Radiation and Land Surface

Geographer Mathias Disney on the Sun’s spectrum, albedo and why plants and animals are sensitive to a very specific part of the overall spectrum

videos | November 20, 2020

I’m going to talk about the Earth’s radiation budget, in particular, how radiation interacts with the surface. One of the reasons why this has become a very important topic is because the radiation that arrives at the Earth’s surface drives our climate: so it’s about understanding how much radiation arrives at the surface, what the nature of that is; obviously, we’re talking about radiation arriving from the Sun, coming through the atmosphere and interacting with the surface, so that’s the oceans, the atmosphere (obviously, that’s not part of the surface, but the atmosphere captures radiation and moves it around), and then the interactions with the surface part, in particular the vegetation: forests, crops, the stuff that drives pretty much all life on Earth and all the resources that we rely on for food and for shelter. Obviously, in the longer term, looking back, solar interaction with the surface has provided us with resources like fossil fuel resources.

So understanding how radiation arriving from the Sun and interacting with the Earth system is a crucial part of climate and also the biogeochemical cycles, the cycles that underpin life on Earth, obviously, is very important to us.

We are part of that life on Earth, and as we all know, we’re having a significant impact on that through all sorts of things: through fossil fuel emissions, through land-use change, through intensive agriculture. So all of those things link back to understanding how much light arrives at the surface, what the properties of that light are, and how it interacts with the Earth system.

The surface of the Sun is at around 6,000 K, and it emits radiation across the ultraviolet into the visible part of the spectrum, into the shortwave infrared, and then the thermal infrared part of the spectrum and longer wavelengths. This is a spectrum, which means it’s a continuous distribution of radiation coming from the Sun. That distribution is described by what we call a ‘blackbody energy distribution’. Theoretically, what that means is we consider the Sun to be a blackbody; all the radiation that falls upon a blackbody is perfectly absorbed and then perfectly reemitted. The reason why we use that approximation is that it allows us essentially to calculate the amount of energy that’s being emitted in this distribution at every different wavelength along this spectrum, and then we sort of divide that spectrum up and give it names. So those names are arbitrary; it’s a continuous distribution, but we call different bits of it different things.

Let’s focus on one very important bit of it to us that we call the visible. The reason why we call it visible is because we can see it, so our eyes are sensitive to a very, very narrow part of the spectrum. If we consider radiation coming from the Sun to be distributed from about 0.2-0.3 microns (millionths of a metre) to many centimetre wavelengths, our eyes are sensitive to a tiny, tiny part of that, from about 0.4 to about 0.7 microns. So it’s a tiny fraction of this overall spectrum, but it’s a bit that we’re sensitive to.

One question that you ought to be asking yourself is why are we sensitive to that bit as a spectrum?

Why is it so important to us? Why have we evolved eye systems that are very sensitive to this tiny part of the spectrum? And it’s not just us, so photosynthesis is also driven by this tiny fraction of radiation in this very very long spectrum. So why just this part of the spectrum? The reason is that when we look at this energy distribution (it is described by something called the Planck function), it has a very, very sharp peak. That peak (not coincidentally) is right in the visible part of the spectrum, so the peak of solar radiation is this very, very narrow range of wavelengths. So when you look at the distribution of the Sun’s radiation overall, if you’re going to evolve a system to be sensitive to a chunk of that spectrum, one chunk to be looking at is the visible part because there is this massive peak of energy there. So, if we sum this up, basically about 40% of the total solar energy is crammed into this 0.4-0.7 micron little tiny slice. Yes, 60% of the sun’s energy is elsewhere, but it’s spread over a much much wider range of wavelengths, a huge range of wavelengths.

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What that means is that there is this very dramatic peak in the solar energy in that part of the spectrum, so it’s not so surprising that evolution has driven optical systems in mammals and photosynthetic mechanisms in plants, chlorophyll. Chlorophyll is sensitive to just this part of the spectrum. What is perhaps less obvious is that the radiation arriving at the top of the atmosphere has this distribution: that’s not really the important bit; the important bit is the bit that hits the bottom here. The atmosphere is full of gases and water vapour and so what the atmosphere does is it tends to absorb and scatter radiation. That absorption is crucial for a couple of reasons. One of the reasons is that the absorption of it keeps the radiation, the warmth; the sunlight that passes through heats up the surface, some of it is absorbed, that heat radiation is then reemitted at long-wave parts of the spectrum, and the atmosphere acts like a blanket keeping that in. That’s the greenhouse effect. Without that greenhouse effect, that radiation would escape, and the Earth’s surface would be completely frozen. So the thing is, how much radiation gets through the atmosphere? What happens to it on the way down?

Sunlight can interact with clouds (water vapour), it can interact with other gases, oxygen, ozone, carbon dioxide, carbon monoxide, methane and various other species and dust and soot as well. How much of that radiation gets through depends on what the atmosphere is made of. There are parts of the spectrum that we call atmospheric windows: what that means is the atmosphere is largely transparent to radiation coming through at those wavelengths. There are large parts of the Sun’s spectrum that arrive at the top of the atmosphere that don’t get through because it’s absorbed by gases and water vapour in the atmosphere.

So it just turns out that there’s a really narrow window in the spectrum right around where the peak of the Sun’s radiation is. That’s fortunate for two reasons.

The Sun’s radiation peaks at 0.4-0.7 microns, and at 0.4-0.7 microns there’s a hole in the atmosphere which allows that radiation through to hit the surface, to be absorbed by plants, to warm up the surface, to evaporate water and to heat the bottom of the atmosphere and drive the whole climate.

So, this combination of a peak in the Sun’s radiation and the atmospheric window then determines what we see here at the bottom of the atmosphere. So we look out the window on a sunny day, and we see that the sky is blue or it’s cloudy; that radiation has to have got through the atmosphere.

So anything that’s going to be evolved to survive on the Earth’s surface has to be sensitive to those things. You go very slightly shorter wavelengths to the ultraviolet, things that our eyes are not sensitive to, but ultraviolet radiation has higher energy; that radiation is potentially very damaging to the tissues of plants and animals; it’s what gives us skin cancer. So you only need to shift to shorter wavelengths from the visible, and you get to the ultraviolet. What protects us there is the ozone in the atmosphere that absorbs most of the ultraviolet radiation coming from the Sun – or it did until we realized that the CFCs, chlorofluorocarbons that we put in the atmosphere, was generating an ozone hole, it was decomposing the ozone and allowing more UV radiation through. So, the composition of the atmosphere, the distribution of the Sun’s radiation, determines entirely what we see here at the bottom of the atmosphere.

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Then the question is, what happens to the radiation at the surface? There are a number of things that can happen. The sunlight can interact with the surface and be absorbed. If it’s absorbed, then the radiation warms things up; if it warms things up, those things give off that radiation as heat energy, and that changes the heat balance of the surface; it changes the heat balance of the atmosphere. The other thing that can happen is that radiation can be reflected. The reflectivity, or what we call the albedo of the surface, is essentially the proportion of that incoming radiation that just bounces back. You can imagine that there are two extremes there: if you have a very black surface, it will absorb everything, and if you have a mirror, it will reflect everything back. Natural surfaces tend to be somewhere in between the two. The brightest things around are things like snow and ice, so they reflect a lot of radiation back into the atmosphere, which is why they stay cold: the fact that they’re so white prevents them from heating up, whereas surfaces that are dark (vegetation is one of those surfaces) absorb a lot of radiation. Vegetation uses it for photosynthesis, but it also heats up. So this balance between the albedo of bright and dark surfaces and the types of the surface has this intimate link with incoming solar radiation, heating up the atmosphere, and the drivers of our climate more generally, as well as the biogeochemical cycles, the life cycle part of the Earth system.

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Professor of Geography, University College London
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