Still, the equation is simple: fewer emissions equal a more hospitable climate. Rising average temperatures make extreme heat more likely, hurricanes and storms more intense and threaten fresh water supplies . Climate impacts have already started to hit.

Halving greenhouse gas emissions by 2030 would be a tall order, to say the least. Changes to infrastructure take a long time. Cars on the road today are on average about 12 years old, and a new car sold in 2020 could still be on the road in 2040 or later. Power plants are built to stay in service much longer. There are a few coal-fired electric power plants in the United States that first began operation in the 1950s and are still producing electricity today. The inherent inertia that society is up against makes a climate action deadline of about a decade not just a sensible option, but an imperative.

We need to speed up the transition to clean and efficient transportation, electricity, industry, agriculture and buildings, and also make infrastructure and human systems more resilient. Achieving this requires much more than business as usual. It demands an enormous public and private undertaking of policy commitments, investments and innovation initiatives. Ten to 12 years is close enough to focus minds and attention. It’s far enough to allow for the necessary fundamental, systemic changes to take effect. None of that guarantees success.

For one, there is plenty of climate hurt already built in, regardless of how much emissions are cut this coming decade. Second, emissions reductions aiming to limit global average temperatures to 2.7 to 3.6 degrees Fahrenheit (1.5 or 2ºC) are not assured of success. Even if the world started out along a path to limit temperatures to, say, 3.6ºF, a chance remains that temperatures climb (much) higher. Some of the I.P.C.C.’s latest “2ºC pathways” go up to a 50-50 chance of exceeding that level. That’s a planetary game of chance no one should want to play, and precisely why deep decarbonization needs to go hand-in-hand with strong and equitable resilience efforts. But it’s not too late to reduce emissions — it will never be too late. It’s hard to imagine a world where we will regret having reduced emissions.

Achieving big reductions in emissions in less than a dozen years requires political action now, or at least soon after the next presidential election. Whether it is the Green New Deal, fundamental green tax reform, or a combination of any of the comprehensive climate plans being proposed now — there are plenty of options that could be taken to bend the emissions trajectory toward zero both in the United States and around the world. The young people, like the Swedish teenager Greta Thunberg, speaking on behalf of millions are correct in calling for bold climate action now.