Posted on 22 September 2010 by alan_marshall
by Alan Marshall from climatechangeanswers.org
If we accept that greenhouse gases are warming the planet, the next concept that needs to be grasped is that it takes time, and we have not yet seen the full rise in temperature that will occur as a result of the CO2 we have already emitted. The Earth’s average surface temperature has already risen by 0.8 degrees C since 1900. The concentration of CO2 in the atmosphere is increasing at the rate of 2 ppm per year. Scientists tell us that even if CO2 was stabilized at its current level of 390 ppm, there is at least another 0.6 degrees “in the pipeline”. If findings from a recent study of Antarctic ice cores is confirmed, the last figure will prove to be conservative [ii]. The delayed response is known as climate lag.
The reason the planet takes several decades to respond to increased CO2 is the thermal inertia of the oceans. Consider a saucepan of water placed on a gas stove. Although the flame has a temperature measured in hundreds of degrees C, the water takes a few minutes to reach boiling point. This simple analogy explains climate lag. The mass of the oceans is around 500 times that of the atmosphere. The time that it takes to warm up is measured in decades. Because of the difficulty in quantifying the rate at which the warm upper layers of the ocean mix with the cooler deeper waters, there is significant variation in estimates of climate lag. A paper by James Hansen and others [iii] estimates the time required for 60% of global warming to take place in response to increased emissions to be in the range of 25 to 50 years. The mid-point of this is 37.5 which I have rounded to 40 years.