The document’s science has four lines of evidence: the physical principles of greenhouse gases, the record of the distant past, measurements from the last century, and climate models that use the other three lines. These models predict a rise of between 2 and 7°C on pre-industrial levels “depending on future greenhouse gas emissions and on the ways that models represent the sensitivity of climate to small disturbances.”
At the lower end, we can expect repercussions in the form of heatwaves, higher global average rainfall, impacts to marine biodiversity and rising sea levels. At the 7°C end things get really nasty. All the 2°C changes will be magnified to a point where the scientists coolly say “such a large and rapid change in climate would likely be beyond the adaptive capacity of many societies and species.”
The report shows we are not in a natural cycle of warming. Nothing in the last 2,000 years is like the last 100 and if we add another 2-7 degrees it will be like nothing in the last 10,000 years. Data over a million years show Earth’s surface has risen and fallen by about 5°C, through 10 major ice age cycles. Feedbacks in the glacial cycle show strong links between global temperature, atmospheric water vapour, polar ice caps and greenhouse gases. In the past million years, the disturbances to the cycle have come from fluctuations in Earth’s solar orbit. In modern times human emissions affecting greenhouse gases reinforce change in the temperature, water vapour and ice caps. Even small influences can amplify into large changes.
Average temperatures have increased over 100 years to 2009 by more than 0.7°C. The global land surface is warming twice as fast as the ocean surface. There has been widespread melting of mountain glaciers and ice caps, particularly since the 1990s. The Greenland ice sheet and West Antarctica are also losing ice. Ocean levels are more than 20 cm higher than in 1870.
In Australia the average surface temperature has increased by 0.7°C in half a century. There is a continent-wide average increase in the frequency of extremely hot days and a decrease in cold days. Rainfall changes are less consistent though it is declining in southwest Western Australia and the southeast coast. In the oceans, there has been a southward shift of the Antarctic Circumpolar Current. Sea level has risen at 1.2 mm per year since 1920, with more frequent coastal inundations.
Atmospheric concentrations of carbon dioxide, methane and nitrous oxide began to rise three hundred years ago and accelerated rapidly in the 20th century. From 2000 to 2007 emissions grew by 3.5 percent per year, exceeding almost all assumed scenarios from the late 1990s. Deforestation, fossil fuel burning, other industrial sources like cement production all contribute. Only 45 percent ends up as atmospheric CO2. Thirty percent is swallowed by increased plant growth and another quarter is making seawater more acidic.
If current levels of emissions continue, the AAS is tipping a doubling of pre-industrial CO2 levels by 2050, and possibly a tripling by 2100. This would produce a warming of around 4.5°C (plus or minus 2.5) to 2100. What this means to climate and sea levels is educated guesswork, but all scenarios are gloomy. “The further climate is pushed beyond the envelope of relative stability that has characterised the last several millennia,” concluded the report, “the greater becomes the risk of passing tipping points that will result in profound changes in climate, vegetation, ocean circulation or ice sheet stability.”
Despite its stark message, the report got little media coverage. Denialist-leaning News Limited muddied its coverage with an unrelated story about New Zealand’s National Institute of Water and Atmospheric Research which faces a legal challenge by sceptics group Climate Science Coalition. The Sydney Morning Herald preferred to highlight there were “still scientific uncertainties about some of the details of climate change”.