• Warming of the climate system is unequivocal and can now be firmly attributed to human activity.
• Numerous long-term changes in climate have been observed at continental, regional and ocean basin scales, including changes in arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones.\
• The 100-year linear warming trend (1906-2005) was 0.74°C, with most of the warming occurring in the past 50 years. The warming for the next 20 years is projected to be about 0.2°C per decade.
• Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
• Projections of future changes in climate indicate for example the following:
• • Increasing atmospheric carbon dioxide concentrations lead to increasing acidification of the oceans;
  • Snow cover projected to contract, widespread increases in thaw depth over most permafrost regions;
  • Sea ice projected to shrink in both Arctic and Antarctic, and arctic late-summer sea ice may disappear almost entirely by the latter part of the 21st century;
  • Hot extremes, heat waves and heavy precipitation events very likely to continue to become more frequent;
  • Changes in precipitation patterns, with increase being very likely in high latitudes, while decreases are likely in most subtropical land regions;
  • Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilized.

Working Group II: Climate change impacts, adaptation and vulnerability

Observed impacts

Many natural systems, on all continents and most oceans, are being affected by regional climate changes, particularly temperature increases.  Observed impacts include:

• Changes in snow, ice and frozen ground (including permafrost);
• Effects on hydrological systems;
• Changes on terrestrial biological systems;
• Trend towards earlier ‘greening’ of vegetation and longer thermal growing season;
• Changes in marine and freshwater biological systems associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation;
• Ocean acidification with an average decrease in pH of 0.1 units.  The associated effects on the marine biosphere were not documented at the time of the assessment.

Projected future impacts

As regards the projected impacts, more specific information is now available on the nature of these impacts, across a wide range of systems and sectors.  Examples of projected impacts include:

• The Arctic, due to impacts of high rates of projected warming on natural systems and human communities;
• Africa, because of low adaptive capacity and projected climate change impacts;
• Small islands, where there is high exposure of population and infrastructure to projected climate change impacts;
• Asian and African megadeltas, due to large populations and high exposure to sea level rise, storm surges and river flooding.
  

• Global greenhouse gas emissions have grown since pre-industrial times, with an increase of 70 percent between 1970 and 2004 (24 percent between 1990 and 2004);
• With current climate change mitigation policies and related sustainable development practices, global GHG emissions will continue to grow over the next few decades.
  

• Global emissions must peak and decline thereafter to meet any long-term GHG;
• concentration stabilisation level; 
• The lower the stabilisation level, the more quickly this peak and decline must occur; 
• The most stringent scenarios could limit global mean temperature increases to 2 - 2.4C above pre-industrial level .This would require emissions to peak by 2015 the latest and decline by 50-85 percent compared to year 2000 emissions by 2050;
• Mitigation efforts over the next two to three decades will determine to a large extent the long-term global mean temperature increase and the corresponding climate change impacts that can be avoided.
  

• Adaptation can reduce vulnerability, both in the short and the long term;
• Vulnerability to climate change can be exacerbated by the presence of other stresses, arising for example from current climate hazards, poverty, unequal access to resources, food insecurity, trends in economic globalisation, conflict and incidence of diseases such as HIV/AIDS;
• Adaptation will be required at regional and local levels to reduce the adverse impacts of projected climate change and variability, regardless of the scale of mitigation undertaken;
• A wide array of adaptation options is available, but more extensive adaptation than is currently occurring is required to reduce vulnerability to future climate change. There are barriers, limits and costs, but these are not fully understood;
• Adaptive capacity is intimately connected to social and economic development, but it is not evenly distributed across and within societies.
  

• There is substantial economic potential for the mitigation of global GHG emissions over the coming decades, that could offset the projected growth of global emissions or reduce emissions below current levels (high agreement and much evidence);
• In all analysed world regions, near-term health co-benefits from reduced air pollution, as a result of actions to reduce GHG emissions, can be substantial and may offset a substantial fraction of mitigation costs (high agreement and much evidence);
• There may be effects from Annex I countries’ action on the global economy and global emissions, although the scale of carbon leakage remains uncertain (high agreement, medium evidence);
• Changes in life style and behaviour patterns can contribute to climate change mitigation across all sectors.  Management practices can also have a positive role (high agreement, medium evidence);
• 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;
• A wide variety of national policies and instruments are available to governments to create the incentives for mitigation action (high agreement and much evidence). Their applicability depends on national circumstances and an understanding of their interactions, but experience form implementation in various countries and sectors shows there are advantages and disadvantages for any given instrument.