'Warming of the climate system is unequivocal': highlights of the fourth IPCC Assessment Report.

The Intergovernmental Panel on Climate Change (IPCC) was established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to recognize the problem of potential global climate change. IPCC has three Working Groups and a Task Force and continues to provide scientific, technical and socio-economic advice to the world community, in particular to the Parties to the United Nations Framework Convention on Climate Change (UNFCCC).

The reports by the three IPCC Working Groups provide a comprehensive assessment of the current state of knowledge on climate change and contribute to the Panel's Fourth Assessment Report, Climate Change 2007, which is coming out in 2007. With 450 lead authors and 800 contributors, the Assessment Report includes findings from more than 2,500 scientists from over 130 countries, summing up the last six years of research.

Building upon past IPCC assessments, Working Group I reports on progress in understanding the human and natural drivers of climate change. Working Group II reviews the current understanding of the climate change impacts on natural, managed and human systems, as well as the vulnerability of socio-economic and natural systems to climate change. Working Group III focuses on the scientific, technological, environmental and socio-economic aspects of mitigation of climate change and the options for limiting greenhouse gas emissions.

THE PHYSICAL SCIENCE BASIS

Human and Natural Drivers of Climate Change

* Carbon dioxide (C[O.sub.2]) is the most important anthropogenic greenhouse gas (GHG). The global atmospheric concentration of C[O.sub.2] has increased from a pre-industrial value of about 280 to 379 parts per million in 2005, exceeding by far the natural range over the last 650,000 years (180 to 300 ppm), as determined from ice cores. The annual C[O.sub.2] concentration growth rate was larger over the period 1995-2005.

Warming of the climate system is unequivocal. This can be found in evidence of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising sea levels.

* Eleven of the last 12 years of 1995 to 2006 rank among the 12 warmest in the instrumental record of global surface temperature since 1850.

* Observations since 1961 show that the average temperature of the ocean has increased to at least 3,000 metres in depth and has been absorbing more than 80 per cent of the heat added to the climate system; such warming causes seawater to expand and contributes to sea-level rise.

* Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea-level rise; the global sea level rose at an average rate of 1.8 mm per year from 1961 to 2003 and the rate was faster over the period 1993-2003.

At continental, regional and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation, ocean salinity, wind patterns and aspects of extreme weather conditions, such as droughts, heavy precipitation, heatwaves and the intensity of tropical cyclones like hurricanes and typhoons.

* Average Arctic temperature increased at almost twice the global average rate in the past 100 years.

* Temperatures at the top of the permafrost layer in the Arctic have generally increased since the 1980s.

* More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Changes in sea surface temperatures and wind patterns, and decreased snowpack and snow cover have also been linked to droughts.

* Widespread changes in extreme temperatures have been observed over the last 50 years. Temperature extremes are likely to have increased due to anthropogenic forces.

Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic GHG concentrations. Discernible human influences extend to other aspects of climate change, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.

For the next two decades, a warming of about 0.2[degrees] Celsius per decade is predicted. Even if the concentrations of all GHGs and aerosols had been kept constant at the 2000 levels, a further warming of about 0.1[degrees] per decade would be expected. Continued GHG emissions at or above current rates would cause further warming and induce many changes in the global climate system, which would very likely be larger than those observed during the twentieth century.

Anthropogenic warming and sea-level rise would continue for centuries due to the timescales associated with climate processes and feedbacks, even if GHG concentrations were to be stabilized.

IMPACTS, ADAPTATION AND VULNERABILITY

Observed Impact of Climate Change on the Natural and Human Environment

* Changes in snow, ice and frozen ground, as well as permafrost, show that natural systems are affected. These include enlargement and increased numbers of glacial lakes, increasing ground instability in permafrost regions, and rock avalanches in mountain regions.

* Increased run-off and earlier Spring peak discharge in many glacier and snow-fed rivers.

* Warming of lakes and rivers in many regions, with effects on thermal structure and water quality.

* Recent warming is strongly affecting terrestrial biological systems, including earlier timing of spring events, such as leaf-unfolding, bird migration and egg-laying; poleward and upward shifts in ranges in plant and animal species.

* Changes in marine and freshwater biological systems are associated with rising water temperatures, as well as in ice cover, salinity, oxygen levels and circulation.

A global assessment of data since 1970 has shown it is likely that anthropogenic warming has had a discernible influence on many physical and biological systems.

* More than 29,000 observational data series from 75 studies show significant change in many physical and biological systems; more than 89 per cent are consistent with the direction of change expected due to warming.

Other effects of regional climate changes on natural and human environments are emerging.

* Effects of temperature increases have impacts on agriculture and forestry management at higher latitudes in the northern hemisphere, such as earlier Spring planting of crops.

* Temperature increases will also influence human health, such as heat-related mortality in Europe.

* Some human activities in the Arctic, such as hunting and travel over snow and ice, will be affected by increasing temperatures.

Recent climate changes and variations are beginning to have effects on other natural and human systems.

* Settlements in mountain regions are at enhanced risk to glacier lake outburst floods caused by melting glaciers.

* Sea-level rise and human development are contributing to losses of coastal wetlands and mangroves, leading to increasing damage from costal flooding in many areas.

Freshwater resources and their management

* Drought-affected areas will likely increase in extent, while heavy precipitation events will likely increase in frequency.

* In the course of the century, water supplies stored in glaciers and snow cover are projected to decline, reducing water availability in regions supplied by meltwater from major mountain ranges, where more than one sixth of the world population currently lives.

Ecosystems

* The resilience of many ecosystems is likely to be exceeded this century by the unprecedented combination of climate change, associated disturbances, such as flooding, drought, wildfire, insects, ocean acidification and other global change drivers like land-use change, pollution and overexploitation of resources.

* Approximately 20 to 30 per cent of plant and animal species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5[degrees] C to 2.5[degrees] C.

Food, fibre and forest product

* Increases in the frequency of droughts and floods are projected to affect local crop production negatively, especially in subsistence sectors at low latitudes

Costal systems and low-lying areas

* Coasts are projected to be exposed to increasing risks, including coastal erosion, due to climate change and sea-level rise, and exacerbated by increasing human-induced pressure on coastal areas

* Millions of people are projected to be flooded every year due to sea-level rise by the 2080s, affecting the mega-deltas of Asia and Africa the most, while small islands are especially vulnerable.

Health

* Climate change-related exposures are likely to affect the health status of millions of people, particularly those with low adaptive capacity.

* Climate change is expected to have some mixed effects, such as the decrease or increase of the range and transmission potential of malaria in Africa.

Industry, settlement and society

* Poor communities can be especially vulnerable, particularly those concentrated in high-risk areas, where they have more limited adaptive capacities.

* When extreme weather events become more intense or frequent, the economic and social costs of those events will increase. Climate change impacts spread directly from affected areas and sectors to other areas and sections through extensive and complex linkages.

Adaptation

* Adaptation will be necessary to address impacts resulting from the warming that is already unavoidable due to past emissions.

* Sustainable development can reduce vulnerability to climate change by enhancing adaptive capacity and increasing resilience.

* One measure of progress towards sustainable development is the Millennium Development Goals. Over the next half-century, climate change could impede the achievements of these goals.

* Many impacts can be avoided, reduced or delayed by mitigation.

MITIGATION OF CLIMATE CHANGE

Global GHG emission trends

* Global GHG emissions have grown since pre-industrial times, with an increase of 70 per cent between 1970 and 2004.

* The largest growth in GHG emissions, between 1970 and 2004, has come from the energy supply sector, 120 per cent from transport, 65 per cent from industry and 40 percent from land-use change and forestry.

* With current climate change mitigation policies and related sustainable development practices, GHG emissions will continue to grow over the next few decades.

Mitigation in the short and medium term (until 2030)

* Changes in lifestyle and behaviour patterns can contribute to climate change mitigation across all sectors.

* Transport-demand development, which includes urban planning and provision of information and educational techniques, can reduce car usage and lead to an efficient driving style.

* Changes in housing occupancy behaviour, cultural patterns, consumer choice and use of technologies can result in considerable reduction in C[O.sub.2] emissions related to energy use in buildings.

* The economic potential in the industrial sector is predominately located in energy intensive industries.

* Agricultural practices collectively can make a significant contribution at low cost to increasing soil carbon sinks and to GHG emissions reduction.

Mitigation in the long term (after 2030)

* Mitigation efforts over the next two to three decades will have a large impact on opportunities to achieve lower stabilization levels.

* Policies that provide a real or implicit price of carbon could create incentives for producers and consumers to significantly invest in low-GHG products, technologies and processes.

* Barriers to the implementation of mitigation options are manifold and vary by country and sector. They can be related to financial, technological, institutional, informational and behavioural factors.

* Notable achievements of the UN Framework Convention on Climate Change and its Kyoto Protocol are the establishment of a global response to the climate problem, stimulation of national policies, the creation of an international carbon market and the establishment of new institutional mechanisms that may provide the foundation for future mitigation efforts.

Sustainable development and climate change mitigation

* Making development more sustainable by changing development paths can significantly contribute to climate change mitigation, but implementation may require recourses to overcome multiple barriers.

* Reducing deforestation and loss of natural habitat can have significant biodiversity, soil and water conservation benefits, and can be implemented in a socially and economically sustainable manner.

* Making development more sustainable can enhance both mitigation and adaptive capacity, and reduce emissions and vulnerability to climate change.

Africa

* By 2020, between 75 million and 250 million people are projected to be exposed to an increase in water stress due to climate change.

* Agricultural production, including access to food, is projected to be severely compromised by climate variability and change. This would further adversely affect food security and exacerbate malnutrition in the continent.

* In some African countries, yields from rain-fed agriculture could be reduced by up to 50 per cent by 2020.

* Toward the end of the twenty-first century, projected sea-level rise will affect low-lying coastal areas with large populations.

Asia

* Freshwater availability in Central, South, East and Southeast Asia, particularly in large river basins, is projected to decrease due to climate change, which could affect more than a billion people by the 2050s.

* Coastal areas, especially the heavily-populated mega-delta regions in South, East and Southeast Asia, will be at greatest risk due to increased flooding from the sea and rivers.

* Climate change is projected to impinge on sustainable development of most developing countries of Asia, with the region's rapid urbanization, industrialization and economic development.

Australia and New Zealand 

* Significant loss of biodiversity is projected to occur by 2020 in some ecologically rich sites, including the Great Barrier Reef and Queensland Wet Tropics.

* Production from agriculture and forestry by 2030 is projected to decline over much of southern and eastern Australia and over parts of eastern New Zealand due to increased drought and fire.

Europe

* In Southern Europe, climate change is projected to worsen conditions due to reduced water availability, hydropower potential, summer tourism and crop productivity.

* Mountain areas will face glacier retreat, reduced snow cover, winter tourism and extensive species losses.

* In Central and Eastern Europe, summer precipitation is projected to decrease, causing higher water stress.

Latin America 

* There is a risk of significant biodiversity loss in many areas of tropical Latin America.

* In drier areas, climate change is expected to lead to salinization and desertification of agriculture lands.

* Sea-level rise is projected to cause increased risk of flooding in low-lying areas.

North America

* Warming in western mountain ranges is projected to cause decreased snowpack, more winter flooding and reduced summer flows.

* Cities that currently experience heatwaves are expected to be further challenged by an increased number, intensity and duration of heatwaves.

* Costal communities and habitats will be increasingly stressed by climate change impacts interacting with development and pollution.

Polar Regions

* In the Arctic, additional impacts include reductions in the extent of sea ice and permafrost, increased coastal erosion and increased depth of permafrost seasonal thawing.

* In both polar regions, specific ecosystems and habitats are projected to be vulnerable, as climatic barriers to species invasions are lowered.

Small Islands

* Small islands, whether located in the tropics or higher latitudes, have characteristics that make them especially vulnerable to the effects of climate change, sea-level rise and extreme events.

* Climate change is projected by the mid-century to reduce water resources in many small islands, such as the Caribbean and the Pacific.

* Deterioration in coastal conditions through erosion of beaches and coral bleaching is expected to affect local resources.

COORDINATED BY YUWEI ZHANG