Why doesn’t the temperature rise at the same rate that CO2 increases?
The amount of CO2 is increasing all the time - we just passed a landmark 400 parts per million concentration of atmospheric CO2, up from around 280ppm before the industrial revolution. That’s a 42.8% increase.
A tiny amount of CO2 and other greenhouse gases, like methane and water vapour, keep the Earth’s surface 30°Celsius (54°F) warmer than it would be without them. We have added 42% more CO2 but that doesn't mean the temperature will go up by 42% too.
There are several reasons why. Doubling the amount of CO2 does not double the greenhouse effect. The way the climate reacts is also complex, and it is difficult to separate the effects of natural changes from man-made ones over short periods of time.
As the amount of man-made CO2 goes up, temperatures do not rise at the same rate. In fact, although estimates vary - climate sensitivity is a hot topic in climate science, if you’ll forgive the pun - the last IPCC report (AR4) described the likely range as between 2 and 4.5 degrees C, for double the amount of CO2 compared to pre-industrial levels.
So far, the average global temperature has gone up by about 0.8 degrees C (1.4 F).
"According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS)…the average global temperature on Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per decade."
Source:
NASA Earth Observatory
The speed of the increase is worth noting too. Unfortunately, as this quote from NASA demonstrates, anthropogenic climate change is happening very quickly compared to changes that occurred in the past (text emboldened for emphasis):
"As the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 years. In the past century alone, the temperature has climbed 0.7 degrees Celsius,
roughly ten times faster than the average rate of ice-age-recovery warming."
Source:
NASA Earth Observatory