CAMBRIDGE, Mass. — ON Tuesday, world leaders will converge at United Nations headquarters in New York for a summit meeting on the climate that will set the stage for global negotiations next year to reduce greenhouse gas emissions and the threat of global climate change. The summit is titled “Catalyzing Action,” a decidedly hopeful characterization.

I wish I were so hopeful.

It is true that, in theory, we can avoid the worst consequences of climate change with an intensive global effort over the next several decades. But given real-world economic and, in particular, political realities, that seems unlikely.

There are emerging hints of a positive path ahead, but first let’s look at the sobering reality.

The world is now on track to more than double current greenhouse gas concentrations in the atmosphere by the end of the century. This would push up average global temperatures by three to eight degrees Celsius and could mean the disappearance of glaciers, droughts in the mid-to-low latitudes, decreased crop productivity, increased sea levels and flooding, vanishing islands and coastal wetlands, greater storm frequency and intensity, the risk of species extinction and a significant spread of infectious disease.

The United Nations has set a goal of keeping global temperatures from rising by no more than two degrees Celsius above preindustrial levels. (The average global temperature has increased by about 0.8 degrees Celsius since 1880, with two-thirds of the warming occurring since 1975.) Meeting this goal would require a worldwide reduction in greenhouse gas emissions of 40 to 70 percent by midcentury, according to the Intergovernmental Panel on Climate Change. That’s an immense challenge.