| Climate change |
Greenhouse gases (especially carbon dioxide) trap heat in the atmosphere and warm the planet. The main source of greenhouse gases is burning fossil fuels (oil, gas, and coal). Other activities such as burning biomass (e.g. wood) and making cement also release greenhouse gases. |
| Biodiversity loss |
Loss of plant and animal species has been more rapid in the past half century than at any time in human history. |
| Ocean acidification |
Carbon dioxide (e.g. from burning fossil fuels) is dissolved into oceans, rivers, and lakes. Corals and planktons struggle to survive in acidic waters — and the effect ripples up the food chain. |
| Ozone depletion |
Thinning of the ozone layer in the atmosphere, allowing more dangerous radiation in. It was caused by the release of CFCs and other chemicals. Following international agreement to limit the use of such chemicals, the hole in the ozone layer has been healing, and should be completely restored by around 2075. However, climate-fuelled wild fires pose a new threat. |
| Nitrogen and phosphorus |
Nitrogen and phosphorus are key ingredients in fertilisers. They also occur in natural biogeochemical flows, but heavy fertiliser use is damaging ecosystems. In particular, runoff into rivers and oceans can have catastrophic effects on marine life. One visible sign is algal bloom — overgrowth of algae or bacteria as a thick froth, scum, or slick, often toxic. |
| Land system change |
This mainly refers to the clearing of wilderness for agriculture, settlements, or other purposes. Forests, wetlands and grasslands are carbon sinks; disturbing them releases carbon back into the atmosphere. Habitat destruction also drives biodiversity loss. |
| Freshwater |
The extraction of water from rivers, lakes, groundwater, and soil moisture. |
| Aerosols |
Aerosols are particles or droplets suspended in the air. They include things like dust, sea spray, wildfire smoke, and volcanic ash. They also come from human sources, such as burning fossil fuels or biomass. Breathing in aerosol pollution harms human health. Some aerosols reflect radiation, so they are actually putting a brake on climate change. A lot of the science around the full impact of aerosols remains uncertain. |
| Novel entities |
This one is a bit of a catch-all for all kinds of toxic, long-lived, and/or unpredictable things we release, or hope to safely store — e.g. radioactive waste, heavy metals, pesticides, genetically engineered organisms. |
