Intergovernmental Panel on Climate Change - Climate Change 2007

Climate Change 2007: The Physical Science Basis - Contribution of Working Group I (WG1) to the Fourth Assessment. Published February 2007.

News Release: IPCC adopts major assessment of climate change science

Paris, 2 February 2007 – Late last night, Working Group I of the Intergovernmental Panel on Climate Change (IPCC) adopted the Summary for Policymakers of the first volume of “Climate Change 2007”, also known as the Fourth Assessment Report (AR4).

“Climate Change 2007: The Physical Science Basis”, 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 some 600 authors from 40 countries.

In this Summary for Policymakers the following levels of confidence have been used to express expert judgments on the correctness of the underlying science:
very high confidence - at least a 9 out of 10 chance of being correct;
high confidence - about an 8 out of 10 chance of being correct.

The following terms have been used to indicate the assessed likelihood, using expert judgement, of an outcome or a result:
Virtually certain > 99% probability of occurrence,
Extremely likely > 95%,
Very likely > 90%,
Likely > 66%,
More likely than not > 50%,
Unlikely less than 33%,
Very unlikely less than 10%,
Extremely unlikely less than 5%.

In general, uncertainty ranges for results given in this Summary for Policymakers are 90% uncertainty intervals unless stated otherwise, i.e., there is an estimated 5% likelihood that the value could be above the range given in square brackets and 5% likelihood that the value could be below that range.

* the Intergovernmental Panel on Climate Change (IPCC) assesses scientific, technical and socio- economic information relevant for the understanding of climate change, its potential impacts and options for adaptation and mitigation.

* Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values (Fig. SPM-1).

* Global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture.

* Carbon dioxide is the most important anthropogenic (def: Caused by humans) greenhouse gas. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005.

* The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores.

* The annual carbon dioxide concentration growth-rate was larger during the last 10 years (1995 – 2005 average: 1.9 ppm per year), than it has been since the beginning of continuous direct atmospheric measurements (1960 – 2005 average: 1.4 ppm per year).

* 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.

* Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.

* The average atmospheric water vapour content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. The increase is broadly consistent with the extra water vapour that warmer air can hold.

* 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 (ice caps do not include contributions from the Greenland and Antarctic ice sheets). Losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003.

* Global average sea level rose at an average rate of 1.8 mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 mm per year.

* There is high confidence that the rate of observed sea level rise increased from the 19th to the 20th century. The total 20th century rise is estimated to be 0.17 m.

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

* since 1978 annual average Arctic sea ice extent has shrunk by 2.7% per decade, with larger decreases in summer of 7.4% per decade.

* the warmth of the last half century is unusual in at least the previous 1300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4-6 metres of sea level rise.

* 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.

* Global average sea level in the last interglacial period (about 125,000 years ago) was likely 4 to 6 m higher than during the 20th century, mainly due to the retreat of polar ice.

* Ice core data indicate that polar temperatures at that time were 3 to 5°C higher than present, because of differences in the Earth’s orbit.

* The Greenland ice sheet and other Arctic ice fields likely contributed no more than 4 m of the observed sea level rise.

* 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.

* Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns (see Figure SPM-4 and Table SPM-2).

* global average surface warming following a doubling of carbon dioxide concentrations is likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded.

* Water vapour changes represent the largest feedback affecting climate sensitivity and are now better understood than in the TAR.

* Cloud feedbacks remain the largest source of uncertainty.

* It is very unlikely that climate changes of at least the seven centuries prior to 1950 were due to variability generated within the climate system alone. A significant fraction of the reconstructed Northern Hemisphere interdecadal temperature variability over those centuries is very likely attributable to volcanic eruptions and changes in solar irradiance, and it is likely that anthropogenic forcing contributed to the early 20th century warming evident in these records.

* 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.

* 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.

* It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent.

* it is likely that future tropical cyclones (typhoons and hurricanes) will become
more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs.

* Increases in the amount of precipitation are very likely in high-latitudes, while decreases are likely in most subtropical land regions

* it is very likely that the meridional overturning circulation (MOC) of the
Atlantic Ocean will slow down during the 21st century. Temperatures in the Atlantic region are projected to increase due to the much larger warming associated with projected increases of greenhouse gases. It is very unlikely that the MOC will undergo a large abrupt transition during the 21st century.

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

* Climate carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain.

* Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100.

* Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance.

* 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.