So: what do we do? How do we respond? On available data? On speculation about what did or did not happen 500 years ago? Who should we listen to?
Good questions. The way I tackle the question of truth in a debate I'm not technically qualified to be an expert in starts with some questions:
Is one side quickly resorting to ad hominem attacks or otherwise trying to shut off debate?
Global warming has been exceptionally prone to this. Anyone opposing one side is assumed to be motivated by short-term greed and willing to connive in the destruction of the planet. There's been attempts to destroy the careers of people on the "denial" side, the latest being a demand to decertify meteorologists who don't conform to the dogma.
Are multiple issues being combined into one to confuse the issue?
Whether the Earth is currently warming is one question (and there's agreement that it's been going up since 1970). What causes this change is a separate question (CO2 is being pushed--but solar cycles and other phenomena are possible). How to deal with the problem is yet another question, but instead debating possibilities one solution is being pushed and evidence for the warming question is being waved about as grounds for instituting the solution.
Is change being defined as bad without discussion?
A thousand years ago cattle raising was practical in Greenland. Then the Earth cooled and the Viking colonists starved. That warm period used to be called the Medieval Climate Optimum, the opposite of the Little Ice Age. Now it's the Medieval Warm Period, if its existence isn't being denied completely. I certainly grant that the world can get too hot, just as it can be too cold. But we haven't tried to figure out what the optimum global average temperature would be (Canadians and Malaysians may disagree on this). If we don't know that, how can we know if we're on the warm or cold side of it?
Is one solution to the problem being pushed instead of debating alternatives?
Assuming atmospheric CO2 levels are driving the global temperature up, there's multiple ways to deal with this. As a space cadet I have some fondness for the concept of putting a shade screen up at the Earth-Sun L1 point to reduce insolation . . . but I have to admit it'd probably be more cost-effective to push reforestation as a way to suck CO2 out of the air. The latter actually came up in the Kyoto Treaty negotiations and was rejected. I referenced some other possibilities here.
Is the solution being advocated the same one those people recommended for an entirely different problem?
High taxes on gasoline and government regulation of industrial choices? Have we heard these before? Oh, yes. For the Arab oil embargo . . . and the 1970s ice age scare . . . and the hole in the ozone layer . . . and instituting creeping socialism. Same suspects, same plan, new pretext. Which may explain why the other options above are being shut out of debate.
Is there a double standard for adhering to the solution?
Clinton didn't submit Kyoto for ratification and nobody cared, while Bush is being savaged for the same thing. The treaty demanded the USA make huge cuts in CO2 emissions, while Europe got retroactive credit for shutting down Warsaw Pact smokestacks and China was exempted. That sounds to me like CO2 isn't what the advocates care the most about.
Those are some logical analysis tools any layman can bring to the issue of global warming. As a technical expert there are couple of questions I also ask:
Are recommendations being based on computer models?
I've written simulations in my day job. It's not easy to make a few (or a million) lines of code match up to the complexities of the real world. "Validating" a model can be harder than building it in the first place, and usually requires multiple tests of the model's predictions against real-world events to shake out the bugs. Climatologists can't run controlled experiments to test their models (and the rest of us would probably object if they tried). Rewinding history a few hundred years and seeing if their model can predict the present could be a good validation . . . if the data needed to initialize the model existed for that date. It doesn't. So any discussion that starts with a prediction from a computer model automatically sets off my bullshit detector.
Is a linear relationship being claimed when past history is chaotic?
Go dig around for graphs of the temperature over the past thousand years. There's a bunch to pick from, all from someone with an axe to grind in the current debate. But the one thing they all have in common is lots of variation in the short term and big long-term fluctuations, none of which have any obvious correspondence to the human population or economy at the time. Now there's predictions coming out that specific levels of CO2 will produce such-and-such temperature increase a hundred years from now. That's claiming that a system which has a history of chaotic behavior will behave linearly from now on. In layman's terms, bullshit.
Now there's one other very important question that I'm not qualified to tackle but has to be dealt with:
Even if all of this is true, is this the most important issue to allocate our resources to?
There is one group that's asked that, the Copenhagen Consensus. When global warming was compared to all the other troubles of the Earth, they ranked it twenty-seventh. I'm not taking their answer as gospel--that kind of decision should be made by elected officials, not self-appointed experts--but I haven't seen other comparisons.