Look at this picture of our own planet, taken out the window of the Apollo 17 spacecraft as it was returning in 1972. Only two dozen people ever saw this; none have seen it since. These few, and the few more who’ve beheld the planet from high in Earth orbit always report something the rest of us can only imagine - its singular beauty and fragility - and its utter, awful, infinite loneliness in the vast blackness that no picture can show.

“ My view of our planet was a glimpse of divinity”, said Edgar Mitchell.

“Suddenly, from behind the rim of the moon, in long, slow-motion moments of immense majesty, there emerges a sparkling blue and white jewel, a light, delicate sky-blue sphere laced with slowly swirling veils of white, rising gradually like a small pearl in a thick sea of black mystery.”

Aleksei Leonov, a Russian astronaut said,

“The Earth was small, light blue, and so touchingly alone, our home that must be defended like a holy relic.”

They don’t send poets into space, but it looks as if space made poets out of them.

The thing is, to understand what climate change means, we all have to find a little bit of this way of looking at Earth, our home. That’s where we’ll start.

How we know about the climate problem ...

a brief & easy introduction to climate science

The next page deals with the single most important, characteristic and controlling feature of Earth's climate system: the atmospheric GREENHOUSE EFFECT

ABOUT THE EARTH

Our planet is a rocky sphere 12,750km in diameter at the equator, orbiting the sun at a mean distance of about 150,000,000 km. It is not alone: its little twin, the moon lies at a mere 384,000 km distance, bound to us by its own Earth orbit. Now it’s often said that Earth, the third planet out from the sun, happens to be at just the right distance - neither too hot nor too cold - to sustain life as we know it; and this is true, but not the whole truth. Consider the moon, which is the same distance from the sun: mean day-time temperature there is 107℃ (it can get to 123℃); mean at night is -153℃ & it can go down to -233℃. There’s no life on the Moon, and almost certainly there would be none here if conditions were the same. What makes Earth different?

1. Geological activity. Earth is hot inside - very hot, enough to hold its interior in a fluid or semi-fluid state - and that means the surface crust has always been changing. Crustal material is being renewed by stuff from deep inside & recycled; surface forms like mountains are being created, and the big crustal plates move around, changing the disposition of water and dry land. All this has vast consequences.


2. The atmosphere. Solid Earth is clothed by an atmosphere of gases clinging to its surface, continually in motion. Because gases weigh something, they are most dense near the surface, where we live; because it has no lid, the atmosphere doesn’t cease at any height, but gradually fades into space. Almost all of it is below 100km; about half is below 5.6km. If it were all at the same pressure as sea level, it would rise no higher than the highest mountains - in fact about 300 metres lower than the peak of Everest. You can imagine its thickness by thinking of a coat of varnish on a school-room globe. But thin though it is, this veil of gases is the main reason Earth is not fried and frozen every day like the moon. It has a mean surface temperature of 14℃ (that is 33℃ warmer than it would be without the atmosphere) and is equable in respect of both the diurnal and seasonal contrasts.


3. Water. About 1.386 billion cubic kilometres of water exist on Earth, some of it as ice, some as vapour, most as the familiar liquid. If this much were made into a sphere, it would be about 1,400km in diameter. If it were spread evenly over the surface of a smooth Earth, it would be about 2.7km deep. But of course it’s not spread like that. 97% is in the oceans which cover three quarters of the surface; of the 3% that’s fresh, two-thirds is ice; and of what’s left, most is in the ground. The part we use the most - the water in rivers is just 0.0002% of the total. Nevertheless, these three factors are what makes possible the fourth.


4. Life. Strange to say, although life could not exist without these three planetary properties, once the biosphere was created, it began to profoundly affect them. The situation can be thought of as a self-regulating complex system with multiple bi-directional feedbacks - that is to say, living things both change and are changed by processes that arise in the systems of the non-living Earth. The more we learn about the history of the Earth and the way everything works, the more profoundly true this seems to be.

5. Modern human activity. Because we all live in the midst of it, we can't easily appreciate how very unusual our time is. Volcanoes have always been the main way the solid Earth recycles carbon dioxide to the air - but human activity now puts more than 100 times as much up there as all the world's volcanoes do. Normal processes of erosion have always sent sediment from mountains and plains down rivers to the sea. But now, human activity - mining, civil works, and building, contributes about 24 times as much as all natural erosion. Human beings plus our domesticated mammals compose 94% of all mammalian biomass on Earth. These astounding facts are even more remarkable when you consider they have mostly come about in the last century.

Only scientists could have discovered the climate problem. We would have noticed something sooner or later, but way too late to do anything about it. You need to see things the way scientists do, to know what's going on. That's what the next couple of pages are for.

ABOUT CLIMATE

The Earth would have no climate at all but for just three circumstances:

the atmosphere,

its spherical shape,

and its axial rotation.

Because it’s a sphere with its equatorial zone facing the sun, this region is always hotter than the rest; because Earth spins (so any point on the surface moves from west to east), the atmosphere is dragged a bit, causing permanent systems of winds that move air away from the equator and westward, opposite to the rotation.

This diagram gives a rough idea what happens as the atmosphere is divided into what climatologists & meteorologists call “cells” by the combined effect of equatorial heat moving toward the poles, and the twisting effect of Earth’s rotation.

But that’s not all. The oceans are also fluid, and although they take up and move heat more slowly than the air, their heat capacity is so much greater, and they are so vast that they act as a kind of heat reservoir in the long term. Heat from the equatorial oceans is set in motion too and moves via vast global currents toward the polar seas. The shape and behaviour of those great currents and “gyres” is set by the physical structure of the ocean basins, and has changed in the past according to the drift of the continents and other more subtle factors. Heat born by the surface ocean waters is readily exchanged with the atmosphere, so winds and currents are constantly interacting.

So climate is something that happens because heat moves around the Earth, conveyed by its air and water (usually called ‘convection’). Purely by habit, we call this story ‘climate’ if it’s about the whole Earth or a large region, and ‘weather’ if it’s local and short-term.

The story, however, is a lot more complicated than this. Many, many things can alter the behaviour of these heat transfer systems at any one time or from time to time - for example, the distribution and type of vegetation on land; the amount of incident sunlight reflected off various Earth surfaces; the kind and distribution of clouds; the intermittent eruption of volcanoes; the slow processes of rock weathering and continental movement; and in recent times, the rapid addition to the atmosphere of heat-trapping gases on a vast scale. But one other thing is characteristic of climate all the time:

The seasons

The Earth has seasons for one big reason: its axis is tilted. This angle, measured with respect to the orbital plane, is about 23.5° & this means that for half the year one hemisphere gets more sunlight (and heat), and then they change places for the other half. Right now (that is, at this moment in Earth’s history) the seasonal maxima (when the difference between the hemispheres is greatest) occur on June 21st (northern mid-summer) and December 21st in the south. But the tilt angle (also called ‘obliquity’) is not constant - it varies from 22.1° to 24.5°, taking around 42,000 years for a cycle, and as it slowly wobbles, these dates will change. This, as it turns out, has a very important effect on Earth climate over time, because the more the planet tilts, the greater the seasonal contrast in both hemispheres - and of course that difference is greatest at higher latitudes, farthest from the equator.

Most water in the atmosphere gets there by evaporation from the oceans. Most evaporation occurs in the tropics and sub-tropics; and most of the rain that falls over the land comes from the ocean. You can see in this diagram that equatorial air holds about ten times as much water vapour as cold air at high latitudes. Because water vapour is the most abundant greenhouse gas in the atmosphere, tropical air is laden with heat. Because heat always moves, this air has a powerful motive force - which is why the biggest storms are made there, and why warm air can travel a long way.

Sun and Earth at June 21st (left) and December 21st (right). These two dates mark the position of the planet in its annual orbit when the axial tilt happens to be exactly in line with the Sun. What we see is the longest day of the year in the northern and southern hemispheres, respectively - when the Sun appears to rise highest in the sky. If it were not for this tilt, the equator would receive the lion's share of sunlight all year, and the poles would be much colder than they are.

Climate and water

We normally think the climate entails two things - temperature, and rain (or snow) - and this is perfectly correct. The movement of heat necessarily involves the movement of water through the air. Typically, the global atmosphere contains about 13,000 cubic kilometres of water at any one time in the form of vapour, enough to drop an inch of rain onto every bit of the surface. Most of it is near the equator, where it is taken up and precipitated much faster than elsewhere; the further north or south, the drier the air becomes (because water vapour condenses out of cold air), so stable polar air is as dry as any on Earth & the only way it can snow there is for water to be transported in moist air currents. The diagram below shows the very uneven distribution of atmospheric water.

This is a picture of Earth's energy balance as it's affected by latitude.

The tropics get the lion's share of solar energy, and the band of solar surplus moves up and down either side of the equator as the seasons alternate. Here, you are seeing the situation in southern summer.

But notice that beyond this band, energy balance is actually negative - that is, the surface radiates more than it receives at high latitudes, for all or part of every year.

This is corrected by atmospheric an ocean heat transport away from the equator.

FOUR THINGS