Earth and Climate — Plates, Atmosphere, Seasons

1.Earth's interior and the movement of plates

Earth is a ball with a radius of about 6,400 km, divided from the outside in into the crust, mantle, outer core, and inner core. The crust we live on is as thin as the skin of an apple; the mantle beneath it is mostly solid but can flow slowly over very long periods of time. The outer core is thought to be liquid and the inner core solid. No one has actually dug down into Earth's interior; this structure was worked out mainly by analyzing how seismic waves travel through the planet.

The rigid layer made up of the crust and the upper part of the mantle (the lithosphere) is broken into several pieces called plates. Plates move slowly, at a rate of a few cm a year. At the boundaries where plates meet, they collide, pull apart, or slide past each other, and earthquakes and volcanic activity are concentrated there. The Himalayas were pushed up where plates collided. The theory that explains crustal changes through the movement of plates like this is called plate tectonics.

2.The atmosphere and the water cycle

The atmosphere, the layer of air surrounding Earth, is about 78% nitrogen and about 21% oxygen, with the rest made up of very small amounts of argon, carbon dioxide, water vapor, and other gases. Based on how temperature changes with altitude, it is divided into the troposphere, stratosphere, mesosphere, and thermosphere. Most weather, such as clouds and rain, happens in the lowest layer, the troposphere, and the stratosphere contains the ozone layer, which absorbs ultraviolet light.

Powered by the Sun's energy, water evaporates from the oceans and land, condenses in the sky into clouds, falls as rain or snow, and flows back to the sea along rivers and through groundwater. This process is called the water cycle. The heat water absorbs when it evaporates is released into the atmosphere when clouds form, so the water cycle also moves heat around. It is the changes of state and heat flow you saw in Lesson 5, happening on a planetary scale.

3.Weather and climate are different

Weather is the state of the atmosphere at a particular place and time. Rain in Seoul today with a temperature of 12 °C is weather. Climate is the average pattern and range of variation of weather in a region over a long period, usually based on 30 years of records.

So the fact that one winter is unusually cold doesn't let you say the climate isn't warming, and conversely, one hot day doesn't let you talk about climate change. Climate is judged by trends over decades. One analogy: weather is the outfit you change every few hours, and climate is your wardrobe.

4.How the greenhouse effect works

Light from the Sun is mainly short-wavelength light, such as visible light, so most of it passes through the atmosphere and warms the surface. The warmed surface then gives off energy of its own, but at temperatures like Earth's, what it gives off is invisible infrared radiation. Gases in the atmosphere such as water vapor, carbon dioxide, and methane absorb this infrared radiation and re-emit it in all directions, and some of it comes back to the surface. As a result, the area near the surface stays warmer. This is the greenhouse effect.

The greenhouse effect itself is a natural phenomenon and essential for life. Without it, Earth's average temperature is estimated to be around −18 °C, whereas the actual average is about 15 °C. The problem is that human activities, such as burning fossil fuels, are raising the concentration of greenhouse gases and strengthening the greenhouse effect. This is called global warming, and based on observational data and climate models, the scientific community assesses that human activity is the main cause of the current warming.

The name comes from greenhouses, but the main reason a real greenhouse is warm is that the glass keeps the warmed air from escaping, so it doesn't work in exactly the same way as the atmosphere's greenhouse effect.

5.Global warming in numbers

The amount of carbon dioxide in the atmosphere is usually expressed in ppm (parts per million). 1 ppm means 1 molecule out of every 1 million molecules of air. Before industrialization, the carbon dioxide concentration was about 280 ppm, but it has risen steadily because of fossil fuel use and other activities, passing 420 ppm in the 2020s. Records measured the same way for decades, such as those from the Mauna Loa Observatory in Hawaii, show this trend.

420 ppm is only 0.042%. Why does such a small amount matter? Nitrogen and oxygen, which make up most of the air, hardly absorb the infrared radiation the surface gives off, but carbon dioxide does. The only gases in the air that play this role are small-quantity gases such as water vapor, carbon dioxide, and methane, so even a small change in their concentration has a large effect.

The Intergovernmental Panel on Climate Change (IPCC) assessed that Earth's average surface temperature in 2011–2020 was about 1.1 °C higher than in 1850–1900. A little over 1 °C is a small difference in a day's weather, but this is different: it is an average over the whole planet across decades. Even a change this size is already having observed effects on the frequency and intensity of heat waves, sea level, and more.

6.The cause of the seasons — tilt, not distance

“It's hot in summer because Earth is closer to the Sun” is a widespread misconception. Earth's orbit is an ellipse very close to a circle, so the difference in distance is small, and in fact Earth is closest to the Sun in early January, when it is winter in the Northern Hemisphere. Also, if distance were the cause, the Southern and Northern Hemispheres would have the same seasons, but in reality their seasons are opposite.

The real cause is that Earth orbits the Sun with its axis of rotation tilted about 23.5° from the direction perpendicular to the plane of its orbit. In summer, when the Northern Hemisphere is tilted toward the Sun, the Sun rides high in the sky (its noon altitude is high), so sunlight is more concentrated on the same area, and the days are longer. In winter, the Sun stays low, so sunlight spreads out at a slant over a wider area, and the days are short. It's the same principle as a flashlight looking brighter when you shine it straight down at the floor than when you shine it at an angle.