Not many years ago most Republican leaders accepted the scientific consensus on human-caused global warming, as this video reminds us:
It's remarkable that the GOP has shifted so far so fast under pressure from the extremists who tend to have disproportionate influence on the party's primaries. Now it's routine for many Republican leaders, even some of the same ones in the video above, to argue that any uncertainty is grounds for doing nothing -- I wouldn't want to be their passenger it a fog -- and even to claim that global warming is a hoax. Which, of course, it isn't; it's basic chemistry and physics.
Of course, a multitude of things influence climate, and climate has been changing since long before humans started burning fossil fuel. No one suggests otherwise. When people bring this up in the context of discussing modern global warming, it suggests a fundamental misunderstanding, an assumption that scientists are guessing when they attribute observed warming to carbon dioxide from burning fossil fuels. In fact, human-caused global warming was predicted in the 1800s before it was observed, based on the straightforward fact that carbon dioxide absorbs some wavelengths of infrared light and thereby traps heat that would otherwise radiate away into space. That's a good thing; without greenhouse gases, the Earth would be mostly frozen over. But adding more carbon dioxide to the atmosphere necessarily means a higher temperature unless something else offsets it, and when we burn fossil fuels we're putting carbon into the carbon cycle that has been locked away underground for tens of millions of years.
(Since fossil fuels also contains lots of hydrogen, burning them also releases water vapor, also a greenhouse gas, but there's already so much water on the surface of the Earth the change is minimal.)
The subject came up in a Bell Telephone science special about weather broadcast in 1958 (see below). A mid-1960s National Academy of Sciences report requested by President Johnson had a section of the subject. I learned about it as a physics major in the early 1970s.
There was some discussion in the 1970s that particulate pollutants might increase the Earth's albedo (reflectivity) enough to cool the planet, but it was soon clear that except during brief periods of major volcanic eruptions, particulates only moderately reduced the rate of global warming and did not cancel it.
Speaking of volcanoes, it's sometime erroneously suggested that they release more carbon dioxide than the use of fossil fuels, but that's not remotely true. In fact, we've been directly monitoring the increase in atmospheric CO2 for decades, and we also have a very good idea how much is being added by the burning of fossil fuel, and it's clear that all of the observed increase in CO2 can be explained by human activity, which is enough to overwhelm the various natural mechanisms that remove carbon dioxide from the air.
Here's a brief clip from that Bell Telephone special. It's a bit alarmist in that today's estimates are that it would take at least a century to produce the drastic rise in sea level described, but it certainly makes it clear that global warming was a return even in the 1950s.
Update 2015 November 8: For a depressing example of how far some of today's Republican candidates have regressed from the sensible positions presented in the video above, see this later post.