A coal-fired power plant in Bow, New Hampshire. The Paris agreement on climate change came into force on Friday, after a year of remarkable success in international efforts to slash man-made emissions of carbon dioxide and other global warming gases. (Jim Cole/AP)

The Paris agreement to combat climate change became international law Friday. The landmark deal aims to tackle global warming amid growing fears that the world is becoming hotter even faster than scientists expected.

So far, 96 countries, accounting for just over two-thirds of the world’s greenhouse gas emissions, have formally joined the accord, which seeks to limit global warming this century to 2 degrees Celsius (3.6 degrees Fahrenheit) above what is was before machines and factories appeared in the late 1700s.

The United States formally entered into the agreement in September, and more countries are expected to come aboard in the coming weeks and months.

United Nations Secretary General Ban-Ki moon commemorated the event, talking with nongovernmental groups at U.N. headquarters in New York to hear their concerns and visions for the future.

“This is an emotional moment for me. It is a credit to all of you. And it is historic for the world,” Ban said in his opening remarks.

He praised the groups for getting hundreds of millions of people to back fighting climate change but warned the outcome remained uncertain.

“We are still in a race against time. We need to transition to a low-emissions and climate-resilient future,” Ban added.

Scientists praised the speed at which the agreement, signed by 192 parties last December in Paris, has come into force, saying it shows a new commitment by the international community to address a problem that is melting polar ice caps, sending sea levels rising and transforming vast areas into desert.

“While the real effect of the agreement after it goes into effect is still uncertain, it is a simple sign that the international society is much more open to alter economic and political behavior to control climate change, which is by all means positive,” said Feng Qi, executive director of the School of Environmental and Sustainability Sciences at Kean University in New Jersey.

Scientists and policymakers say the agreement is the first step of a much longer and complicated process of transitioning away from fossil fuels, which currently supply the bulk of the planet’s energy needs and also are the primary drivers of global warming.

“Climate change is a marathon, not a sprint, and the agreement sets a course for the marathon in the years ahead,” said David Sandalow, of the Columbia Center on Global Energy Policy.

Countries that joined the agreement pledged to reduce greenhouse gas emissions by a certain percentage and to report on their progress. But if they don’t succeed, they will not be punished. The hope is that making the information public will push countries to adopt clean, renewable energy sources such as wind, solar and hydropower (electricity powered by water).

The agreement also requires governments to develop climate action plans that will be periodically revised and replaced with new, even more ambitious plans. Many of these details will begin to be addressed at the COP 22 climate change meeting that begins next week in Marrakesh, Morocco.