SkS Analogy 4 - Ocean Time Lag

Posted on 18 May 2017 by Evan

Tag Line

Greenhouse gases (GHG) determine amount of warming, but oceans delay the warming.

Elevator Statement

To see how the oceans delay warming of the atmosphere, try the following thought experiment.

Imagine a pot that holds about 8 liters/quarts.

Hang a thermometer from the center of the lid so that it hangs in the middle of the pot.

Put the pot on the stove with no water, just air.

Turn the burner on your stove on very low heat.

Measure the time it takes for the thermometer to reach 60°C (about 140°F).

Remove the pot from the stove, let it cool, fill with water, and place it on the stove on very low heat.

How much longer does it take to reach 60°C (about 140°F) with water instead of air in the pot? A lot longer!

If you wanted to heat the water to 90°C (about 195°F) in the same amount of time, you would need to start this experiment with the burner on higher heat.

The longer time it takes to heat the pot of water than a pot of air explains why there is a delay between GHG emissions and a rise in temperature of the atmosphere: the oceans absorb a lot of heat, requiring a long time to heat up. This is why scientists such as James Hansen refer to global warming as an inter-generational issue, because the heating due to our emissions are only fully felt by the next generation, due to the time lag created by the oceans.

Climate Science

The earth is covered mostly in water. The large heat capacity of the oceans mean they soak up a lot of energy and slow down the heating of the atmosphere. Just how long is the delay between the time we inject CO 2 and other GHG's into the atmosphere and when the effect is felt? The CO 2 concentration is like the burner setting in the above example: more CO 2 is like a higher burner temperature. However, even though turning up the heat creates hotter water, it takes a while for the water to heat up.



We can estimate the final temperature the atmosphere will reach for a given CO 2 concentration by using the average IPCC estimate of 3°C warming for doubling CO 2 concentration (this is called the “climate sensitivity”). Using the estimate of pre-industrial CO 2 concentration of 280 ppm (parts per million), a climate sensitivity of 3°C implies that CO 2 concentrations of 350, 440, and 560 ppm yield 1, 2, and 3°C warming, respectively. Using this estimate of climate sensitivity together with measurements of CO 2 from 1970 to today, we can estimate the warming that has been locked in due to recent CO 2 emissions. That is, knowing the burner setting, we can estimate the final temperature of the pot of water, even though we will have to wait some time for it to heat up.



We also use the GISS (Goddard Institute for Space Studies) data to plot measured global mean temperature above preindustrial to estimate the time lag between the temperature anomaly suggested by a particular CO 2 concentration and the time when that temperature is observed. This and CO 2 concentrations for 4 selected years are shown in the following figure.