"The carbon dioxide concentration in air tells two stories. It varies like a clock, rhythmically marking the annual cycle of the biosphere (plants) breathing in CO2 in the spring, and exhaling in the fall. There is also a general upward tick from year to year. The trend is from society exhaling CO2 as the waste product of energy and agriculture: it feeds us, heats us and moves us. We borrow CO2 from the past (fossil fuels), and give it to the future (in the atmosphere). The laws of physics say that has consequences. Warmer temperatures on average. Shifts in rain patterns. Extreme wet and dry conditions we have not seen in recorded climate. We can choose how much CO2 we emit. As we slowly push it up, we could slowly push it down. Someday, our choices will be recorded in the CO2 record itself. Where will the trend go? How hard will the planet be breathing? That is the story we continue to write into the record."
Jeffrey Kiehl, Senior Scientist, National Center for Atmospheric Research
"In 1978, when I began my graduate career in climate science, atmospheric carbon dioxide levels were around 335 ppm. I was working on a simple climate model to study the effects of a doubling of carbon dioxide on temperature. Scientists did this thought experiment to gauge the sensitivity of Earth's climate, as is still done today. Back then, most of us viewed this as a theoretical problem. Clearly, we would be off of fossil fuels in 30 or 40 years given all the talk of nuclear and solar power. After all, fossil fuels were going to run out. In the very near future we will cross the 400 ppm level hurtling our way towards 560 ppm (a doubling of the pre-industrial levels). Continuing on our current energy path will lead to projected carbon dioxide levels of around 1,000 ppm by 2100, a mere 90 years in the future. At that point, we will have returned Earth's carbon dioxide levels to that of the extremely warm Eocene a time more than 30 million years ago, in which there were no large ice sheets. We now realize that the increase in carbon dioxide is not just a theoretical problem, but one of immense importance for the well-being of all life on Earth. Do we now have the wisdom to avoid a return to Earth's deep warm past?"
Prashant Sardeshmukh, Senior Research Scientist, Cooperative Institute for Research in Environmental Sciences, University of Colorado
"Reaching the 400 parts per million milestone for carbon dioxide should not by itself affect our general concern about rising carbon dioxide levels in the atmosphere. It would be cause for special alarm if 400 ppm were a climate tipping point or close to a tipping point, but no evidence exists for such a claim. Still, something's gotta give as carbon dioxide levels continue to rise. And if that something makes the occurrence of an ice-free Arctic or a mega drought or a monster Nino or a jumbo storm virtually certain in the next several decades, then a tipping point with serious consequences would indeed have been reached. It's rather like a Chinese water torture, not knowing precisely when or even if this will happen. Both natural climate variability and climate modeling errors, not to mention uncertainty in future climate forcing, prevent us from answering the question at present."