Threats and action and quick revision

4.1 A balance under pressure

Earth's life rests on a fragile balance between the living and the non-living, and human activity unsettles it. Shifts in global temperature, oxygen or ozone, even modest ones, can endanger living things.

Today the biggest environmental challenges are climate change, biodiversity loss and pollution. These three are known together as the triple planetary crisis.

4.2 Climate change

Burning fossil fuels such as coal and oil releases greenhouse gases, including carbon dioxide and methane. These gases trap more heat in the atmosphere and cause global warming. Normally Earth keeps a balance because trees, plants and tiny ocean plankton absorb carbon dioxide as they grow. But burning fossil fuels releases carbon that was locked underground for millions of years, and Earth cannot absorb it fast enough, so heat builds up.

Even a small temperature rise can melt ice caps, raise sea levels, flood coastal cities, cause extreme weather and make many plants and animals disappear. Such lasting shifts in temperature, rain and weather are what we call climate change.

4.3 Biodiversity loss

Destroying natural habitats can wipe out species and disrupt ecosystems. If grasses disappear, deer and grasshoppers that feed on them struggle. Without herbivores, predators such as tigers and foxes lose their food. Each species matters, and losing even a handful reduces nature's capacity to sustain life.

4.4 Pollution

Pollution makes the problem worse. Smoke from factories, vehicles and fuel burning hurts people and nature: it causes breathing trouble, harms crops and produces smog and acid rain. Factory, farm and plastic waste foul water and soil, harming aquatic life and making water unsafe. Too much fertiliser and careless disposal degrade soil, cut harvests and carry toxins up the food chain.

4.5 Agreements and action

Nations have signed international agreements to safeguard the environment. The Montreal Protocol of 1987 reduced harmful chemicals such as CFCs, allowing the ozone layer to recover slowly. The 1992 Earth Summit spurred international work on climate and biodiversity. The Kyoto Protocol of 2005 and the Paris Agreement of 2015 committed countries to cut greenhouse gas emissions. Paris then set a goal of limiting warming to 1.5 °C, yet in 2025 the world remains off track.

Protecting the climate means cutting greenhouse gases by using renewable energy such as solar and wind, improving energy efficiency and choosing greener travel. Preserving biodiversity keeps ecosystems stronger and more balanced, and local communities can use natural resources sustainably. Everyone can help by reusing, repairing and recycling, saving energy and water, learning more and sharing ideas.

4.6 Common mistakes to avoid

• Thinking the greenhouse effect is always harmful: the mild natural effect keeps Earth warm enough for liquid water.
• Treating ozone as the same as oxygen: ozone is a three-atom molecule, while oxygen has two atoms.
• Believing Venus is hottest because it is closest to the Sun: its thick carbon-dioxide atmosphere traps heat.
• Assuming one factor makes Earth habitable: distance, size, atmosphere and magnetic field all matter.
• Saying asexual reproduction creates variety: it produces exact copies, while sexual reproduction mixes instructions.
• Believing only animals reproduce sexually: flowering plants do too.
• Treating climate change as a future problem: its effects are already visible.
• Thinking one person cannot help: small actions add up when many people take them.