“As a dam built across a river causes a
local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a
local heightening of the temperature at the Earth’s surface.” Thus in 1862 John Tyndall described the key to
climate change. He had discovered in his laboratory that certain gases, including water vapor and carbon dioxide
(CO2), are opaque to heat rays. He understood that such gases high in the air help keep our planet warm by
interfering with escaping radiation.
This kind of intuitive physical reasoning had already appeared in the earliest speculations on how atmospheric
composition could affect climate. It was in the 1820s that Joseph Fourier first explained that the Earth’s
atmosphere retains heat radiation. He had asked himself a
deceptively simple question, of a sort that physics theory was just then beginning to learn how to attack: what
determines the average temperature of a planet like the Earth? When light from the Sun strikes the Earth’s
surface and warms it up, why doesn’t the planet keep heating up until it is as hot as the Sun itself? Fourier’s
answer was that the heated surface emits invisible infrared radiation, which carries the heat energy away into
space. But when he calculated the effect with his new theoretical tools, he got a temperature well below freezing,
much colder than the actual Earth.
The difference, Fourier recognized, was due to the Earth’s atmosphere. Somehow it kept part of the heat
radiation in. He tried to explain this by comparing the Earth with its covering of air to a box with a glass
cover. That was a well-known experiment — the box’s interior warms up when sunlight enters while the heat cannot
escape. This was an over-simple explanation, for it is quite different physics that keeps heat inside an actual
glass box, or similarly in a greenhouse. (The main effect of the glass is to keep the air, heated by contact with
sun-warmed surfaces, from wafting away, although the glass does also keep heat radiation from escaping.)
Nevertheless, trapping of heat by the atmosphere eventually came to be called “the greenhouse effect.”
Early attempts at explanation
Not until the mid-20th century would scientists fully grasp, and calculate with some precision, just how the
effect works. A rough explanation goes like this. Visible sunlight penetrates easily through the air and warms
the Earth’s surface. When the surface emits invisible infrared heat radiation, this radiation easily passes
through oxygen and nitrogen, the main gases of the air. But as Tyndall found, even a trace of CO2, no more than
it took to fill a bottle in his laboratory, is almost opaque to heat radiation. Thus a good part of the radiation
that rises from the surface is absorbed by CO2 in the middle levels of the atmosphere. Its energy transfers into
the air itself rather than escaping directly into space. Not only is the air thus warmed, but also some of the
energy trapped there is radiated back to the surface, warming it further.
That’s a shorthand way of explaining the greenhouse effect — seeing it from below, from “inside” the
atmosphere. Unfortunately, shorthand arguments can be misleading if you push them too far. Fourier, Tyndall and
most other scientists for nearly a century used this approach, looking at warming from ground level, so to speak,
asking about the radiation that reaches and leaves the surface of the Earth. So they tended to think of the
atmosphere overhead as a unit, as if it were a single sheet of glass. (Thus the “greenhouse” analogy.) But this
is not how global warming actually works, if you look at the process in detail.
The modern view
What happens to infrared radiation emitted by the Earth’s surface? As it moves up layer by layer through the
atmosphere, some is stopped in each layer. (To be specific: a molecule of carbon dioxide, water vapor or some
other greenhouse gas absorbs a bit of energy from the radiation. The molecule may radiate the energy back out
again in a random direction. Or it may transfer the energy into velocity in collisions with other air molecules,
so that the layer of air where it sits gets warmer.) The layer of air radiates some of the energy it has absorbed
back toward the ground, and some upwards to higher layers. As you go higher, the atmosphere gets thinner and
colder. Eventually the energy reaches a layer so thin that the radiation can escape into space.
What happens if we add more carbon dioxide? In the layers so high and thin that much of the heat radiation from
lower down slips through, adding more greenhouse gas means the layer will absorb more of the rays. So the place
from which most of the heat energy finally leaves the Earth will shift to even higher layers. Those are colder
layers, so they do not radiate heat as well. The planet as a whole is therefore now taking in more energy than it
radiates (which is in fact our current situation).
As the higher levels radiate some of the excess downwards, all the lower levels down to the surface warm up.
The imbalance must continue until the high levels get warmer and radiate out more energy. As in Tyndall’s analogy
of a dam on a river, the barrier thrown across the outgoing radiation forces the levels below to accumulate heat
(ie., their temperature to rise) until there is enough radiation pushing out to balance what the Sun sends in.
While that may sound fairly simple once it’s explained, the process is not obvious if you have started by thinking
of the atmosphere from below as a single slab.
The non-issue of “saturation”
The correct way of thinking eluded nearly all scientists for more than a century after Fourier. Physicists
learned only gradually how to describe the greenhouse effect. To do so, they had to make detailed calculations of
a variety of processes in each layer of the atmosphere. Early on, most experts stuck by the old objection to the
greenhouse theory of climate change — in the parts of the spectrum where infrared absorption took place, the CO2
plus the water vapor that were already in the atmosphere sufficed to block all the radiation that could be blocked.
In this “saturated” condition, raising the level of the gas could not change anything. But this argument was
falling into doubt. The discovery of quantum mechanics in the 1920s had opened the way to an accurate theory for
the details of how absorption took place, and precise laboratory measurements studies during WWII and after
confirmed a new outlook. In the frigid and rarified upper atmosphere where the crucial infrared absorption takes
place, the nature of the absorption is different from what scientists had assumed from the old sea-level
Take a single molecule of CO2 or H2O. It will absorb
light only in a set of specific wavelengths, which show up as thin dark lines in a spectrum. In a gas at sea-level
temperature and pressure, the countless molecules colliding with one another at different velocities each absorb
at slightly different wavelengths, so the lines are broadened considerably. With the primitive infrared instruments
available earlier in the 20th century, scientists saw the absorption smeared out into wide bands. And they had no
theory to suggest anything else.
A modern spectrograph, however, shows a set of peaks and valleys superimposed on each band, even at sea-level
pressure. In cold air at low pressure, each band resolves into a cluster of sharply defined lines, like a picket
fence. There are gaps between the H2O lines where radiation can get through unless blocked by CO2 lines. That
showed up clearly in data compiled for the U.S. Air Force, drawing the attention of researchers to the details of
the absorption, especially at high altitudes. Moreover, researchers working for the Air Force had become acutely
aware of how very dry the air gets at upper altitudes — indeed the stratosphere has scarcely any water vapor at all.
By contrast, CO2 is fairly well mixed all through the atmosphere, so as you look higher it becomes relatively
What if water vapor did entirely block any radiation that could have been absorbed by adding CO2 in the lower
layers of the atmosphere? It was still possible for CO2 to make a difference in the thin, cold upper layers. With
the new absorption data in hand, Lewis D. Kaplan ground through some extensive numerical computations. In 1952, he
showed that in the upper atmosphere the saturation of CO2 lines should be weak. Thus adding more of the gas would
certainly change the overall balance and temperature structure of the atmosphere.
Neither Kaplan nor anyone else of the time was thinking clearly enough about the greenhouse effect to point out
that it will operate regardless of the details of the absorption. The trick, again, was to follow how the
radiation passed up layer by layer. Consider a layer of the atmosphere so high and thin that heat radiation from
lower down would slip through. Add more gas, and the layer would absorb some of the rays. Therefore the place from
which heat energy finally left the Earth would shift to a higher layer. That would be a colder layer, unable to
radiate heat so efficiently. The imbalance would cause all the lower levels to get warmer, until the high levels
became hot enough to radiate as much energy back out as the planet received. Adding carbon dioxide will make for
a stronger greenhouse effect regardless of saturation in the lower atmosphere. (And actually, there is no
saturation. The primitive infrared techniques of the laboratory measurements made at the turn of the century had
given a misleading result. Studies from the 1940s on have shown that there is not nearly enough CO2 in the
atmosphere to block most of the infrared radiation in the bands of the spectrum where the gas absorbs it,
particularly in deserts where the air is extremely dry.)
This view is still a simplification: in the mid-60s scientists began to realize that a significant amount of
energy leaves the surface not as radiation but through convection (in the rising of warm air) and as latent
energy in the form of water vapor (for example in the columns of humid air that climb into thunderclouds). The
energy eventually reaches thin levels near the top of the atmosphere, and is radiated out into space from there,
and it is in the modulation of that radiation that the greenhouse gases play their starring role.
(This page is a brief adaptation from Spencer Weart’s superb
Discovery of Global Warming web-site,
specifically here and