The biggest challenge faced by Solomon: ruling out the effects of solar storms, which skeptics of man-made climate change have proposed might be causing global warming. Solomon and others have refuted this notion but confirmed the storms’ dominance as a temperature driver in the thermosphere. “Solar-driven changes, while they’re extremely large above 100 kilometers, get progressively smaller and smaller as you get lower into the atmosphere, and are extremely small once you reach the surface,” says Solomon. Other scientific studies have identified human influence in everything from declining mountain glaciers and snow cover in both hemispheres, to increases in ocean salinity, to the growing frequency and range of severe droughts.
And as the science has peered into more areas, the models have gotten sharper, too. New modeling techniques pioneered by the Oxford University-based climateprediction.net program, a distributed computing effort, helped the IPCC express its predictions more precisely. The early draft of the IPCC report leaked last week predicts a far tighter range of temperature and sea-level increase over the next century than the previous IPCC report did. For example, the draft projects that sea level will gain between 28 and 43 centimeters by 2100, compared with the 9-to-88-centimeter rise forecast in 2001.Probabilistic models are also helping the IPCC come to consensus over the most likely figure within their predicted range. In the case of temperature, the leaked draft says that if carbon dioxide concentrations stabilize at 550 parts per million, the planet will warm 2 to 4.5 °C, but it adds a “best estimate” of 3 °C, which is slightly below the mean for that range. Of course, the future trajectory of greenhouse gas emissions and consequent warming depends greatly on future energy and transportation policies and trends. Weaver estimates that uncertainty in emissions contributes about 50 percent of the uncertainty in most climate-change predictions. The rest comes from imperfect understanding of how the climate’s physical and biological components interact, such as how much cloud cover will result from changes in temperature.
What is most clear is that the increasing scientific certainty around global warming has emboldened climate scientists as much as it has isolated the small band of skeptics swimming against the scientific mainstream. “There is no credible alternative hypothesis,” says Mahlman. “It simply doesn’t exist.” Weaver predicts that the skeptics will soon fade away as funding sources increasingly pull their funding from organizations and researchers leading the anti-IPCC charge, as oil and gas giant ExxonMobil announced that it had done last month in pulling funding from the Washington-based Competitive Enterprise Institute, a conservative think tank. It was the same story with the “smoking doesn’t cause cancer” crowd, says Weaver: “Once the funding breaks down, they’ll break down.”