The science of climate change
Greenhouse gases comprise principally carbon dioxide (CO2), but also methane, ozone and nitrous oxide. Their rate of occurrence in the atmosphere is measured in parts per million (ppm) and the effect of the different components is calculated as a carbon dioxide equivalent (CO2e). The pre-industrial level was 280 ppm CO2e and the anthropogenic impact (emissions as a consequence of human activity) has pushed this level now to 450 ppm CO2e.
The measure of how much increasing levels of greenhouse gases affect temperature is known as "climate sensitivity", and this is now well-estimated at 3 degree in the short term. That is, a doubling of atmospheric carbon dioxide from the the pre-industrial level of 280 ppm to 560 ppm will produce an increase of around 3 degree. Each year the level of level of greenhouse gases increases about 22.5 ppm, so that every fourfive years the temperature is being pushed up 0.1 degree, but but this may rise to 3-4 ppm by mid-century if the world continues produces emissions "business as usual", ultimately resulting in a one degree temperature increase every 25 years. However in the long term when "slow" feedbacks are included, climate sensitivity is higher, in the range of 4.5-6C.
The world is now annually producing double the atmospheric carbon that the biosphere's carbon sinks can absorb: production of 7.9 GtC compared to the earth's total land and ocean carbon sink capacity (capacity to absorb carbon) of 4 GtC..
In other words, to stabilise atmospheric CO2 at its present levels, which are already dangerous, would require an immediate average global 50 per cent reduction in emissions.
Anthropogenic CO2 emissions are rising at an increasing rate. Between 2000 and 2005, global emissions grew two-and-a-half times faster than in the preceding 10 years, according to the Global Carbon Project. Others suggest a four-fold rate of increase: from growth rates of 0.8% for 19901999 to 3.2% for 20002005. Half of all anthropogenic emissions have been in the last 30 years, as has three-quarters of the temperature rise so far.
Concentrations of CO2 rose by 2.6 ppm during 2006, giving an average rise of 2.2 ppm since 2001, compared to about 1.5 ppm for 1970-2000. In a worrying development, "The spike in the last five years does not appear to match the pattern of steady increases in human emissions… the finding could indicate that global temperatures are making forests, soils and oceans less able to absorb carbon dioxide a shift that would make it harder to tackle global warming" .
With a current world population of 6.2 billion people, global atmospheric carbon emissions average 1.27 tonnes per capita; in Australia the rate is 5.63 tonnes per capita**. In comparison, the earth's current capacity to absorb carbon is 0.62 tonnes per capita, decreasing to 0.32 by 2030. The maths is simple and devastating: emissions need to be reduced to well below the earth's carbon sink capacity to reduce the already dangerous levels of atmospheric carbon, but just to get current emissions in Australia down to the level of the earth's current carbon sink capacity requires a cut of 90% of the current level of emissions.
[** Australian Government "National Greenhouse Gas Inventory 2004": total emissions 564.7 MtCO2e, of which 73.5% were CO2; total population 20.2 million; 1 tonne carbon = 3.65 tonnes CO2. So 564.7m X 73.5%/20.2 X 3.65 = 5.63 tonnes of carbon emissions per head.]
Global CO2 emissions, the principal component of greenhouses gases, are increasing at an average of about two per cent per year, in accordance with the worst-case "business-as-usual" (BAU) scenarios described in the third report of the International Panel on Climate Change (IPCC). "Business as usual" is the most pessimistic of the various "scenarios" considered by the IPCC and implies that little is done to prevent increasing emissions.
CO2 emissions are predicted to increase by 63% over 2002 levels by 2030 to 1314 GtC annually; Asia's greenhouse gas emissions will treble over the next 25 years, according to a report commissioned by the Asian Development Bank.
The earth's carbon sink capacity is expected to fall to 2.7 GtC by 2030 due to deceased capacity of the earth's carbon sinks due to both human activity and as a consequence of higher temperatures, so that if we continue as we are, in another 25 years we will globally be producing five times the amount of atmospheric carbon that the earth can absorb. This is a path to uncontrollable, runaway climate change.
The Greenhouse Office predicts Australia's emissions by 2020 will have swollen 22 per cent from their 1990 levels. Transport would be emitting 78 per cent more gases than in 1990, power generation 70 per cent more, industry 75 per cent more. ABARE predicts energy emissions will be more than 50-60 per cent higher over the next 25 years if we continue with "business as usual".
The global means temperature rise has been 0.8 degree (Hansen 2006a) since the late 1880s; in Australia "annual mean temperatures have increased by approximately 0.9°C since 1910" . But some of the effects of increasing greenhouses gases on temperature are delayed and take time to work their way through the climate system; this process presently results in a "thermal imbalance" ** of about 0.6 degree, so there is latent temperature rise still to come for the present level of greenhouse gases. That is, our present level of greenhouse gases will produce a temperature rise of 1.4 degree.
On current trends and if emissions increase as predicted, by 2025 the implied (actual plus thermal imbalance "in the system") rise will be around 2 degree, comprising 1.4 degree now and "in the pipeline" plus at least another 0.4 degree (0.1 degree for every fourfive years atmospheric levels increase at the current annual rate of 22.5 ppm, and more if that rate rises as predicted).
As a point of reference, China on 27 December 2006 released its first comprehensive scientific review on global warming, predicting rises of 1.32.1°C by 2020, 1.52.8°C by 2030 and 2.33.3°C by 2050. Leaks from the IPCC 2007 summary report say it is likely to foresee a rise in temperatures of 2 to 4.5°C this century, with about 3°C most likely.
The calm before the storm
Since the 1880s our greenhouse gas emissions have produced an actual rise of 0.8 degree. The impacts have been significant, including the widespread changes in global climate patterns being felt around the globe and the imminent end of the Arctic floating ice, but the day-by-day impact in the developed nations has not been perceived as dangerous in the way that an approaching bush fire would. It is the calm before the storm, because in another 20 years the implied rise will be around 2 degree and the consequences will be all around us as the impacts hit home, and catastrophic climate change triggered.