Well, Robert Tracinski at The Federalist offers a detailed response to just that question. He prefaces his answer with this:
Recently, Reason‘s Ronald Bailey asked what it would take to convince conservatives and libertarians that global warming is real. If generally rising temperatures, decreasing diurnal temperature differences, melting glacial and sea ice, smaller snow extent, stronger rainstorms, and warming oceans are not enough to persuade you that man-made climate [change] is occurring, what evidence would be?After a brief elaboration on the unfortunate shortcomings exhibited by Messers Bailey and Adler, Tracinski lays out what it would take to convince him that global warming or climate change is real, anthropogenic (caused by humans), and catastrophic. Needless to say, the current state of both rhetoric and evidence falls far short of a rigorous demonstration:
This has since been picked up by Jonathan Adler at the Washington Post‘s token right-leaning blog, the Volokh Conspiracy. There’s no pressure: Bailey and Adler merely insinuate that you are “obscurantist”—that is, you hate new knowledge—if you don’t agree.
That, by the way — the smug insistence of global warming alarmists on presenting themselves as the embodiment of scientific knowledge as such — is one of the reasons I stopped taking them seriously. In fact, I have thought about what it would take to convince me global warming is real. And it’s pretty clear that Bailey has not thought about it.
1) A clear understanding of the temperature record.In other words, small increases in global mean temperatures since 1970 are really pretty meaningless and are certainly not proof that human activity is the cause of those increases which brings Tracinski to his second criterion:
The warmists don’t just have to show that temperatures are getting warmer, because variation is normal. That’s what makes “climate change” such an appallingly stupid euphemism. The climate is always changing. The environmentalists are the real climate-change “deniers” because they basically want global temperatures to maintain absolute stasis relative to 1970—not coincidentally the point at which environmentalists first began paying any attention to the issue.
That’s what makes ‘climate change’ such an appallingly stupid euphemism. The climate is always changing.
So to demonstrate human-caused global warming, we would have to have a long-term temperature record that allows us to isolate what the normal baseline is, so we know what natural variation looks like and we can identify any un-natural, man-made effect.
A big part of the problem is that we only have accurate global thermometer measurements going back 135 years — a blink of an eye on the time-scales that are relevant to determining natural variation of temperature. Within that, we only have a few decades of warming that could conceivably be blamed on human emissions of carbon dioxide: a minor run up in temperatures from the 1970s to the late 1990s. Since then, warming has leveled off (despite strenuous attempts to pretend otherwise). I think it’s impossible to claim, on that basis, that we even know what natural temperature variation is, much less to demonstrate that we’ve deviated from it.
2) A full understanding of the underlying physical mechanisms.If I understand this correctly climatologists don't know how much the temperatures are rising and disagree on how long it will take to find out. In other words, this is not at all the settled science we've been repeatedly told that it is.
We have to know what physical mechanisms determine global temperatures and how they interact. The glibbest thing said by environmentalists — and proof that the person who says it has no understanding of science — is that human-caused global warming is “basic physics” because we know carbon dioxide is a greenhouse gas. Carbon dioxide is a very weak greenhouse gas and there is no theory that claims it can cause runaway warming all on its own. The warmists’ theory requires feedback mechanisms that amplify the effect of carbon dioxide. Without that, there is no human-caused global warming. But those feedback mechanisms are dubious, unproven assumptions.
Basic questions about the “sensitivity” of the climate to carbon dioxide have never been answered. Even Bailey admits this:In recent years, there has [been] a lot of back and forth between researchers trying to refine their estimates of climate sensitivity. At the low end, some researchers think that temperatures would increase a comparatively trivial 1.5 degrees Celsius; on the high end, some worry it could go as high as high 6 degrees Celsius…. In a 2014 article in Geophysical Research Letters, a group of researchers calculated that it would take another 20 years of temperature observations for us to be confident that climate sensitivity is on the low end and more than 50 years of data to confirm the high end of the projections.
3) The ability to make forecasting models with a track record of accurate predictions over the very long term.So given all this what reason is there for the urgent demands to act now? Why are the Obama administration and so many others, particularly on the left, so desperate to do something now to reverse a trend that may not even exist or, if it does, may not be at all deleterious? Perhaps one answer is that if enough people are convinced there is an imminent crisis they'll be willing to cede more of their individual freedom and more power to the government to resolve it.
We don’t know whether current warming departs from natural variation, nor have scientists proven the underlying mechanisms by which humans could cause such an increase. But even if we did know these things, we would have to be able to forecast with reasonable accuracy how big the effect is going to be. A very small warming may not even be noticeable or may have mostly salutary effects, such as a slightly longer growing season, whereas the impact of a much larger warming is likely to cause greater disruption.
I should also point out that the “catastrophic” part of “catastrophic anthropogenic global warming” is a much larger question that is even harder to forecast. For example, global warming was supposed to lead to more hurricanes, which is why movie posters for Al Gore’s An Inconvenient Truth featured a hurricane emerging from an industrial smokestack. Then hurricane activity in the Atlantic promptly receded to historical lows.
It’s pretty clear that scientists aren’t any good yet at making global climate forecasts. Current temperatures are at or below the low range of all of the climate models. Nobody predicted the recent 17-year-long temperature plateau. And while they can come up with ad hoc explanations after the fact for why the data don’t match their models, the whole point of a forecast is to be able to get the right answer before the data comes in.
Given the abysmal record of climate forecasting, we should tell the warmists to go back and make a new set of predictions, then come back to us in 20 or 30 years and tell us how these predictions panned out. Then we’ll talk.
In any case, Tracinski closes with this:
So yes, I know exactly what it would take to convince me that catastrophic anthropogenic global warming is really happening. And no, the warmists haven’t even come close.Read the whole essay at the link, especially if climate change is an issue that, one way or another, interests you.