General · 24th August 2015
The agreed political consensus for keeping climate change within tolerable limits is to reduce greenhouse gas emissions sufficiently so the average global temperature does not rise above 2°C. If we can achieve this emissions goal, so the thinking goes, all will be well. The issue, however, is more complicated than a simple number.
Strangely enough, the 2°C limit was initially proposed in 1975 by an economist, William Nordhaus. He considered it little more than “a first intuition”, a temperature rise that made him nervous because it was “outside the range of observations” describing “the last several hundred thousand years.”
The 2°C number was mentioned again in 1990 by the scientists of the Stockholm Environment Institute. In their climate report, they wrote of two levels of risk. "Temperature increases beyond 1.0°C may elicit rapid, unpredictable, and non-linear responses that could lead to extensive ecosystem damage." Their next highest level of risk was a less “safe” 2°C. But with global temperatures already up 0.85°C and with no hope of stopping at 1.0°C, the 2°C limit entered negotiations as a practical target. Simple to remember and politically comfortable, it was formally adopted in the UN's Copenhagen Agreement of 2009. Dr. John Holdren, a former Science and Technology assistant to US President Obama, described 2.0°C as “the best we can do, while being the worst we can tolerate.”
More recently, the 2.0°C target has been getting a sober evaluation from Dr. James Hansen, one of the world's foremost climatologists and co-author of a recent report, Ice Melt, Sea Level Rise and Superstorms. When temperatures were last this high during the Eemian interglacial period 120,000 years ago, the report notes, sea levels were 5 to 9 metres higher than they are today.
“It is unlikely that coastal cities or low-lying areas such as Bangladesh, European lowlands, and large portions of the United States eastern coast and northeast China plains could be protected against such large sea level rise,” the report adds. “If the ocean continues to accumulate heat and increase melting of marine-terminating ice shelves of Antarctica and Greenland, a point will be reached at which it is impossible to avoid large-scale ice sheet disintegration with sea level rise of at least several meters. The economic and social cost of losing functionality of all coastal cities is practically incalculable.”
Dr. Hansen's solution is to use a “simple, honest, price on carbon emissions” to cut greenhouse gases. When combined with an end to all subsidies on fossil fuels, a rapid transition to renewable energy, carbon sequestration, intelligent forestry and efficiencies in agriculture to achieve a sustained six percent per year reduction in emissions, we have a possible chance of avoiding the worst effects of a much hotter planet.
Calculated another way, of a total budget of 2,900 billion tonnes of carbon emissions, we have already used 1,900, and we cannot use more than another 1,000 without sending our global average temperature above 2°C. If we don't start cutting emissions immediately, we will have exhausted our entire budget by about 2045.
As urgency is forced to the edge of desperation, huge expectations are being placed on all nations and the UN's COP21 climate conference scheduled for this December in Paris.