Is climate change raising sea levels, as Al Gore has argued -- or are climate scientists doctoring the data?

The University of Colorado’s Sea Level Research Group decided in May to add 0.3 millimeters -- or about the thickness of a fingernail -- every year to its actual measurements of sea levels, sparking criticism from experts who called it an attempt to exaggerate the effects of global warming.

"Gatekeepers of our sea level data are manufacturing a fictitious sea level rise that is not occurring," said James M. Taylor, a lawyer who focuses on environmental issues for the Heartland Institute.

Steve Nerem, the director of the widely relied-upon research center, told FoxNews.com that his group added the 0.3 millimeters per year to the actual sea level measurements because land masses, still rebounding from the ice age, are rising and increasing the amount of water that oceans can hold.

"We have to account for the fact that the ocean basins are actually getting slightly bigger... water volume is expanding," he said, a phenomenon they call glacial isostatic adjustment (GIA).

Taylor calls it tomfoolery.

"There really is no reason to do this other than to advance a political agenda," he said.

Climate scientist John Christy, a professor at the University of Alabama in Huntsville, said that the amount of water in the ocean and sea level were two different things.

"To me… sea level rise is what's measured against the actual coast," he told FoxNews.com. "That's what tells us the impact of rising oceans."

Taylor agreed.

"Many global warming alarmists say that vast stretches of coastline are going to be swallowed up by the sea. Well, that means we should be talking about sea level, not about global water volume."

In e-mails with FoxNews.com, Nerem indicated that he considered "sea level rise" to be the same thing as the amount of water in the ocean.

"If we correct our data to remove [the effect of rising land], it actually does cause the rate of sea level (a.k.a. ocean water volume change) rise to be bigger," Nerem wrote. The adjustment is trivial, and not worth public attention, he added.

"For the layperson, this correction is a non-issue and certainly not newsworthy… [The] effect is tiny -- only 1 inch over 100 years, whereas we expect sea level to rise 2-4 feet."

But Taylor said that the correction seemed bigger when compared with actual sea level increases.

"We’ve seen only 7 inches of sea level rise in the past century and it hasn’t sped up this century. Compared to that, this would add nearly 20 percent to the sea level rise. That's not insignificant," he told FoxNews.com.

Nerem said that the research center is considering compromising on the adjustment.

"We are considering putting both data sets on our website -- a GIA-corrected dataset, as well as one without the GIA correction," he said.

Christy said that would be a welcome change.

"I would encourage CU to put the sea level rate [with] no adjustment at the top of the website," he said.

Taylor’s takeaway: Be wary of sea level rise estimates.

"When Al Gore talks about Manhattan flooding this century, and 20 feet of sea level rise, that’s simply not going to happen. If it were going to happen, he wouldn’t have bought his multi-million dollar mansion along the coast in California."