During Tuesday night's Republican debate, Rand Paul delivered what has always been, to my mind, one of the strangest climate denialist skeptic doubter talking points. Here he is:

While I do think man may have a role in our climate, I think nature also has a role. The planet's 4.5 billion years old. We've been through geologic age through geologic age. We've had times when the temperature’s been warmer, we’ve had times when the temperature’s been colder, we’ve had times when the carbon in the atmosphere has been higher.

Paul deserves credit for knowing the age of Earth, which is a matter of some controversy and confusion among the other GOP candidates. But as for the rest, I am left as befuddled as ever. I've heard variations on this point approximately a bazillion times on the internets, always delivered with great satisfaction, as though that settles that.

But ... settles what?

Earth: ch'-ch'-ch'-changin'

As far as I am aware, zero out of 10 scientists deny that Earth's climate is always changing and has always changed. Science aficionados (and Buddhists) are hip to the fact that everything is always changing. That's just, like, life, man.

More to the point, geologists are well aware that Earth used to be a giant molten hellstone, and then it cooled, reaching a very rough equilibrium that nonetheless contained within it giant swings between eras of ice and eras when most of the surface of the planet was covered in water. It's in all there in the textbooks.

Scientists are also aware that atmospheric carbon dioxide concentrations have been higher at points in Earth's past. According to NOAA, we recently hit 400 parts per million of CO2 in the atmosphere. The last time concentrations were that high was, depending on who you believe, either 2 or 10 million years ago. Prior to that, there were periods when it was much, much higher. Around 440 million years ago, in the late Ordovician, they think it may have topped 5,000 ppm. (Skeptical Science has a nice mini-splainer on this.)

Earth's temperature has always changed as well. Wikipedia has a great graphic that stitches together temperature reconstructions from various data sets (sediment cores, ice cores, etc.) into a 500-million-year record of Earth's average temperature, from around the time multicellular life first emerged:

I'm definitely seeing some change there. Paul has a point.

Now let's zoom way in. Here's the last 5 million years or so.

Changier and changier! For the past 3 million years or so, there have been frequent glacial and interglacial cycles, while overall we've moved deeper into an ice age (part of a longer ice age that started around 40 million years ago).

The Holocene epoch: settling down a bit

Enough geologic scene setting. Now let's home in on a time period we can wrap our heads around: the past 100,000 years. This covers roughly the time that modern Homo sapiens have been bopping around.

I would draw your attention to the right side of this graph, the past 12,000 years or so. That is what's known as the Holocene epoch, featuring such highlights as the development of agriculture and all subsequent human civilization.

Looks a bit different, doesn't it? As it happens, the Holocene has been characterized by unusually stable temperatures. Freakishly stable, really.

This is not to say that change has stopped, of course. (Change, like, never stops.) Let's zoom in further.

As you can see, temperature has fluctuated quite a bit even during this period. There were variations substantial enough to make a difference to humans — the Medieval Warm Period during the 10th to 14th centuries, the Little Ice Age from the 14th to the 19th — but they were mild enough that human culture persisted and grew.

Long story short: While Earth's climate has experienced wide fluctuations throughout its history, human civilization, in its wee, geologically insignificant 10,000 years, has developed entirely within the comfortable confines of the Holocene. Agriculture, cities, the written word, industrial civilization, the iPhone — everything we know has unfolded during this period of stability. There's been climate change, but spookily little relative to the geological record.

The question — the existential question — is this: How much of our welfare is tied to the atmospheric conditions in which we developed advanced civilization? Can we prosper outside those conditions?

Paul seems confident that we can. Heck, there was more carbon dioxide in the atmosphere 2 million years ago! It was hotter only 100,000 years ago! What's the bigz?

Saying goodbye to the Holocene

We're about to find out. Here's one final zoom, onto the past 1,000 years.

As you'll note, temperatures are heading up, quickly, pulled along by the extraordinary recent spike in atmospheric carbon dioxide.

There is no other explanation for the recent, comparatively sudden spike in temperature. Not only does "man have a role," as Paul says, but scientists are as close to certain as they ever get about anything that human greenhouse gas emissions are the primary driver.

Now, if you take the long view, as Paul does, this might not seem like a big deal. Whatever changes humanity's massive CO2 emissions might cause, they're likely to be temporary.

Maybe temperatures will spike for a few thousand years and then settle back into the Holocene groove. Or maybe the atmosphere, once knocked out of its temporary Holocene equilibrium, will resume the rapid glacial-interglacial spikes of the past 100,000 years. Or maybe something else will happen.

Who knows? What we do know, thanks to Paul, is that Earth's climate has always changed; this too shall pass.

Of course, some of us suffer from a more limited temporal perspective and are preoccupied with the next 100 years or so. Geologically, that is nothing, the blink of an eye. But then again, it does encompass the lives of every extant human and their children and grandchildren.

On our current trajectory, during those next 100 years we are on course to drive temperatures higher than human civilization has ever experienced, with a small but nontrivial chance of driving it so high that civilization becomes impossible to sustain. This is now accepted by the entire global scientific community and just about every major political party on Earth save the US Republican Party.

If our descendants suffer under more intense storms, heat waves, food shortages, forced migrations, and rising sea levels, they are unlikely to find Paul's perspective — "Hey, you think it's hot now, you shoulda see the late Ordovician!" — of any great comfort.