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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Climate Hustle

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 CO2 and other GHG's into the atmosphere and when the effect is felt? The CO2 concentration is like the burner setting in the above example: more CO2 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 CO2 concentration by using the average IPCC estimate of 3°C warming for doubling CO2 concentration (this is called the “climate sensitivity”). Using the estimate of pre-industrial CO2 concentration of 280 ppm (parts per million), a climate sensitivity of 3°C implies that CO2 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 CO2 from 1970 to today, we can estimate the warming that has been locked in due to recent CO2 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 CO2 concentration and the time when that temperature is observed. This and CO2 concentrations for 4 selected years are shown in the following figure.

Time lag between CO2 and final warming

This figure therefore shows the temperature anomaly starting in 1970, the year when the temperature increase due to greenhouse gases began to emerge from the background noise. This figure indicates 3 things: (1) the time lag between emitting greenhouse gases and when we see the principle effect is about 30 years, due mostly to the time required to heat the oceans, (2) the rate of temperature increase predicted by a climate sensitivity of 3°C tracks well with the observed rate of temperature increase, and (3) we have already locked in more than 1.5°C warming. As of 2017 we have reached 406 ppm CO2. At the current increase of 2 ppm CO2/yr., this implies that we will reach 440 ppm and lock in 2°C warming by 2035 … if we don’t act now.

So whereas the experiment at home with a pot of water on low heat yields a time lag of something like 10’s of minutes to heat the water, to heat an Earth-sized pot of water the time lag is about 30 years.

What this figure does not show, however, is that as other complex feedback mechanisms kick in, the rate of warming may begin to exceed the IPCC average climate sensitivity. Therefore, for future trends, this plot likely represents the minimum temperature increase that we can expect for a given CO2 concentration.
 

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Comments

Comments 1 to 2:

  1. How come CO2eq is not used?

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  2. I agree with you that CO2eq is the appropriate data to use. However, I have not been able to identify a source to use that melds all of the GHGs together into CO2eq, so for now I am using CO2. Once I identify such a source, I will update the plot in the analogy and begin using CO2eq. I don't think it will change the message, and at most make a small adjustment to the time delay. But I do agree that CO2eq is the appropriate metric to use.

    Thanks for pointing this out.

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