CO₂ added to the atmosphere mounts quickly but can only be drawn down slowly, underscoring the need to get to work fast. Even if emissions from human activities stopped tomorrow, it would take about a century for the land and the ocean to bring CO₂ levels 60 percent of the way down toward preindustrial levels. Getting rid of the other 40 percent will, unfortunately, take much longer.
The faster we can cut emissions of CO₂, the more we can limit the harm from its consequences, including more extreme weather, ocean acidification and sea-level rise and their harmful effects on our health, infrastructure and the environment, which could last generations.
An individual CO₂ molecule typically spends around 10 years in the atmosphere after being released. So how can emissions from human activities be creating an essentially irreversible rise in atmospheric CO₂ concentrations? If atmospheric carbon was like money in a bank account, the answer would be that while some natural processes are busy withdrawing money at one ATM, other natural processes are constantly taking that money and redepositing it at another ATM.
To compare the rates at which CO₂ is absorbed and released by natural processes, it helps to look at CO₂ concentrations recorded at Mauna Loa Observatory, located at the summit of Hawaii’s Mauna Loa mountain. These measurements, which began in 1958, are collected more than 11,000 feet above sea level in the middle of the Pacific Ocean and represent global average values.
The black curve, shown below, plots monthly average CO₂ concentrations against time, with seasonal variations removed; it shows a dramatic upward trend produced by burning fossil fuels, deforestation and other human activities.
The red saw-toothed curve in this same chart includes seasonal variations and reveals an up-down annual cycle with a minimum early in the northern hemisphere fall. Back in October of 1958, the monthly CO₂ level dropped below 313 ppm — it never fell that low again.
The annual up-down dance of atmospheric CO₂ comes mostly from plants, both living and dead. It is dominated by the seasons in the northern hemisphere where there is more land, and more foliage, than in the south. In spring, plants are active and absorb a lot of CO₂, using the carbon to grow larger. In the fall, plant growth slows as daylight dwindles, and CO₂ uptake ebbs. In contrast, organic material on the ground decays year-round, and that decay returns carbon to the atmosphere as CO₂.
Over a full year, the rate at which CO₂ is returned to the atmosphere from decay on land nearly balances the rate at which it is taken up by plant growth. To estimate the impact of the biosphere on atmospheric CO₂ we need to look at changes to this balance, not at the average lifetime of an individual molecule in the atmosphere. Atmospheric CO₂ levels are actually determined by three different balances involving the land biosphere, the oceans, and the Earth’s crust.