Let’s face the truth about climate change mitigation (I)

This post will be the first of a series, aiming at giving you a fair idea of what would be needed to effectively mitigate climate change. Let me first emphasize our core belief: global warming and the resultant climate change are anthropogenic, meaning human-induced. Some might argue that no, everything is a question of natural cycles, cosmic rays, God’s hand and who knows. For those, I’d advise some easy reading: the “Start here” page on a blog written by climatologists. But I’ll try to give you a brief description of the phenomena. The sources I use are wikipedia and the IPCC working group I report, 2007.

Sun radiates electromagnetic waves in a variety of wavelengths. For example, it radiates visible light, but also infrared and ultraviolet. Quantum mechanics predict that the exact distribution of the emitted radiation is given according to the sun’s temperature (5800 K), following a mathematical formula called Planck’s law. In reality, the black-body is just a model and the real spectra is slightly different. The figure on the left shows the solar radiation spectrum as seen from space (in yellow) and from the surface of the Earth (in red). The black body approximation is also shown, and one can notice that it fits well to the yellow curve.

So when the sunlight arrives on Earth, some of the energy is reflected to space because the albedo of the Earth (albedo is comprised between 1 -total reflection of the incoming light- and 0 -complete absorption. Earth’s is now 0.31, but bound to decrease because of the polar caps melting), while some is transmitted through the atmosphere. The different components of our atmosphere fill absorb some energy in different frequencies, which explains that when you look at the red spectrum, measured at sea level, you do not have the same pattern as before.

The light then hits the ground and gives some energy to it, thereby decreasing its frequency (an electromagnetic wave with high energy has a high frequency, ie a short wavelength). It is then reflected to the atmosphere. A fraction will be emitted in space, but the major part of this reflected energy will be trapped and absorbed by the atmosphere and converted in heat. You will ask why this did not occur earlier, when the light went through the atmosphere the first time: that is because then it had a high energy content, with high frequencies! And molecules like CO2 are really effective at absorbing electromagnetic waves of low frequencies, rather than high ones.

So you get what is called the greenhouse effect. Without atmosphere, the temperature on Earth would be around -19 degrees Celsius, and of course no life could ever have developed in these conditions…

The concentration of CO2 in the atmosphere has increased during the last 150 years, because of human activity. Of course, the CO2 concentration follows cycles; sometimes it goes up, sometimes it goes down. The following figure shows the evolution of temperature and CO2 concentration during the last 450,000 years.

What is striking is that temperature is strongly correlated to the concentration of CO2. Moreover, an increase in CO2 leads to an increase in temperature through radiative forcing, and a feedback exists: an increase in temperature leads for example to higher evaporation of water, which is by far the largest greenhouse gas present in the atmosphere, so in turn higher temperature, etc… Until a new equilibrium is reached.

There are several such positive feedbacks, although for us living species their effects might be considered as negative. Since it takes some time for the CO2 recently released in the atmosphere to have an effect, the worst is yet to come. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. Indeed, and as can be seen on the graph, the atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores. There is not only CO2 as greenhouse gas; the most common ones, besides carbon dioxide, are CH4 and N2O. Scientists have tried to put some “equivalent global warming potentials” (GWP) on these other molecules, compared to CO2. For example, CH4 has a GWP of 21, meaning that it is potentially 21 times more effective than CO2. And guess how much is GWP of N2O? 310.

Now, you mqybe wonder why the global atmospheric concentration of CO2 has increased recently. But you know the answer. Our society is based on the consumption of fossil fuels. Fossil fuels (coal, oil, gas) are remainings from carbon wastes such as plants and living organisms like animals, which died millions of years ago and progressively decayed and decomposed in the soil. Under certain temperature and pressure conditions, time helping, these wastes are transformed into fossil fuels. Indeed, they are composed of molecules based of carbon. Burning them in presence of oxygen leads to formation of CO2 and water (H2O), among other substances. And that’s where the extra atmospheric CO2 comes from.

This introduction was just a short explanation of the basic physical principles behind the greenhouse effect and its extension to anthropogenic global warming (AGW). My next post will expose the solutions the scientific community recommends for an “effective” mitigation of climate change. I put it between brackets, because it is not possible to stop AGW so easily. CO2 has a half-life of 12 years in the atmosphere, N2O of 120; that means that it will take maybe centuries for natural processes to come back to our original situation, just 150 years ago. Yeah, isn’t it powerful, destroy a 12,000 years long climatic equilibrium in 150 years?

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