“Everyone should understand such basic ‘bathtub dynamics,’ but in fact many do not recognize that the atmosphere is accumulating greenhouse gases just as your bathtub accumulates water,” says Professor Sterman.
Because human activity now pumps greenhouse gases into the atmosphere at more than twice the natural removal rate, it's not sufficient to merely stabilize emissions in order to stabilize atmospheric greenhouse gas concentrations. Instead, emissions must fall to the rate at which greenhouse gases are removed from the atmosphere. The best estimates, Sterman says, are that stabilizing the concentration of atmospheric greenhouse gases at about 500 parts per million of CO² — still roughly double the preindustrial level — requires emissions to fall about 80 percent from current rates by about 2050. Given the long delays in changing the economy, there's no time to lose.