A new central plan for the climate: representatives of 195 countries hammer out an emissions accord in Paris.

• Author: Bailey, Ronald
• Position: Columns: - Climate change conference in Paris, France

WHEN A UNIVERSAL agreement was adopted at the Paris climate change conference on December 12,2015, President Barack Obama hailed the pact as "the best chance we have to save the one planet that we've got." But the approbation was not universal. The activist and climatologist James Hansen, often described as the "father of climate change awareness," rebuked the Paris negotiators. "It's just bullshit for them to say, 'We'll have a 2[degrees]C warming target and then try to do a little better every five years,"' he told The Guardian. "It's just worthless words. There is no action, just promises."

Hansen dismissed the agreement because its cuts to greenhouse gas emissions are not mandatory. In fact, the key to getting 195 countries to approve the Paris Agreement was that--unlike earlier climate pacts--it is based on a non-zero-sum bottom-up process. Countries were not told what to do; instead, each one proposed for itself the steps it would take to combat man-made global warming. Eventually, 186 countries voluntarily submitted "nationally determined contributions" outlining their plans for managing future emissions.

The new agreement sets the objective of "holding the increase in the global average temperature to well below 2[degrees]C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5[degrees]C above pre-industrial levels." Its long-term goal is to peak global emissions of greenhouse gases (chiefly carbon dioxide released by burning fossil fuels) as soon as possible. Thereafter, countries are to pursue rapid reductions so as to achieve net zero emissions by the second half of this century.

The future temperature goals were set based on computer climate models in which the global average temperature could increase as much as 5 degrees Celsius by 2100 if countries continue to ramp up their use of fossil fuels. Various groups calculate that the emissions cuts pledged under the Paris Agreement would still boost average temperatures between 2.7 and 3.7 degrees Celsius by that year. The differences in the estimates depend on assumptions about whether countries will speed up their rates of decarbonization.

For context, keep two figures in mind. First, the global average temperature at the coldest point of the last ice age, 20,000 years ago, is estimated to have been only about 4 degrees Celsius below pre-industrial temperatures. Glaciers then covered about 25 percent of the earth's land surface, and the sea...