Why global warming affects the poles more than the equator

Friends, welcome to Episode 137 in the occasional series of “Things I really should have known a long time ago, but have somehow only realised just now“.

In this case, the focus of my ignorance is the observation that the warming resulting from our emissions of carbon dioxide affects higher latitudes more than lower latitudes.

This is a feature both of our observations and models. But what I learned this week from reading the 1965 White House Report on Environmental Pollution (link) was the simple reason why.

[Note added after feedback: In this article I am describing an effect that makes the direct effect of increase CO2 levels more important at high latitudes. There are also many feedback effects that amplify the direct effect and some of these are also more  important at high latitudes. Carbon Brief has an excellent article on these feedback effects here, but that is not what I am talking about in this article.

Why?

The two gases responsible for majority of greenhouse warming of the Earth’s surface are water vapour and carbon dioxide. But the distribution of these two gases around the planet differs significantly.

  • The concentration of water vapour in the atmosphere depends on the temperature of the liquid surfaces from which the water evaporates. Because the Equator is much hotter than the poles, there is much more water vapour in the atmosphere at the Equator compared with the poles.
  • The concentration of carbon dioxide in the atmosphere is pretty uniform around the globe.

This is illustrated schematically in the figure below.

Click on the image for a larger version. There is more water vapour in the atmosphere in lower latitudes (near the Equator) that at higher latitudes (near the poles). In contrast, carbon dioxide is rather uniformly distributed around the globe.

Of course the truth is a bit more complex than the simplistic figure above might imply.

  • Water vapour from the Equator is transported throughout the atmosphere. But nonetheless, the generality is correct. And the effect is large: the atmosphere above water at 15 °C contains roughly twice as much moisture as the atmosphere above water at 5 °C.
  • Carbon dioxide is mainly emitted in the northern hemisphere, and is then uniformly mixed in the northern hemisphere within a year or so. The mixing with the Southern Hemisphere usually takes two or three years. The variability around the globe is usually within ±2%.

The uniformity of the carbon dioxide distribution can be seen in the figure below from Scripps Institute showing the carbon dioxide concentrations measured at (a) Mauna Loa in the Northern hemisphere, and (b) the South Pole.

Click on the image for a larger version. The carbon dioxide concentrations measured at (a) Mauna Loa in the Northern hemisphere, and (b) the South Pole. Notice that the data from South Pole shows only small seasonal variations and lags behind the Northern Hemisphere data by a couple of years.

Because of this difference in geographical distribution, the greenhouse effect due to carbon dioxide is relatively more important at higher latitudes where the water vapour concentration is low.

And that is why the observed warming at these latitudes is inevitably higher.

Click on the image for a larger version. The observed temperature anomalies shown as a function of location around the Earth for four recent years. Notice the extreme warming at the highest latitudes in the Northern Hemisphere (Source: UEA CRU).

Once I had read this explanation it seemed completely obvious, and yet somehow I had neither figured it out myself nor knowingly read it almost 20 years of study!

One Response to “Why global warming affects the poles more than the equator”

  1. Article 400 – Decoding The Global Warming Book | Data Blends Says:

    […] reason the largest temperature changes are occurring at the poles most likely has to do with the ratio between atmospheric CO2 levels and the total water vapor in the air. The poles have less atmospheric water vapor, which causes exaggerated heating due to the […]

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