Australian Academy of Science on the science of climate change
The document’s science is based on four major lines of evidence: the known 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 of evidence. These models are currently predicting a rise of between 2 and 7°C on pre-industrial levels depending on “depending on future greenhouse gas emissions and on the ways that models represent the sensitivity of climate to small disturbances.”
Even at the 2°C lower end, we can expect nasty repercussions in the form of heatwaves, higher global average rainfall, impacts to marine biodiversity and rising sea levels. But it is at the 7°C end where things get really nasty. All of 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 is at pains to show we are not in some 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 in that time. As well, feedbacks in the glacial cycle mean there are 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 it is human emissions affecting greenhouse gases which reinforce change in the temperature, water vapour and ice caps. Even small influences can amplify into large changes.
The pace of change is also picking up. Average temperatures have increased over the 100 years to 2009 by more than 0.7°C. However the rates of observed near-surface warming has increased since the mid-1970s with the global land surface warming at double the rate of the ocean surface. There has been widespread melting of mountain glaciers and ice caps, particularly noticeable since the 1990s. The Greenland ice sheet and West Antarctica are also losing ice. Ocean levels are now more than 20 cm higher than in 1870.
Australia is not immune to these global trends. Here 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 the frequency of cold days. Rainfall changes are less consistent though it is noticeably declining in southwest Western Australia and the southeast coast. In the oceans, there has been a there has been a southward shift of the Antarctic Circumpolar Current and sea level has risen at a rate of about 1.2 mm per year since 1920, resulting in more frequent coastal inundation events.
Humans are the cause of the problem. Atmospheric concentrations of carbon dioxide, methane and nitrous oxide began to rise two to three hundred years ago at the start of industrialisation and accelerated rapidly in the 20th century. The problem is worsening in the 21st century. From 2000 to 2007 emissions grew by 3.5 percent per year, exceeding almost all assumed scenarios generated in the late 1990s. Deforestation, fossil fuel burning, other industrial sources such as cement production all contribute. Only 45 percent ends up as atmospheric CO2. 30 percent is swallowed by increased plant growth and another quarter is making seawater more acidic.
If “business as usual” 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 at best educated guesswork, but all the scenarios put forward are unremittingly 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 or perhaps because of its stark message, the report got short shrift in the media. In the few stories that were there, the message was diluted. The 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 did look at the report in more detail but preferred to highlight there were “still scientific uncertainties about some of the details of climate change”.
The conclusion of the report itself should be a surprise to no-one: greenhouse gases from human activities are the main cause of temperature increases for the last 100 years and if nothing is done, they will continue to increase significantly. Yet the way the AAS document is reported continues to fly in the face of such evidence. The SMH said the report “unambiguously” supported the conclusion that a continued reliance on fossil fuels would lead to a warmer world while a similar thing happened over at the ABC with their headline about climate change “misinformation”. Subeditors will say that putting words like unambiguously and misinformation in quotes is done to show who is using the words. But the effect is the opposite: the quotes undermine the words, causing the reader to doubt the source. The media is doing a great disservice to climate change science by the way it reports the issues and an even greater one by the way it doesn’t.