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The Science of Climate Change

The greenhouse effect

The Earth is warmed by radiation from the sun. Some of this radiation is absorbed, but much is radiated back. The atmosphere prevents some of this radiation from escaping (infrared radiation included), acting like a blanket. If there were no atmosphere, the Earth would be much colder. This entrapment of radiation is known as 'the greenhouse effect'.

Within a greenhouse, some of the radiation from the plants and their surroundings is trapped and reflected back by the glass. This makes the greenhouse warmer than the ambient environment. The Earth's atmosphere acts in a similar way, trapping heat and making our planet warm enough to be hospitable to life.

But the greenhouse effect is getting stronger, and the atmosphere is becoming warmer in consequence. This change is associated with an increase of various 'greenhouse gases', of which carbon dioxide is the foremost by volume. For the period 1000-1750 ce (and for much of the time since the last ice age), the concentration of carbon dioxide remained at around 280 parts per million (ppm). But by 2000 ce, it had reached 368 ppm, and it currently stands at well over 400 ppm (Houghton 2015: 70; according to Caldecott and his distinguished sources, at 421 ppm in 2022 (Caldecott 2022a: 3: UCSD and SIO 2022). Across the same period (since the year 1750), average temperatures have risen by over 1°C (Caldecott 2022a: 72, Fig. 4.4), most of this increase being attributable to human activity. That may not sound much, but superficially small increases can have huge impacts, as will shortly be seen. (A shorter version of Caldecott's essay was published later in 2022: see Caldecott 2022b.)

Nor is carbon dioxide the only greenhouse gas. Methane (a gas generated by the decay of organic matter) is another, and one that is much more potent, volume for volume, at that. The average concentration of methane from 1000 to 1750 ce was 700 parts per billion (ppb), but this had increased to 1,750 ppb in the year 2000, and to 1,809 ppb in 2012 (Houghton 2015: 70). According to Caldecott and Tollefson, this average level reached over 1,900 ppb in 2022 (Caldecott 2022a: 3; Tollefson 2022), after a record increase during 2020-2021; this total amounts to 262 per cent of the pre-industrial level (Horton 2022). One human contribution to methane levels is made through the manufacture of plastics, for microplastics (the tiny fragments into which plastic bottles and wrappings are shredded by the action of seas and oceans) emit methane as they break down (Winters 2022: 11). Surprisingly, reservoirs contributed 5.2 per cent of global anthropogenic methane emissions in 2020 (Soued et al. 2022). Methane is particularly significant (among other reasons) because 'vast amounts of methane . . . are trapped in permafrost on land and under the sea around the Arctic' (Broome 2012: 75). If the warming of the atmosphere causes much of this methane to escape, then the greenhouse effect will be hugely accelerated.

A third significant greenhouse gas is nitrous oxide, generated through the use of nitrogen fertilizers and from the use of diesel in the engines of cars and lorries. From 1000 to 1750, its concentration in the atmosphere was 270 ppb; by 2012, this had risen to 325 ppb (Houghton 2015: 70), and, by 2022, to 335 ppb (Carrington 2022: 4). As we shall see in the next chapter, as well as being a potent greenhouse gas, nitrous oxide is also a source of air pollution and a widespread direct threat to human health.

Water vapour is also a greenhouse gas, but no one suggests that its presence is substantially due to human activity or significantly subject to human control. However, there are other greenhouse gases, such as CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons), used till recently as refrigerants, which are greenhouse gases and were found to be destroying stratospheric ozone. That layer of ozone is vital for preserving most life on Earth from ultraviolet radiation. So the world's nations agreed to ban the sale and purchase of CFCs and HCFCs through the Montreal Protocol of 1987 and subsequent agreements (Broome 2012). Since then, the ozone layer has been recovering, although recent wildfires (of the 2020s) seem to be casting this recovery into question.

However, HFCs (hydrofluorocarbons), which were introduced as a substitute for these banned substances, were found to be another greenhouse gas and equally harmful to the ozone layer as well. So they too were banned in an agreement made at Kigali in 2016; if this is implemented, then HFCs will cease to be emitted, and, come to that, to be generated. The Montreal and Kigali agreements supply grounds for hope that international collaboration over environmental threats has proved achievable, and could prove so again, serving to rebut the view that planetary collapse is inevitable.

Ozone at low altitudes (largely a result of human-generated pollution) is yet another greenhouse gas itself, and has almost doubled since pre-industrial levels (Broome 2012: 24). It was estimated in 2012 that the combined impact of all the greenhouse gases, other than carbon dioxide (and water vapour), was equivalent to 11.5 per cent of the (then) level of carbon dioxide. That would bring the total of carbon-equivalent greenhouse gases now (2023) to around 450 ppm.

All the various greenhouse gases absorb warming infrared radiation, and this well explains the phenomenon of global warming, which is itself readily observable from rising average air temperatures and has been found to be affecting the oceans to a depth of at least 3 km (Broome 2012: 27). This warming also explains most of the melting of ice caps and glaciers. This in turn explains the observed rises in sea levels, which amounted to 17 cm in the twentieth century. Sea levels continue to rise at an average of 3 cm per decade (ibid.).

There is plenty of empirical evidence for all this. For example, the increases of greenhouse gases in the atmosphere are evidenced by the proportions present in air bubbles trapped in ancient ice cores from Antarctica, and comparisons of these with average proportions in the current atmosphere. Nor is there significant room for doubt that these increases are due to human activity in the period from the Industrial Revolution onwards. The various greenhouse gases have resulted from emissions from domestic and industrial fires, and from the exhaust pipes of vehicles, ships and aircraft. As Broome adds, atmospheric carbon consists of a mixture of different carbon isotopes, and study of these isotopes shows that a sizeable proportion of this carbon comes from human sources (or is anthropogenic) (Broome 2012: 26).

Other theories have been advanced to explain global warming (as it used to be called), or global heating (an improved description). For example, sunspot activity has been presented as possibly explaining variations in average planetary temperatures across recent years. Perhaps this solar activity really makes a (very) small contribution. But scientific studies show that solar output since at least 1978 has been virtually constant, with fluctuations of only 0.1 per cent between its maximum and minimum extent (Houghton 2015: 158-9). The anthropogenic component turns out to be enormously larger, with sunspot activity playing a relatively tiny supporting role (see Houghton's diagrams, ibid.: 72). The alternating southern Pacific weather systems of Il Niño and La Niña complicate matters, and have the effect that some years are not as hot as their predecessor; but this does not detract from the anthropogenic greenhouse gas theory being substantially vindicated. Indeed, no theory explains the data as well as the anthropogenic one (Broome 2012: 28).

Scientific knowledge of the climate is summarized every five or six years in the reports of the Intergovernmental Panel on Climate Change (IPCC), a body which consults virtually the whole community of climate scientists. These reports give access to the latest scientific findings and conclusions to non-scientists such as Broome and myself. (Broome's conclusion, expressed at the end of the last paragraph, was closely based on IPCC reports.) A reference is given in the full list of references for the section of the IPCC's Sixth Assessment Report that relates to Europe and its climate problems (Bednar-Friedl, Biesbroek and Schmidt 2022), and another to the IPCC report presented in March 2023, urging rapid action to counter increases in greenhouse gas emissions (IPCC 2023). Non-scientists are also fortunate to have previous IPCC findings explained and summarized in John Houghton's Global Warming: The Complete Briefing (2015). Houghton is the retired chair of one of the IPCC panels, and his book supplies a comprehensive and up-to-date overview of this whole field of science. While there remains a theoretical possibility that the IPCC stance could be undermined (on climate scepticism, see Lejano and Nero 2020), the evidence that it adduces for global warming, its causes and its threats is close to conclusive.

It should be added that in 2009 a group of climate scientists published in Nature a paper showing that, for a 50 per cent chance of avoiding a 2°C rise in temperatures above pre-industrial levels, humanity is limited to emitting (from 1750, the dawn of the Industrial Revolution) just one trillion tonnes of carbon. But, by 2009, more than 55 per cent of this total had been emitted already, and at the rates of emission then current, this 'carbon budget' stood to be used up completely by February 2044...