1.5 to stay alive

LE BOURGET, France — The Paris climate-change conference was supposed to be about the needs of big countries and what they are willing to do to slow the warming of Earth’s atmosphere. But in the end, the two weeks of sometimes round-the-clock negotiations have focused at least as much on some of the smallest, most defenseless nations whose very existence could hinge on the outcome of the talks.

The result could be a tougher set of policy goals than anyone originally thought could emerge from the conference. While the ultimate agreement is expected to embrace a goal of limiting global warming to less than two degrees Celsius, it also is likely to recognize a far more challenging and aspirational goal of 1.5 degrees Celsius.

That tougher language might not be legally binding for countries such as the United States, but the fact that it is in…