Humans now steer climate bus

Climate scientists at the Jackson School of Geosciences address common myths about climate change in this eight part series.

Myth No. 8: In the past, global temperatures rose first and then carbon dioxide levels rose later. Therefore, rising temperatures cause higher CO2 levels, not the other way around.

Photo of Kerry Cook
Kerry CookPhoto: Sasha Haagensen

Ice cores from Dome C in Antarctica record surface temperatures and atmospheric concentrations of CO2 going back over 800,000 years. During that time, several ice ages came and went. After each ice age ended, temperatures rose first and then several centuries later, CO2 concentrations rose. This lag, some skeptics conclude, proves that CO2 increases are caused by global warming, not the other way around.

According to Kerry Cook, professor of climate systems science, it isn't an either/or proposition. Climate variations can have many different causes, which are known as "climate forcing factors." The climate forcings for ice ages and warmer periods are well-known variations in Earth's orbital parameters (the eccentricity of its orbit around the sun, the tilt of its axis of rotation, and the season during which Earth is closest to the sun).  These factors can be accurately calculated for any past or future time, and they vary on time scales ranging from 23,000 years to 400,000 years. The amount and distribution of solar radiation that reaches Earth changes with the orbital parameters, causing climate variations on these same time scales (tens of thousands of years).

During glacial (cool) periods on Earth, atmospheric CO2 levels are lower, and during interglacial (warm) periods they are higher. That's true in part because during warmer periods, the oceans store less dissolved CO2, just as your glass of soda releases its CO2 and goes flat when it warms to room temperature. Warming soils also release more CO2. So it makes perfect sense that in the past, CO2 variations followed temperature variations. The role of CO2 in natural glacial/interglacial climate change is to amplify the climate change, making warm periods somewhat warmer.  So changes in Earth's orbital parameters likely kicked off the warmer periods and rising CO2 likely boosted the effects.

That doesn't prove that CO2 can't cause global warming. In fact, since past natural warmings took about 5,000 years to complete, CO2 and temperature actually rose together for about 90 percent of the time. Climate scientists think much of the warming in ice core records during interglacials is due to CO2.

Graph showing CO2 lag
Data from Antarctic ice cores show that temperatures (brown) have changed before CO2 concentrations (blue) over a series of recent ice ages. That trend has been upset during the past 100 years, as a rapid increase in CO2 preceded the current warming. Credit: Michael Ernst/Woods Hole Research Center

The game has changed dramatically in recent decades. There is no lag between rising temperatures and rising CO2. Both have spiked dramatically in the past 50 years. The rate of change and the lack of a lag are exactly what climate scientists theorize should happen if CO2 takes over as the main climate forcing.

There are several reasons that it is useful to refer to the ice core records when we are thinking about CO2-induced climate change.  One is to simply understand that global climate can change, and that a change in the globally-averaged surface temperature of just a few degrees implies a huge change in climate. The difference between a glacial and an interglacial climate is only about 9 degrees Fahrenheit (5 degrees Celsius), which is comparable to predicted changes in globally-averaged temperature over the coming decades due to increasing greenhouse gas concentrations.

We also learn from the ice core record that these natural, long-period changes take CO2 levels from about 200 parts per million by volume (ppmv) during a glacial (cool) period to about 280 ppmv during an interglacial (warm). This provides perspective on the current atmospheric CO2 concentration of about 390 ppmv, showing that it is far outside the range of natural variations even on time scales of tens of thousands of years.

You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.