This week's New Scientist has an in-depth cover story by Anil Ananthaswamy on the science of sea-level rises. Famously, back in 2007, the IPCC forecast a sea-level rise of between 19 and 59 centimeters by 2100 as a result of global warming. That doesn't sound so terrible... except that the IPCC explicitly refrained from factoring in all sorts of important phenomenon, such as "future rapid dynamical changes in ice flow." There just wasn't enough information at the time to include that stuff. In reality, sea-level rises are almost certain to be much higher. Ananthaswamy offers a helpful metaphor:
Crudely speaking, [the IPCC] estimates assume ice sheets are a bit like vast ice cubes sitting on a flat surface, which will stay in place as they slowly melt. But what if some ice sheets are more like ice cubes sitting on an upside-down bowl, which could suddenly slide off into the sea as conditions get slippery? "Larger rises cannot be excluded but understanding of these effects is too limited to assess their likelihood," the IPCC report stated.
Even before it was released, the report was outdated. Researchers now know far more. And while we still don't understand the dynamics of ice sheets and glaciers well enough to make precise predictions, we are narrowing down the possibilities. The good news is that some of the scarier scenarios, such as a sudden collapse of the Greenland ice sheet, now appear less likely. The bad news is that there is a growing consensus that the IPCC estimates are wildly optimistic.
Read the whole piece, it's terrific, and explains well some of the recent research into ice sheets and glaciers—and, in particular, how much we need to worry about parts of the West Antarctic Ice Sheet collapsing. Although plenty of smaller, niggling questions persist, the consensus is inching closer to the view that we're facing at least a one-meter rise by 2100 if emissions aren't tamed, with continuous rises thereafter.
--Bradford Plumer