First Assessment Report - IPCC

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XXIV Policymakers Summary ESTIMATES FOR CHANGES BY 2030 (IPCC Business-as-Usual scenario; changes from pre-industrial) The numbers given below are based on high resolution models, scaled to be consistent with our best estimate of global mean warming of 1.8°C by 2030. For values consistent with other estimates of global temperature rise, the numbers below should be reduced by 30% for the low estimate or increased by 50% for the high estimate. Precipitation estimates are also scaled in a similar way. Confidence in these regional estimates is low Central North America (35 '-50 W 85 °-105 °W) The warming varies from 2 to 4°C in winter and 2 to 3°C in summer. Precipitation increases range from 0 to 15% in winter whereas there are decreases of 5 to 10% in summer. Soil moisture decreases in summer by 15 to 20%. Southern Asia (5 °-30 W 70 °-105 °E) The warming varies from 1 to 2°C throughout the year. Precipitation changes little in winter and generally increases throughout the region by 5 to 15% in summer. Summer soil moisture increases by 5 to 10%. Sahel (10 °-20 °N 20 °W-40 °E) The warming ranges from 1 to 3°C. Area mean precipitation increases and area mean soil moisture decreases marginally in summer. However, throughout the region, there are areas of both increase and decrease in both parameters throughout the region. Southern Europe (35°-50°N 10°W- 45°E) The warming is about 2°C in winter and varies from 2 to 3°C in summer. There is some indication of increased precipitation in winter, but summer precipitation decreases by 5 to 15%, and summer soil moisture by 15 to 25%. Australia (12°-45°S 110°-115°E) The warming ranges from 1 to 2°C in summer and is about 2°C in winter. Summer precipitation increases by around 10%, but the models do not produce consistent estimates of the changes in soil moisture. The area averages hide large variations at the sub-continental level. Figure 10: Map showing the locations and extents of the five areas selected by IPCC
Policymakers Summary xxv WHAT TOOLS DO WE USE TO PREDICT FUTURE CLIMATE, AND HOW DO WE USE THEM? The most highly developed tool which we have to predict future climate is known as a general circulation model or GCM. These models are based on the laws of physics and use descriptions in simplified physical terms (called parameterisations) of the smaller-scale processes such as those due to clouds and deep mixing in the ocean. In a climate model an atmospheric component, essentially the same as a weather prediction model, is coupled to a model of the ocean, which can be equally complex. Climate forecasts are derived in a different way from weather forecasts. A weather prediction model gives a description of the atmosphere's state up to 10 days or so ahead, starting from a detailed description of an initial state of the atmosphere at a given time. Such forecasts describe the movement and development of large weather systems, though they cannot represent very small scale phenomena; for example, individual shower clouds. To make a climate forecast, the climate model is first run for a few (simulated) decades. The statistics of the model's output is a description of the model's simulated climate which, if the model is a good one, will bear a close resemblance to the climate of the real atmosphere and ocean. The above exercise is then repeated with increasing concentrations of the greenhouse gases in the model. The differences between the statistics of the two simulations (for example in mean temperature and interannual variability) provide an estimate of the accompanying climate change. The long term change in surface air temperature following a doubling of carbon dioxide (referred to as the climate sensitivity) is generally used as a benchmark to compare models. The range of results from model studies is 1.9 to 5.2°C. Most results are close to 4.0°C but recent studies using a more detailed but not necessarily more accurate representation of cloud processes give results in the lower half of this range. Hence the models results do not justify altering the previously accepted range of 1.5 to 4.5°C. Although scientists are reluctant to give a single best estimate in this range, it is necessary for the presentation of climate predictions for a choice of best estimate to be made. Taking into account the model results, together with observational evidence over the last century which is suggestive of the climate sensitivity being in the lower half of the range, (see section: "Has man already begun to change global climate?") a value of climate sensitivity of 2.5°C has been chosen as the best estimate. Further details are given in Section 5 of the report. In this Assessment, we have also used much simpler models, which simulate the behaviour of GCMs, to make predictions of the evolution with time of global temperature from a number of emission scenarios. These so-called box-diffusion models contain highly simplified physics but give similar results to GCMs when globally averaged. A completely different, and potentially useful, way of predicting patterns of future climate is to search for periods in the past when the global mean temperatures were similar to those we expect in future, and then use the past spatial patterns as analogues of those which will arise in the future. For a good analogue, it is also necessary for the forcing factors (for example, greenhouse gases, orbital variations) and other conditions (for example, ice cover, topography, etc.) to be similar; direct comparisons with climate situations for which these conditions do not apply cannot be easily interpreted. Analogues of future greenhouse-gas-changed climates have not been found. We cannot therefore advocate the use of palaeo-climates as predictions of regional climate change due to future increases in greenhouse gases. However, palaeo-climatological information can provide useful insights into climate processes, and can assist in the validation of climate models. Will storms increase in a warmer world? Storms can have a major impact on society. Will their frequency, intensity or location increase in a warmer world? Tropical storms, such as typhoons and hurricanes, only develop at present over seas that are warmer than about 26°C. Although the area of sea having temperatures over this critical value will increase as the globe warms, the critical temperature itself may increase in a warmer world. Although the theoretical maximum intensity is expected to increase with temperature, climate models give no consistent indication whether tropical storms will increase or decrease in frequency or intensity as climate changes; neither is there any evidence that this has occurred over the past few decades. Mid-latitude storms, such as those which track across the North Atlantic and North Pacific, are driven by the equator-to-pole temperature contrast. As this contrast will
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Page 13 and 14: Foreword Many previous reports have
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Page 53 and 54: EXECUTIVE SUMMARY The Earth's clima
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36 Gi eenhouse Gases and Act osols
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38 Gi eenhouse Gases and Aerosols 1
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2 Radiative Forcing of Climate K.P.
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EXECUTIVE SUMMARY 1 The climate of
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2 Radiali\ e Foicinq of Climate 47
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2 Radiative Foicing of Climate 49 a
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2 Radiative Fo>cim> of Climate 51 S
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2 Radiatn e Fo> c im> of Climate 53
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2 Radiative Foi c ing of Climate 55
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2 Radiatne Foi cinq of Climate 57 T
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2 Radiatn e Foi c mq of Climate 59
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2 Radiatn e Foi c mg of Climate 61
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2 Radunn e h01 c uiv, of Climate 63
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2 Radiatne Foicin of Climate 67 eru
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CONTENTS Executive Summary 73 3.1 I
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3 Pi ocesses and Modelling 75 3.1 I
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3 Pioc esses and Modelling 77 away
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3 Pi OC es tes and Modellm v, 79 wh
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Table 3.2(a): Summai\ of tesults po
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3 Pi OC esses and Modelling 83 3.5.
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3 Pi ocesses and Modelling 85 many
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3 Pi oc esses and Modelling 87 glob
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3 Pi OC esses and Modelling 89 from
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3 Pi OC esses and Modellm ? 91 Rava
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Executive Summary 1 The validation
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100 Validation of Climate Models 4
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I OH Validation of Climate Models 4
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(a) DJF PRECIPITATION: OBSERVED (b)
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4 Validation of Climate Models IIS
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5 Equilibrium Climate Change - and
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EXECUTIVE SUMMARY 1. All models sho
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nx Equihbi mm Climate Change 5 temp
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5 Equilibrium Climate Change 143 di
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150 Ec/uilibi mm Climate Change 5 1
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H4 Ec/uilibiiiim Climate Clninqe 5
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756 Equilibi mm Climate Change 5 Ta
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1SS Ec/mlibi nun Climate Change 5 S
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160 Eqmlibnum Climate Chcuis,e 5 im
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162 Equilibrium Climate Change 5 Or
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164 Cquilibi mm Climate Change 5 Sc
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CONTENTS Executive Summary 177 6.1
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6 Time-Dependent Gi eenhoute-Gas In
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6 Time Dependent Gi eenhouse Gas-In
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6 Time-Dependent Gieenhouse Gas-Ind
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6 Time-Dependent Gieenhouse Gas Ind
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6 Time-Dependent Greenhouse Gas-Ind
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CONTENTS Executive Summary 199 7.1
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7 ObseivedClimate Variation andChan
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7 Obsei ved Climate Vai tation and
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7 Observed Climate Variation and Ch
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7 Obsei \ eel Climate Vai iation an
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7 Obsei \ed Climate Vailatum and Ch
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7 Obsei \ ed Climate Vai lation and
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7 Obsei ved Climate Va> mtion and C
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7 Obsencd Climate Vernation and Cha
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7 Obsened Climate Vai lation and Ch
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2M) Obsenid Climate Vaimtion and Ch
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7 Obsened Climate Van at ion and Ch
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7 Obsei i ed Climate Vai lation and
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7 Obsei \ ed Climate Vai wtion and
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8 Detection of the Greenhouse Effec
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EXECUTIVE SUMMARY Global-mean tempe
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246 Detec tion of the Gi eenhouse E
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248 Dctec tion of the Gieenhouse Ef
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250 Detec turn of the Gieenhouse Ef
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252 Detec tion of the Gi eenhouse E
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2S4 Detec tion of the Gi eenhouse E
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9 Sea Level Rise R. WARRICK, J. OER
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EXECUTIVE SUMMARY This Section addr
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264 Sea Level Rise 9 In addition, s
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266 Sea Le\ el Rise 9 meteoiologica
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270 Sea Level Rise 9 Table 9.4 Esti
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272 Sea Level Rise 9 Table 9.6 Anta
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274 Sea Level Rise 9 Table 9.8 Esti
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276 Sea Le\ el Rise 9 each scenario
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278 Sea Level Rise 9 O 50 Stop chan
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2S0 Sea Level Rise 9 Gornit/, V and
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10 Effects on Ecosystems J.M. MELIL
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EXECUTIVE SUMMARY Ecosystem Metabol
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10 Effects on Ecosystems 289 10.0 I
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7 0 Effec ts on Eco system s 291 in
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10 Effects on Ecosystems 293 Table
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296 Effects on Ecosystems 10 will h
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29h Effet ts on Ecosys'ems 10 10 2
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mo Effec ti on Ec osystem s 10 Tabl
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302 Effects on Ecosystems 10 (N2O)
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M)4 Effec ts on Ecosystems 10 poten
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106 Effects on Ecosystems 10 38,000
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308 Effects on Ecosystems 10 Hardy,
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11 Narrowing the Uncertainties: A S
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EXECUTIVE SUMMARY The IPCC has the
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?/ the Unt ei tamties 11 Analysis o
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uo 11.2 3 Precipitation and Evapora
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322 Narr owing the Uncertainties 11
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U4 Nai i cm ing the Unc ei tamties
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Narrowing the Uncertainties 327 199
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Annex 331 A.l Introduction Modellin
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Annex 333 900 Ball 4000 .-• 3500
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Annex 335 the current atmospheric b
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Annex 337 E UJ w ui > UJ < ui u BaU
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Appendix 1 EMISSIONS SCENARIOS FROM
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Appendix 3 CONTRIBUTORS TO IPCC WG1
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Appendix 3 347 Contributors: P.A. A
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Appendix 3 P Ya Groisman A Gruber S
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Appendix 3 351 J.P. Grime R. Giffor
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354 Appendix 4 H. Kenway M. Manton
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356 Appendix 4 JAPAN H. Akimoto H.
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358 Appendix 4 B. Flannery M.Ghil I
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Appendix 6 ACRONYMS: PROGRAMMES & M
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364 Appendix 7 Special Names and Sy
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First Assessment Report - IPCC

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