That statement is either confused or disingenuous. It would indeed be arrogant to say that the science is settled on what percentage of present and future warming is due to global warming gasses and particulate matter released into the atmosphere. With evidence still being collected and assessed, the extent to which natural changes are affecting warming trends is still debated. That said, there is consensus in the scientific community that the warming is due largely to human activity. As a rhetorical device, Bush is extending the idea that a hard ratio between man-made and natural causes hasn't been determined to ally himself with those that want to say the overall picture of human role in climate change is still heavily debated. It isn't, outside of politics.
You'll be unsurprised to learn that Bush's answer makes sense for a Republican politician. In April, Gallup looked at attitudes on climate change by political ideology. Conservative Republicans were most likely to say that global warming's effects would never be felt and most likely to say that any warming is due mostly to natural causes.
Saying the world is warming but that humans aren't entirely to blame has become something of the party line in the Republican Senate. Earlier this year, the body voted overwhelmingly to say the world is getting warmer, but carving out a rhetorical caveat as to cause.
That's also what George W. Bush said when he was president.
"[W]e know the surface temperature of the Earth is warming," he said in a speech in June 2001, five months after taking office. He cites data from the National Academy of Sciences to bolster his point, and notes that "the increase is due in large part to human activity."
And then, the carve-out. "Yet, the Academy's report tells us that we do not know how much effect natural fluctuations in climate may have had on warming," he said. "We do not know how much our climate could, or will change in the future. We do not know how fast change will occur, or even how some of our actions could impact it." Which, again, is true. It was more true then, since we now have the benefit of 14 more years of science which has sanded down some of the rougher edges of those questions.
But it is true, and it offered Bush (George) enough wiggle room to not crack down on the main cause of warming — carbon dioxide emissions from burning fossil fuels, especially coal — as president. His 2002 Clear Skies & Global Climate Change Initiatives addressed other pollutants from electricity generation, but left carbon dioxide reductions as voluntary. The idea was introduced in Congress, but not passed. (Close observers of the issue will note that the congressional bill was sponsored by Sen. Jim Inhofe (R-Okla.), which neatly summarizes how much it would have addressed climate change.)
What Jeb said in New Hampshire on Wednesday accomplishes the same political goal, no matter what his eventual policies as president might be. (Bush isn't officially a candidate yet, of course.) His position is the same as his brother's: Agreeing to the problem but not entirely the cause.
"While scientific uncertainties remain," George Bush said in 2001, "we can begin now to address the factors that contribute to climate change." Jeb agrees with that.
At least the first part.