely” causing most of the rise in temperatures since 1950.

The United States, with about 5 percent of the world’s population, contributes about a quarter of greenhouse gas emissions, more than any other country.