When the Earth Systems Research Laboratory announced that atmospheric carbon dioxide concentrations reached more than 400 ppm, it made the news. As well it should! That's the highest levels of CO2 recorded in all of human history.
But why is it that the 400 ppm mark gets our attention? Why wasn't it newsworthy when we hit record levels of 399 ppm? Or 398 ppm? Or every other record level as CO2 concentrations rose rapidly over the last half-century of monitoring?
I guess it's the psychological significance we ascribe to round numbers. Like when the temperature hits 100 degrees. Or when gas hits $4.00/gallon. That gets our attention in a way that temperature of 97 degrees or gas prices of $3.929/gallon don't.
Not coincidentally, now that CO2 levels have exceeded 400 ppm, you are probably going to see more 100 degree days. Higher CO2 concentrations have the effect of turning up the temperature on Earth's thermostat. Hot days will get hotter and hit the century mark more frequently.
The only way to turn back the planetary thermostat is to dramatically reduce emissions of carbon dioxide and other greenhouse gases. That means transitioning our energy systems from fossil fuels to renewable sources. We can take immediate steps to reduce greenhouse gas emissions by protecting and restoring the world's forests, and by investing in cost-saving energy efficiencies such as more efficient lighting and refrigeration.
With commitment and perseverance to take these and more steps in the right direction, we might one day read headlines about CO2 concentrations returning below 400 ppm.