Last week in Washington, D.C., Congress began debate over global warming legislation that would override state laws, including California's new measure to lower emissions to 1990 levels by the year 2020. This debate, and all others over global warming, should reference a recent exchange on the subject.

"I have no doubt that a trend of global warming exists. I am not sure that it is fair to say that it is a problem we must wrestle with. To assume that it is a problem is to assume that the state of the Earth's climate today is the optimal climate, the best climate that we could have or ever have had, and that we need to take steps to make sure that it doesn't change."

This was not a senator or congressman from an industrial state, concerned about the effect of legislation on the economy. Nor was it a global warming skeptic, a caller to the Rush Limbaugh show, or a guest on Fox News. The speaker was Michael Griffin, administrator of NASA, the National Aeronautics and Space Administration, on National Public Radio's  "Morning Edition" on May 31. 

Mr. Griffin remains uncertain that global warming is a problem we must wrestle with. Such a stand does not exactly conform to the orthodoxies of climate change as brokered to the public. According to those orthodoxies, we need to wrestle with it a great deal, regardless of damage to the economy. Mr. Griffin's statement so jarred NPR staff that they turned immediately to James Hansen, NASA's chief scientist on climate change.

"I almost fell off my chair," he told NPR's Madeleine Brand.  "It's remarkably uninformed. You know, civilization developed with the current climate. And we've got an infrastructure along coastlines that assumes that our climate is going to stay roughly what it is now. But if we are going to simply allow human emissions to greatly change climate, I think that's extremely arrogant of our species. It will be devastating to many other species on the planet, not to mention many of our own species."

On the same program, Mr. Hansen said, "I think it's very timely to get together and start to do something where time is really running out. . . We are at a tipping point. If we don't begin to make some changes in our emissions – reducing greenhouse gas emissions – we're going to get some really large climate changes."

This exchange highlights a few points worthy of note. There are, after all, differences of opinion on the subject of climate change, and what it might mean. In other words, even if there is agreement that there is some trend of global warming, there is room for honest disagreement about what this means and what our public policy should be. Readers can explore this subject in PRI's latest Index of Leading Environmental Indicators, and by viewing the PRI documentary An Inconvenient Truth . . . Or Convenient Fiction.

There can also be differences of opinion whether, as Mr. Hansen says, "time is really running out" and whether "we are at a tipping point." As Index author Steven Hayward notes, we may have reached a tipping point on tipping points. One hopes he is right.

There is no room in the climate debate for claims of infallibility, prophetic speculation, and charges that those who hold doubts about current orthodoxies are arrogant. The debate should center on hard scientific data, and observable phenomena. It can, for example be observed that Sacramento, usually a blast furnace by now, has been comparatively mild of late. Maybe NASA can explain the temperature drop, and whether we need to wrestle with it.