WASHINGTON — The historic announcement by President Obama and President Xi Jinping of China that they will commit to targets for cuts in their nations’ carbon emissions has fundamentally shifted the global politics of climate change. The agreement has given a fresh jolt of optimism to negotiations aimed at reaching a new international climate treaty next year in Paris, where the American and Chinese targets are expected to be the heart of the deal.

“For the world’s biggest emitters to be coming together and announcing concrete numbers, serious numbers, sends a signal to the world,” said David B. Sandalow, who was Mr. Obama’s assistant secretary of energy for policy and international affairs until May 2013. Nearly two decades ago, the world’s first climate change treaty, the 1997 Kyoto Protocol, failed to stop the rise of planet-warming carbon pollution in large part because of a standoff between China and the United States, which never signed the deal.

But experts and negotiators cautioned that the emissions reductions targets now put forth by the two countries will not be enough to prevent an increase in global atmospheric temperature of 2 degrees Celsius, or 3.6 Fahrenheit. That is the point where scientists say the planet will tip into a future of dangerous and irreversible warming, which will include the loss of vast stretches of arable land, rapid melting of Arctic sea ice, rising sea levels, extreme droughts, storms and flooding.

Under the Kyoto plan, developed economies, including the United States, were to slash their fossil fuel emissions, while developing countries like China were exempt. The United States refused to ratify the treaty, while China went on to become the world’s largest carbon polluter. In the following years, the superpowers remained at an impasse over climate change. Many other governments also refused to cut emissions, arguing that if the world’s top two polluters were not acting, they shouldn’t have to, either.