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Unit 9
15–20% of examThis unit is about changes that affect the whole planet: the thinning ozone layer, rising greenhouse gases and climate change, warmer and more acidic oceans, and the loss of species. You'll learn what causes each change, what it means for living things, and what people are doing about it. It's the most heavily weighted unit on the exam.
Longer videos that cover the whole unit. Good for a first pass or a final review.
The ozone layer in the stratosphere absorbs most of the sun's harmful ultraviolet (UV) radiation. Chlorofluorocarbons (CFCs), once common in refrigerators, air conditioners and spray cans, release chlorine high in the atmosphere that destroys ozone; a thinner ozone layer lets more UV reach the ground, raising the risk of skin cancer and cataracts.
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The Montreal Protocol (1987) phased out CFCs worldwide, and the ozone layer is slowly recovering. CFCs were replaced first by HCFCs, which harm ozone less, and then by HFCs, which don't harm ozone but many of which are powerful greenhouse gases.
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Sunlight warms Earth's surface, which gives off heat as infrared radiation. Greenhouse gases such as carbon dioxide, methane, nitrous oxide, water vapor and CFCs absorb some of that heat and send part of it back toward the surface, which keeps Earth warm enough for life. Global warming potential (GWP) compares how strongly each gas traps heat against CO₂, which is set at 1: CFCs are highest, then nitrous oxide, then methane.
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Since the Industrial Revolution, burning fossil fuels, clearing forests, farming, raising livestock and filling landfills have raised the amount of CO₂, methane and nitrous oxide in the air, as measurements like the Keeling Curve from Mauna Loa show. The extra warming melts ice sheets and makes seawater expand, raising sea levels, and lets disease-carrying insects spread out of the tropics into cooler regions, which can force people and wildlife to move.
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Earth's climate has warmed and cooled many times, and ice cores, which trap bubbles of ancient air, record past CO₂ levels and temperatures. Today's warming melts glaciers, sea ice and permafrost, raises sea levels, displaces coastal communities and can shift the winds and ocean currents that carry heat around the planet. The poles warm fastest because melting ice and snow reflect less sunlight, a positive feedback loop, and thawing permafrost releases even more greenhouse gases.
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The ocean has absorbed most of the extra heat trapped by greenhouse gases. Warmer water causes coral bleaching, where corals push out the algae that feed them, shifts where marine species can live, and adds to sea level rise.
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The ocean absorbs CO₂ from the air, and the CO₂ forms carbonic acid, lowering the water's pH. More acidic water makes it harder for corals, shellfish and some plankton to build shells and skeletons out of calcium carbonate.
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Invasive species are non-native species that spread quickly and harm the ecosystems they're brought into, often because nothing there eats them or competes well with them. They tend to be generalists that reproduce fast (r-selected); examples include zebra mussels, kudzu, cane toads and Burmese pythons.
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A species is endangered when it is at serious risk of extinction, often because it has a small population, a small range, specialized needs or slow reproduction. Laws like the US Endangered Species Act and the international treaty CITES, which controls trade in wildlife, protect species, and protecting habitat is one of the best tools.
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Habitat loss and fragmentation, where roads, farms and logging break habitat into small, isolated patches, are the biggest threats to biodiversity; the main threats are often summed up as HIPPCO: habitat destruction, invasive species, population growth, pollution, climate change and overexploitation. Conservation tools include protected areas, wildlife corridors that link habitat patches, habitat restoration and sustainable land use.
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