1.6 Haven’t natural fluctuations in carbon dioxide affected the Earth’s temperature?
"At present rates of human emissions, there will be more CO2 in Earth’s atmosphere by 2025 than at any time in at least the last 3.3 million years."
Dr Elwyn De La Vega, University of Southampton
In the absence of human activities, the levels of carbon dioxide in our atmosphere tend to remain fairly stable over time. Carbon dioxide is constantly being absorbed into the oceans, and locked into carbon compounds in trees and plants by a process called photosynthesis. Other living creatures feed off these trees and plants, taking the carbon compounds into their own bodies. These carbon compounds are then turned back into carbon dioxide through a process called respiration, and the carbon dioxide is released back into the atmosphere. Carbon dioxide is also released when dead matter decays and rots, or is burnt. This process of the constant removal and replacement of carbon dioxide in our atmosphere is known as the carbon cycle.
Whilst analysis of air bubbles trapped in Antarctic ice sheets reveals that over the past 800,000 years there also have been periods in the Earth’s history where the carbon dioxide levels have naturally risen and fallen, this has taken place extremely gradually. These changes are brought about by natural variations in the way in which the Earth travels around the Sun (known as Milankovitch cycles), which lead to changes in the absorption of sunlight on Earth. For example, a small increase in the amount of sunlight being absorbed causes the oceans to warm slightly, which results in some of the carbon dioxide dissolved within them being released back into the atmosphere. The increase in atmospheric carbon dioxide leads to more ocean warming, which causes more carbon dioxide to be released, which causes more warming... and so on and so forth. The changes in temperature caused by this feedback loop, and others like it, drive the transitions into ice ages (glacial periods) and back out of them (interglacial periods). The last time this happened was around 17,000 years ago, when we began to transition out of our most recent ice age.
The key point here is that these natural changes in carbon dioxide levels, triggered by cycles in the Earth’s orbit and modulated by feedback cycles within the Earth system, take place over tens of thousands of years. In contrast, the extremely rapid increases in carbon dioxide levels that we have been seeing over the past 60 years, due to human actions, have been taking place about 100 times faster than any of these previous natural increases.
Carbon dioxide levels today are higher than they have been in more than 3 million years. Back then, global average temperatures were 2-3°C higher than in pre-industrial times and sea levels were a whopping 16 m higher. Studying these past changes has revealed to climate scientists just how sensitive the Earth system is to changes in greenhouse gas concentrations. This is one of the key reasons why scientists are so alarmed about the current rapid rise in their concentrations.