Is history. The Greenland Ice Core Project (GRIP) drills into the glacier and analyzes the width of the rings. Like trees with annual growth rings, each year sees the deposition of a new layer onto the glacier. This is, in fact, the reason that glaciers exist at all - each year there is net new ice.
The rings go back for years and years. So we can use these rings as a proxy for temperature - thicker rings mean colder and thinner rings mean warmer (more ice in colder winters, duh). So what does the last millennium look like?
See the modest spike at the right hand side? That's today, the Warmest! Year! Ever! See the much bigger spike at the left? That's what used to be called the Medieval Warm Period when Europe built all of those quaint Cathedrals and vinyards were to be seen in Scotland and Sweden. The trough in between? We used to call that the Little Ice Age with the Black Death.
Of course, the Hockey Stick paper from 1998 said that there was no Medieval Warm Period or Little Ice Age. Because Science! And shut up, wingnut.
So what if we extend this back further? The GRIP data go way, way back.
See that big spike in the first century BC (middle of the chart)? That's called the "Roman Climate Optimum" and is clearly way bigger than today's Hottest! Year! Ever! spike. It's even higher than the Medieval Warm Period. And the Roman Climate Optimum wasn't even the warmest - look at that massive spike on the left, around 1400 BC. This was when Ramesses the Great and the Crete of Knossos flourished.
The temperature of our own time is a poor reflection compared to these. Remember, Civilization was at its peak (for the time) - harvests were good and there was such a large surplus that vast armies and navies, and palaces full of artists were all present in large numbers.
But wait, we're not done. Let's spin the GRIP record back even more:
This goes all the way back to the end of the last Ice Age (more or less). What happened at the peak around 8000 BC (note that it's much higher than today's temperatures)? This was the dawn of agriculture and the emergence of the first cities. Just for grins, here's 400,000 years of GRIP temperature readings (note the Ice Ages and Interglaciation periods which were even warmer than the 1400 BC peak). Remember this chart because we'll come back to it:
The common theme here? Temperature peaks are not catastrophic, rather the reverse. Civilization emerged and flourished during these ZOMGTHERMAGEDDON!!! times. Civilizations collapsed (or retrenched) when the temperatures fell, like 1100 BC when the Egyptian New Kingdom collapsed, like 500 AD when Western Civilization collapsed, like 1250 AD when Europe began to starve and die.
You can get a lot more detail (and look at much, much longer views of GRIP) here, but you get the point. The point is that we hear that today it's all about the CO2, it's only about the CO2, and nothing else counts. There's no easy way to square that claim with the GRIP data.
Because we're not talking theory, like the warming catastrophists are. Rather, we're looking at data. Facts are stubborn things.
Their argument is that Greenland is only one place on the Globe, and tells us little about temperature elsewhere. Interestingly, Greenland isn't the home of the world's largest glaciers - those are in Antarctica, where the Vostok project has drilled over 3000 feet (!) into the primeval ice. Their ice cores track temperature back over 650,000 years:
Compare with the last chart. Pretty close, right? And today's temperature isn't as high as the post Ice Age peak even though CO2 has been increasing steeply (you can actually extract CO2 concentration from the ice cores; neato, huh?).
The it's all CO2 and it's only CO2 guys simply don't have an explanation to any of this. Why was it hotter in the past? Dunno. Why did it get cooler in the past? Dunno. Why did civilization flourish when it was warmer than today? Dunno. What's going on today? ZOMGTHERMAGEDDON!!!
Ooooh kaaaay. They don't need no steenking explanation. Because Science! And shut up, wingnut.
Now remember, these are the scientists who "hid the decline" in the ClimateGate email releases. This short (and very accessible) video explains precisely how they manipulated their proxy data to hide the decline. Cliff Notes version: they wouldn't have manipulated their data if their proxy data wasn't showing that temperatures were going down.
The data that got hidden is shown at about 2:30.
Quite frankly, I believe that this is the strongest evidence against the theory that rising CO2 is significantly changing the climate. Of course, looking at all of this data is very anti-scientific of me, but you know what a beastly Denier I am.