Earth's climate destiny finally seen more clearly

It seems like such a simple question: How hot is Earth going to get? For 40 years, climate scientists have repeated the same unsatisfying answer: If humans double atmospheric carbon dioxide from preindustrial levels, the planet will eventually warm between 1.5°C and 4.5°C—a temperature range that encompasses everything from a merely troubling rise to a catastrophic one.

In a landmark effort, a team of scientists has significantly narrowed the bounds on this critical factor, known as climate sensitivity. The assessment relies on three strands of evidence: trends indicated by contemporary warming, the latest understanding of the feedback effects that can slow or accelerate climate change, and lessons from ancient climates. They support a likely warming range of between 2.6°C and 3.9°C. The work will feed into the next major U.N. report on climate change and, ultimately, inform projections of sea-level rise, economic damage, and much else.