Since 2001, greatly improved computer models and an abundance of data of many kinds strengthened the conclusion that human emissions are very likely to cause serious climate change. The IPCC’s conclusions were reviewed and endorsed by the national science academies of every major nation from the United States to China, along with leading scientific societies and indeed virtually every organization that could speak for a scientific consensus. Specialists meanwhile improved their understanding of some less probable but more severe possibilities. On the one hand, a dangerous change in ocean circulation seemed unlikely in the next century or two. On the other hand, there were signs that disintegrating ice sheets could raise sea levels faster than most scientists had expected. Worse, new evidence suggested that the warming was itself starting to cause changes that would generate still more warming.
In 2007 the IPCC reported that scientists were more confident than ever that humans were changing the climate. Although only a small fraction of the predicted warming had happened so far, effects were already becoming visible in some regions — more deadly heat waves, stronger floods and droughts, heat‑related changes in the ranges and behavior of sensitive species. But the scientists had not been able to narrow the range of possibilities. Depending on what steps people took to restrict emissions, by the end of the century we could expect the planet’s average temperature to rise anywhere between about 1.4 and 6°C (2.5–11°F).