The "Paris agreement" aims to keep global temperatures from rising another degree Celsius (1.8 Fahrenheit) between now and 2100, a key demand of poor countries ravaged by rising sea levels and other effects of climate change.
In the pact, the countries pledge to limit the amount of greenhouse gases emitted by human activity to the levels that trees, soil and oceans can absorb naturally, beginning at some point between 2050 and 2100.
In practical terms, achieving that goal means the world would have to stop emitting greenhouse gases - most of which come from the burning of oil, coal and gas for energy - altogether in the next half-century, scientists said. That's because the less we pollute, the less pollution nature absorbs.
Achieving such a reduction in emissions would involve a complete transformation of how people get energy, and many activists worry that despite the pledges, countries are not ready to make such profound, costly changes.
The deal now needs to be ratified by individual governments - at least 55 countries representing at least 55% of global emissions - before taking effect. It is the first pact to ask all countries to join the fight against global warming, representing a sea change in U.N. talks that previously required only wealthy nations to reduce their emissions.