Did Someone Leave the Oven On?

If we suddenly stopped emitting carbon into the atmosphere, global temperatures would continue to increase.

How is this possible?  Think about using your oven to heat a loaf of bread. 

Say you put a loaf of bread in the oven, close the door, and set it at 100°F.  If the bread is in there long enough, it will eventually reach an equilibrium temperature of 100°F.  It will then remain at this temperature as long as the oven setting does not change.  However, if we slowly start to increase the temperature, say by 5°F per minute, the bread will begin to warm again.  Because it takes time for the bread to warm up, we would not expect the bread’s temperature to equal the oven setting as it changes. 

For example, after 10 minutes, the oven will be set at 150°F, but the bread may still be 120°F.  If we stop increasing the temperature and wait long enough, the bread will eventually reach 150°F.  The bread will then stay at that temperature until we turn the oven back down.

In this example, the bread is the earth and the oven settings represent the amount of greenhouse gases in our atmosphere.  The earth’s temperature used to be in relative equilibrium (which was the case for about ten thousand years), partially because the amount of carbon in our atmosphere had changed very little.  This is like leaving the oven set at a constant temperature.  Now, by continuously adding carbon dioxide to our atmosphere through the use of fossil fuels, we are turning the temperature knob up.  The earth is warming in response, but it will take time to catch up to the current temperature setting.  This is just like the loaf of bread when we first put it in the oven.  The current temperature setting on our climate is higher than actual temperatures today. 

“The temperature setting on our climate is higher than actual temperatures today. ”

If we completely stop emitting carbon dioxide, atmospheric concentrations will remain elevated for thousands of years.  This is like turning the oven up faster than the bread’s temperature can change, leaving it at a constant setting until the bread catches up, and then leaving it at that temperature for thousands of years. 

In reality, natural sinks do remove carbon from the atmosphere (similar to turning the oven back down), but this process can take thousands, hundreds of thousands, or even millions of years.  Once we increase the temperature setting on our climate through the use of fossil fuels, we are locked into that setting for a very long time.  

 For a more detailed scientific explanation, see Irreversible climate change due to carbon dioxide emissions (Solomon et al. 2008) at http://www.pnas.org/content/106/6/1704.full