Most people tend to think of inertia as applying to something at rest or stationary, such as a person on a couch. However, it also applies to bodies in motion. In physics, the term inertia refers to the resistance of any object to any change in its velocity. A body will stay stationary or will continue moving in a straight line at a constant speed unless a force acts on it. Isaac Newton clearly defined this in his first law of motion1. A similar thing is true of the climate, as well as ecosystems and socio-economic systems.
Climate inertia refers to the slow reaction of climate systems to changes in greenhouse gas concentrations. Not only was the climate slow to react to the increase in greenhouse gases, but it will be slow to react to a decrease in greenhouse gases. As a consequence, if we reduce our CO2 emissions to zero relatively quickly, its concentration in the atmosphere will be elevated for many hundreds of years2. This is because the long-term sinks for CO2 operate very slowly. These sinks are part of the global carbon cycle and include soils, the deeper ocean and rocks. Some of these operate on very long time frames, such that up to 40% of CO2 emitted now, will remain in the atmosphere for up to 2,000 years3. This is termed its residence time; the length of time it resides in the atmosphere. If we stabilise emissions of shorter-lived greenhouse gases, such as methane (CH4), their atmospheric concentrations will stabilise within a decade or so. However, that is dependent on humanity being able to control CH4 emissions. Here is the catch. The long residence time of CO2 and its cycling into the deeper ocean may make methane clathrates unstable, releasing an enormous amount of CH4. Given that there are indications that some methane clathrates are already becoming unstable, thereby releasing methane into the ocean and atmosphere4, it may already be too late.
If these feedbacks are already under way, it is unlikely that we can ever stop them, because of the inertia of the system. It didn’t have to be this way. We perhaps could have done something to prevent this from happening if we had started in 1990, when the first Intergovernmental Panel on Climate Change report came out5. However, vested interests such as fossil fuel companies, their investors and their purchased politicians started the denialism industry and prevented any meaningful action. That is a crime against humanity, and they should pay for it.