In the 1820s, French scientist Joseph Fourier was trying to understand how the Earth could maintain its observed average temperature. A simple calculation based on the amount of light received from the sun at Earth’s distance and the amount radiated back to space by the planet would suggest an equilibrium temperature of -18 degrees Celsius, much lower than what we actually observe. Fourier proposed that, like the panes of glass in a greenhouse, atmospheric gases such as water vapour and carbon dioxide could trap radiation from the sun, warming the planet. His was the first model of the well-known ‘greenhouse effect’, which we now know warms the planet by around 33 degrees to provide the pleasant ambient temperatures necessary for life.
Actually, although widely accepted, ‘greenhouse effect’ isn’t a very accurate name. In a greenhouse, the air is heated because the sun’s radiation can get in, warming first the surfaces inside. The walls and ceiling prevent convective cooling, causing the temperature to gradually rise. The way in which ‘greenhouse’ gasses keep Earth warm is fundamentally different and more complicated. Essentially, and somewhat counterintuitively, these gases lower the ‘effective temperature’ of the planet – that is, the temperature that an outside observer would measure for the Earth given the amount of radiation emitted from the top of the atmosphere. Less radiation leaving Earth means that more must have been trapped in the lower atmosphere – hence, for the same input from the sun, a lower effective temperature actually means a warmer surface for the planet.
What makes this picture more complicated is the wide variety of different ‘greenhouse’ gasses, all of which can change the effective temperature in different ways depending on how strongly they reflect incoming radiation from the sun – a cooling effect – and how effectively they trap radiation from Earth’s surface – a warming effect. Whilst radiation from the sun is predominantly short-wave visible light, that re-radiated by Earth is largely long-wave infrared light, which is of course why the ground doesn’t glow at night. Different gasses have different capacities to scatter, absorb and re-radiate these different types of light. Carbon dioxide affects incoming short-wave radiation hardly at all, so its effects are relative easy to predict. But clouds are a different matter, and this is what makes them the most difficult contributor towards the ‘greenhouse’ effect to model.
Everyone knows that clouds block incoming sunlight, and that a cloudy night tends to be warmer than a clear one because they also block heat from escaping Earth’s surface. But the strength of each of these effects depends not only on the size and shape of the cloud but also on its type and height. All clouds of a given size will tend to absorb long-wave radiation from Earth and re-radiate it in all directions by a similar amount. But low clouds tend to be thicker than high ones, and are much more effective at reflecting solar short-wave radiation. On its day side, therefore, Earth’s warming is greater if there are more high clouds relative to low ones. High clouds are also colder than low clouds, which are almost at the same temperature as the surface. So during the night, an observer would see the same ‘effective temperature’ for a part of the planet covered with low cloud as they would for a region with no cloud at all. Although the ground will be kept warm over the course of the night – there’s seldom a frost under low cloud cover – the cloud itself is radiating energy out to space, so the planet as a whole doesn’t warm. High clouds, meanwhile, would show up as cold spots, meaning less radiation is leaving the planet and more heat must have been trapped on Earth.
Hence, low clouds tend to exert an overall cooling effect on the planet whilst high clouds tend to warm it. Since most clouds are too small to explicitly resolve in global models of the climate, this makes it very difficult to predict what the cloud contribution to global warming will be in the future. We also don’t know how the climate change spurred on by man-made greenhouse gasses will alter the abundance of different types of cloud, since their formation depends on the biology as well as the temperature of the land and sea surfaces. If climate change encourages more high clouds to form, there’s likely to be a ‘positive’ feedback whereby clouds amplify the warming effect, unless this is counteracted by increased amounts of low cloud. If last winter and this spring’s record-breaking sunny skies are anything to go by, we could see a decline in both types. What’s clear, though, is that both the number and effect of clouds in the future is very difficult to predict. We’ve moved on in our understanding from Fourier’s original model. But when it comes to actually predicting his ‘greenhouse’ effect into the future, modern science remains far from achieving definitive answers.