151.2.1 Setting the Stage for the Assessment

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The IPCC was set up in 1988 by the World Meteorological Organization and the United Nations Environment Programme to provide governments with a clear view of the current state of knowledge about the science of climate change, potential impacts, and options for adaptation and mitigation through regular assessments of the most recent information published in the scientific, technical and socio-economic literature worldwide. The WGI contribution to the IPCC AR5 assesses the current state of the physical sciences with respect to climate change. This report presents an assessment of the current state of research results and is not a discussion of all relevant papers as would be included in a review. It thus seeks to make sure that the range of scientific views, as represented in the peer-reviewed literature, is considered and evaluated in the assessment, and that the state of the science is concisely and accurately presented. A transparent review process ensures that disparate views are included (IPCC, 2012a).[1]

As an overview, Table 1.1 shows a selection of key findings from earlier IPCC assessments. This table provides a non-comprehensive selection of key assessment statements from previous assessment reports— IPCC First Assessment Report (FAR, IPCC, 1990), IPCC Second Assessment Report (SAR, IPCC, 1996), IPCC Third Assessment Report (TAR, IPCC, 2001) and IPCC Fourth Assessment Report (AR4, IPCC, 2007)— with a focus on policy-relevant quantities that have been evaluated in each of the IPCC assessments.[2]

Scientific hypotheses are contingent and always open to revision in light of new evidence and theory. In this sense the distinguishing features of scientific enquiry are the search for truth and the willingness to subject itself to critical re-examination. Modern research science conducts this critical revision through processes such as the peer review. At conferences and in the procedures that surround publication in peer-reviewed journals, scientific claims about environmental processes are analysed and held up to scrutiny. Even after publication, findings are further analysed and evaluated. That is the self-correcting nature of the scientific process (more details are given in AR4 Chapter 1 and Le Treut et al., 2007).[3]

Science strives for objectivity but inevitably also involves choices and judgements. Scientists make choices regarding data and models, which processes to include and which to leave out. Usually these choices are uncontroversial and play only a minor role in the production of research. Sometimes, however, the choices scientists make are sources of disagreement and uncertainty. These are usually resolved by further scientific enquiry into the sources of disagreement. In some cases, experts cannot reach a consensus view. Examples in climate science include how best to evaluate climate models relative to observations, how best to evaluate potential sea level rise and how to evaluate probabilistic projections of climate change. In many cases there may be no definitive solution to these questions. The IPCC process is aimed at assessing the literature as it stands and attempts to reflect the level of reasonable scientific consensus as well as disagreement.

To assess areas of scientific controversy, the peer-reviewed literature is considered and evaluated. Not all papers on a controversial point can be discussed individually in an assessment, but every effort has been made here to ensure that all views represented in the peer-reviewed literature are considered in the assessment process. A list of topical issues is given in Table 1.3.

The Earth sciences study the multitude of processes that shape our environment. Some of these processes can be understood through idealized laboratory experiments, by altering a single element and then tracing through the effects of that controlled change. However, as in other natural and the social sciences, the openness of environmental systems, in terms of our lack of control of the boundaries of the system, their spatially and temporally multi-scale character and the complexity of interactions, often hamper scientists’ ability to definitively isolate causal links. This in turn places important limits on the understanding of many of the inferences in the Earth sciences (e.g., Oreskes et al., 1994).[4] There are many cases where scientists are able to make inferences using statistical tools with considerable evidential support and with high degrees of confidence, and conceptual and numerical modelling can assist in forming understanding and intuition about the interaction of dynamic processes.

Table 1.1 Historical overview of major conclusions of previous IPCC assessment reports. The table provides a non-comprehensive selection of key statements from previous assessment reports—IPCC First Assessment Report (FAR; IPCC,1990), IPCC Second Assessment Report (AR2; IPCC, 1996), IPCC Third Assessment Report (AR3; IPCC, 2001) and IPCC Fourth Assessment Report (AR4; IPCC, 2007)—with a focus on global mean surface air temperature and sea level change as two policy relevant quantities that have been covered in IPCC since the first assessment report.

TopicAR2 SPM StatementAR3 SPM StatementAR4 SPM StatementAR5 SPM Statement
Human and Natural Drivers of Climate ChangeThere is a natural greenhouse effect which already keeps the Earth warmer than it would otherwise be. Emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases carbon dioxide, methane, chlorofluorocarbons and nitrous oxide. These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface.
Continued emissions of these gases at present rates would commit us to increased concentrations for centuries ahead.
Greenhouse gas concentrations have continued to increase. These trends can be attributed largely to human activities, mostly fossil fuel use, land use change and agriculture.
Anthropogenic aerosols are shortlived and tend to produce negative radiative forcing.
Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate. The atmospheric concentration of CO2 has increased by 31% since 1750 and that of methane by 151%.
Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing by their direct effect. There is more evidence for their indirect effect, which is negative, although of very uncertain magnitude.
Natural factors have made small contributions to radiative forcing over the past century.
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 determined from ice cores spanning many thousands of years. The 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.
Very high confidence that the global average net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W m–2.
Direct Observations of Recent Climate Change - TemperatureGlobal mean surface air temperature has increased by 0.3°C to 0.6°C over the last 100 years, with the five global-average warmest years being in the 1980s.Climate has changed over the past century. Global mean surface temperature has increased by between about 0.3 and 0.6°C since the late 19th century. Recent years have been among the warmest since 1860, despite the cooling effect of the 1991 Mt. Pinatubo volcanic eruptionAn increasing body of observations gives a collective picture of a warming world and other changes in the climate system.
The global average temperature has increased since 1861. Over the 20th century the increase has been 0.6°C.
Some important aspects of climate appear not to have changed.
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.
Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). The updated 100-year linear trend (1906 to 2005) of 0.74°C [0.56°C to 0.92°C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6°C [0.4°C to 0.8°C].
Some aspects of climate have not been observed to change.
Direct Observations of Recent Climate Change - Sea level riseOver the same period global sea level has increased by 10 to 20 cm These increases have not been smooth with time, nor uniform over the globe.Global sea level has risen by between 10 and 25 cm over the past 100 years and much of the rise may be related to the increase in global mean temperature.Tide gauge data show that global average sea level rose between 0.1 and 0.2 m during the 20th century.Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003: about 3.1 [2.4 to 3.8] mm per year. The total 20th century rise is estimated to be 0.17 [0.12 to 0.22] m.
A Palaeoclimatic PerspectiveClimate varies naturally on all timescales from hundreds of millions of years down to the year-to-year. Prominent in the Earth’s history have been the 100,000 year glacial–interglacial cycles when climate was mostly cooler than at present. Global surface temperatures have typically varied by 5°C to 7°C through these cycles, with large changes in ice volume and sea level, and temperature changes as great as 10°C to 15°C in some middle and high latitude regions of the Northern Hemisphere. Since the end of the last ice age, about 10,000 years ago, global surface temperatures have probably fluctuated by little more than 1°C. Some fluctuations have lasted several centuries, including the Little Ice Age which ended in the nineteenth century and which appears to have been global in extent.The limited available evidence from proxy climate indicators suggests that the 20th century global mean temperature is at least as warm as any other century since at least 1400 AD. Data prior to 1400 are too sparse to allow the reliable estimation of global mean temperature.New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year. Because less data are available, less is known about annual averages prior to 1,000 years before present and for conditions prevailing in most of the Southern Hemisphere prior to 1861.Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 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 to 6 m of sea level rise.
Understanding and Attributing Climate ChangeThe size of this warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability; alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming. The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.The balance of evidence suggests a discernible human influence on global climate. Simulations with coupled atmosphere–ocean models have provided important information about decade to century timescale natural internal climate variability.There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. There is a longer and more scrutinized temperature record and new model estimates of variability. Reconstructions of climate data for the past 1,000 years indicate this warming was unusual and is unlikely to be entirely natural in origin.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. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
Projections of Future Changes in Climate- TemperatureUnder the IPCC Business-as-Usual emissions of greenhouse gases, a rate of increase of global mean temperature during the next century of about 0.3°C per decade (with an uncertainty range of 0.2°C to 0.5°C per decade); this is greater than that seen over the past 10,000 years.Climate is expected to continue to change in the future. For the midrange IPCC emission scenario, IS92a, assuming the ‘best estimate’ value of climate sensitivity and including the effects of future increases in aerosols, models project an increase in global mean surface air temperature relative to 1990 of about 2°C by 2100.Global average temperature and sea level are projected to rise under all IPCC SRES scenarios. The globally averaged surface temperature is projected to increase by 1.4°C to 5.8°C over the period 1990 to 2100.
Confidence in the ability of models to project future climate has increased.
Anthropogenic climate change will persist for many centuries.
For the next two decades, a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected.
There is now higher confidence in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation and some aspects of extremes and of ice.
Anthropogenic warming and sea level rise would continue for centuries, even if greenhouse gas concentrations were to be stabilised.
Projections of Future Changes in Climate- Sea level riseAn average rate of global mean sea level rise of about 6 cm per decade over the next century (with an uncertainty range of 3 to 10 cm per decade) is projected.Models project a sea level rise of 50 cm from the present to 2100.Global mean sea level is projected to rise by 0.09 to 0.88 m between 1990 and 2100.Global sea level rise for the range of scenarios is projected as 0.18 to 0.59 m by the end of the 21st century.


  1. IPCC procedures">PRINCIPLES AND PROCEDURESAppendix A to the Principles Governing IPCC Work, Geneva, Switzerland, 6-9 June 2012, 29 pp.
  2. IPCC ReportsCambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  3. Le Treut, H., R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson and M. Prather, 2007: Historical Overview of Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  4. Verification, Validation, and Confirmation of Numerical Models in the Earth Sciences Science 4 February 1994 Vol. 263 no. 5147 pp. 641-646 DOI: 10.1126/science.263.5147.641
ES 1.1 1.2.1 1.2.2 1.2.3 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4.1 1.4.2 1.4.3 1.4.4 1.5 1.5.1 1.5.2 1.6 Box 1 FAQ Refs

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