Every second, sunlight pours onto Earth, and every second Earth sends energy back out to space. Climate is, at bottom, the story of that exchange. Climate science studies the long-term patterns of temperature, rain, ice and wind across the planet, and why they change. This course follows the evidence from nineteenth-century lab benches to satellites, ocean robots and ice drilled from Antarctica, and ends by asking what the findings mean for the people who share this planet.

Weather is not climate

Weather is what happens on a given day: a storm on Tuesday, a heat wave next week. Climate is the statistics of weather over decades, usually summarized as 30-year averages along with how often extremes occur. An old saying captures the difference: climate is what you expect, weather is what you get. One cold winter in one city tells us little about climate, just as one roll of a die tells you little about whether the die is loaded. To see climate change, scientists look at long records over large areas.

Earth's energy budget

Sunlight arrives mostly as visible light. About 30 percent is reflected straight back to space by clouds, ice, snow and other bright surfaces. This reflectivity is called albedo. The rest is absorbed and warms the land, oceans and air. Every warm object glows, and Earth glows too, but at its temperature it gives off infrared light, which our eyes cannot see. When incoming and outgoing energy balance, the planet's temperature holds steady. When they do not, the planet warms or cools until a new balance is reached.

A simple calculation reveals something odd. If Earth's air did not absorb infrared, the energy balance would settle at an average surface temperature of about minus 18°C (0°F). The actual average is about 15°C (59°F). That gap of roughly 33 degrees is the natural greenhouse effect. Without it, Earth would be a frozen ball, and we would not be here to study it.

How the greenhouse effect works

Nitrogen and oxygen, which make up about 99 percent of dry air, let infrared pass straight through. A few trace gases do not. Water vapor, carbon dioxide (CO₂), methane and nitrous oxide have molecules that bend and stretch in ways that absorb infrared at particular wavelengths, then re-emit it in all directions, including back toward the ground. The upshot is that heat finally escapes to space from higher, colder layers of the atmosphere. Colder layers radiate less, so the surface has to warm until the outgoing flow matches the incoming sunlight again. Adding more CO₂ pushes that escape level higher still, which is why more of the gas means a warmer surface.

The name is a little misleading. A glass greenhouse stays warm mainly by stopping warm air from blowing away, while the atmosphere works by absorbing and re-emitting infrared. The label stuck anyway.

The discoverers

In the 1820s the French mathematician Joseph Fourier realized that Earth was warmer than sunlight alone could explain and suggested the atmosphere holds in heat. In 1856 the American scientist Eunice Foote filled glass cylinders with different gases, set them in the sun, and found that the one holding carbon dioxide grew hottest. She wrote that an atmosphere with more of this gas would give Earth a higher temperature. Three years later the Irish physicist John Tyndall used more precise equipment to measure how water vapor and CO₂ absorb infrared. In 1896 the Swedish chemist Svante Arrhenius spent months on hand calculations and estimated that doubling CO₂ would warm the planet by several degrees. The core physics is well over a century old.

Lessons from our neighbors

Venus is a natural experiment. Its thick atmosphere is about 96 percent CO₂, and its surface sits near 465°C, hotter than Mercury even though Mercury is closer to the Sun. Mars has a thin CO₂ atmosphere, a weak greenhouse effect and a frigid surface. The same physics explains all three worlds, and the same physics applies to ours.

Worked example: does a snowball disprove warming?

In February 2015 a United States senator carried a snowball onto the Senate floor and offered it as evidence that global warming was a hoax. How should we weigh that?

• Identify the scale: one snowball is weather, in one place, on one day. Climate concerns decades and the whole globe.
• Check the bigger picture: weeks earlier, NASA and NOAA had reported 2014 as the warmest year in the instrumental record, and new records followed in 2015 and 2016.
• Recall the energy budget: warming means the planet as a whole is holding more energy, not that every place is warmer every day.
• Ask for the right evidence: long-term global averages, which later lessons explore.

Cold days will keep happening in a warming world. What shifts is how often record cold arrives compared with record heat, and that pattern only shows up when you look at the whole record rather than a single winter afternoon.