Earth's Climate System

You are standing inside the largest machine in the world, and it never switches off. Scientists call it the climate system, and it has five moving parts: the atmosphere (the air), the hydrosphere (all the liquid water, mostly ocean), the cryosphere (everything frozen — ice, snow, glaciers), the biosphere (every living thing, from redwoods to plankton to you), and the geosphere (the rock and soil underneath). All five swap heat, water and carbon with each other, all day, every day. And one thing powers the whole lot: the Sun.

Of the sunlight that reaches Earth, about 29% bounces straight back to space off clouds, ice and bright deserts. The other 71% gets soaked up and turns into heat. That heat has to escape again as invisible infrared — otherwise Earth would just keep heating up forever — and on the way out some of it gets caught by greenhouse gases: water vapour, carbon dioxide, methane and a few others. That catching is the greenhouse effect, and it is the reason you are alive. Without it Earth's average temperature would be about −18 °C instead of the comfortable 15 °C we actually get. Not "a bit chilly" — a frozen planet.

One last thing to get straight before anything else: weather is not climate. Weather is what the sky is doing right now, where you are. Climate is what the sky usually does there, averaged over about 30 years. Weather is your mood today. Climate is your personality.

The climate system is always trying to even things out. The equator gets far more sunshine than the poles do, and the atmosphere spends its whole life shifting that spare heat away from the middle and towards the ends. Storms are what that looks like when it happens in a hurry.

Inside a thundercloud, ice crystals and soft hail pellets crash into each other in a violent updraught and rub electric charge off one another — the same static that makes your hair stick to a balloon, but on a scale of kilometres. The bottom of the cloud goes negative, the ground below goes positive, and eventually the air gives up and lightning punches through. A bolt heats the air to about 30,000 K — five times hotter than the surface of the Sun. That superheated air explodes outwards, and the bang it makes is thunder. Light gets to you almost instantly; sound crawls at about 340 metres a second. So count the seconds between the flash and the bang, divide by three, and that is how many kilometres away the storm is.

Hurricanes are a completely different animal. They need an ocean at least 26.5 °C — and warm to a depth of about 45 metres, not just at the surface — because warm water evaporating is their fuel. The rising air spirals because Earth is spinning (the Coriolis effect), which is also why a hurricane can never form right on the equator: there is no spin there to get it started. Call the same storm a hurricane in the Atlantic, a typhoon in the northwest Pacific, or a cyclone around India and Australia — it is one kind of storm with three passports. Tornadoes are smaller, faster and far more local: usually under a kilometre across, born from a single thunderstorm over land, gone in minutes — and holding the record for the fastest winds ever measured anywhere on Earth, about 480 km/h.

Here is the fact that reorganises everything you think you know about climate. When people say "global warming", they are almost entirely talking about the ocean. Of all the extra heat humans have trapped since the 1970s, about 91% went into the sea. Land took 5%, melting ice took 3%, and the air — the bit we actually live in and complain about — took 1%.

The ocean can do that because water is astonishingly good at holding heat. NASA puts it like this: the top few metres of the ocean store as much heat as the entire atmosphere above it. Water covers 71% of the planet, so this enormous blue flywheel is what stops Earth from roasting by day and freezing by night, and it is why coastal places have gentler weather than the middle of a continent.

The ocean also moves that heat around, in two different ways. Near the surface, wind pushes the water: the Gulf Stream carries tropical warmth up towards north-western Europe, which is why Britain is far milder than Canada at the same latitude. Far below, something stranger happens. Near Greenland and Antarctica, surface water gets very cold, and as sea ice forms it leaves its salt behind in the water underneath. Cold plus extra salty equals dense — so that water sinks, kilometres down, and starts crawling along the sea floor. This is the thermohaline circulation (from thermo = heat and haline = salt), nicknamed the global conveyor belt. One drop of water can take about 1,000 years to go all the way round.

And every few years the Pacific throws a switch. In an El Niño, the trade winds weaken, warm water sloshes back east across the Pacific, and the whole planet's weather rearranges itself — droughts in some places, floods in others, and global temperature nudged upward for a year or two. Its cool twin is La Niña. Together they are called ENSO, and they are the single biggest reason one year is warmer than the next.

Wear a black T-shirt on a hot day and you will regret it. Wear a white one and you are fine. That is the entire physics of the cryosphere — Earth's frozen parts — and scientists have a word for it: albedo, which just means how reflective something is.

The open ocean has an albedo of about 0.06: it soaks up 94 rays out of every 100 and turns them into heat. Sea ice with fresh snow on top has an albedo of up to 0.9: it bounces 90 rays straight back to space, unused. The Arctic and Antarctic are the planet's white T-shirt. Overall, Earth reflects about 29% of the sunlight that hits it.

Now notice the trap. Warm the Arctic a little, and some ice melts. Where the ice was white, there is now dark water. Dark water absorbs the sunlight the ice used to bounce away. That extra heat melts more ice. Which makes more dark water. This is the ice–albedo feedback, and it is why the Arctic is warming several times faster than the rest of the planet. Feedbacks are the most important idea in climate science: a small push that makes itself bigger.

The two ends of the Earth are not twins — they are opposites. The Arctic is an ocean surrounded by land, capped by floating sea ice a few metres thick. Antarctica is a continent surrounded by ocean, buried under an ice sheet nearly 5 km thick sitting on solid rock. That difference decides what melting does: floating sea ice is already displacing its own weight, so melting it does not directly raise sea level (drop an ice cube in a full glass and watch). Land ice is a different story — Greenland alone holds about 7.4 m of sea level, and Antarctica about 58 m.

Ice also remembers. Snow falling on Antarctica traps tiny bubbles of air, and those bubbles get buried and squeezed for hundreds of thousands of years. Drill a core, pull it up, and you are holding actual samples of ancient sky. The published record goes back 800,000 years — and a team has now recovered ice around 1.2 million years old and is reading it as you sit here.

Every tree, every blade of grass and every speck of plankton is part of the climate system, because all of them are in the carbon business. In photosynthesis, a plant pulls CO₂ out of the air, uses sunlight to strip the carbon off, builds itself out of the carbon, and lets the oxygen go. When it is eaten, burned or rots, the carbon goes back. That round trip is the carbon cycle, and it has been running for billions of years.

You can actually see the planet breathing. Because most of the world's land is in the Northern Hemisphere, when northern forests leaf out in spring the whole atmosphere's CO₂ dips, and when the leaves fall it rises again — a saw-tooth wiggle in the global CO₂ graph, once a year, every year.

These living systems are also doing us an enormous unpaid favour. Of all the CO₂ humans release, plants and soils absorb about 21% and the ocean absorbs about 29% — so roughly half of our pollution is quietly mopped up and only half stays in the air. The catch, discovered recently, is that these sinks are getting tired: the land sink is about 25% weaker than it would have been without climate change. And in the far north, the frozen ground called permafrost holds something like 1,500 billion tonnes of carbon — about twice as much as the whole atmosphere — locked in ancient plants that never rotted because they never thawed.

So what about us? Humans currently release around 38 billion tonnes of CO₂ a year from fossil fuels, plus about 4 billion more from cutting down forests. The biggest slice by far is making electricity and heat (about 34%), then industry (24%), then farming and land use (22%), then transport (15%) and buildings (6%). CO₂ in the air has gone from about 280 parts per million before the industrial revolution to over 425 today — higher than at any point in the 800,000-year ice-core record. 2024 was the warmest year ever measured, about 1.6 °C above pre-industrial. The sea is rising about 4.4 mm a year, roughly twice as fast as in the 1990s.

Now the part that most climate pages leave out, and shouldn't. We have already fixed a problem exactly like this one. In the 1980s we discovered chemicals in fridges and spray cans were eating a hole in the ozone layer. Every country on Earth agreed to ban them. The 2025 ozone hole was the fifth smallest since 1992, and the layer is on track to fully recover by about 2066. Acid rain, same story — US power-station sulphur pollution is down 95% since 1990. And clean energy stopped being expensive: solar power costs about a tenth of what it did in 2010, batteries about a twentieth, and one in every four new cars sold in the world is now electric. None of that means the problem is solved. It means it is the kind of problem that can be solved, by people who decide to.