Even moderate action isn't likely to help. Follow-up work by these same researchers published this year in MIT's annual Energy and Climate Outlook found that if countries achieve the emission cuts they promised at international climate negotiations, the global temperature would still increase by over 4 degrees Celsius (7.2 degrees Fahrenheit), with a significant chance of a 5 degree Celsius rise by century's end
For some scientists, however, the IPCC's findings are extreme.
"I'm surprised there are those who think the IPCC is too conservative," says John Christy, atmospheric scientist at the University of Alabama in Huntsville, IPCC author in 2001 and a well-known skeptic of human-caused climate change. "I think the simple evidence is very clear—the IPCC models overestimate the warming of the climate system." The IPCC declined to comment on the record.
Missing Ice Sheets and Slow Timing
Perhaps the biggest controversy surrounding the IPCC scenarios is that they omit the rapid melting of the Greenland and Antarctic ice sheets in sea-level rise projections.
Several researchers, including Thompson, the polar ice expert from Ohio State University, and James Hansen, head of the NASA Goddard Institute for Space Studies, have been vocal critics of that omission, which they say dramatically skews the IPCC scenarios. If the Greenland and Antarctic ice sheets, the only two in the world, continue to melt at their current pace, Thompson and other scientists believe sea levels could rise several feet and swamp coastlines this century, not the 8 to 17 inches projected in the IPCC mid-range scenarios.
"Those [ice sheets] are the big elephants in the room," says Thompson. "They are going to play a big role, yet they aren't taken into account." (The IPCC left them out because of uncertainty about how to predict effects of ice-sheet meltdowns in climate models.)
Some scientists say the very nature of the IPCC process means its reports can never be truly up to date. Research must be published at least two years before the release of an IPCC assessment to be considered. That lag time also means the projections will be on the conservative side, Thompson says. He argues that as scientific understanding of climate change improves, and as CO2 emissions continue to rise, the predictions grow more dire.
Reilly, the MIT scientist, says most scientists studying climate change today are viewing "the seemingly unstoppable rise in global greenhouse emissions" with "increasing alarm."
Why Aren't Scientists More Vocal?
So, if climate scientists are convinced that the Earth is warming faster than expected, then why aren't more speaking out?
The researchers interviewed for this story said many have retreated into silence to avoid the small but vocal band of climate skeptics. "Researchers find it hard to raise significant questions even within the climate science community for fear that it will be exploited by the skeptics," says Sarewitz, the science and society professor from Arizona State University.
"Climate science is a huge, sprawling area of discussion," explains Sarewitz, and skeptics are known to seize on arguments as proof that the science linking human activity to global warming is dubious.
Indeed, there are still many points not understood in climate science. Long-term changes in solar activity and their effects on the climate system are not well known. The effect of aerosols on global temperature is still uncertain, because they all react differently to atmospheric heat. Sulfates, for example, block sunlight, which in turn can cool the climate, while black carbon absorbs sunlight and can accelerate warming. Few doubt that sea levels will rise, but how fast and by how much is hotly contested.
There are also major limitations with climate models. They can predict whole-Earth scenarios better than localized scenarios, meaning regional trends still can't be predicted with much accuracy. They also don't reflect the physics of cloud formation well, an issue the IPCC has made a research priority.
While none of these undermine the consensus that climate change is human-caused, Sarewitz says, any dissension helps skeptics chisel away at the perception of scientific agreement. "It all makes it hard for the disinterested citizen ... to actually know how to untangle the conversation and who to trust."
Is silence the answer? Not according to Thompson of Ohio State, who admits to being "frustrated' by skeptic tactics and scientists' lack of response to them. "If they want to be more than just a historian documenting the change—if they want to make a difference—[scientists] have to speak out about these issues." Thompson himself regularly speaks about climate change, even allowing TV and print journalists to join his polar ice expeditions.
Reilly agrees. "Without interaction [with the public], it becomes too easy for people to vilify or defy those who disagree or agree with them, and there is little chance for real understanding."