The Climate

The global economy’s soaring greenhouse gas emissions, chiefly fueled by burning coal and gasoline, have already triggered significant climate change. The warming atmosphere is now causing damage that includes dying forests, collapsing sea ice, heat waves and torrential rains, according to a November report by the Intergovernmental Panel on Climate Change. Last week, scientists at the National Oceanic and Atmospheric Administration declared that 2014 was the warmest year on earth since record-keeping began in 1880. If emissions continue unchecked, the November report warned, society in the coming decades could face food shortages, refugee crises, the flooding of major cities and entire island nations, and mass extinction of plants and animals.

There is uncertainty about each individual consequence, but the overall picture is not a happy one. In response, governments around the world are negotiating a United Nations accord, expected to be signed in Paris in December, which would for the first time commit every nation to cutting its carbon pollution. Mr. Obama’s pledge depends on the enactment of a suite of Environmental Protection Agency regulations, but those rules alone won’t reduce emissions enough to meet the 2025 target. Mr. Obama’s pledge has effectively committed the next president to enact further pollution cuts; whether he or she will actually do so is a question for the 2016 presidential candidates.

Many climate scientists have praised Mr. Obama’s E.P.A. rules, and are pushing for completion of a U.N. deal. But they also warn that by this point, it’s probably impossible to stave off many of the dangerous and costly impacts of global warming. A global accord signed this year won’t take effect until 2020, and given the levels of greenhouse gases already in the atmosphere and the rapidly rising rate of global emissions, the planet is set to tip past an average temperature rise of 3.6 degrees Fahrenheit. That’s the point at which, scientists say, the world will be locked into a future of irreversible climate change — and the best that governments can do after that is to adapt to the change and prevent it from becoming far worse.

One big question is how rapidly clean energy will be able to compete with the forms that produce carbon emissions. And while there have been significant breakthroughs in both the technology and the economics of low-carbon energy, the transition could take decades. CORAL DAVENPORT

Health Care

The American health care system has arguably changed more in the last year than in any of the last 50. Yet some of its problems remain stubbornly the same.

In 2014, the percentage of Americans without health insurance plummeted, as more people signed up for an expanded Medicaid program and bought insurance on new online marketplaces — both the results of the Affordable Care Act.

The people who got that insurance tended to be those with the largest economic disadvantages: young people, blacks, Hispanics, people living in rural areas, and people with the lowest incomes. It’s too early to know exactly what that change means for people’s health — medical care isn’t the main determinant of people’s health — but there’s some early evidence that expanded access to health insurance is improving many people’s financial well-being by making the care they need more affordable.