What's in a Number? New Carbon Dioxide Level Unseen in Human History

Michael Mann, Director, Earth System Science Center, Pennsylvania State University

"The soon-to-be-breached atmospheric CO2 concentration of 400 ppm has important symbolic significance. It is a reminder of just how uncontrolled this dangerous experiment we're playing w/ the planet really is. 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. This was a time when global temperatures were substantially warmer than today, and there was very little ice around anywhere on the planet. And so sea level was considerably higher -- around 100 feet higher -- than it is today. It is for this reason that some climate scientists, like James Hansen, have argued that even current-day CO2 levels are too high. There is the possibility that we've already breached the threshold of truly dangerous human influence on our climate and planet."

James Butler, Director, Global Monitoring Division, NOAA Earth System Research Laboratory

"Atmospheric carbon dioxide (CO2) reaching 400 ppm at Mauna Loa is not in itself a significant event. It is, however, a noteworthy marker of what is significant -- the accelerating growth and persistence of CO2 in the atmosphere. CO2, far and away the dominant greenhouse gas emitted by humans, is responsible in good part for recent climate change, and, once emitted, will remain in the ocean-atmosphere system for thousands of years, warming the planet, changing climate, and driving acidification of the oceans. Atmospheric CO2 has been about 280 ppm through almost all of human civilization, yet, primarily in the past century, humans have driven it up to around 400 ppm mainly by burning fossil fuels. These emissions continue to accelerate unabatedly. Last year, NOAA reported that CO2 at all arctic sites reached 400 ppm for the first time. This month, the iconic Mauna Loa site reports 400 ppm, with the global average not far behind. South Pole, the last to reach this marker, will do so in 4-5 years."

Pieter Tans, Climate Scientist, NOAA Earth System Research Laboratory

The modern measurements of atmospheric CO2 started in 1958 near the summit of Mauna Loa, Hawaii. The station is still the best known global "benchmark station" because the site was very carefully chosen to be representative of most of the northern hemisphere. The round number of 400 ppm can be seen as a milestone. It reminds us that CO2 is now higher than in the last several million years. In addition, the rate of increase, over the last decade more than 2 ppm per year, is at least 100 times faster than what has been observed in ice cores over the last 800,000 years. We know for a fact that today's CO2 increase is entirely due to human activities, especially the burning of coal, oil and natural gas.

Mark Serreze, Director, National Snow and Ice Data Center

"It seems that we've made our decision regarding what to do about climate change. We are going to do nothing and hope that the effects will be manageable. The history of humanity is filled with shortsighted, illogical choices. We can now add another one to our quiver."

Robert Corell, Principal, Global Environment and Technology Foundation Center for Energy and Climate Solutions

"For the past 800,000 years, the maximum average concentration of C02 in the atmosphere has been about 270 ppm, but with fossil fuels use accelerating, CO2 concentration will now exceed 400 ppm. The CO2 levels in the atmosphere will fly through 400 ppm towards 500 ppm by 2060, leading to serious food shortages for the poorest of the poor, water scarcity for half of the world and extreme floods and droughts. At current rates of fossil fuel emissions, the CO2 levels in the atmosphere will double over the maximum level of CO2 in the atmosphere experienced during the past 800,000 years with devastating consequences. At current rates of fossil fuel emissions, sea level globally will very likely rise 1.5 feet by 2050 and global average surface temperatures will pass the international target of 2 degrees centigrade by 2050."

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