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''Climate Change 2007'', the Fourth Assessment Report (AR4) of the United Nations '''Intergovernmental Panel on Climate Change''' ( IPCC ), is the fourth in a series of such reports. The IPCC was established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess scientific, technical and socio-economic information relevant for the understanding of Climate Change , its potential impacts and options for adaptation and mitigation.

Two of the three Fourth Assessment reports (working groups I and II) have been been published so far.


''CLIMATE CHANGE 2007'': REPORT OVERVIEW


The Fourth Assessment Report (Climate Change 2007) {Link without Title} is released in four distinct sections:


For each section, the IPCC will release the main report and a summary version, known as the Summary for Policymakers. To date, the full WGI report and the ''Summary for Policymakers'' of the WGII and WGIII reports have been completed and released. Author lists and a chapter outline of the reports are available. WG I Fourth Assessment Report (AR4)


WORKING GROUP I (WGI): THE PHYSICAL SCIENCE BASIS


The Working Group I Summary for Policymakers (SPM) was published on 2 February , 2007 and revised on 5 February , 2007 . The full WGI report was published in March (its front matter, index, and some supplementary material was still pending as of 4 May 2007).

''Climate Change 2007: The Physical Science Basis'', the report of Working Group I, "assesses the current scientific knowledge of the natural and human drivers of climate change, observed changes in climate, the ability of science to attribute changes to different causes, and projections for future climate change".

The report was produced by around 600 authors from 40 countries, and reviewed by over 620 experts and governments. Before being accepted, the summary was reviewed line-by-line by representatives from 113 governments during the 10th Session of Working Group I, IPCC adopts major assessment of climate change science which took place in Paris , France , between 29 January and 1 February 2007 .

On the issue of global warming and its causes, the SPM states that:
  • "Warming of the climate system is ."

  • "Most of the observed increase in globally averaged temperatures since the mid-20th century is'' very likely'' due to the observed increase in Anthropogenic greenhouse gas concentrations."


Footnotes on page 4 of the summary indicate ''very likely'' and ''likely'' mean "the assessed likelihood, using expert judgment", are over 90% and 66% respectively.


Observations


The report notes many observed changes in the Earth's climate including atmospheric composition, global average temperatures, ocean conditions, and other climate changes.


Changes in the atmosphere


Carbon Dioxide , Methane , and Nitrous Oxide are all long-lived Greenhouse Gases .
  • "Carbon dioxide, methane, and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values."

  • The amount of carbon dioxide in the atmosphere in 2005 (379 Ppm ) exceeds by far the natural range of the last 650,000 years (180 to 300 ppm).

  • The amount of methane in the atmosphere in 2005 (1774 Ppb ) exceeds by far the natural range of the last 650,000 years (320 to 790 ppb).

  • The primary source of the increase in carbon dioxide is Fossil Fuel use, but land-use changes also make a contribution.

  • The primary source of the increase in methane is very likely to be a combination of human Agricultural Activities and fossil fuel use. How much each contributes is not well determined.

  • Nitrous oxide concentrations have risen from a pre-industrial value of 270 ppb to a 2005 value of 319 ppb. More than a third of this rise is due to human activity, primarily agriculture.



Warming of the planet


Cold days, cold nights, and frost events have become less frequent. Hot days, hot nights, and heat waves have become more frequent. Additionally:
  • Eleven of the twelve years in the period (1995-2006) rank among the top 12 warmest years in the instrumental record (since 1850).

  • Warming in the last 100 years has caused about a 0.74 °C increase in global average temperature. This is up from the 0.6 °C increase in the 100 years prior to the Third Assessment Report.

  • Urban Heat Island effects were determined to have negligible influence (less than 0.0006 °C per decade over land and zero over oceans) on these measurements.

  • Observations since 1961 show that the ocean has been absorbing more than 80% of the heat added to the climate system, and that ocean temperatures have increased to depths of at least 3000m (9800 ft).

  • "Average Arctic temperatures increased at almost twice the global average rate in the past 100 years."

  • It is likely that greenhouse gases would have caused more warming than we have observed if not for the cooling effects of volcanic and human-caused aerosols. See Global Dimming .

  • Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years.



Ice, snow, permafrost, rain, and the oceans


The SPM documents increases in wind intensity, decline of permafrost coverage, and increases of both drought and heavy precipitation events. Additionally:
  • "Mountain Glacier s and snow cover have declined on average in both hemispheres."

  • Losses from the land-based Ice Sheets of Greenland and Antarctica have very likely (>90%) contributed to sea level rise between 1993 and 2003.

  • Ocean warming causes seawater to expand, which contributes to sea level rising.

  • Sea level rose at an average rate of about 1.8mm/year during the years 1961-2003. The rise in sea level during 1993-2003 was at an average rate of 3.1mm/year. It is not clear whether this is a long-term trend or just variability.

  • Antarctic Sea Ice shows no significant overall trend, consistent with a lack of warming in that region.



Hurricanes


  • There has been an increase in Hurricane intensity in the North Atlantic since the 1970s, and that increase correlates with increases in sea surface temperature.

  • The observed increase in hurricane intensity is larger than Climate Model s predict for the sea surface temperature changes we have experienced.

  • There is no clear trend in the number of hurricanes.

  • Other regions appear to have experienced increased hurricane intensity as well, but there are concerns about the quality of data in these other regions.

  • It is more likely than not (>50%) that there has been some human contribution to the increases in hurricane intensity.

  • It is likely (>66%) that we will see increases in hurricane intensity during the 21st Century .


Table SPM-2 lists recent trends along with certainty levels for the trend having actually occurred, for a human contribution to the trend, and for the trend occurring in the future. In relation to changes (including increased hurricane intensity) where the certainty of a human contribution is stated as "more likely than not" footnote f to table SPM-2 notes "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies."


Factors that warm or cool the planet


AR4 describes warming and cooling effects on the planet in terms of Radiative Forcing — the rate at which energy is being added to the system, measured as power per unit area (in SI units, W/m&2). The report shows in detail the individual warming contributions of carbon dioxide, methane, nitrous oxide, Halocarbons , other human warming factors, and the warming effects of changes in solar activity. Also shown are the cooling effects of Aerosols , Land-use changes, and other human activities. All values are shown as a change from pre-industrial conditions.
  • Total radiative forcing from the sum of all human activities is a warming force of about +1.6 watts/m2

  • Radiative forcing from an increase of Solar Intensity since 1750 is about +0.12 watts/m2

  • Radiative forcing from carbon dioxide, methane, and nitrous oxide combined is very likely (>90%) increasing more quickly during the current era (1750-present) than at any other time in the last 10,000 years.



Climate sensitivity


Climate Sensitivity is defined as the amount of global average surface warming following a doubling of carbon dioxide concentrations. It is likely to be in the range of 2 to 4.5 °C, with a best estimate of about 3 °C. This range of values is not a projection of the temperature rise we will see in the 21st century.


Model-based projections for the future

for a range of stabilization scenarios (the coloured bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of ]]
Model projections are made based on an analysis of various computer climate models running within different SRES Scenarios . As a result it is predicted that, during the 21st century:
  • Surface air warming in the 21st century:

  • ---Best estimate for a "low scenario""''... a convergent world with the same global population, that peaks in mid-century and declines thereafter, ... but with rapid change in economic structures toward a service and information economy, with reductions in material intensity and the introduction of clean and resource efficient technologies. The emphasis is on global solutions to economic, social and environmental sustainability, including improved equity, but without additional climate initiatives.''" is 1.8 °C with a likely range of 1.1 to 2.9 °C (3.2 °F with a likely range of 2.0 to 5.2 °F)

  • ---Best estimate for a "high scenario""''... a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. ... technological emphasis: fossil intensive''" is 4.0 °C with a likely range of 2.4 to 6.4 °C (7.2 °F with a likely range of 4.3 to 11.5 °F)

  • ---A temperature rise of about 0.1 °C per decade would be expected for the next two decades, even if greenhouse gas and aerosol concentrations were kept at year 2000 levels.

  • ---A temperature rise of about 0.2 °C per decade is projected for the next two decades for all SRES scenarios.

  • ---Confidence in these near-term projections is strengthened because of the agreement between past model projections and actual observed temperature increases.

  • Based on multiple models that all exclude will be:

  • ---in a low scenario 18 to 38 cm (7 to 15 inches)

  • ---in a high scenario 26 to 59 cm (10 to 23 inches)

  • It is ''very likely'' that there will be an increase in frequency of warm spells, Heat Wave s and events of heavy rainfall.

  • It is ''likely'' that there will be an increase in areas affected by Drought s, intensity of Tropical Cyclone s (which include hurricanes and Typhoon s) and the occurrence of extreme High Tide s.

  • " Sea Ice is projected to shrink in both the Arctic and Antarctic … In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century."


Scenario specific projections are based on analysis of multiple runs by multiple climate models, using the various SRES Scenarios . "Low scenario" refers to B1, the most optimistic scenario family. "High scenario" refers to A1FI, the most pessimistic scenario family.


Temperature and sea level rise for each SRES scenario family

There are six families of SRES Scenarios , and AR4 provides projected temperature and sea level rises for each scenario family.

  • Scenario B1

  • ---Best estimate temperature rise of 1.8 °C with a likely range of 1.1 to 2.9 °C (3.2 °F with a likely range of 2.0 to 5.2 °F)

  • ---Sea level rise likely range to 38 cm (7 to 15 inches)

  • Scenario A1T

  • ---Best estimate temperature rise of 2.4 °C with a likely range of 1.4 to 3.8 °C (4.3 °F with a likely range of 2.5 to 6.8 °F)

  • ---Sea level rise likely range to 45 cm (8 to 18 inches)

  • Scenario B2

  • ---Best estimate temperature rise of 2.4 °C with a likely range of 1.4 to 3.8 °C (4.3 °F with a likely range of 2.5 to 6.8 °F)

  • ---Sea level rise likely range to 43 cm (8 to 17 inches)

  • Scenario A1B

  • ---Best estimate temperature rise of 2.8 °C with a likely range of 1.7 to 4.4 °C (5.0 °F with a likely range of 3.1 to 7.9 °F)

  • ---Sea level rise likely range to 48 cm (8 to 19 inches)

  • Scenario A2

  • ---Best estimate temperature rise of 3.4 °C with a likely range of 2.0 to 5.4 °C (6.1 °F with a likely range of 3.6 to 9.7 °F)

  • ---Sea level rise likely range to 51 cm (9 to 20 inches)

  • Scenario A1FI

  • ---Best estimate temperature rise of 4.0 °C with a likely range of 2.4 to 6.4 °C (7.2 °F with a likely range of 4.3 to 11.5 °F)

  • ---Sea level rise likely range to 59 cm (10 to 23 inches)



Selected quotes from the WGI Summary for Policymakers


  • "Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere."



Reaction to WGI


In the weeks before publication of the first report, controversy broke out about the report's projections of sea-level change, which in the new report was estimated at less than previous estimates. The now published text gives a warning that the new estimation of sea-level could be too low: "Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise." The mid-points of the sea level rise estimates are within +/- 10% of those from the TAR ; but the range has narrowed.

Lord Rees , the president of the Royal Society , said, "This report makes it clear, more convincingly than ever before, that human actions are writ large on the changes we are seeing, and will see, to our climate. The IPCC strongly emphasises that substantial climate change is inevitable, and we will have to adapt to this. This should compel all of us - world leaders, businesses and individuals - towards action rather than the paralysis of fear. We need both to reduce our emissions of greenhouse gases and to prepare for the impacts of climate change. Those who would claim otherwise can no longer use science as a basis for their argument."1

U.S. Energy Secretary Samuel Bodman told a news conference that the report was "sound science" and "As the president has said, and this report makes clear, human activity is attributing to changes in our earth's climate and that issue is no longer up for debate."2 Kurt Volker, Principal Deputy Assistant Secretary for European and Eurasian Affairs, said, "We support the recent IPCC report, in which U.S. scientists played a leading role."http://www.state.gov/p/eur/rls/rm/80465.htm]

Based on the report, 46 countries in a "Paris Call for Action" read out by French President Chirac , have called for the creation of a United Nations Environment Organization (UNEO), which is to have more power than the current United Nations Environment Programme (UNEP), and is to be modelled after the more powerful World Health Organization . The 46 countries included the European Union nations, but notably did not include the United States , China , Russia , and India , the top four emitters of greenhouse gases.3


WORKING GROUP II (WGII): IMPACTS, ADAPTATION AND VULNERABILITY


The summary for policymakers for the Working Group II {Link without Title} report was released on April 6, 2007.

WGII states that "evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases."


Observations

Some observed changes have been associated with climate change at varying levels of confidence.

With a High Confidence (about an 8 in 10 chance to be correct) WGII asserts that climate change has resulted in:
  • More and larger glacial lakes.

  • Increasing ground instability in permafrost regions.

  • Increasing rock avalanches in mountain regions.

  • Changes in some Arctic and Antarctic ecosystems.

  • Increased run-off and earlier spring peak discharge in many glacier and snow-fed rivers.

  • Changes affecting algae, plankton, fish and zooplankton because rising water temperatures and changes in:

  • ---ice cover

  • ---salinity

  • ---oxygen levels

  • ---water circulation


With a Very High Confidence (about a 9 in 10 chance to be correct) WGII asserts that climate change is affecting terrestrial biological systems in that:
  • Spring events such as the unfolding of leaves, laying of eggs, and migration are happening earlier.

  • There are poleward and upward (to higher altitude) shifts in ranges of plant and animal species.


WGII also states that the ocean has become more acidic because it has absorbed human caused CO2. Ocean pH has dropped by 0.1, but how this affects marine life is not documented.


Attribution of Changes

WGII acknowledges some of the difficulties of attributing specific changes to human caused global warming, stating that "Limitations and gaps prevent more complete attribution of the causes of observed system responses to anthropogenic warming." but found that the agreement between observed and projected changes was "Nevertheless ... sufficient to conclude with high confidence that anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems."


Projections

WGII describes some of what might be expected in the coming century, based on studies and model projections.


Fresh Water

It is projected with High Confidence that:
  • Dry regions are projected to get drier, and wet regions are projected to get wetter.


"By mid-century, annual average river runoff and water availability are projected to increase by 10-40% at high latitudes and in some wet tropical areas, and decrease by 10-30% over some dry regions at mid-latitudes and in the dry tropics..."

  • Drought affected areas will become larger.

  • Heavy precipitation events are very likely to become more common and will increase flood risk.

  • Water supplies stored in glaciers and snow cover will be reduced over the course of the century.



Ecosystems

It is projected with High Confidence that:
  • The resilience of many ecosystems is likely to be exceeded this century by a combination of climate change and other stressors.

  • Carbon removal by terrestrial ecosystems is likely to peak before mid-century and then weaken or reverse. This would amplify climate change.



Food

It is projected with Medium Confidence (about 5 in 10 chance to be correct) that globally, potential food production will increase for temperature rises of 1-3 °C, but decrease for higher temperature ranges.


Coastal Systems

It is projected with Very High Confidence that:
  • Coasts will be exposed to increasing risks such as coastal erosion due to climate change and sea-level rise.

  • "Increases in sea-surface temperature of about 1-3 °C are projected to result in more frequent coral bleaching events and widespread mortality unless there is thermal adaptation or acclimatisation by corals."

  • "Many millions more people are projected to be flooded every year due to sea-level rise by the 2080s."




Reaction to WGII


The Washington Post reported that U.S. negotiators managed to eliminate language calling for cuts in greenhouse gas emissions, according to Patricia Romero Lankao, a lead author from the National Center For Atmospheric Research (NCAR). The original draft read: "However, adaptation alone is not expected to cope with all the projected effects of climate change, and especially not over the long run as most impacts increase in magnitude. ''Mitigation measures will therefore also be required.''" The second sentence does not appear in the final version of the report.4

China objected to wording that said "based on observed evidence, there is very high confidence that many natural systems, on all continents and in most oceans, are being affected by regional climate changes, particularly temperature increases." When China asked that the word "very" be stricken, three scientific authors balked, and the deadlock was broken only by a compromise to delete any reference to confidence levels.


WORKING GROUP III (WGIII): MITIGATION OF CLIMATE CHANGE

See Also: Mitigation of global warming