We are pumping out more greenhouse gases – principally CO2, methane and nitrous oxide – more quickly than they can be absorbed by the planet. Above this limit, they accumulate in the atmosphere and cause the temperature to rise. We’ve been doing this with CO2 since the start of the industrial revolution (around 1800) with ever increasing excesses. The accumulated excesses have risen the CO2 in the atmosphere from 280 ppm to over 410 ppm over the last 180 years and warmed the planet by a little less than 1C.
The temperature rise associated with higher CO2 concentrations isn’t immediate; if we were to stop emitting CO2 today, scientists predict continued warming of around 0.6C over the next 40 years.
Although it took 180 years to go from 280 ppm to 410 ppm, the process is accelerating. The first half of that rise took 155 years, but the second half only 25 years. Every year we pump out more CO2. If we stabilize our CO2 levels or even cut them to 70% of our current levels, we’re still emitting more than the planet can absorb – CO2 concentrations will continue to rise and temperatures will follow on.
To limit global warming we have to cut the planet’s CO2 emissions to around 50% of what they are today. Even then temperatures will rise (due to the committed warming) for around 40 years before stabilizing.