On the right is the spectrum of the Sun's radiation as it arrives on Earth. You can see it has a peak right in the visible range. That's not a coincidence of course, it simply means that the Sun's radiation hasn't changed much in 4 billion years, and creatures with eyes have evolved to use the most abundant radiation in their environment.

The chunks missing from the spectrum - the yellow bits - are absorption bands, where various gases in the air are heated by specific wavelengths that react with those photons. You can see the important absorption of UV around 250 nm by stratospheric ozone on the left.

The great majority of sunlight is in the visible and UV. Notice how nitrogen & oxygen, the main components of the atmosphere, have no effect on sunlight.

Once you examine the way energy comes and goes on Earth, you'll discover some very interesting things.

For example, if the Earth returned from its surface exactly the quantity of radiation it receives from the Sun, straight to space, the mean surface temperature would be -18℃, and the planet would be frozen and lifeless. In fact, the temperature is +15℃ - a big difference. If it were not for this warming, there'd be no liquid water here, and no life.

What's going on?

In this short video, based on the diagram opposite, I explain that the atmosphere acts as a radiation source close to Earth's surface. It does this by capturing most of the outgoing I-R radiation and releasing it again, so that there's a big transfer of energy between the surface and lower atmosphere, just as if the world were covered by a blanket.

Earth doesn't radiate from its surface at all, but from the top of its atmosphere, keeping the bottom warm enough for life to flourish.

How Earth stays warm: the atmospheric greenhouse

This positive energy balance (about 1.0 W/m2) is the whole cause of the warming in the climate system.

It doesn't look like very much ... it's about 0.2% of Earth's energy budget - but that's still 250 trillion Watts. It is the equivalent of adding the energy of 800,000 Hiroshima bombs to the surface systems every day.

If it were sustained long enough, in about a half a million years, the oceans would all boil away.

The fact is, Earth is normally close to energy balance. It doesn't take a very big perturbation, especially if it comes fast, to cause large effects.

How does the greenhouse effect work?

Here's a photograph taken on a dark night, using a camera that can "see" infra-red. Notice that, apart from there being no colours, the lanscape looks pretty normal ... everything in it - trees, grass, dirt, water, sky, clouds - are all visible in much the same way they would be under reflected sunlight.

They can't be reflecting any light, because there isn't any, so they must be emitting it. The picture shows Earth's I-R radiation as it happens - from every surface, all the time, day and night.

Here is Earth's radiation spectrum compared with the Sun's.

The peak of the solar spectrum occurs at about 500nm; peak of the Earth's at about 10,000 nm, in the middle I-R.

The blue bit of the graph is the part of Earth's radiation that escapes straight through to space - all the rest, at least 80% - is absorbed by greenhouse gases - mostly CO2 & water vapour.

Whereas the Sun's radiation passes down through the atmosphere almost unhindered by the gas molecules it encounters, Earth radiates much longer wavelengths (nearly all I-R), and this does interact with the gases. Not the nitrogen & oxygen, but only some of the trace gases - water vapour, CO2, methane, nitrous oxide, ozone, and some others. The first two have by far the most effect ... 80-90% of Earth radiation gets absorbed by these molecules before it reaches space. What actually happens is the energy photon hits a CO2 molecule (or one of the others), and is captured. The now energised molecule vibrates or rotates faster, and it jumps around faster. The photon only stays with its host a tiny fraction of a second, then it is released. It can travel in any direction, and is highly likely to collide with another molecule and get captured all over. Since most of this happens close to Earth's surface, where the gases are most abundant, more photons travel down toward the surface than escape upwards. It's this net downward re-radiation that warms the near-surface air, keeping the Earth at the temperature we're accustomed to.

Solar radiation passes through the atmosphere to the surface.

Earth's I-R radiation is absorbed by greenhouse gas molecules on its way upward, and re-radiated in all directions ... more near the surface. Photons work their way upward through this trap to be eventually sent to space from the cold top of the atmosphere.

This little video will explain in a bit more detail how the greenhouse gases do their stuff.

It's important to know that this is not a contested field of physics. If you look for "greenhouse effect" in a Google search, you'll be reading in no time how scientists have got the theory all wrong & how this or that blogger knows everything about it.

This is rubbish.

Greenhouse theory took a century to work out in detail, but it is now as solid and complete as anything can be. Claiming otherwise is just like saying germs don't cause disease.

If water vapour is the most abundant greenhouse gas, with the biggest effect, why is CO2 said to control the climate?

On the left you can see roughly the contribution of each of the four principal gases to greenhouse warming.

The large variation in the case of water vapour is because the amount of water vapour in the air is highly variable from place to place and time to time - typically 0-9% of the atmosphere.

Variation in the case of CO2 is because radiation from Earth's surface varies by latitude and region.

Furthermore, as scientists realised over 100 years ago, the greenhouse efficacy of CO2 is greatly amplified by the "water vapour feedback" ... if CO2 rises, the air gets warmer. That evaporates more water, and warm air holds more vapour. That in turn causes more warming; a warmer ocean releases more CO2 to the air, causing more warming ... and so it goes.

As well, CO2 is a "long-lived" component of the atmosphere. Even though any given molecule will move around from the air into the sea or land, or plants, and back; the only way it is naturally removed from these interacting reservoirs (and so effectively taken out of the air) works very slowly over something like 100,000 years.

Water vapour, on the other hand, stays in the atmosphere about 9 days. On average, about 500,000 km3 of water are evaporated every year, mostly from the ocean, but each water molecule makes the trip up to the air and back about 40 times in that year.

That's why water vapour is understood as a "feedback" - a powerful secondary cause of greenhouse warming, closely determined by the effects of the long-lived gases.

The turbulent troposphere

We've already seen how equatorial heating causes air to move toward the poles, creating big permanent circulation cells in the atmosphere. The greenhouse effect gives it vertical motion too.

The reason is that it is heated from below, just like a saucepan on a stove, and hot air must rise. That's why, as anyone who watches the weather knows, the lower atmosphere is never still. In fact, you can see the effect if you watch an eagle or a pelican rise on a thermal, or feel it if you ride in a glider.

This is the reason, too, why the troposphere (the bottom atmospheric layer) has a well-defined upper boundary, called the tropopause, separating it from the stratosphere above. At this boundary, there's a sharp thermal discontinuity.

From the ground to the tropopause, the air gets colder as you go up; above that, it gets warmer as you rise. The two layers are physically distinct as well. Nearly all the weather happens in the troposphere.

The height of the tropopause is about 17km at the equator; and about 9km at the poles. The boundary is so clear that scientists have been able to measure a small but significant increase in its altitude due to expansion of the troposphere caused by twentieth century warming - a precise confirmation of the reality of greenhouse warming because it can have no other cause.

At 10km altitude, typical air temperature is around -40°C. This the zone where most of Earth’s radiation loss takes place, where CO2 & water molecules heated from below emit I-R photons to space.

At 5 km typical temperature is about -8°C

Mean temperature at the surface is 15°C. The upside-down temperature profile of the troposphere is what keeps it always agitated.

Three ways you've already experienced the greenhouse effect

• Everyone knows humid nights are hot and sticky. It doesn't cool down on sultry nights because the extra water vapour strengthens the greenhouse effect.

• If you ever camped in the desert, you'll know that, no matter how hot it's been during the day, it cools down fast soon after the sun goes down. That's because there's little water vapour in the air and the greenhouse effect is weak. If you could experience the going down of the Sun on the Moon, where there's no atmosphere at all, you'd see the day time temperature of 100℃ or so drop to -150℃ in a matter of hours, as all the surface heat radiates straight to space.

• When you climb a mountain you know it will get colder as you ascend. When you think about it, you might expect to get warmer, because Earth is warmed from above by sunlight. The whole reason this doesn't happen is because greenhouse warming is greatest close to the surface. In the troposphere, where greenhouse warming rules, the normal temperature gradient is upside-down.

If you feel up to a bit more detail, use this link to an excellent review article by one of the world's experts on the greenhouse effect

Earth only receives a tiny part of the Sun's total output: 1,366 Watts per square metre [W/m2] of 63,000,000 W/m2, because we are 150 million kilometres away, and the Sun sends its heat and light in all directions. Not all the radiation reaching the top of Earth's atmosphere gets to warm the planet. About 30% reflects from clouds and haze in the air, or snow and ice on the surface; some more is absorbed in the atmosphere or scattered. The total available at the planet's surface, averaged over the whole sphere, day and night, is 239W/m2.

Everything that is hotter than its surroundings radiates energy ... that is to say, a stream of energy in the form of photons or waves (depending on how you look at them) is sent out from the body, until its surface is the same temperature as its environment. Hotter things send shorter wavelengths; cooler ones, longer wavelengths.

In this diagram, the shortest (most energetic) radiation is on the left; longer on the right. The rainbow in the middle is the piece of this spectrum that people can see, with the familiar colours at their respective wavelengths from 400 nm to 700. Notice that the bands next door to the visible are UV on the short side, and I-R on the long.

there wouldn't be any need to explain this, and we'd all understand the greenhouse effect as well as we understand reflection in a mirror.

But we can't see it, and we need to understand this because otherwise it's impossible to know what's happening to the climate system. So we'll begin at the beginning.

GET THE FULL-SIZE VIDEO AT YOUTUBE

If you've ever been puzzled by the question why a couple of degrees of warming is such a big deal (it's really about the energy imbalance), click here

GET THE FULL-SIZE VIDEO AT YOUTUBE

ABOUT THE GREENHOUSE EFFECT

If we could see infra-red (I-R)

If you want to read an excellent account of how the greenhouse theory was discovered and then developed to its present state, use this link to physicist Spencer Weart's site.