The Earth is truly remarkable in its ability to maintain life-friendly surface temperatures for billions of years despite drastic changes in the amount of solar energy reaching it. The mechanisms in place for this thermal regulation have always been (apparently) extremely efficient.The complexity of the thermal regulation about which I'm talking is mind-boggling, so I'll try to keep it simple. During the tenure of life on Earth (some 3.86 billion years), the solar energy reaching Earth has increased by 30 percent. And yet, the Earth's surface temperatures have stayed within a very narrow life-friendly range.*
If you've read much about the global warming issue, you're undoubtedly familiar with some of the factors that go into determining the Earth's surface temperature. A short list includes the atmosphere and its make-up (amounts and proportions of greenhouse gasses and such), the amount of energy coming from the sun (natch), and the Earth's albedo (read 'reflectivity'). But besides the obvious factors that play into these things, a host of other factors have played a role in maintaining this delicate balance throughout life's history. These include the amounts and forms of life on Earth, the timing and extent of extinction events, the varying rate at which once-living material has been stored in the Earth's crust and mantle, the timing of the re-release of such carbon deposits to the atmosphere,** and more.
As scientists continue to research these things, more factors are added to this list. The point is that all of these very different things act--and always have acted--in concert to keep Earth at a life-supporting temperature even though the energy input has varied drastically through time.
Now it is possible that the humans alive today have the capacity to throw this efficient, time-tested thermal regulatory mechanism out of whack. It should be noted, however, that some historical occurrences would seem to have involved perturbations to this system that would have dwarfed--in terms of abruptness and magnitude--even the sum of current human perturbations. (As just one example, think of Chicxulub, the meteorite that wiped out the last of the dinosaurs when it struck the coast of the Yucatan, sending debris 3000 miles in every direction and blotting out the sun for two years or more.) It is also noteworthy that every once in awhile (though with seemingly less fanfare than the dire predictions receive), a scientist will go on record as being amazed by the unexpected resiliency of the atmosphere (explaining, in part, why the predictions made by the models have failed to actualize).
Again, what I have shared in this latest certain and confident statement does not prove that anthropogenic global warming is false, nor does it mean that it's true. It seems, however, that it should play a part in the search for the truth on this issue, but it just doesn't seem to come up much; perhaps it's another of those 'inconvenient truths.'
* There have been periods--though not while humans were alive--when the Earth was quite a bit warmer than today, but no so warm as to preclude the flourishing of living things.
** You'll notice that these last two factors have to do with plate tectonics and the resulting earthquakes and volcanoes. This is yet another aspect of the Earth that is necessary for life's existence, and one that is believed to be unique to Earth (among other identified planets both in our solar system and outside it).