National Committees

SUMMARY OF THE IPCC FOURTH ASSESSMENT SYNTHESIS REPORT

The Intergovernmental Panel on Climate Change (IPCC) has prepared a synthesis of the conclusions of the three Working Groups. Each Working Group released their major reports earlier in 2007 summarising the worldwide research findings up to 2006. These reports cover the physical science of climate change; impacts adaptation and vulnerability to climate change; and mitigation of climate change.

The IPCC overall synthesis summary concludes that there is unequivocal observation of accelerating warming of the global surface atmosphere and oceans. This is leading to widespread melting of snow and ice, and sea level rising by over 3mm per year. It is considered very likely – with more than 90% certainty – that the main cause of this climatic change over the last 50 years is the increasing global atmospheric concentration of carbon dioxide, methane and nitrous oxide as a result of human burning of fossil fuels and usages of vegetated land. This discernible human influence on global and regional average temperatures now extends to other aspects of climate including temperature extremes, wind patterns and distribution of land rainfall. There is medium confidence that this warming has been the cause of numerous changes such as earlier springs in higher northern hemisphere latitudes, and increased heat-related mortality in Europe. The other anticipated direct effects of the increasing atmospheric CO₂ concentration, acting via observable increases in ocean acidity and plant photosynthesis rates, on marine processes and land ecosystems have not yet been quantified on a global basis.

The increasing emission of greenhouse gases is an expression of rapidly increasing human population size (69% increase between 1970 and 2004) combined with rising average income per person (77% average increase), offset a little by an overall decrease in energy intensity of economic activity (33% decrease), which has reversed since 2000. Some future impacts already appear inevitable owing to the inertia of the climate system. For the range of plausible future greenhouse gas emission scenarios, continued global average warming of 0.2^oC per decade is projected for the next couple of decades no matter what emission policies are taking effect in that period. Even if greenhouse gas concentrations in the atmosphere were somehow to be stabilised immediately (impossible in practice), some warming (starting at 0.1^oC per decade) and sea-level rise would continue for centuries at a gradually diminishing annual rate of increase, owing to time-lags in the climate system. The pace at which continued warming levels off depends on emission reduction policies world wide that are implemented in the near future. Uncertainty about the precise speed of continued temperature increase beyond a couple of decades, for any given scenario of greenhouse gas emissions, relates most strongly to our relatively poor current understanding of long term carbon cycle feedbacks, cloud feedbacks, the role of man-made and natural aerosols, the speed of polar ice-melt, the rate of ocean heat uptake, and the possibility of large scale changes in oceanic circulation. In addition, abrupt irreversible changes ('trigger points'), which can arise in complex chaotic systems like the climate system, are unpredictable possibilities but are considered very unlikely this century. However, even without abrupt, irreversible climate change occurring, irreversible impacts on ecosystems from gradual climate change are likely.

To hold the global average temperature increase down to 2–2.4^oC (above the pre-industrial average) at equilibrium, requires mankind to stabilise greenhouse gas concentration at 445–490 ppm CO₂ equivalent. This requires reducing greenhouse gas emissions by 50 to 85% of 2000AD levels by 2050. A temperature increase of 2^oC above the pre-industrial average has been widely mooted as an increase that could be dangerous to vulnerable systems such as low lying coastal regions, Mediterranean-type ecosystems, water resources in the dry tropics and subtropics, and human health in areas of low adaptive capacity. Water security problems are projected with high-confidence to intensify in southern and eastern Australia.

There is high confidence that in the short term there are adaptation measures that can be taken to reduce vulnerability to the continuing climate change but more extensive adaptation measures than are currently occurring are needed to reduce vulnerability. Adaptation measures can include improved water storage and conservation techniques in areas of increasing water scarcity, changed planting dates and diversification in agricultural enterprises, sea-wall and storm surge barriers on vulnerable coastal areas, and heat wave alerts for human health. In the longer term, financial, technological, cognitive, behavioural, social and cultural constraints, that are not well understood, will pose barriers to the implementation and effectiveness of adaptation measures. Adaptive capacity is unevenly distributed being closely connected to social and economic development.

There is substantial economic potential, and a wide variety of policies and instruments available to governments for creating incentives, to implement mitigation measures to reduce greenhouse gas emissions over the coming decades. Such measures include changes in lifestyle, behavior patterns and management practices. An effective carbon-price signal could realise significant mitigation potential in all economic sectors. No single technology can provide all the mitigation potential in any sector. However, stabilisation can be achieved by deploying a range of technologies that are either existing or will be commercialised in coming decades assuming that appropriate and effective incentives are put in place. Without substantial investment flows and effective technology transfer, it may be difficult to achieve significant emission reduction. However, on a global average basis, the necessary investments to achieve stabilisation at between 445 and 710ppm CO₂ equivalent would slow annual global GDP growth by less than 0.12 percentage points.

Roger M. Gifford
Chairman, National Committee for Earth Systems Science
November 2007