Explainer: How tropical oceans and El Niños heat the land worldwide

A robust feature of global warming is that the surface of the land warms more than the surface of the oceans. We call this the land/ocean temperature contrast.
The reason is partly because the oceans are slow to warm up compared to land; the oceans have a much larger heat capacity than land which means lots more energy is required to change their temperature. But that’s not the whole story.
If we were to warm earth’s oceans by \(1^{\circ}\)C and then stop (in our real world “experiment” we haven’t stopped yet), the land would warm by about \(1.5^{\circ}\)C and then stay warmer than the oceans, even after the ocean has ’caught up’ to the land. This is due to the relationship between the surface and the atmosphere above it; the air in the few kilometers above land is much dryer than the air above oceans. This dryer air is more sensitive to change so its temperature increases more than the air above oceans, leading to a land/ocean temperature contrast. The land and ocean surface temperatures have a similar relationship in the warming and cooling of year-to-year variability.

Observed temperature change in the 20C. Note the increased warming over the continents. Figure from (Compo 2008)

From one year to the next, the global temperature fluctuates up and down around an average temperature. If we compare the fluctuations of land surface temperatures to the fluctuations of the ocean surface temperatures, we see that the land temperature fluctuates more than the ocean. Why? We might expect that the oceans are slower to respond to changes than the land. This is certainly part of the story but we can do an experiment to test how important this is.

In our experiment, we take a global climate model and we specify the ocean surface temperatures; we make them vary up and down each year and see how the land responds. The results show that the land varies more than the ocean temperatures. The ocean variability – which we have input into the model – is being amplified over the land. Interesting! But let’s pull it apart a bit more.