Imagine you are riding a bike. You are pedaling at a constant speed, but sometimes you have the wind at your back, and at other times you’re pedaling into a headwind. You go up hills and down hills. Even though there’s no change to your pedaling, you will slow down or speed up at various points, because pedaling is not the only factor in your forward motion.
This is how the climate works too. There are a variety of different forces affecting the temperature of the atmosphere. The sun can be stronger or weaker as it goes through its cycle. There are weather cycles like El Nino, and long running regional climate variations. Other human activity has an effect, such as aerosol loading that slows down warming. There are other greenhouse gases – methane or nitrous oxide, and these interact with other natural processes such as cloud formation, another factor in the climate.
The existence of these many factors does not undermine the main argument that CO2 from human activity is warming the planet. In fact, they are vital to understanding the climate out there in the real world. Just as you can speed up and slow down while pedaling at the same rate, so the earth can warm faster when different cycles coincide, and slow down when they work against each other. CO2 emissions continue to rise, but warming will move faster or slower as all these various effects inter-relate.
This isn’t particularly complicated, but it’s surprising how often it has to be repeated. This post was prompted by hearing James Hansen having to explain it on Radio 4 this morning. Despite being among the more senior journalists in the country, the Today programme team didn’t seem to understand it.
Specifically, warming moved faster in the 80s and 90s than it has done over the last ten years. This doesn’t change the importance of CO2. To suggest it does is like riding your bike uphill, and concluding that since you’re slowing down even though you’re pedaling, pedaling doesn’t work.
The Earthbound Report 