Global warming

The greenhouse effect. The atmosphere lets most incoming short-wave sunlight through but absorbs most of the long-wave radiation the earth gives off. Gases that absorb long-wave radiation are greenhouse gases (GHGs), and the warming they cause is the greenhouse effect. The name comes from a glass greenhouse used in cold countries: glass lets sunlight in but blocks outgoing heat, so it is warmer inside.

The main GHGs: carbon dioxide (CO₂), chlorofluorocarbons (CFCs), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃). Nitric oxide and carbon monoxide react with GHGs and change their amounts.

• A GHG's effect depends on how much its concentration rises, its lifetime in the air, and the wavelengths it absorbs. The longer a gas stays, the longer the system takes to recover. CFCs are highly effective. Ozone, which shields us from ultra-violet in the stratosphere, traps heat when it is in the lower troposphere.
• CO₂ has the largest concentration. It comes mainly from burning fossil fuels (oil, gas, coal). Forests and oceans are sinks that absorb it, so deforestation also raises it. It takes 20-50 years for CO₂ to adjust to changes in sources and sinks; the book says it is rising about 0.5 per cent a year. Doubling of pre-industrial CO₂ is used as a benchmark in climate models.
• CFCs are made by people. Drifting into the stratosphere, they destroy ozone. The large thinning over Antarctica is the ozone hole, which lets more ultra-violet through.

How much has the earth warmed? The world's average near-surface air temperature is about 14°C. In the 20th century it rose by about 0.6°C, mostly in 1901-44 and 1977-99. The book names 1998 as the warmest year of the century; several later years have been warmer still.

Effects: once warming sets in it is hard to reverse, and its effects are uneven. Sea level rises as glaciers and ice caps melt and sea water expands, threatening coasts and islands.

International action: the Kyoto Protocol (1997, in force 2005) committed industrialised countries to cut emissions to 5 per cent below 1990 levels by 2012. Since 2015, the Paris Agreement has asked all countries to set and meet their own targets to limit warming.

Example (new): a cotton blanket on a cold night does not make heat; it slows the escape of your body's heat. Adding more greenhouse gas is like adding another blanket to the earth.