Common Climate Questions

Q: What does the ice core record tell us about the relationship between temperature and carbon dioxide in the past?

A: We know from ice core records that temperature and carbon dioxide (CO2) levels are closely correlated. In the distant past, warming episodes appear to have been initiated by cyclical changes in Earth’s orbit around the Sun that caused more summer sunlight to fall in the northern hemisphere. This caused snow and ice on land and sea to melt, revealing darker land and water, which caused more warming, in a self-reinforcing cycle. As the planet continued to warm, more CO2 was released from the oceans, and this increase in heat-trapping gas caused even more warming. Thus, while CO2 did not initiate those warming episodes, it did contribute to them.

In the current warming episode, it is clear that CO2 and other human-induced heat-trapping gases are driving the warming. We know with certainty that the increase in CO2 concentrations since the industrial revolution is caused by human activities because the isotopes of carbon show that it comes from fossil fuel burning and the clearing of forests.

So even though past warm episodes may have been initiated by orbital changes that caused warming and thus caused CO2 to rise, which then led to more warming, we know that the current warm episode is being driven by increasing CO2 due to the burning of fossil fuels and the clearing of forests. The orbital changes that caused the ice ages are far too weak and slow to cause a warming as rapid as the current one.