Gaia's Five Daughters: the Factors Affecting Earth's Temperatures

A test of your knowledge of climate science.

How knowledgeable are the people who talk about climate? On the one side, there are many people whom we may call “believers” or “activists” who don’t seem to think that it is important for them to know anything about the mechanisms of global warming. If pressed, they may say something like “Scientists say so,” and sometimes, “In Valencia, it rained a lot.”

On the other side of the debate, the skeptics, aka “deniers,” often know something more, but normally only about some narrow sectors of climate science — so narrow to be totally irrelevant. For instance, whether some Alpine mountain paths were or were not open in Medieval times. Others are a little less focused on irrelevant points but still tend to consider only some specific sides of the multi-faceted (actually, hyper-faceted) entity that we call “climate.” They may tell you, for instance, that “CO2 concentrations trailed temperature during the Pleistocene” — which is true, but just as irrelevant as how many of Hannibal’s elephants managed to cross the Alps during the Punic wars.

Are you a believer or a denier? It doesn’t matter: if you are interested in climate, I propose a test to evaluate your understanding of climate science. It consists of answering the question, “Can you list five factors that directly affect Earth’s surface temperatures?”

Before going on, try to answer the question yourself. Then, we may discuss the results.

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Have you worked on the questions? Now, let’s see some possible answers,

1. Solar Irradiation. This is an obvious factor: the fact that Earth has a temperature higher than the average one of the universe (2.7 K (−270.4 °C)) is because it is irradiated by the Sun. Note that the Sun is a huge ball of plasma: it does change a little its brightness, but that’s a minor factor in affecting Earth’s climate. Mostly, the effects are due to the Earth's exposing or not exposing some parts of its surface to the sun as a result of orbital perturbations. Then, there are long-term effects: one is that solar irradiation increases by about 8% every billion years. That will eventually sterilize Earth, but we don’t have to worry about that for now.

2. Albedo. It is defined as the ratio of reflected to absorbed temperature. If Earth had an albedo=1, it would reflect all the radiation it receives, and its temperature would be around -240°C, the result of internal heat only. The current average albedo of Earth is about 0.3, meaning it reflects approximately 30% of incoming sunlight, contributing to an average surface temperature around 15 °C (288 K). Albedo is an important factor in climate change since it depends on such factors as cloud cover, atmospheric particulate, vegetation type, and more.

3. Radiative forcing. Also called the “Greenhouse Effect” (not a good name). It has to do with the atmospheric capture of heat emitted from the surface. This heat is partly re-transferred to lower layers and it causes a heating of the surface.

4. The Hydrological Cycle. Liquid water is continuously evaporated in the form of water vapor from the planet's surface to condense into water droplets (clouds). This movement only transfers heat from a low height to a higher height, but it affects the planet’s temperature because the heat released by water condensation has a shorter path to reach the outer atmosphere, where it is emitted into space. In more technical terms, you can say that a moist atmosphere has a lower temperature lapse rate.

5. Heat Transfer by Atmospheric Cells and Oceanic currents. (aka Horizontal thermal advection). There exist three major cells in Earth’s atmosphere: Hadley cells, Ferrel cells, and Polar cells. These cells transfer heat from low latitudes (toward the Equator) to high latitudes (toward the poles). The cells do not directly cool the Earth, but they move heat from high-moisture regions to low-moisture regions. Heat escapes into space more easily from these regions because the atmosphere is drier in absolute terms, so the lapse rate is steeper. The same effect occurs with the oceanic “thermohaline” currents.

And that’s my take. Note that the matter is complex, and there may be more than five answers or different answers — these answers are not supposed to be the absolute truth, nor is the test supposed to shame anyone. The idea is to stimulate a discussion and learn something.

So, you may have thought of volcanoes as factors that affect Earth’s temperature. They do, but not directly. The contribution of heat from a volcano is insignificant; volcanoes affect temperatures indirectly by generating greenhouse gases or atmospheric particulate, which in turn affect the greenhouse parameters and the albedo. The same is true for clouds, which you may have listed among the answers. Clouds, in themselves, do not affect temperatures, but they do affect the albedo and they do generate a greenhouse effect by blocking infrared emission to space. There are many other effects, for instance cosmic rays, that were proposed by Svensmark as indirectly affecting Earth’s temperature by causing cloud nucleation. Similar considerations hold for James Lovelock’s proposal that Earth’s temperature was regulated by DMS (Dimethyl Sulfide) emissions. That, too, didn’t find confirmation; if it exists it is a minor effect.

I have no statistical data to tell us what people understand about climate science. So, if you let me know in the comments how you fared with the test, I would appreciate it. I can tell you that I tried the test with my students; undergraduates with a solid knowledge of physics and chemistry, but no specific exposure to climate science. The results were normally dismaying. At best, they knew something about albedo (vaguely) but couldn’t describe in an intelligible way how the greenhouse effect works, even though it was described for the first time by a chemist, Svante Arrhenius. Testing in informal situations with climate “activists” didn’t provide better results.

So, it may well be that the debate is carried on with the same degree of knowledge and understanding as we could imagine it for fish discussing bicycling. Climate Science is not an easy matter that can be solved using political slogans (“CO2 is plant food”). It is so complicated that even the goddess Gaia herself may get confused sometimes. That has to be the reason why we have mass extinctions every once in a while.