As spring turns ever-so-slowly to summer in the Northern Hemisphere, a worrying milestone was just passed: atmospheric carbon dioxide, measured at the Mauna Loa observatory, averaged 401.25ppm for the entire month of April. This is the first time that the monthly average measurement has exceeded 400 ppm, and it suggests that atmospheric CO2 levels are higher than they’ve been for the last 800,000 years or so.
Why does this matter?
The 400 ppm level has been identified by the IPCC as the concentration of CO2 likely to keep global climate change below 5°C. James Hansen, former NASA scientist, has gone further, saying that 350 ppm, well below 400 ppm, is the safe level of CO2 concentrations. Exceeding these concentrations for an extended period of time sets the world on a path of significant warming that will be difficult to reverse once begun.
Natural variation in atmospheric carbon means the level of atmospheric CO2 probably won’t stay this high for long. As the northern summer rolls around and trees and plants begin growing after the long winter, they pull CO2 from the atmosphere for photosynthesis. This is why last year’s high mark of above 400 ppm, set in May, rapidly disappeared.
However, this measurement is concerning. Carbon emissions are a stock, not a flow problem. Most of the CO2 making up this latest measurement was emitted years ago. Most of the effects of emissions released today won’t be felt for years, so there is some rise in atmospheric CO2 already locked in. This suggests that, despite recent progress in cutting carbon emissions, we are likely already on track for some level of significant global warming.
If we stopped emitting tomorrow we might be able to avoid exceeding 400 ppm for an extended period of time. But how likely is it that we stop emitting carbon even by the end of the decade? Exceeding 400 ppm for an extended period is now likely a reality. If we are to keep it even close to that level, we must take action now. This is only the latest warning sign – how many more will it take?
Image Credit: Joe Parks via Wikimedia Commons