Sunlight arrives unevenly

Word	What it means
Weather	Conditions over hours and days.
Climate	The long-term average pattern for a place — decades, not weeks.
Albedo	How much radiation a surface reflects. Fresh snow is high; dark ocean is low.
Greenhouse effect	Atmospheric gases absorbing the infrared Earth re-emits, warming the surface. Without it Earth would average about −18 °C.
Enhanced greenhouse effect	The extra warming from added greenhouse gases. This is the part that is a problem.
Biome	A large region defined by its climate and the community adapted to it.
Feedback loop	A change that affects itself. Positive means self-reinforcing, not beneficial; negative means self-correcting.
Proxy record	Evidence of past climate from something else: ice cores, tree rings, sediment layers, coral.

1. Sunlight arrives, mostly as visible light. Some is reflected straight back by clouds, ice and bright surfaces — that is albedo.
2. The surface absorbs the rest and warms up.
3. The surface re-radiates as longer-wavelength infrared, because it is far cooler than the Sun.
4. Greenhouse gases absorb that infrared and re-emit it in all directions, including back down.
5. The remainder escapes to space. Over the long run, in must equal out or the planet's temperature changes.

Why the equator gets more. Sunlight strikes it nearly head-on, so a given beam covers a small area. Near the poles the same beam arrives at a shallow angle and spreads over far more ground, and passes through more atmosphere on the way. The difference in distance to the Sun is negligible.

Air. Warm air rises at the equator, moves towards the poles, cools and sinks. Those convection cells set the latitudes of deserts and rainforests before any local detail is considered.

Water. Ocean currents carry warm water polewards and cold water back. The Gulf Stream is why western Europe is far milder than its latitude alone would suggest.

What decides a local climate. Latitude, elevation, proximity to a large body of water, ocean currents and mountain ranges. Time of day is not one — it changes the weather and averages out.

Biomes follow. Plot average temperature against annual precipitation and the biome is largely predictable: tundra cold and dry, grassland warm and dry, rainforest hot and wet. Climate decides the biome, not the other way round.

How past climate is known. Ice cores trap bubbles of ancient atmosphere, one layer per year, going back hundreds of thousands of years. Tree rings, sediment layers and coral give overlapping records. They agree with each other, which is what makes the reconstruction trustworthy.

The greenhouse gases. Carbon dioxide is released in by far the largest quantity. Methane traps more heat per molecule but far less is emitted — quantity and potency are separate questions and a good answer keeps them apart.

Feedback loops. Melting Arctic ice exposes darker ocean, which absorbs more energy, which melts more ice: a positive feedback, meaning self-reinforcing rather than good. The name describes the direction of the loop, and saying so explicitly is worth doing.

Why burning fossil fuels raises CO₂. That carbon was taken out of circulation over millions of years. Returning it over a couple of centuries is faster than photosynthesis and ocean uptake can absorb it. Nothing creates carbon; a slow store is being emptied quickly.

• Treating a cold week as evidence about climate. Different timescales.
• Saying the greenhouse effect is the problem. The enhanced effect is; the natural one keeps Earth habitable.
• Reading positive feedback as beneficial.
• Saying the equator is closer to the Sun. It is the angle, not the distance.