August 20, 2014

The Physical Science Basis

Intergovernmental Panel On Climate Change: AR5 Working Group I

Detection and Attribution of Climate Change

Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes (Figure SPM.6).
This evidence for human influence has grown since AR4.
[There is 99% confidence] that human influence has been the dominant cause of the observed warming since the mid-20th century.
(p 12)

There has been further strengthening of the evidence for human influence on temperature extremes since the SREX.
It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century, and likely that human influence has more than doubled the probability of occurrence of heat waves in some locations. …

There is high confidence that changes in total solar irradiance have not contributed to the increase in global mean surface temperature over the period 1986 to 2008, based on direct satellite measurements of total solar irradiance.

No robust association between changes in cosmic rays and cloudiness has been identified.
(p 13)

Future Global and Regional Climate Change

Relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is projected to …
  • likely … exceed 2°C for RCP6.0 and RCP8.5 (high confidence) …
  • as likely as not … exceed 4°C for RCP8.5 (medium confidence).
(p 15)

Adapted from Table SPM.2, Table SPM.3 and Box SPM.1

Cumulative CO2 Emissions (b) (GtC)Surface Temperature (c) (°C)Sea Level Rise (d) (m)
Scenario (a)
(ppm CO2e)
Mean (Range)
Global Mean Increase (likely range)
(Relative to 1986-2005)
RCP2.6 (475)270 (14-410)1.0 (0.4-1.6)1.0 (0.3-1.7)0.24 (0.17-0.32)0.40 (0.25-0.55)
RCP4.5 (630)780 (595-1005)1.4 (0.9-2.0)1.8 (1.1-2.6)0.26 (0.19-0.33)0.47 (0.32-0.63)
RCP6.0 (800)1060 (840-1250)1.3 (0.8-1.8)2.2 (1.4-3.1)0.25 (0.18-0.32)0.48 (0.33-0.63)
RCP8.5 (1313)1685 (1415-1910)2.0 (1.4-2.6)3.7 (2.6-4.8)0.30 (0.22-0.38)0.63 (0.45-0.82)

Representative Concentration Pathways (RCP) (a) are identified by their approximate total radiative forcing in year 2100 relative to 1750:
  • 2.6 W/m² for RCP2.6,
  • 4.5 W/m² for RCP4.5,
  • 6.0 W/m² for RCP6.0 and
  • 8.5 W/m² for RCP8.5.

Cumulative CO2 emissions for the 2012–2100 period (b) compatible with the RCP atmospheric concentrations simulated by the CMIP5 Earth System Models.
  • Most of the CMIP5 and Earth System Model (ESM) simulations were performed with prescribed CO2 concentrations reaching 421 ppm (RCP2.6), 538 ppm (RCP4.5), 670 ppm (RCP6.0), and 936 ppm (RCP 8.5) by the year 2100.
  • Including also the prescribed concentrations of CH4 and N2O, the combined CO2-equivalent concentrations are 475 ppm (RCP2.6), 630 ppm (RCP4.5), 800 ppm (RCP6.0), and 1313 ppm (RCP8.5).

Projected change in global mean surface air temperature (c) and global mean sea level (d) rise for the mid- and late 21st century relative to the reference period of 1986–2005.
  • The total increase [in combined land and ocean surface temperature] between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available.
    (p 3)
  • Based on current understanding, only the collapse of marine-based sectors of the Antarctic Ice Sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century.
    There is medium confidence that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century.

[The decrease in surface ocean pH due to ongoing ocean acidification] by the end of 21st century is in the range of
  • 0.06 to 0.07 for RCP2.6,
  • 0.14 to 0.15 for RCP4.5,
  • 0.20 to 0.21 for RCP6.0 and
  • 0.30 to 0.32 for RCP8.5 (see Figures SPM.7 and SPM.8).
(p 19)

Carbon budget required to limit global surface temperature rise to less than 2°C

(531 [446 to 616] GtC was already emitted by 2011)
LikelihoodMaximum cumulative CO2 emissions since 1861-80 (GtC)

Climate Change Research Centre:
In 2000-2009, about 350 [GtC of CO2 were] emitted …
(Abrupt Change, Tipping Points and Past and Future Climate, The Copenhagen Diagnosis, 2009)

[Annual emissions of CO2 alone] have grown between 1970 and 2004 by about 80%, from 21 to 38 gigatonnes (Gt), and represented 77% of total anthropogenic GHG emissions in 2004.
(Climate Change 2007, Synthesis Report, p 36)

A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale …
{[Up] to 40% of emitted CO2 will remain in the atmosphere longer than 1,000 years.}

Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions.
[Ocean] warming will continue for centuries.

It is virtually certain that global mean sea level rise … due to thermal expansion [will] continue for many centuries.
[For] scenario RCP8.5, the projected rise [by 2300] is 1 m to more than 3 m (medium confidence).

Sustained mass loss by ice sheets would cause larger sea level rise …
There is high confidence that sustained warming greater than some threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a global mean sea level rise of up to 7 m.
Current estimates indicate that the threshold is greater than about 1°C (low confidence) but less than about 4°C (medium confidence) global mean warming with respect to pre-industrial.
(p 20, emphasis added)

Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic Ice Sheet in response to climate forcing is [also possible.]

[With respect to geoengineering, there] is insufficient knowledge to quantify how much CO2 emissions could be partially offset by [Carbon Dioxide Removal] on a century timescale.

[Solar Radiation Management, if realizable, has] the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification.
If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing.
(p 21)

Would you like to know more?

(Working Group I, Summary For Policy Makers, 27 September, 2013)



Observed Changes in the Climate System

Drivers of Climate Change

Understanding the Climate System and its Recent Changes

Future Global and Regional Climate Change

IPPC Fifth Assessment Report: Working Group I

  • Climate Change 2013: The Physical Science Basis — Summary for Policymakers, Working Group I, IPCC Fifth Assessment Report, 27 September, 2013.

    Drafting Authors:
    Lisa Alexander (Australia), Simon Allen (Switzerland/New Zealand), Nathaniel L Bindoff (Australia), François-Marie Bréon (France), John Church (Australia), Ulrich Cubasch (Germany), Seita Emori (Japan), Piers Forster (UK), Pierre Friedlingstein (UK/Belgium), Nathan Gillett (Canada), Jonathan Gregory (UK), Dennis Hartmann (USA), Eystein Jansen (Norway), Ben Kirtman (USA), Reto Knutti (Switzerland), Krishna Kumar Kanikicharla (India), Peter Lemke (Germany), Jochem Marotzke (Germany), Valérie Masson-Delmotte (France), Gerald Meehl (USA), Igor Mokhov (Russia), Shilong Piao (China), Gian-Kasper Plattner (Switzerland), Qin Dahe (China), Venkatachalam Ramaswamy (USA), David Randall (USA), Monika Rhein (Germany), Maisa Rojas (Chile), Christopher Sabine (USA), Drew Shindell (USA), Thomas F Stocker (Switzerland), Lynne Talley (USA), David Vaughan (UK), Shang-Ping Xie (USA).

    Draft Contributing Authors:
    Myles Allen (UK), Olivier Boucher (France), Don Chambers (USA), Jens Hesselbjerg Christensen (Denmark), Philippe Ciais (France), Peter Clark (USA), Matthew Collins (UK), Josefino Comiso (USA), Viviane Vasconcellos de Menezes (Australia/Brazil), Richard Feely (USA), Thierry Fichefet (Belgium), Arlene Fiore (USA), Gregory Flato (Canada), Jan Fuglestvedt (Norway), Gabriele Hegerl (UK/Germany), Paul Hezel (Belgium/USA), Gregory Johnson (USA), Georg Kaser (Austria/Italy), Vladimir Kattsov (Russia), John Kennedy (UK), Albert Klein Tank (Netherlands), Corinne Le Quéré (UK/France), Gunnar Myhre (Norway), Tim Osborn (UK), Antony Payne (UK), Judith Perlwitz (USA/Germany), Scott Power (Australia), Michael Prather (USA), Stephen Rintoul (Australia), Joeri Rogelj (Switzerland), Matilde Rusticucci (Argentina), Michael Schulz (Germany), Jan Sedláček (Switzerland), Peter Stott (UK), Rowan Sutton (UK), Peter Thorne (USA/Norway/UK), Donald Wuebbles (USA).

    A. Introduction

    The degree of certainty in key findings in this assessment is based on the author teams’ evaluations of underlying scientific understanding and is expressed as a qualitative level of confidence (from very low to very high) and, when possible, probabilistically with a quantified likelihood (from exceptionally unlikely [0-1%] to virtually certain [99-100%).
    • Confidence in the validity of a finding is based on the type, amount, quality, and consistency of evidence (eg, data, mechanistic understanding, theory, models, expert judgment) and the degree of agreement.
    • Probabilistic estimates of quantified measures of uncertainty in a finding are based on statistical analysis of observations or model results, or both, and expert judgment.

    B. Observed Changes in the Climate System

    Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia.
    The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased (see Figures SPM.1, SPM.2, SPM.3 and SPM.4).

    (p 2, emphasis added)

    B.1 Atmosphere

    Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.
    In the Northern Hemisphere, 1983–2012 was
    likely the warmest 30-year period of the last 1400 years (medium confidence).

    Figure SPM.1
    (a) Observed global mean combined land and ocean surface temperature anomalies, from 1850 to 2012 from three data sets.
    • Top panel: annual mean values …
    • [Bottom] panel: decadal mean values including the estimate of uncertainty for one dataset (black).
    Anomalies are relative to the mean of 1961-90.

    (b) Map of the observed surface temperature change from 1901 to 2012 derived from temperature trends determined by linear regression from one dataset (orange line in panel a).
    Trends have been calculated where data availability permits a robust estimate (ie, only for grid boxes with greater than 70% complete records and more than 20% data availability in the first and last 10% of the time period).
    Other areas are white.
    Grid boxes where the trend is significant at the 10% level are indicated by a + sign.
    • The globally averaged combined land and ocean surface temperature data as calculated by a linear trend, show a warming of 0.85 [0.65 to 1.06] °C [3], over the period 1880–2012, when multiple independently produced datasets exist.
    (p 4)

    Figure SPM.2
    Maps of observed precipitation change from 1901 to 2010 and from 1951 to 2010 (trends calculated using the same criteria as in Figure SPM.1b) from one data set.

    B.2 Ocean

    Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence).
    • It is virtually certain that the upper ocean (0-700 m) warmed from 1971 to 2010 (see Figure SPM.3), and …
    • likely [that it] warmed between the 1870s and 1971.
    (p 5)

    B.3 Cryosphere

    Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent (high confidence). …
    (p 5)
    • The average rate of ice loss from the Greenland ice sheet has very likely substantially increased
      • from 34 [–6 to 74] Gt yr^–1 over the period 1992–2001
      • to 215 [157 to 274] Gt yr^–1 over the period 2002–2011. …
    • The average rate of ice loss from the Antarctic ice sheet has likely increased
      • from 30 [–37 to 97] Gt yr^–1 over the period 1992–2001
      • to 147 [72 to 221] Gt yr^–1 over the period 2002–2011. …

    B.4 Sea Level

    The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence).
    Over the period 1901–2010, global mean sea leve(see Figure SPM.3)17 to 0.21] m (see Figure SPM.3). …
    • It is very likely that the mean rate of global averaged sea level rise was
      • 1.7 [1.5 to 1.9] mm yr^–1 between 1901 and 2010,
      • 2.0 [1.7 to 2.3] mm yr^–1 between 1971 and 2010 and
      • 3.2 [2.8 to 3.6] mm yr^–1 between 1993 and 2010.
    (p 6)

    Figure SPM.3
    Multiple observed indicators of a changing global climate:
      (a) Extent of Northern Hemisphere March-April (spring) average snow cover,
      (b) Extent of Arctic July-August-September (summer) average sea ice,
      (c) change in global mean upper ocean (0–700 m) heat content aligned to 2006-2010, and relative to the mean of all datasets for 1971,
      (d) global mean sea level relative to the 1900–1905 mean of the longest running dataset, and with
      all datasets aligned to have the same value in 1993, the first year of satellite altimetry data.
    All time-series (coloured lines indicating different data sets) show annual values, and where assessed, uncertainties are indicated by coloured shading.
    (p 5)

    B.5 Carbon and Other Biogeochemical Cycles

    The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years.
    CO2 concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions.
    The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification (see Figure SPM.4).

    (p 7)

    Figure SPM.4
    Multiple observed indicators of a changing global carbon cycle:
      (a) atmospheric concentrations of carbon dioxide (CO2) from Mauna Loa (19°32′N, 155°34′W — red) and South Pole (89°59′S, 24°48′W — black) since 1958;
      (b) partial pressure of dissolved CO2 at the ocean surface (blue curves) and in situ pH (green curves), a measure of the acidity of ocean water.
      Measurements are from three stations from
      • the Atlantic (29°10′N, 15°30′W — dark blue/dark green; 31°40′N, 64°10′W — blue/green) and
      • the Pacific Oceans (22°45′N, 158°00′W — light blue/light green).

    C. Drivers of Climate Change

    Total radiative forcing is positive, and has led to an uptake of energy by the climate system.
    The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750 (see Figure SPM.5).

    Figure SPM.5
    Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climate change.
    Values are global average radiative forcing (RF15) partitioned according to the emitted compounds or processes that result in a combination of drivers.
    The best estimates of the net radiative forcing are shown as black diamonds with corresponding uncertainty intervals; the numerical values are provided on the right of the figure, together with the confidence level in the net forcing (VH — very high, H — high, M — medium, L — low, VL — very low).
    Albedo forcing due to black carbon on snow and ice is included in the black carbon aerosol bar.
    Small forcings due to contrails (0.05 W/m², including contrail induced cirrus), and HFCs, PFCs and SF6 (total 0.03 W/m²) are not shown.
    Concentration-based RFs for gases can be obtained by summing the like-coloured bars.
    Volcanic forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms.
    (p 8)

    D. Understanding the Climate System and its Recent Changes

    Human influence on the climate system is clear.
    This is evident from the
    • increasing greenhouse gas concentrations in the atmosphere,
    • positive radiative forcing,
    • observed warming, and
    • understanding of the climate system.

    D.1 Evaluation of Climate Models

    Climate models have improved since the AR4.
    Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions (
    very high confidence).
    • The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence).
      There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (eg, 1998 to 2012).

    • The observed reduction in surface warming trend over the period 1998–2012 as compared to the period 1951–2012, is due in roughly equal measure to
      • a reduced trend in radiative forcing {primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle,} and
      • a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). …
    • [There] is high confidence that regional-scale surface temperature is better simulated than at the time of the AR4.

    • There has been substantial progress in the assessment of extreme weather and climate events since AR4.
      Simulated global-mean trends in the frequency of extreme warm and cold days and nights over the second half of the 20th century are generally consistent with observations.
    (p 10)

    • There has been some improvement in the simulation of continental-scale patterns of precipitation since the AR4. …

    • There is high confidence that the statistics of monsoon and El Niño-Southern Oscillation (ENSO) based on multi-model simulations have improved since AR4.

    • Climate models now include more cloud and aerosol processes, and their interactions, than at the time of the AR4, but there remains low confidence in the representation and quantification of these processes in models.

    • There is robust evidence that the downward trend in Arctic summer sea ice extent since 1979 is now reproduced by more models than at the time of the AR4 …
      Most models simulate a small downward trend in Antarctic sea ice extent, albeit with large inter-model spread, in contrast to the small upward trend in observations.

    • Many models reproduce the observed changes in upper-ocean heat content (0–700 m) from 1961 to 2005 (high confidence)

    • Climate models that include the carbon cycle (Earth System Models) simulate the global pattern of ocean-atmosphere CO2 fluxes, with outgassing in the tropics and uptake in the mid and high latitudes.
      In the majority of these models the sizes of the simulated global land and ocean carbon sinks over the latter part of the 20th century are within the range of observational estimates.

    D.2 Quantification of Climate System Responses

    Observational and model studies of temperature change, climate feedbacks and changes in the Earth’s energy budget together provide confidence in the magnitude of global warming in response to past and future forcing.
    • The net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate.
      The net radiative feedback due to all cloud types combined is likely positive.
      Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds.

    • Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence) …
      The lower temperature limit … is thus less than the 2°C in the AR4, but the upper limit is the same.
      {No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.}
    (p 11, emphasis added)

    • The transient climate response quantifies the response of the climate system to an increasing radiative forcing on a decadal to century timescale.
      It is defined as the change in global mean surface temperature at the time when the atmospheric CO2 concentration has doubled in a scenario of concentration increasing at 1% per year.
      The transient climate response is likely in the range of 1.0°C to 2.5°C (high confidence) …

    • [The transient climate response to cumulative carbon emissions (TCRE)] quantifies the transient response of the climate system to cumulative carbon emissions.
      TCRE is defined as the global mean surface temperature change per 1000 GtC emitted to the atmosphere.
      TCRE is likely in the range of 0.8°C to 2.5°C per 1000 GtC and applies for cumulative emissions up to about 2000 GtC until the time temperatures peak (see Figure SPM.10).

    Figure SPM.10
    Global mean surface temperature increase as a function of cumulative total global CO2 emissions from various lines of evidence.
    Multi-model results from a hierarchy of climate-carbon cycle models for each RCP until 2100 are shown with coloured lines and decadal means (dots).
    Some decadal means are indicated for clarity (eg, 2050 indicating the decade 2041-2050).
    Model results over the historical period (1860–2010) are indicated in black.
    The coloured plume illustrates the multi-model spread over the four RCP scenarios and fades with the decreasing number of available models in RCP8.5.
    The multi-model mean and range simulated by CMIP5 models, forced by a CO2 increase of 1% per year (1% per year CO2 simulations), is given by the thin black line and grey area.
    For a specific amount of cumulative CO2 emissions, the 1% per year CO2 simulations exhibit lower warming than those driven by RCPs, which include additional non-CO2 drivers. …

    D.3 Detection and Attribution of Climate Change

    • It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. …

    • Greenhouse gases contributed a global mean surface warming likely to be in the range of 0.5°C to 1.3°C over the period 1951-2010, with the contributions from other anthropogenic forcings, including the cooling effect of aerosols, likely to be in the range of −0.6°C to 0.1°C.
      The contribution from natural forcings is likely to be in the range of −0.1°C to 0.1°C, and from internal variability is likely to be in the range of −0.1°C to 0.1°C.
      Together these assessed contributions are consistent with the observed warming of approximately 0.6°C to 0.7°C over this period.
    (p 12, emphasis added)

    • Over every continental region except Antarctica, anthropogenic forcings have likely made a substantial contribution to surface temperature increases since the mid-20th century (see Figure SPM.6).
      For Antarctica, large observational uncertainties result in low confidence that anthropogenic forcings have contributed to the observed warming averaged over available stations.
      It is likely that there has been an anthropogenic contribution to the very substantial Arctic warming since the mid-20th century.

    • It is very likely that anthropogenic influence, particularly greenhouse gases and stratospheric ozone depletion, has led to a detectable observed pattern of tropospheric warming and a corresponding cooling in the lower stratosphere since 1961.

    • It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content (0–700 m) observed since the 1970s (see Figure SPM.6). …

    • It is likely that anthropogenic influences have affected the global water cycle since 1960.
      Anthropogenic influences have contributed
      • to observed increases in atmospheric moisture content in the atmosphere (medium confidence),
      • to global-scale changes in precipitation patterns over land (medium confidence),
      • to intensification of heavy precipitation over land regions where data are sufficient (medium confidence), and
      • to changes in surface and subsurface ocean salinity (very likely).
    • Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979.
      There is low confidence in the scientific understanding of the small observed increase in Antarctic sea ice extent due to the incomplete and competing scientific explanations for the causes of change and low confidence in estimates of internal variability in that region (see Figure SPM.6).

    • Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s and to the increased surface mass loss of the Greenland ice sheet since 1993.
      Due to a low level of scientific understanding there is low confidence in attributing the causes of the observed loss of mass from the Antarctic ice sheet over the past two decades.

    • It is likely that there has been an anthropogenic contribution to observed reductions in Northern Hemisphere spring snow cover since 1970.

    • It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s.
      This is based on the high confidence in an anthropogenic influence on the two largest contributions to sea level rise, that is thermal expansion and glacier mass loss.

    Figure SPM.6
    Comparison of observed and simulated climate change based on three large-scale indicators in the atmosphere, the cryosphere and the ocean:
    • change in continental land surface air temperatures (yellow panels),
    • Arctic and Antarctic September sea ice extent (white panels), and
    • upper ocean heat content in the major ocean basins (blue panels). …
    For temperature panels, observations are dashed lines if the spatial coverage of areas being examined is below 50%.
    For ocean heat content and sea ice panels the solid line is where the coverage of data is good and higher in quality, and the dashed line is where the data coverage is only adequate, and thus, uncertainty is larger.
    Model results shown are Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensemble ranges, with shaded bands indicating the 5 to 95% confidence intervals.
    (p 13)

    E. Future Global and Regional Climate Change

    A new set of scenarios, the Representative Concentration Pathways (RCPs), was used for the new climate model simulations carried out under the framework of the Coupled Model Intercomparison Project Phase 5 (CMIP5) of the World Climate Research Programme.
    (p 14)
    Box SPM.1

    [The] four RCPs include one mitigation scenario leading to a very low forcing level (RCP2.6), two stabilization scenarios (RCP4.5 and RCP6), and one scenario with very high greenhouse gas emissions (RCP8.5).
    The RCPs can thus represent a range of 21st century climate policies, as compared with the no-climate-policy of the Special Report on Emissions Scenarios (SRES) used in the Third Assessment Report and the Fourth Assessment Report.
    • For RCP6.0 and RCP8.5, radiative forcing does not peak by year 2100;
    • for RCP2.6 it peaks and declines; and
    • for RCP4.5 it stabilizes by 2100. …

    RCPs are based on a combination of integrated assessment models, simple climate models, atmospheric chemistry and global carbon cycle models.
    While the RCPs span a wide range of total forcing values, they do not cover the full range of emissions in the literature, particularly for aerosols.
    (p 22, emphasis added)

    Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.
    Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
    • Projected climate change based on RCPs is similar to AR4 in both patterns and magnitude, after accounting for scenario differences. …
      Projections of sea level rise are larger than in the AR4, primarily because of improved modelling of land-ice contributions.

    Figure SPM.7
    CMIP5 multi-model simulated time series from 1950 to 2100 for
      (a) change in global annual mean surface temperature relative to 1986–2005,
      (b) Northern Hemisphere September sea ice extent (5 year running mean) and
      (c) global mean ocean surface pH.
    Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red).
    Black (grey shading) is the modelled historical evolution using historical reconstructed forcings.
    The mean and associated uncertainties averaged over 2081-2100 are given for all RCP scenarios as colored vertical bars.
    The numbers of CMIP5 models used to calculate the multi-model mean is indicated.

    For sea ice extent (b), the projected mean and uncertainty (minimum-maximum range) of the subset of models that most closely reproduce the climatological mean state and 1979‒2012 trend of the Arctic sea ice is given (number of models given in brackets).
    For completeness, the CMIP5 multi-model mean is also indicated with dotted lines.
    The dashed line represents nearly ice-free conditions (i.e., when sea ice extent is less than 10^6 km^2 for at least five consecutive years).
    (p 14)

    Figure SPM.8
    Maps of CMIP5 multi-model mean results for the scenarios RCP2.6 and RCP8.5 in 2081 — 2100 of
      (a) annual mean surface temperature change,
      (b) average percent change in annual mean precipitation,
      (c) Northern Hemisphere September sea ice extent and
      (d) change in ocean surface pH.
    Changes in panels (a), (b) and (d) are shown relative to 1986–2005.
    The number of CMIP5 models used to calculate the multi-model mean is indicated in the upper right corner of each panel.
    For panels (a) and (b), hatching indicates regions where the multi-model mean is small compared to internal variability (ie, less than one standard deviation of internal variability in 20-year means).
    Stippling indicates regions where the multi-model mean is large compared to internal variability (ie, greater than two standard deviations of internal variability in 20-year means) and where 90% of models agree on the sign of change.

    In panel (c), the lines are the modelled means for 1986-2005; the filled areas are for the end of the century.

    The CMIP5 multi-model mean is given in white colour, the projected mean sea ice extent of a subset of models (number of models given in brackets) that most closely reproduce the climatological mean state and 1979‒2012 trend of the Arctic sea ice extent is given in light blue colour.

    E.1 Atmosphere: Temperature

    Global surface temperature change for the end of the 21st century is likely to exceed 1.5°C relative to 1850 to 1900 for all RCP scenarios except RCP2.6.
    It is
    likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5.
    Warming will continue beyond 2100 under all RCP scenarios except RCP2.6.
    Warming will continue to exhibit interannual-to-decadal variability and will not be regionally uniform (see Figures SPM.7 and SPM.8).
    • The global mean surface temperature change for the period 2016–2035 relative to 1986–2005 will likely be in the range of 0.3°C to 0.7°C (medium confidence). …
      [Near-term] increases in seasonal mean and annual mean temperatures are expected to be larger in the tropics and subtropics than in mid-latitudes (high confidence).

    • It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas …
      It is very likely that heat waves will occur with a higher frequency and duration.
      Occasional cold winter extremes will continue to occur.
    (p 15)

    E.2 Atmosphere: Water Cycle

    Changes in the global water cycle in response to the warming over the 21st century will not be uniform.
    The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions (see Figure SPM.8).
    • In many midlatitude and subtropical dry regions, mean precipitation will likely decrease, while in many midlatitude wet regions, mean precipitation will likely increase by the end of this century under the RCP8.5 scenario (see Figure SPM.8).

    • Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century …

    • [It] is likely that the area encompassed by monsoon systems will increase over the 21st century [and that] monsoon precipitation is likely to intensify due to the increase in atmospheric moisture. …
      Monsoon retreat dates will likely be delayed, resulting in lengthening of the monsoon season in many regions. …

    • Due to the increase in moisture availability, ENSO-related precipitation variability on regional scales will likely intensify.

    E.3 Atmosphere: Air Quality

    • There is high confidence that globally, warming decreases background surface ozone.
      High CH4 levels (RCP8.5) can offset this decrease, raising background surface ozone by year 2100 on average by about 8 ppb (25% of current levels) relative to scenarios with small CH4 changes (RCP4.5, RCP6.0) (high confidence).

    • Observational and modelling evidence indicates that … locally higher surface temperatures in polluted regions will trigger regional feedbacks in chemistry and local emissions that will increase peak levels of ozone and PM2.5 (medium confidence).
      {PM2.5 refers to particulate matter with a diameter of less than 2.5 micrometres, a measure of atmospheric aerosol concentration.}
    (p 16, emphasis added)

    E.4 Ocean

    The global ocean will continue to warm during the 21st century.
    Heat will penetrate from the surface to the deep ocean and affect ocean circulation.
    • The strongest ocean warming is projected for the surface in tropical and Northern Hemisphere subtropical regions.
      At greater depth the warming will be most pronounced in the Southern Ocean (high confidence). …

    • It is very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century.
      Best estimates and range for the reduction … are
      • 11% (1 to 24%) in RCP2.6 and
      • 34% (12 to 54%) in RCP8.5.
      It is likely that there will be some decline in the AMOC by about 2050 …

    • It is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century …
      However, a collapse beyond the 21st century … cannot be excluded.

    E.5 Cryosphere

    It is very likely that the Arctic sea ice cover will continue to shrink and thin and that Northern Hemisphere spring snow cover will decrease during the 21st century as global mean surface temperature rises.
    Global glacier volume will further decrease.
    • [A] nearly ice-free Arctic Ocean in September before mid-century is likely for RCP8.5 (medium confidence).
      ["Nearly ice-free" being defined as a sea ice extent of less than 10^6 km^2 for at least five consecutive years.]

    • By the end of the 21st century, the global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease
      • by 15 to 55% for RCP2.6, and
      • by 35 to 85% for RCP8.5
      (medium confidence). …

    • By the end of the 21st century, the area of [near-surface permafrost] is projected to decrease by between 37% (RCP2.6) to 81% (RCP8.5) … (medium confidence).
    (p 17)

    E.6 Sea Level

    Global mean sea level will continue to rise during the 21st century (see Figure SPM.9).
    Under all RCP scenarios the rate of sea level rise will
    very likely exceed that observed during 1971–2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets.
    • The increase in surface melting of the Greenland ice sheet will exceed the increase in snowfall, leading to a positive contribution from changes in surface mass balance to future sea level (high confidence).
      While surface melting will remain small, an increase in snowfall on the Antarctic ice sheet is expected (medium confidence), resulting in a negative contribution to future sea level from changes in surface mass balance.
      Changes in outflow from both ice sheets combined will likely make a contribution in the range of 0.03 to 0.20 m by 2081-2100 (medium confidence).

    Figure SPM.9 Projections of global mean sea level rise over the 21st century relative to 1986–2005 from the combination of the CMIP5 ensemble with process-based models, for RCP2.6 and RCP8.5.
    The assessed likely range is shown as a shaded band.
    The assessed likely ranges for the mean over the period 2081–2100 for all RCP scenarios are given as coloured vertical bars, with the corresponding median value given as a horizontal line.
    Many semi-empirical model projections of global mean sea level rise are higher than process-based model projections (up to about twice as large), but there is no consensus in the scientific community about their reliability and there is thus low confidence in their projections.
    (p 18)

    E.7 Carbon and Other Biogeochemical Cycles

    Climate change will affect carbon cycle processes in a way that will exacerbate the increase of CO2 in the atmosphere (high confidence).
    Further uptake of carbon by the ocean will increase ocean acidification.
    • [While a] majority of models project a continued land carbon uptake … some models simulate a land carbon loss due to the combined effect of climate change and land use change.

    • Based on Earth System Models, there is high confidence that the feedback between climate and the carbon cycle is positive in the 21st century …
      As a result more of the emitted anthropogenic CO2 will remain in the atmosphere.
      A positive feedback between climate and the carbon cycle on century to millennial time scales is supported by paleoclimate observations and modelling. …

    • The release of CO2 or CH4 to the atmosphere from thawing permafrost carbon stocks over the 21st century is assessed to be in the range of 50 to 250 GtC for RCP8.5 (low confidence).

    E.8 Climate Stabilization, Climate Change Commitment and Irreversibility

    Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond (see Figure SPM.10).
    Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped.
    This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.

    (p 19)

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