So the IPCC team will probably use reality—the actual warming of the world over the past few decades—to constrain the CMIP projections. Several papers have shown how doing so can reduce the uncertainty of the model projections by half, and lower their most extreme projections. For 2100, in a worst-case scenario, that would reduce a raw 5°C of projected warming over preindustrial levels to 4.2°C. It’s good news for the modelers—but also a clear, and dismaying, sign that global warming has gone on long enough to help chart its own path, says Aurélien Ribes, a climate scientist at France’s National Centre for Meteorological Research. “Observations now provide a clear view for what climate change will be.”