Windows & Warming
The "Greenhouse" Effect ...
After a talk I gave to a U3A group in Cromwell last week, one of the audience asked why we describe Climate Change as a “Greenhouse Effect”. The short answer is that the glass in a greenhouse acts a bit like the Earth’s atmosphere. It allows most of the incoming sunlight to pass through it, but blocks most of the outgoing infrared radiation from escaping. As a result, it’s much warmer inside than out.
It’s an interesting story, so I’ll try to explain it in a bit more detail. Two helpful explanatory pictures can be found in the excellent book by Jose Piexoto and Abraham Oort, The Physics of Climate, published in 1992. They’re on pages 92 and 93, and slightly modified versions of them are reproduced below.
The first shows the spectrum of incoming sunlight - over the wavelength range to 3 µm (microns) - and compares it with that reaching Earth’s surface for overhead sun under clear skies. That second spectrum is lower at all wavelengths because of scattering losses when the radiation bounces off air molecules (it’s called Rayleigh scattering) which become more and more important at shorter and shorter wavelengths. The sharper remaining shaded areas are absorptions by gases that impede sunlight from getting through the atmosphere to the surface. For visible light (wavelengths 0.4 to 0.7 microns with colours highlighted), the atmosphere is virtually transparent (luckily, because otherwise there would be little for our eyes to see). Because of its transparency at visible wavelengths, we call that range - returning to the greenhouse analogy - an atmospheric “window”. The only significant absorption there is a slight reduction due to ozone.
Ozone absorption becomes much more important at UVB wavelengths, where it absorbs virtually all the available sunlight. That tiny piece circled in red is what’s important for skin cancer. There’s not much of it, but each photon at those wavelengths packs a big punch. Big enough to damage our DNA, the building blocks of life. At longer wavelengths, beyond the visible in the infrared region, absorption by water vapour (H2O), and carbon dioxide (CO2) start to become important. At Lauder we use the depth of those absorptions to measure the amount of each gas in the atmosphere.
The second plot - the multi-panel one below - shows how the balance between incoming solar radiation and outgoing heat radiation is affected by the concentrations of greenhouse gases like CO2.
The upper panel compares the incoming solar spectrum (at left) with the outgoing radiation emitted by the Earth’s surface (at right). According to a well-known law of physics, the radiation distribution depends on the temperature of the emitting surface and varies with wavelength as shown. The effective surface of the Sun has temperature of 6000K, and to balance that the effective temperature of Earth’s surface without any greenhouse effect would be about 255K (that’s about -18°C). Notice that the x-axis scale is not linear, so the sunlight curve is distorted compared with that shown in the first figure. The logarithmic scale squashes longer wavelengths to allow an easier comparison between incoming and outgoing energy streams. There’s little overlap between them. The only significant region of overlap is at wavelengths near 5 microns.
The next two panels (labelled b and c) show the overall gaseous absorption from space down to ground level, and down to an altitude of 11 km - the approximate boundary between the troposphere and the stratosphere. The atmospheric window in the visible can be clearly seen as the large white area at wavelengths around 0.5 microns. There’s a smaller atmospheric window between 10 and 12 microns, and it’s the slight closing of that window as trace gases increase that causes the problem.
The remaining panels show separately the absorption contribution of each of the major greenhouse gases, including water. It’s clear that the variable amount of water in the atmosphere is very important. It exerts a strong positive feedback to any induced warming by the other greenhouse gases because warmer air can hold more water vapour.
Without those greenhouse gases, the infrared atmospheric window would be more transparent, allowing more energy to escape to space. Earth’s mean temperature would be about 255K (-18°C), and the whole planet would be covered by ice. Their trapping of outgoing radiation is a major contributor to increasing the mean temperature by around 30°C to the much more comfortable levels we currently enjoy.
The problem is that those concentrations are currently increasing at the rate of about 3 percent per decade, and that increases their radiation trapping. The infra-red escape window is closing. Because of that, temperatures have already increased by around 1.3°C since pre-industrial times (as I showed in my last post), and continue to increase rapidly.
Thanks for reading this. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack. Click below to subscribe for occasional free update