Global carbon dioxide emissions would need to be 50–85 per cent below 2000 levels by 2050 to keep eventual global warming to about two degrees above pre-industrial levels.

Even such drastic cuts are still achievable with a mix of new and emerging technologies, according to the latest report from the Intergovernmental Panel on Climate Change (IPCC), which includes several Australian and New Zealand scientists.

The IPCC’s final instalment of its Fourth Assessment was approved on 17 November 2007 in Valencia, Spain. This “Synthesis Report” summarises the key policy-relevant findings from the three telephone-book-sized volumes of science released earlier this year.

The report also considers what limits to emissions would be required to cap temperature rises at various other levels. For all these, the “price tag” is a small reduction in the rate of global economic growth (by less than 0.12 percentage points annually in 2050 for least-cost trajectories).

The European Union has formally adopted a two degree target, but the IPCC itself does not define a level at which climate change becomes “dangerous”. Instead, the report says the five “reasons of concern” raised by the IPCC in 2001 are now “stronger”, with some risks “projected to be larger or to occur at lower increases in temperature”.

The five reasons of concern are:

• Risks to unique and threatened systems;
• Risks of extreme weather events;
• Distribution of impacts and vulnerabilities, with climate change expected to hit many poor communities and with low-latitude and less-developed areas generally facing the greatest risk;
• Net costs of impacts of increased warming are projected to increase over time, with some initial economic benefits of climate change no longer expected to last as long as previously thought;
• Risks of large-scale, long-term, or irreversible changes. For example, sea level rises over coming centuries are projected to be much larger than over the 20th Century, with resulting loss of coastal area.

The report suggests that mitigation efforts and investments in the next 20–30 years will “have a large impact” on opportunities to stabilise greenhouse gas concentrations, and hence temperature increases, at lower levels. For all scenarios considered by the IPCC, “60–80% of the reductions would come from energy supply and use, and industrial processes, with energy efficiency playing a key role in many scenarios.”

The report says that “neither adaptation nor mitigation alone can avoid all climate change impacts”; however, together “they can significantly reduce the risks of climate change”. An increasing risk of species extinction and coral reef damage is projected as warming proceeds. Many semi-arid areas (e.g., the Mediterranean Basin, western United States, southern Africa, and northeast Brazil) are expected to suffer a decrease in water resources due to climate change. Also, increasing vulnerability of “indigenous communities in the Arctic and small island communities to warming” is projected.

The findings will be presented at the United Nations Climate Change Conference in Bali (3–14 December) where countries will continue negotiations on what will follow the end (in 2012) of the first commitment period under the Kyoto Protocol. The first commitment period requires industrialised countries to reduce their emissions by an average of 5 per cent below 1990 levels.

To download the Summary for Policymakers visit www.ipcc.ch