There's plenty more evidence in the Met Office report to support global warming. But the question from critics remains: how can we be sure this isn't just a natural phenomenon? Scientists haven't done a good enough job of communicating how they distinguish human versus natural influences, says Hegerl. The answer lies in climate models — massive computer simulations that allow the scientists to project climate effects in various scenarios, including those in which humans do not emit any greenhouses at all. "We go out of our way to check out other explanations — by assuming it's all explained by solar activity, or by solar activity plus volcanoes, or by combinations of any of the other natural forcings known to affect climate," says Hegerl. According to the models, none of those combinations can produce the climate patterns currently being observed in the real world. Add the greenhouse gases that we know humans are generating (and which we've known since the 1800s tend to warm the Earth, all other things being equal), and the simulations finally come close to matching the real world. Its possible, albeit far-fetched, that the simulations are defective. It is even less possible that all of them (and there are many) are defective in the direction of overstating humanity's contribution to warming. Report: The Case for Global Warming Stronger Than Ever - TIME So much for the skeptic/denier meme that climate scientists aren't considering other sources of warming.