Met the moment'
"The Paris agreement establishes the enduring framework the world needs to solve the climate crisis," U.S. President Barack Obama said.
"We came together around the strong agreement the world needed. We met the moment."
German Chancellor Angela Merkel added: "Paris will always be connected with this historic turning point in climate policy."
The crux of the fight entails slashing or eliminating the use of coal, oil and gas for energy, which has largely powered prosperity since the Industrial Revolution.
The burning of those fossil fuels releases invisible greenhouse gases, which cause the planet to warm and disrupt Earth's delicate climate system.
Ending the vicious circle requires a switch to cleaner sources, such as solar and wind, and improving energy efficiency. Some nations are also aggressively pursuing nuclear power, which does not emit greenhouse gases.
The Paris accord sets a target of limiting warming of the planet to "well below" 2.0 degrees Celsius (3.6 Fahrenheit) compared with the Industrial Revolution, while aiming for an even more ambitious goal of 1.5C.
To do so, emissions of greenhouse gases will need to peak "as soon as possible," followed by rapid reductions, the agreement states.
The world has already warmed by almost 1C, which has caused major problems in dry developing countries, according to scientists.