Global warming and climate agreements

The greenhouse effect and global warming

The Sun's short-wave radiation passes through the atmosphere and warms the Earth, which re-emits long-wave infrared radiation. Greenhouse gases (GHGs) absorb this infrared and re-radiate part of it back, keeping the planet warm enough for life. This natural greenhouse effect becomes a problem when human activities add extra GHGs, causing global warming and wider climate change.

Main GHGs: carbon dioxide (fossil fuels, deforestation, cement), methane (rice paddies, livestock, landfills, coal mines), nitrous oxide (fertilisers), and fluorinated gases (HFCs, PFCs, SF₆). Water vapour is also a GHG but acts mainly as a feedback. Gases differ in global warming potential (GWP); methane and nitrous oxide trap far more heat per molecule than CO₂, and fluorinated gases much more again, though CO₂ is the largest contributor overall because of its volume.

Impacts include rising sea levels from thermal expansion and melting ice, retreating glaciers, more intense heat waves, droughts and floods, changes in monsoon patterns, coral bleaching, ocean acidification and threats to food and water security.

Climate agreements

• IPCC (1988): set up by the WMO and UNEP to assess climate science.
• UNFCCC (1992, Rio Earth Summit): framework treaty with the principle of common but differentiated responsibilities (CBDR). Parties meet every year at the Conference of the Parties (COP); COP1 was in Berlin in 1995.
• Kyoto Protocol (adopted at COP3 in 1997, in force 2005): set legally binding emission-reduction targets for developed (Annex I) countries, on average about 5.2% below 1990 levels in 2008–2012, covering six GHGs. It created three market mechanisms: emissions trading, the Clean Development Mechanism and Joint Implementation. Developing countries had no binding targets.
• Paris Agreement (COP21, 2015, in force 2016): aims to keep warming well below 2 °C above pre-industrial levels and to pursue efforts to limit it to 1.5 °C. Every country sets its own Nationally Determined Contributions (NDCs), updated every five years, with a global stocktake to check progress.

Key points

• GHGs trap outgoing infrared radiation.
• Kyoto: binding targets only for developed countries; three flexible mechanisms.
• Paris: all countries set NDCs; goal well below 2 °C, aiming for 1.5 °C.
• CBDR recognises developed countries' historical responsibility.

Common mistakes

• Saying the Kyoto Protocol bound India to cut emissions; developing countries had no binding targets.
• Confusing the ozone hole with the greenhouse effect.