There seems to be a lot of things that could go into skepticism about climate change issues beyond stereotypical flat-earther ignorance that John Kerry was trying to joke about this week. Too often, the fans of a climate deal tend to assume any critics are either ignorant boobs or in the pockets of carbon-centric robber-barons, often trotting out the pejorative "climate-denier" designed to rhyme with "Holocaust-denier" casting the critics in with folks on the SPLC's radar or a Muslim demagogue.
That doesn't help matters.
Here's an incomplete set of bullet points that seem to be worth considering for folks weighing policy.
(1) The temperature increase (the 1oC uptick in the last century seems legit, agreeing to that fact up front) seems to have plateaued somewhat. If you're prone to be fearful, then it seems more like a momentary pause before things rise again down the line, but if you see things half-full, it argues against radical immediate action.
(2) Climate-change mavens tend to be liberal and have a higher regard for the effectiveness of government. Conservatives would tend to be skeptical about how much good expenditures will do and how much of the money will go to actually solving the problem rather than just creating more paper-pushers and rent-seeking from friends of the administration of the day.
(3) When looking at transferring big money from the developed to developing world, especially to developing-world governments, corruption looms as a factor. The bureaucratic dead-weight loss of government tends to magnify greatly when set in a third-world milieu, where the government's allies (or The Big Man's relatives) get overpaid for services.
(4) Big, showy projects tend to be under-productive, even when factoring out corruption issues. That's something the World Bank has seen up close as they've tried to help the developing world develop. Big factories, power plants and other large projects far too often come in over-budget and don't have the impact promised. One recent example of that was big solar plants in India who aren't delivering nearly as much power as advertised.
(5) The negative impact of regulations are often understated, creating an off-budget cost to implementation. Rules that look good on paper can prove problematic in the field. In addition, any taxes on offending products can be a drag on the economy, often hitting the poor harder than the better-off.
(6) Any pledges made in Paris are merely pledges. Some governments might make good on them, but others might not. The more volatile the political situation is in a country, the more variance you'll see in voluntary compliance. Countries with green-sounding delegations might not be in office down the line to deliver and more disingenuous ones may have no intention of following through on their end of the bargain. For instance, any US pledge for financial assistance to poorer countries would need to get through Congress, which would be a hard sell.
(7) To a Michigander, another degree or two of temperature rise seems more like a feature than a bug if you dislike winter and would trade a warmer winter for a warmer summer. In areas where things are already too hot or where melting polar ice could induce extra flooding, a temperature increase is a bigger deal, but quite a few voters in more temperate climes might not be all that worried for their neighborhood. Selfish, yes, but a factor nonetheless.
These are all questions outside the hard science, questions of politics, geopolitics and economics, that lend to a critical take on any climate agreements. One can accept the scientific evidence and still have questions as to if the efforts are worth taking, especially if what efforts one country (even a big one like the US) can make won't make a dent in the big picture.