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  • Nigel Hargreaves


Global warming is the biggest existential threat to life on earth, threatening biodiversity and all other forms of geosystems that it depends upon. It has been a long time, since the 'greenhouse effect' was arguably first identified by Joseph Fourier in 1824. And in 1981 James Hanson alerted the world to faster warming than predicted due to trapped carbon dioxide in the Earth's atmosphere.

Sustainability: A new approach? reviewed the science behind global warming, estimating the impact of carbon emissions on global temperature in 2009 and explored the potential for solar power to supply the world's energy demand.

Carbon emissions were reported in 2007 to be increasing in the range of 1 to 3% per annum. Hansen (2009) had proposed a target to limit carbon dioxide concentration in the atmosphere to 350ppm and Allen (2009) calculated this was equivalent to releasing 1000 giga tonnes of carbon into the atmosphere as a result of anthropogenic activities in order to limit global warming to 2ºC by 2100 compared to preindustrial times.

By 2007 around half of the 1000Gt carbon budget had already been released, locking Earth into radiative forcing resulting in a temperature anomaly of +0.7ºC.

There is no measure of sustainability from limiting global temperatures to a 2ºC increase, a target in uncharted territory. In any case, Earth’s sensitivity to climate forcing may be higher than estimated, making the 2ºC target even harder to attain. So the question of what is the acceptable probability of attaining that target and the associated carbon emitted becomes further abstracted from what is safe or sustainable (Victor, 2009). Even in 2009, based on a relationship between linear amounts of carbon released to the atmosphere and the total carbon budget above, to achieve an 80% probability of not exceeding warming of 2ºC by 2100, ALL carbon emission had to be halted by 2031. For a 75% probability the date could be extended until 2034.

In 2015, this target was lowered to +1.5ºC in the Paris Climate Agreement of CoP21. And since then the UNEP have reviewed the gap between aspiration and actuality, which in 2020 was pointing us towards 3ºC of warming. In 2022 the carbon dioxide concentration in Earth's atmosphere has risen to 417ppm and the temperature anomaly is 1.2ºC.

Source: Fossil fuel reserves, BP (2009); Carbon conversion factors, CDIAC (2009).

World proven fossil fuel reserves and burn limits to conserving global warming to 2ºC with probabilities given by Meinhausen et al. (2009). The stacked bar on the left is an estimate of carbon that would be emitted from burning known world fossil fuel reserves and the bars on the right show emitted carbon to conform to probabilities of achieving a 2ºC limit on global warming by 2100.

These calculations were carried out in 2011 and used probabilities derived by Meinhausen (2009) to show the potential trajectory of the World Cumulative Warming Commitment based on the above science.

Non-renewable energy supplies, reserves and potential resources (not yet economically or geologically possible to access), in 2006, compared with desert solar resource over 26M sqkm at insolation of 2100kWh/sqm/yr.

Incident solar energy on the Earth's surface is more than total global energy demand from human activities. For example, in 2008, solar radiation falling on deserts in under three hours was equivalent to total world electricity demand.


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