On Thursday, something changed.

Did you notice?

Was it harder to breathe? Feel a little warmer? Did the seas rise a little higher?

Didn’t you see that? Me either.

The change was too subtle. And yet, significant.

On Thursday, the research facility atop the Mauna Loa volcano on the island of Hawaii recorded an average carbon dioxide reading of more than 400 parts per million.

That’s higher than the CO2 levels for the past 500 million years. According to Bill McKibben, co-founder of 350.org, an environmental activist, author and journalist, we’re in new territory. Unpredictable territory.

But the fact that the change was subtle is a reflection on how far we’re pushing into this unknown.

Already we’re seeing the deadly effects of climate change in the form of rising seas, wildfires and extreme weather of all kinds, and passing 400 ppm is an ominous sign of what might come next.

Last summer, the majority of the ice sheet on Greenland registered a melt over four days. My friend Chris is up in Greenland now collecting ice cores to try to understand why that melt occurred. (You can follow his progress here.)

I like this quote from Richard B. Alley, a climate scientist at Pennsylvania State University.

He told the New York Times: “If you start turning the Titanic long before you hit the iceberg, you can go clear without even spilling a drink of a passenger on deck. If you wait until you’re really close, spilling a lot of drinks is the best you can hope for.”

Have we already decided it’s full steam ahead?