Even if greenhouse emissions stopped overnight the concentrations already in the atmosphere would still mean a global rise of between 0.5 and 1C. A shift of a single degree is barely perceptible to human skin, but it’s not human skin we’re talking about. It’s the planet; and an average increase of one degree across its entire surface means huge changes in climatic extremes.
Six thousand years ago, when the world was one degree warmer than it is now, the American agricultural heartland around Nebraska was desert. It suffered a short reprise during the dust- bowl years of the 1930s, when the topsoil blew away and hundreds of thousands of refugees trailed through the dust to an uncertain welcome further west. The effect of one-degree warming, therefore, requires no great feat of imagination.