There is general scientific agreement that the globe has been warming, in fits and starts of course, for the last three centuries or so. And since it has been thusly warming for centuries, the obvious null hypothesis would have to be that the half-degree of warming we experienced in the 20th century was a continuation of some long-term ongoing natural trend.
But that’s not what Dr. Trenberth is doing here. Keep your eye on the pea. He has smoothly segued from the IPCC saying “global warming is ‘unequivocal’”, which is true, and stitched that idea so cleverly onto another idea, ‘and thus humans affect the climate’, that you can’t even see the seam.
The pea is already under the other walnut shell. He is implying that the IPCC says that scientists have “unequivocally” shown that humans are the cause of weather ills, and if I don’t take that as an article of faith, it’s my job to prove that we are not the cause of floods in Brisbane.