GRASSLEY: Well, I’d be foolish if I didn’t give—I’d be foolish if I didn’t give it some consideration because there’s a massive amount of scientists that feel that it does. But there’s also an increasing number of scientists that have doubt about it.
And so, not being a scientist, I don’t know exactly where to say only those things that are really quantifiable, and temperature has risen. But the scientific aspect that I still reserving judgment on is the extent to which it’s manmade or natural.
Now, a lot of members of Congress and most environmentalists are—are absolutely convinced manmade is the—is the factor—chief factor here. But I—I want to, before I vote on it, be more conclusive in my judgment, and I haven’t reached that conclusion at this point.
Roberts is justified in feeling frustrated, and the arguments do read like multiple excuses for not doing the right thing. OTOH, Grassley's not using the "global warming is a hoax" language that Inhofe does. I think that it would at least be worth a shot to try and convince Grassley that there's no reasonable doubt about human-caused change.
One of the most persuasive arguments for why climate change is human-caused, I think, is stratospheric cooling. The reason why greenhouse gases cause stratospheric cooling is complicated (one explanation here, I'd welcome better references) and I don't think I've got a good grasp on it yet, but the key aspect is that solar influences can't explain why the stratosphere is cooling.
If you look at the first two images in this RealClimate post, one shows warming from greenhouse gases, and the other from solar radiation. At the top of each image, only greenhouse gases also show stratospheric cooling. If Grassley wants evidence, there it is.
What do the denialists have to explain the cooling? Nothing, that I can tell. There's a hilarious attempt by Steve Milloy to actually deny cooling through cherry-picked start and end dates that a fifth grader could spot (and even then I'm not sure his trend line fits the data). Not sure if anyone else has tried.
Milloy has no interest in the truth that I can tell, but there are some skeptics out there that actually believe what they say. I should ask them what they think.