The Ups and Downs of CO2
Evidence: Ice Cores
For hundreds of thousands of years, the concentration of CO2 in the atmosphere remained fairly stable. But not anymore. Today, the atmosphere contains more CO2 than at any time in at least the past 800,000 years.
Scientists studying ice cores from Antarctica now know that the big jump in CO2 in the atmosphere of the past century is extraordinary: in the past 800,000 years, atmospheric CO2 has never risen this much, this fast. Earth's atmosphere is in uncharted territory.
Since 1958, scientists at Mauna Loa Observatory in Hawaii have watched CO2 in the atmosphere increase more and more rapidly, but with important seasonal fluctuations.
In spring, plants in the Northern Hemisphere begin to grow and absorb CO2 from the atmosphere, so atmospheric CO2 concentration decreases—the line dives down.
In fall, plants begin to decay and release their CO2 back into the atmosphere, so atmospheric CO2 concentration increases—the line shoots up.
Nonetheless, the overall direction of the graph is clear: CO2 levels are increasing.
Where Is All The CO2 Coming From?
It's mainly from the burning of fossil fuels. We can tell because the CO2 produced when we burn oil, natural gas and coal has a particular isotopic signature. Since about 1850, the ratio of this "manmade" CO2 to "natural" CO2 has increased dramatically.
As snow falls and turns to ice, it traps tiny bubbles of air. These atmospheric "time capsules" preserve samples of the different gases that made up the air when the ice solidified. And since ice cores—and the bubbles trapped within—can be dated, scientists can learn not just how much CO2 was in the atmosphere at any one time, but also how CO2 concentration has changed over the years.
Analyzing The Atmosphere
Scientists crush small sections of ice cores to release trapped gas and determine the concentration of CO2 and other gases in the ancient atmosphere.
CO2 in the atmosphere is measured in parts per million, or ppm. For instance, the present-day concentration of CO2 is over 385 ppm. That means that for every 1,000,000 gas molecules in the air, 385 are CO2. Put another way, the air is 0.0385% CO2.
For at least 800,000 years, atmospheric CO2 concentration never rose above 300 parts per million. Today it is over 385 ppm and rising at about 2ppm per year.
CO2 concentration in the atmosphere has never fallen below about 180 ppm for at least the past 650,000 years.
A Dynamic Duo
Temperature and CO2 march in lockstep. For hundreds of thousands of years, the temperature and CO2 content of the atmosphere have risen and fallen together. But past changes in temperature do not appear to have been caused by changes in CO2. Rather, variations in Earth's orientation to the Sun repeatedly caused the planet to warm or cool. The changes in CO2 followed. Since the ocean plays a key role in controlling atmospheric CO2 concentration, the warming and cooling likely affected the ocean's ability to absorb CO2, which in turn may have intensified the warming or cooling through the greenhouse effect.