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400 PPM: Carbon Dioxide Levels Cross a Sobering New Threshold

Last week we passed an unfortunate marker when it comes to climate change: concentrations of carbon dioxide (CO2) in the atmosphere have hit 400 parts per million (ppm) near the Arctic.

What Does it Mean and Why Should We Care?

This level was discovered by scientists at the National Oceanic and Atmospheric Administration (NOAA), who have long measured CO2 concentrations at stations around the world through two ways: (1) volunteers collect air samples and send them to NOAA’s Earth System Research Laboratory in Colorado for analysis; and (2) half a dozen baseline observatories continuously monitor CO2 levels. One of these observatories is located in Barrow, Alaska. The observatory in Barrow, as well as air samples from several other northern locations including Canada, Finland, and Norway, show that 400 ppm was surpassed sometime this spring. Carbon dioxide is the greenhouse gas most responsible for global warming, and its concentration in the atmosphere provides a strong signal of how close we are in moving toward irreversible climate change. Because the projected impacts of higher CO2 concentrations are so significant, many advocate that we need to stay around 350 ppm in order to maintain a stable climate system.

Present global average atmospheric CO2 concentrations, however, are 393.9 ppm. If current trends continue, it should take roughly four years for global levels to reach 400 ppm, according to NOAA. Part of the variation in regional CO2 levels is a result of the vegetation in mid-latitudes, which absorb CO2 during the spring and summer, causing somewhat lower concentrations in these areas. Levels then rise again during the fall, when CO2 is emitted from decaying plant matter. Despite these seasonal ups and downs, growth of global atmospheric concentrations of carbon dioxide has accelerated over the past half century, increasing roughly 2 ppm annually.

To put this data into context, scientific models show that CO2 concentrations are greater today than at any time in the last 800,000 years (check out for a powerful demonstration of the unprecedented rising levels of CO2). This trend has accelerated rapidly in the post-industrial age, leading scientists to draw the connection between human activity and the heightened CO2 levels.

IEA: Record-High Emissions

The news of surpassing the 400 ppm marker was made more troubling as it coincided with new data from the International Energy Agency (IEA), which indicates that global CO2 emissions increased 3.2 percent over the past year, reaching a record high of 31.6 gigatonnes (Gt). The IEA suggests that the world is now just 1 Gt away from the level at which CO2 emissions must stay if we are to have a 50 percent chance of keeping the rise in global average temperature to 2°C above preindustrial levels. And most scientists suggest that even a 2°C increase is too high, as some parts of the world—such as the polar regions—would face temperature increases of two-to-three times the global average.

Globally, temperatures have risen 0.8°C since the late 1880s, and we are already seeing climate-related impacts take hold. Global temperature increases have already led to: earlier springtime and shifts in animal migration patterns; increased glacial runoff and warming of many rivers; enlargement of glacial lakes; changes to food chains; and shifts in ranges and abundance of plankton and fish. All of these have significant impacts on people, ecosystems, and economies around the world.

Some would argue that surpassing 400 ppm is only noteworthy because it is a round number. But the figure serves as an alarm that we need to urgently find lasting solutions to turn these current climate trends around. Otherwise, society will continue to move into unchartered territory as our activities lead us into a new and more uncertain world.

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