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Grade 10-11 (age 15-17)

high school Climate Change

Climate is not day-to-day weather but the state of the atmosphere averaged over 30 years or more, and since industrialization, burning fossil fuels has added CO₂ to the air and raised the global average temperature by about 1.1 °C. Greenhouse gases absorb the infrared radiation the ground gives off and send part of it back, keeping the Earth warm; with no greenhouse gases at all, the average temperature would sit near -18 °C. Natural factors such as the Milankovitch cycles also change the climate, but they act over tens of thousands of years, so the rapid warming of recent decades is explained by the greenhouse gases people have added. In the diagram, moving the year shows CO₂ and temperature rising together, and after 2020 the future splits into two paths depending on whether emissions are cut.

The Earth is warming

🌡️ What is climate change?
①Climate is the state of the atmosphere in a region, such as temperature and rainfall, averaged over 30 years or more, unlike weather, which changes from day to day
②Since the Industrial Revolution, burning fossil fuels such as coal and oil on a huge scale has raised atmospheric CO₂ from about 285 ppm to about 413 ppm in 2020
③Over the same period, the global average temperature has risen about 1.1 °C above the 1850–1900 average (2011–2020 average)
④Along with the warming come rising sea levels, more extreme weather such as heat waves and downpours, and changes in ecosystems

CO₂ concentration and temperature

2020
💡 Reading the graph
①CO₂ in the upper panel and the temperature anomaly in the lower panel rise together after 1850, and the rise speeds up around 1950
②The orange lines up to 2020 are observations, and the two branches after that are projections that depend on how much greenhouse gas is emitted from now on
③On the low-emission path, which cuts emissions sharply, warming levels off at about 1.8 °C by 2100; on the high-emission path, where emissions keep growing, it reaches about 3.6 °C
④Both paths cross the 1.5 °C line in the early 2030s, a sign that the CO₂ already emitted keeps its effect for a long time

The greenhouse effect

How the greenhouse effect works
solar radiation → ground warms → ground emits infrared → greenhouse gases absorb and re-emit it → ground ends up warmer
Without greenhouse gases the global average temperature would be about -18 °C; today it is about 15 °C.

Main greenhouse gases

Greenhouse gases and their sources
CO₂ (carbon dioxide)
burning fossil fuels, cutting forests
the largest increase, so the largest contribution to warming
CH₄ (methane)
rice paddies, livestock digestion, wetlands, natural gas leaks
about 28 times CO₂ per unit mass over 100 years
N₂O (nitrous oxide)
nitrogen fertilizers, industrial processes
about 270 times CO₂ per unit mass over 100 years
CFCs (chlorofluorocarbons)
once used in refrigerants and sprays; they also destroy the ozone layer and are now banned
thousands of times CO₂ per unit mass or more

Milankovitch cycles (natural climate change)

Milankovitch cycles
orbital eccentricity (about 100,000 years) + axial tilt (about 41,000 years) + precession (about 26,000 years)
The way sunlight is spread across seasons and latitudes changes in cycles, so ice ages and interglacial periods take turns.
🔬 Natural and human causes
①Natural causes include the Milankovitch cycles, large volcanic eruptions, and changes in solar activity; volcanoes and the Sun act over a few years to a few centuries, and the Milankovitch cycles over tens of thousands of years
②Human causes include greenhouse gas emissions, deforestation, and urbanization, which act within decades to centuries
③Over recent decades solar activity has barely increased, yet temperatures have risen quickly
④So today's warming cannot be explained by natural causes alone, and the greenhouse gases people have added are identified as the main cause

Worked Examples and Exam Practice

Example 1
If the atmosphere had no greenhouse gases at all, roughly what would the Earth's average temperature be? (It is about 15 °C today.)
1
Greenhouse gases absorb the infrared the ground gives off and re-emit it, warming the surface.
2
Without greenhouse gases that warming disappears, and the temperature drops to about -18 °C, where the energy the Earth absorbs from the Sun equals the energy it gives off.
▸ about -18 °C
The greenhouse effect itself is a natural process that makes life possible. The problem is the extra warming from the greenhouse gases people have added.
Example 2
Why can't the Milankovitch cycles explain the rapid warming of recent decades?
1
The Milankovitch cycles act very slowly, over tens of thousands to a hundred thousand years.
2
The recent rise happened within decades and lines up with the rapid increase in CO₂, so it is attributed to human causes.
▸ The timescales differ: the Milankovitch cycles take tens of thousands of years, while the recent rise took decades.
To separate natural from human causes, look at both how fast a change happened and what it lines up with in time.
CSAT-style problem
Which statement about climate change and the greenhouse effect is correct?
①Climate means the state of the atmosphere on a given day
②CO₂, because it has increased so much, is the greenhouse gas that contributes most to warming
③The greenhouse effect is something people created
④The eccentricity cycle of the Milankovitch cycles is about 26,000 years
⑤Without greenhouse gases the Earth would be hotter than it is now
▸ ② CO₂, because it has increased so much, is the greenhouse gas that contributes most to warming
1
Methane and nitrous oxide are stronger molecule for molecule, but CO₂ has increased by far the most, so it contributes most to overall warming.
2
Climate is the average over 30 years or more (①), the greenhouse effect is a natural process (③), the eccentricity cycle is about 100,000 years (④), and without greenhouse gases the Earth would cool to about -18 °C (⑤).

Summary

Climate change in brief
burning fossil fuels → more CO₂ → stronger greenhouse effect → global warming
Natural causes can change the climate too, but the rapid warming of recent decades comes mainly from greenhouse gases added by people.
🎯 Exam Points
①The greenhouse effect is a natural process in which greenhouse gases absorb infrared from the ground and re-emit it, warming the surface; the problem is that people have strengthened it
②The main greenhouse gases are CO₂, CH₄, N₂O, and CFCs, and CO₂, which has increased the most, contributes most to warming
③The Milankovitch cycles are eccentricity (about 100,000 years), axial tilt (about 41,000 years), and precession (about 26,000 years)
④Effects of global warming include rising sea levels, extreme weather, and changes in ecosystems
⑤Without the greenhouse effect, the global average temperature would be about -18 °C
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