"It’s been at least 800,000 years — probably more — since Earth saw carbon dioxide levels in the 400s…."

Without following this closely, one might think "well, 800,000 years ago we were in the middle of an ice age, so what's the big deal...?"

Reading the linked article, though, you see this:

“We are able, for the first time, to accurately reproduce the ice-core record for the last 800,000 years “” the record of atmospheric C02 based on measurements of carbon dioxide in gas bubbles in ice,” Tripati said. “This suggests that the technique we are using is valid.

“We then applied this technique to study the history of carbon dioxide from 800,000 years ago to 20 million years ago,” she said. “We report evidence for a very close coupling between carbon dioxide levels and climate. When there is evidence for the growth of a large ice sheet on Antarctica or on Greenland or the growth of sea ice in the Arctic Ocean, we see evidence for a dramatic change in carbon dioxide levels over the last 20 million years.

“A slightly shocking finding,” Tripati said, “is that the only time in the last 20 million years that we find evidence for carbon dioxide levels similar to the modern level of 387 parts per million was 15 to 20 million years ago, when the planet was dramatically different.”

So what's really going on is that the data for the last 800k years is very solid, but that's the only special thing about that number. Perhaps slightly less solid are the numbers going back 20 million years, but they show that it's been at least 15 million years since we were at this level. Kinda sloppy reporting in the first article...

Of course it should be noted that even the 15 million number might seem a bit reassuring, since the world wasn't all that different then, other than having no people... until you think about the Co2 growth accelerating. Truly uncharted territory, coming up!

• The apparent basis for the 800k number