IPCC Fourth Assessment Report




Climate Change 2007, the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change, was published in 2007 and is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for adaptation and mitigation. The report is the largest and most detailed summary of the climate change situation ever undertaken, produced by thousands of authors, editors, and reviewers from dozens of countries, citing over 6,000 peer-reviewed scientific studies. People from over 130 countries contributed to the IPCC Fourth Assessment Report, which took six years to produce. Contributors to AR4 included more than 2,500 scientific expert reviewers, more than 800 contributing authors, and more than 450 lead authors.
"Robust findings" of the Synthesis report include:
  • "Warming of the climate system 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".
  • Most of the global average warming over the past 50 years is "very likely" due to human activities.
  • "Impacts will very likely increase due to increased frequencies and intensities of some extreme weather events".
  • "Anthropogenic warming and sea level rise would continue for centuries even if GHG emissions were to be reduced sufficiently for GHG concentrations to stabilise, due to the time scales associated with climate processes and feedbacks". Stabilization of atmospheric greenhouse gas concentrations is discussed in climate change mitigation.
  • "Some planned adaptation is occurring now; more extensive adaptation is required to reduce vulnerability to climate change".
  • "Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt".
  • "Many impacts can be reduced, delayed or avoided by mitigation".

    Overview

Like previous assessment reports, it consists of four reports:
Global warming projections from AR4 are shown below. The projections apply to the end of the 21st century, relative to temperatures at the end of the 20th century. Add 0.7 °C to projections to make them relative to pre-industrial levels instead of 1980–1999.. Descriptions of the greenhouse gas emissions scenarios can be found in Special Report on Emissions Scenarios.
Emissions
scenario		Best estimate
"Likely" range
B11.81.1 – 2.9
A1T2.41.4 – 3.8
B22.41.4 – 3.8
A1B2.81.7 – 4.4
A23.42.0 – 5.4
A1FI4.02.4 – 6.4

"Likely" means greater than 66% probability of being correct, based on expert judgement.

Sections

The report was released in four principal sections:
  • Contribution of Working Group I : Climate Change 2007: The Physical Science Basis.
  • Contribution of Working Group II : Climate Change 2007: Impacts, Adaptation and Vulnerability.
  • Contribution of Working Group III : Climate Change 2007: Mitigation of Climate Change.
  • Contribution of Working Groups I, II, and III: The Synthesis Report.

    Working Group I: The Physical Science Basis

The full WGI report was published in March 2007, and last updated in September of that year. It includes a Summary for Policymakers, which was published in February 2007, and a Frequently Asked Questions section.
This section of the report, Climate Change 2007: The Physical Science Basis, assessed current scientific knowledge of "the natural and human drivers of climate change" as well as observed changes in climate. It looked at the ability of science to attribute changes to different causes, and made projections of future climate change.
It was produced by 676 authors from 40 countries, then reviewed by over 625 expert reviewers. More than 6,000 peer-reviewed publications were cited.
Before being approved, the summary was reviewed line by line by representatives of 113 governments during the 10th session of WGI, in January to February 2007.
On the issue of global warming and its causes, the SPM states that:
  • "Warming of the climate system is unequivocal."
  • "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."
Very likely and likely mean "the assessed likelihood, using expert judgment" are over 90% and over 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

, 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 exceeds by far the natural range of the last 650,000 years.
  • The amount of methane in the atmosphere in 2005 exceeds by far the natural range of the last 650,000 years.
  • 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 rank among the top 12 warmest years in the instrumental record.
  • 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 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 3000 m.
  • "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 glaciers and snow cover have declined on average in both hemispheres."
  • Losses from the land-based ice sheets of Greenland and Antarctica have very likely 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 1.8 mm/year during the years 1961–2003. The rise in sea level during 1993–2003 was at an average rate of 3.1 mm/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 models 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 that there has been some human contribution to the increases in hurricane intensity.
  • It is likely 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 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 of change of energy in the system, measured as power per unit area. 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 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 increasing more quickly during the current era than at any other time in the last 10,000 years.