"The last time the atmosphere had 400 parts per million of carbon dioxide was most likely between 3 and 5 million years ago, long before humans like us inhabited the earth. It was a geological epoch known as the Pliocene. The planet was many degrees warmer and scientists estimate sea level was about 80 feet higher.
Reaching 400 parts per million for the first time in human history is a wake up call for all of us. The science is clear."
