We have long known every tonne of carbon dioxide matters to the climate. But governments keep approving new fossil fuel projects—even as climate change worsens.

Until now, it's been difficult to say how much damage and extra warming a specific project will cause. Fossil fuel companies—and government agencies approving their projects—have often relied on what's known as the "drop in the ocean" defense.

While backers concede a new project will add more carbon dioxide emissions to the atmosphere, they say those emissions are tiny compared with the global total and that it's almost impossible to track damage attributable to an individual project.

This defense won't work anymore. In recent years, attribution science has advanced rapidly, allowing scientists to pinpoint the role of climate change in making disasters and extreme weather more likely.

Our new open-source climate tool—the Carbon Impacts Tracer—goes one step further. It shows how much a new project will warm the planet and how much damage it will cause across eight areas, from crop losses to heat wave deaths in Europe.

This will help hold fossil fuel companies and governments approving these projects accountable for the damage the projects will cause...