An upcoming climate summit is supposed to save the planet. But pessimism is building.

Earlier this year, John F. Kerry, the United States’ climate envoy, described the summit as the “last, best chance” to get more pledges of emissions cuts, aid to less-wealthy countries vulnerable to climate change and investments in renewable energies to wean the world off fossil fuels. As it is, unless mammoth reforms are implemented across the world in the coming years, it looks unlikely that governments will be able to ward against the 1.5 degree Celsius rise in preindustrial global temperatures that scientists believe marks a kind of red line — a rate of planetary warming that would lead to catastrophic climate effects, the disruption of economic life and the destabilization of whole communities.