The milestone, hit on May 9, may be symbolic, notes Climate Central, but manmade CO2 emissions from the burning of fossil fuels continue to rise, bringing greater atmospheric warming and exacerbating the effects of climate change.
"The last time we're confident that CO2 was sustained at these levels is more than 10 million years ago, during the middle of the Miocene period," climate scientist Michael Mann told The Huffington Post in an email. "This was a time when global temperatures were substantially warmer than today, and there was very little ice around anywhere on the planet."
"Take this day and the milestone it represents to reflect on the fragility of our civilization and and the planetary ecosystem on which it depends," Al Gore urged on Friday. "We must take immediate action to solve this crisis. Not tomorrow, not next week, not next year. Now."
350.org founder Bill McKibben said in a statement emailed to HuffPost, "The only question now is whether the relentless rise in carbon can be matched by a relentless rise in the activism necessary to stop it."