IPCC Fourth Assessment Report (2007)

The IPCC Fourth Assessment Report (AR4) “Climate Change 2007” comprehensively assess policy-relevant scientific, technical and socioeconomic information relevant for the understanding of human induced climate change, potential impacts of climate change and options for mitigation and adaptation. It comprises three Working Group Reports, plus a Synthesis Report bringing together information relevant to the policy needs of governments and the UN Framework Convention on Climate Change. The IPCC Fourth Assessment Report involved over 1200 scientific authors and over 2500 expert reviewers from more than 130 countries. These people are not employed by the IPCC; most work for independent scientific research organisations. The Fourth Assessment broadly supports the direction of the Third Assessment. In most areas, however, the scientific conclusions are now more certain.

The Fourth Assessment Report (AR4) comprised three Working Group Reports, plus a Synthesis Report. The Synthesis Report synthesized and integrated information from the Working Group Reports and other approved IPCC Reports to address a range of policy-relevant questions.

Major points from the three Working Group reports are summarised below, using "headlines" from the Summaries for Policymakers.

The Physical Science Basis (Working Group I)

An increasing body of observations gives a collective picture of a warming world and other changes in the climate system:

  • Warming of the climate is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level.
  • The global average surface temperature has increased over the past 100 years by about 0.74°C, which is larger than the 0.6°C given in the Third Assessment Report (TAR). 
  • Eleven of the last twelve years rank among the warmest years since global surface temperature measurements commenced in 1850.
  • The linear warming trend over the last 50 years is nearly twice that for the last 100 years.
  • The average atmospheric water vapour content has increase since at least the 1980s and is broadly consistent with the extra water vapour that warmer air can hold.
  • Snow cover and ice extent have decreased on average in both hemispheres.
  • Global average sea level has risen at an average rate of 1.8 mm per year (1961 – 2003) and the rate was even faster over 1993 – 2003 at approximately 3.1 mm per year.
  • Ocean heat content has increased. The ocean has been absorbing more than 80% of the heat added to the climate system.
  • Arctic temperatures increased at almost twice the global average in the past 100 years.
  • Annual average arctic sea ice extent has shrunk by 2.7% per decade since 1978.
  • Increased precipitation has been observed in eastern parts of North and South America, northern Europe and northern and central Asia.
  • Drying has been observed in the Sahel, the Mediterranean, southern Africa and parts of southern Asia.
  • More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics.
  • Frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour.
  • Cold days, cold nights and frost have become less frequent, while hot days hot nights and heat waves have become more frequent.

Changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation due to human activities continue to alter the atmosphere in ways that are expected to affect the climate:

  • Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activities
    Explanatory note: Radiative forcing is a measure of the importance of a potential climate change mechanism. A positive radiative forcing tends to warm the surface and a negative radiative forcing tends to cool the surface.
  • Global greenhouse gas emissions due to human activities have grown since pre-industrial times by 70% between 1970 and 2004.
  • Global atmospheric concentrations of CO2, methane (CH4) and nitrous oxide (N2O) have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values.
  • Annual emissions of CO2 grew by about 80% between 1970 and 2004.
  • The combined radiative forcing due to increases in carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) is +2.30 W m-2. The CO2 radiative forcing increased by 20% from 1995 to 2005.
  • Global atmospheric concentration of CO2 has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005.
  • The global atmospheric concentration of the greenhouse gas methane (CH4) has increased from a pre-industrial value of about 715 ppb to 1774 ppb in 2005.
  • The atmospheric concentration of CO2 and CH4 in 2005 greatly exceed the natural range over the last 650,000 years (180 to 300 ppm).
  • The annual growth rate of CO2 was larger during the last 10 years (1.9 ppm per year) since continuous direct measurements began in 1960.
  • Primary source of increased atmospheric concentration of CO2 results from fossil fuel use. Land-use change provides another significant, but smaller contribution.
  • The global atmospheric concentration of the greenhouse gas N2O has increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005.
  • Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing producing a cooling effect, with a total direct radiative forcing of -0.5 W m-2 and an indirect cloud albedo forcing of -0.7 W m-2.
  • Tropospheric ozone changes due to emissions of ozone-forming chemicals contribute +0.34 W m-2 to radative forcing.
  • Changes in halocarbons contribute + 0.34 W m-2 towards radiative forcing.
  • Changes in surface albedo, due to land cover changes and deposition of black carbon aerosols on snow, exert forcings of -0.2 and +0.1 W m-2 respectively.
  • There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
  • Human influences will continue to change atmospheric composition throughout the 21st century.

Projections of future changes in climate:

  • The report summarises projections for changes in emissions and concentrations of greenhouse gases and aerosols through the 21st century, based on emission scenarios from the IPCC Special Report on Emission Scenarios (SRES).
  • Confidence in the ability of models to project future climate has increased.
  • There is now a large number of simulations available from a broader range of models. These models taken together with additional information from observations provide a quantitative basis for estimating likelihoods for many aspects of future climate change.
  • Global average temperature and sea level are projected to rise under all IPCC SRES scenarios.
  • Temperature: Based on the mid-range A1B scenario temperature is projected to increase by about 2.8 °C, within a likely range of 1.7 to 4.4 °C, over the period 1990 to 2100. This assessment relies on a large number of climate models of increasing complexity and realism.
  • Precipitation: Global average water vapour concentration and precipitation are projected to increase during the 21st century. Increases in the volume of precipitation are very likely in high latitudes, while decreases are likely in most subtropical land regions.
  • Extreme events: Click here for a table summarising observed and projected changes in extreme events. For some other extreme phenomena which may have important impacts there is currently insufficient information to assess recent trends, and climate models currently lack the spatial detail required to make confident projections (e.g. for changes in thunderstorms, tornadoes, hail and lightning).
  • Hot extremes, heat waves and heavy precipitation events will continue to become more frequent. Typhoons and hurricanes will become more intense with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical sea surface temperatures.
  • El Niño: Current projections show little change or a small increase in the amplitude of El Niño events over the next 100 years, but there are some shortcomings in how well El Niño is simulated in complex models. Global warming is likely to lead to greater extremes of drying and heavy rainfall and increase the risk of droughts and floods that occur with El Niño events over many regions, even if there is little or no change in El Niño amplitude.
  • Monsoons: An increase in Asian summer monsoon precipitation variability is likely.
  • Thermohaline circulation: Most models show weakening of the ocean thermohaline circulation, which leads to a reduction of the heat transport into high latitudes of the Northern Hemisphere.
  • Snow and ice: Northern Hemisphere snow cover and sea-ice extent are projected to decrease further. Glaciers and ice caps are expected to continue retreating during the 21st century. The Antarctic ice sheet is likely to gain mass because of greater precipitation, while the Greenland ice sheet is expected to lose mass.
  • Sea level: Based on the A1B scenario level is projected to rise by 0.21 – 0.48 metres between 1990 and 2100. This is due primarily to thermal expansion and loss of mass from glaciers and ice caps.
  • Acidification of the ocean: Increasing atmospheric CO2 concentrations lead to increasing acidification of the ocean of between 0.14 and 0.35 pH units over the 21st century.
  • Anthropogenic climate change will persist for many centuries.
  • Further action is required to address remaining gaps in information and understanding.

Impacts, Adaptation and Vulnerability (Working Group II)

This report addresses the current scientific understanding of the impacts of climate change on natural, managed and human systems, the capacity for these systems to adapt and their vulnerability.


  • Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases.
  • 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.
  • Other effects of regional climate changes on natural and human environments are emerging, although many are difficult to discern due to adaptation and non-climate drivers.
  • More specific information is now available across a wide range of systems and sectors concerning the nature of future impacts, including for some fields not covered in previous assessments.
  • Magnitudes of impact can now be estimated more systematically for a range of possible increase in global average temperature.
  • Natural systems are vulnerable to climate change, and some will be irreversibly damaged.
  • Impacts due to altered frequencies and intensities of extreme weather, climate and sea-level events are very likely to change.
  • Some large-scale climate events have the potential to cause very large impacts, especially after the 21st century.
  • The potential for large scale and possibly irreversible impacts poses risks that have yet to be reliably quantified.
  • Examples mentioned include: increased ground instability in permafrost regions and rock avalanches in mountain regions, water supplies in glaciers and snow cover and likely to decline, warming of lakes and rivers in many regions, with effects on thermal structure and water quality, effects on aspects of human health, such as heat-related mortality in Europe and infectious disease in some areas, increases deaths, disease and injury due to heatwaves, floods, storms, fires and droughts, increases in the frequency of droughts and floods are projected to affect local crop production negatively, sea-level rise and human development contributing to losses of coastal wetland areas and increasing damage from coastal flooding, millions more people are projected to be flooded every year due to sea-level rise, more frequent coral bleaching events on coral reefs, accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and some weather events will become more frequent, more widespread and/or more intense.
  • The likelihood of many of these changes in Earth systems is not well-known, but is probably very low; however their likelihood is expected to increase with the rate, magnitude and direction of climate change.


  • Adaptation is a necessary strategy at all scales to complement climate change mitigation efforts.
  • Adaptation will be necessary to address impacts resulting from the warming which is already unavoidable due to past emissions.
  • Some adaptation is occurring now, to observed and projected future climate, but on a limited basis.
  • A wide range of adaptation options is available, but more extensive adaptation than is currently occurring is required to reduce vulnerability to future climate change.
  • Adaptation alone is not expected to cope with all projected effects of climate change, and especially not over the long term as most impacts increase in magnitude.
  • Many impacts can be avoided, reduced or delayed by mitigation.


  • Vulnerability to climate change can be exacerbated by the presence of other stresses.
  • Sustainable development can reduce vulnerability to climate change, and climate change could impede nations’ abilities to achieve sustainable development pathways.
  • Those with the least resources have the least capacity to adapt and are the most vulnerable.

Mitigation (Working Group III)

This report focuses on new literature addressing the scientific, technological, environmental, economic and social aspects of the mitigation of climate change published since the IPCC Third Assessment Report (TAR).

Greenhouse gas emission trends:

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

Mitigation in the short and medium term (until 2030):

  • There is a substantial economic potential for the mitigation of global greenhouse gas emissions over the coming decades that could offset the projected growth of global emissions or reduce emissions below current levels.
  • No one sector or technology can address the entire mitigation challenge.
  • Changes in lifestyle and behaviour patterns can contribute to climate change mitigation across all sectors. Management practices can also have a positive role.
  • Near-term health co-benefits from reduced air pollution as a result of actions to reduce greenhouse gas emissions can be substantial and may offset a substantial fraction of mitigation costs.
  • New energy infrastructure investments in developing countries, upgrades of energy infrastructure in industrialised countries, and policies that promote energy security, can create opportunities to achieve greenhouse gas emissions reductions compared to baseline scenarios.
  • There are multiple mitigations options in the transport sector, but their effects may be counteracted by growth in the sector. Mitigation options are faced with many barriers, such as consumer preferences and lack of policy framework.
  • Energy efficiency options for new and existing buildings could considerably reduce CO2 emissions with net economic benefit. Many barriers exist against tapping this potential, but there are also large co-benefits.
  • Agricultural practices collectively can make significant contribution at low cost to increasing soil carbon sinks, to greenhouse gas emission reductions, and by contributing biomass feedstocks for energy use.
  • Forest-related mitigation activities can considerably reduce emissions from sources and increase CO2 removals by sinks at low cost, and can be designed to create synergies with adoption and sustainable development.

Mitigation in the long term (after 2030):

  • The report states that in order to stabilise the concentration of greenhouse gas in the atmosphere, emissions would have to peak and decline thereafter. The lower the stabilisation level, the more quickly this peak and decline would need to occur.
  • Mitigation efforts over the next two to three decades will have a large impact on opportunities to achieve lower stabilisation levels.
  • The range of stabilisation levels assessed can be achieved by deployment of a portfolio of technologies that are currently available and those that are expected to be commercialised in coming decades. This assumes that appropriate and effective incentives are in place for development, acquisition, deployment and diffusion of technologies and for addressing related barriers.
  • Decision-making about the appropriate level of global mitigation over time involves an iterative risk management process that includes mitigation and adaptation, taking into account actual and avoided climate change damages, co-benefits, sustainability, equity, and attitudes to risk.
  • Choices about the scale and timing of greenhouse gas mitigation involve balancing the economic costs of more rapid emission reductions now against the corresponding medium-term and long-term climate risks of delay.

Policies, measures and instruments to mitigate climate change:

  • The report suggests that there is a wide variety of national policies and instruments available to governments to create the incentives for mitigation action. Their applicability depends on national circumstances and an understanding of their interactions, but experience from implementation in various countries and sectors shows there are advantages and disadvantages for any given instrument.
  • Policies that provide a real or implicit price of carbon could create incentives for producers and customers to significantly invest in low-greenhouse gas products, technologies and processes. Such policies could include economic instruments, government funding and regulation.
  • Government support through financial contributions, tax credits, standard setting and market creation is important for effective technology development, innovation and deployment. Transfer of technology to developing countries depends on enabling conditions and financing.
  • Notable achievements of the UNFCCC and its Kyoto Protocol are the establishment of a global response to the climate problem, stimulation of an array 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.
  • The literature identifies many options for achieving reductions of global GHG emissions at the international level through cooperation. It also suggests that successful agreements are environmentally effective, cost-effective, incorporate distributional considerations and equity, and are institutionally feasible.

Sustainable development and climate change mitigation:

  • Making development more sustainable by changing development paths can make a major contribution to climate change mitigation, but implementation may require resources to overcome multiple barriers.
  • There is a growing understanding of the possibilities to choose and implement mitigation options in several sectors to realize synergies and avoid conflicts with other dimensions of sustainable development.

Gaps in knowledge:

  • The report notes that advances have been made since previous IPCC assessments in the understanding of the scientific, technical, environmental, and economic and social aspects of climate change. However it also points out that there are still relevant gaps in currently available knowledge regarding some aspects of mitigation of climate change, especially in developing countries.
  • The report states that additional research addressing those gaps would further reduce uncertainties and thus facilitate decision-making related to mitigation of climate change.