While the Times may know exactly how this conclusion has been made, the rest of us don't. Before we base future plans on Earth's temperature rising 5-9 degrees, it might be worthwhile to look at the source material. The article did not mention rising ocean levels. According to New Scientist, some hydroelectric dams have been found to release more greenhouse gasses into the atmosphere than their coal-fired counterparts. The source of the emissions was the mass of rotting vegetation that accumulated in the reservoir.
Hydroelectric reservoirs cover an area of the world the size of France.
They release carbon dioxide and methane. Stagnant water produces the worst emissions because the decaying vegetation generates methane. This is 20 times as potent a greenhouse gas as CO2, which is produced when there is oxygen in the water. So a reservoir will produce more methane than the river did before the dam was built. Dams can also be lethal to salmon .