By Dr. Robert Thorson

Last week, the world got a case of sticker shock. For the first time in recorded history, the price of our energy-rich lifestyle hit the magic number of 400 parts-per-million carbon dioxide in our atmosphere.

There’s no politics involved in this number. No propaganda. No conspiracy. It’s a fact as plain as the nose on my face. It’s based on well-calibrated measurements taken at the summit observatory on Mauna Loa, Hawaii, since 1958 and published as the Keeling Curve.

Reading this fact is equivalent to a typical person stepping on a high-quality bathroom scale and realizing you hit 200 pounds for the first time in your life. Biologically, there’s no significant difference between one who reaches this weight and one who’s slightly under it. But the psychological difference is enormous because our brains are programmed to think in categories, rather than continuums.

This categorical thinking is then reinforced by language, and then later by law. One is either a legal adult or one is not. One is over the blood alcohol limit or one is not. One qualifies for Social Security or one does not. Though completely arbitrary, we adopt the categories with ease.

Hence, I suggest we take the arbitrary number of 400 parts per million, hold it up in our minds and transform it into a major psychological distinction. One that puts Earth’s climate into a wholly different category, a realm not yet named. What word should we use?

“Meltdown.” That’s my candidate. One either has ice or does not have ice in any particular place. And the global volume of that disappearing ice at any moment can be accurately estimated, as with the concentration of carbon dioxide in the air.

Yes, it’s true that mountain glaciers, the sea ice and zones of discontinuous permafrost were disappearing during the old realm, which I will leave unnamed. But in the inaugural year of the new meltdown realm, the target has shifted away from the snows of Mount Kilimanjaro, the possibility of a Northwest Passage, and some instability in northern soils to some really “ginormous” things. I refer to: massive losses from the Greenland Ice Sheet and its counterpart in West Antarctica; a completely ice-free Arctic Ocean during summer; and the vast regions of continuous permafrost now holding gigatons of methane, a greenhouse gas far more potent than carbon dioxide.

Let’s rewind back to about 3 million years ago to get some perspective. The carbon dioxide concentration on planet Earth is close to that of today. Our largest ice sheets exist, but are much, much smaller. Mountain glaciers and pack ice are restricted to the highest elevations and latitudes. Permafrost is negligible. Sea level is 50 to 80 feet higher than at present. We’re in a named geological epoch called the Pliocene.

Now let’s fast-forward this pre-history again. The carbon dioxide concentration begins to drop. Progressive refrigeration sets in. Ice sheets oscillate in time and space. Sea levels rise and fall accordingly. Our genus of Homo, which includes all great apes (including us), emerges in Africa. Eventually, our species of Homo sapiens evolves. We are indeed children of the ice age. For without it, we likely would not be here at all.

We’ve reached the present. Homo sapiens has, almost single-handedly, changed Earth’s climate. During the meltdown, we’ll be saying bye-bye to the conditions in which we evolved, and presumably were best suited to our needs. What an irony. To foul a nest on a scale that is global in space and geological in time.

What’s done is done. The impacts of our carbon pollution will be with us for centuries. But despair accomplishes nothing. Having passed 400 ppm, we have a new task at hand: to prevent reaching the next — and even more psychologically significant — magic number of 500 parts per million.

I don’t expect to live long enough to witness this next threshold. But I do wonder what name will be picked for the new category of greenhouse heat.