Technical Summary. A report accepted by Working Group I of the IPCC but not approved in detail

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1 Techncal Summary A report accepted by Workng Group I of the IPCC but not approved n detal Acceptance of IPCC Reports at a Sesson of the Workng Group or Panel sgnfes that the materal has not been subject to lne by lne dscusson and agreement, but nevertheless presents a comprehensve, objectve and balanced vew of the subject matter. Co-ordnatng Lead Authors D.L. Albrtton (USA), L.G. Mera Flho (Brazl) Lead Authors U. Cubasch (Germany), X. Da (Chna), Y. Dng (Chna), D.J. Grggs (UK), B. Hewtson (South Afrca), J.T. Houghton (UK), I. Isaksen (Norway), T. Karl (USA), M. McFarland (USA), V.P. Meleshko (Russa), J.F.B. Mtchell (UK), M. Noguer (UK), B.S. Nyenz (Tanzana), M. Oppenhemer (USA), J.E. Penner (USA), S. Pollonas (Trndad and Tobago), T. Stocker (Swtzerland), K.E. Trenberth (USA) Contrbutng Authors M.R. Allen, (UK), A.P.M. Baede (Netherlands), J.A. Church (Australa), D.H. Ehhalt (Germany), C.K. Folland (UK), F. Gorg (Italy), J.M. Gregory (UK), J.M. Haywood (UK), J.I. House (Germany), M. Hulme (UK), V.J. Jaramllo (Mexco), A. Jayaraman (Inda), C.A. Johnson (UK), S. Joussaume (France), D.J. Karoly (Australa), H. Kheshg (USA), C. Le Quéré (France), L.J. Mata (Germany), B.J. McAvaney (Australa), L.O. Mearns (USA), G.A. Meehl (USA), B. Moore III (USA), R.K. Mugara (Zamba), M. Prather (USA), C. Prentce (Germany), V. Ramaswamy (USA), S.C.B. Raper (UK), M.J. Salnger (New Zealand), R. Scholes (S. Afrca), S. Solomon (USA), R. Stouffer (USA), M-X. Wang (Chna), R.T. Watson (USA), K-S. Yap (Malaysa) Revew Edtors F. Joos (Swtzerland), A. Ramrez-Rojas (Venzuela), J.M.R. Stone (Canada), J. Zllman (Australa) 21

2 Techncal Summary of the Workng Group I Report A. Introducton A.1 The IPCC and ts Workng Groups The Intergovernmental Panel on Clmate Change (IPCC) was establshed by the World Meteorologcal Organsaton (WMO) and the Unted Natons Envronment Programme (UNEP) n The am was, and remans, to provde an assessment of the understandng of all aspects of clmate change 1, ncludng how human actvtes can cause such changes and can be mpacted by them. It had become wdely recognsed that human-nfluenced emssons of greenhouse gases have the potental to alter the clmate system (see Box 1), wth possble deleterous or benefcal effects. It was also recognsed that addressng such global ssues requred organsaton on a global scale, ncludng assessment of the understandng of the ssue by the worldwde expert communtes. At ts frst sesson, the IPCC was organsed nto three Workng Groups. The current remts of the Workng Groups are for Workng Group I to address the scentfc aspects of the clmate system and clmate change, Workng Group II to address the mpacts of and adaptatons to clmate change, and Workng Group III to address the optons for the mtgaton of clmate change. The IPCC provded ts frst major assessment report n 1990 and ts second major assessment report n The IPCC reports are () up-to-date descrptons of the knowns and unknowns of the clmate system and related factors, () based on the knowledge of the nternatonal expert communtes, () produced by an open and peer-revewed professonal process, and (v) based upon scentfc publcatons whose fndngs are summarsed n terms useful to decson makers. Whle the assessed nformaton s polcy relevant, the IPCC does not establsh or advocate publc polcy. The scope of the assessments of Workng Group I ncludes observatons of the current changes and trends n the clmate system, a reconstructon of past changes and trends, an understandng of the processes nvolved n those changes, and the ncorporaton of ths knowledge nto models that can attrbute the causes of changes and that can provde smulaton of natural and human-nduced future changes n the clmate system. A.2 The Frst and Second Assessment Reports of Workng Group I In the Frst Assessment Report n 1990, Workng Group I broadly descrbed the status of the understandng of the clmate system and clmate change that had been ganed over the precedng decades of research. Several major ponts were emphassed. The greenhouse effect s a natural feature of the planet, and ts fundamental physcs s well understood. The atmospherc abundances of greenhouse gases were ncreasng, due largely to human actvtes. Contnued future growth n greenhouse gas emssons was predcted to lead to sgnfcant ncreases n the average surface temperature of the planet, ncreases that would exceed the natural varaton of the past several mllenna and that could be reversed only slowly. The past century had, at that tme, seen a surface warmng of nearly 0.5 C, whch was broadly consstent wth that predcted by clmate models for the greenhouse gas ncreases, but was also comparable to what was then known about natural varaton. Lastly, t was ponted out that the current level of understandng at that tme and the exstng capabltes of clmate models lmted the predcton of changes n the clmate of specfc regons. Based on the results of addtonal research and Specal Reports produced n the nterm, IPCC Workng Group I assessed the new state of understandng n ts Second Assessment Report (SAR 2 ) n The report underscored that greenhouse gas abundances contnued to ncrease n the atmosphere and that very substantal cuts n emssons would be requred for stablsaton of greenhouse gas concentratons n the atmosphere (whch s the ultmate goal of Artcle 2 of the Framework Conventon on Clmate Change). Further, the general ncrease n 1 Clmate change n IPCC usage refers to any change n clmate over tme, whether due to natural varablty or as a result of human actvty. Ths usage dffers from that n the Framework Conventon on Clmate Change, where clmate change refers to a change of clmate that s attrbuted drectly or ndrectly to human actvty that alters the composton of the global atmosphere and that s n addton to natural clmate varablty observed over comparable tme perods. For a defnton of scentfc and techncal terms: see the Glossary n Appendx I. 2 The IPCC Second Assessment Report s referred to n ths Techncal Summary as the SAR. 22

3 global temperature contnued, wth recent years beng the warmest snce at least The ablty of clmate models to smulate observed events and trends had mproved, partcularly wth the ncluson of sulphate aerosols and stratospherc ozone as radatve forcng agents n clmate models. Utlsng ths smulatve capablty to compare to the observed patterns of regonal temperature changes, the report concluded that the ablty to quantfy the human nfluence on global clmate was lmted. The lmtatons arose because the expected sgnal was stll emergng from the nose of natural varablty and because of uncertantes n other key factors. Nevertheless, the report also concluded that the balance of evdence suggests a dscernble human nfluence on global clmate. Lastly, based on a range of scenaros of future greenhouse gas abundances, a set of responses of the clmate system was smulated. A.3 The Thrd Assessment Report: Ths Techncal Summary The thrd major assessment report of IPCC Workng Group I bulds upon these past assessments and ncorporates the results of the past fve years of clmate research. Ths Techncal Summary s based on the underlyng nformaton of the chapters, whch s cross-referenced n the Source Notes n the Appendx. Ths Summary ams to descrbe the major features (see Fgure 1) of the understandng of the clmate system and clmate change at the outset of the 21st century. Specfcally: What does the observatonal record show wth regard to past clmate changes, both globally and regonally and both on the average and n the extremes? (Secton B) What changes have occurred? How well are the past and present clmates understood? What changes could le ahead? Observatons: temperatures precptaton snow / ce cover sea level crculaton extremes Smulatons: natural varaton forcng agents global clmate regonal clmate hgh mpact events stablsaton Observatons vs-à-vs Smulatons Tmelne: Palaeo & Instrumental Perods The Present The Future Ths Summary: Sectons B + C Sectons D + E Sectons F + G Fgure 1: Key questons about the clmate system and ts relaton to humanknd. Ths Techncal Summary, whch s based on the underlyng nformaton n the chapters, s a status report on the answers, presented n the structure ndcated. 23

4 How quanttatve s the understandng of the agents that cause clmate to change, ncludng both those that are natural (e.g., solar varaton) and human-related (e.g., greenhouse gases) phenomena? (Secton C) What s the current ablty to smulate the responses of the clmate system to these forcng agents? In partcular, how well are key physcal and bogeochemcal processes descrbed by present global clmate models? (Secton D) Based on today s observatonal data and today s clmate predctve capabltes, what does the comparson show regardng a human nfluence on today s clmate? (Secton E) Further, usng current predctve tools, what could the possble clmate future be? Namely, for a wde spectrum of projectons for several clmate-forcng agents, what does current understandng project for global temperatures, regonal patterns of precptaton, sea levels, and changes n extremes? (Secton F) Fnally, what are the most urgent research actvtes that need to be addressed to mprove our understandng of the clmate system and to reduce our uncertanty regardng future clmate change? The Thrd Assessment Report of IPCC Workng Group I s the product of hundreds of scentsts from the developed and developng world who contrbuted to ts preparaton and revew. What follows s a summary of ther understandng of the clmate system. Box 1: What drves changes n clmate? The Earth absorbs radaton from the Sun, manly at the surface. Ths energy s then redstrbuted by the atmospherc and oceanc crculatons and radated back to space at longer (nfrared) wavelengths. For the annual mean and for the Earth as a whole, the ncomng solar radaton energy s balanced approxmately by the outgong terrestral radaton. Any factor that alters the radaton receved from the Sun or lost to space, or that alters the redstrbuton of energy wthn the atmosphere and between the atmosphere, land, and ocean, can affect clmate. A change n the net radatve energy avalable to the global Earth-atmosphere system s termed here, and n prevous IPCC reports, a radatve forcng. Postve radatve forcngs tend to warm the Earth s surface and lower atmosphere. Negatve radatve forcngs tend to cool them. Increases n the concentratons of greenhouse gases wll reduce the effcency wth whch the Earth s surface radates to space. More of the outgong terrestral radaton from the surface s absorbed by the atmosphere and re-emtted at hgher alttudes and lower temperatures. Ths results n a postve radatve forcng that tends to warm the lower atmosphere and surface. Because less heat escapes to space, ths s the enhanced greenhouse effect an enhancement of an effect that has operated n the Earth s atmosphere for bllons of years due to the presence of naturally occurrng greenhouse gases: water vapour, carbon doxde, ozone, methane and ntrous oxde. The amount of radatve forcng depends on the sze of the ncrease n concentraton of each greenhouse gas, the radatve propertes of the gases nvolved, and the concentratons of other greenhouse gases already present n the atmosphere. Further, many greenhouse gases resde n the atmosphere for centures after beng emtted, thereby ntroducng a long-term commtment to postve radatve forcng. Anthropogenc aerosols (mcroscopc arborne partcles or droplets) n the troposphere, such as those derved from fossl fuel and bomass burnng, can reflect solar radaton, whch leads to a coolng tendency n the clmate system. Because t can absorb solar radaton, black carbon (soot) aerosol tends to warm the clmate system. In addton, changes n aerosol concentratons can alter cloud amount and cloud reflectvty through ther effect on cloud propertes and lfetmes. In most cases, tropospherc aerosols tend to produce a negatve radatve forcng and a cooler clmate. They have a much shorter lfetme (days to weeks) than most greenhouse 24

5 gases (decades to centures), and, as a result, ther concentratons respond much more quckly to changes n emssons. Volcanc actvty can nject large amounts of sulphur-contanng gases (prmarly sulphur doxde) nto the stratosphere, whch are transformed nto sulphate aerosols. Indvdual eruptons can produce a large, but transtory, negatve radatve forcng, tendng to cool the Earth s surface and lower atmosphere over perods of a few years. The Sun s output of energy vares by small amounts (0.1%) over an 11-year cycle and, n addton, varatons over longer perods may occur. On tme-scales of tens to thousands of years, slow varatons n the Earth s orbt, whch are well understood, have led to changes n the seasonal and lattudnal dstrbuton of solar radaton. These changes have played an mportant part n controllng the varatons of clmate n the dstant past, such as the glacal and nter-glacal cycles. When radatve forcng changes, the clmate system responds on varous tme-scales. The longest of these are due to the large heat capacty of the deep ocean and dynamc adjustment of the ce sheets. Ths means that the transent response to a change (ether postve or negatve) may last for thousands of years. Any changes n the radatve balance of the Earth, ncludng those due to an ncrease n greenhouse gases or n aerosols, wll alter the global hydrologcal cycle and atmospherc and oceanc crculaton, thereby affectng weather patterns and regonal temperatures and precptaton. Any human-nduced changes n clmate wll be embedded n a background of natural clmatc varatons that occur on a whole range of tme- and space-scales. Clmate varablty can occur as a result of natural changes n the forcng of the clmate system, for example varatons n the strength of the ncomng solar radaton and changes n the concentratons of aerosols arsng from volcanc eruptons. Natural clmate varatons can also occur n the absence of a change n external forcng, as a result of complex nteractons between components of the clmate system, such as the couplng between the atmosphere and ocean. The El Nño-Southern Oscllaton (ENSO) phenomenon s an example of such natural nternal varablty on nterannual tme-scales. To dstngush anthropogenc clmate changes from natural varatons, t s necessary to dentfy the anthropogenc sgnal aganst the background nose of natural clmate varablty. B. The Observed Changes n the Clmate System Is the Earth s clmate changng? The answer s unequvocally Yes. A sute of observatons supports ths concluson and provdes nsght about the rapdty of those changes. These data are also the bedrock upon whch to construct the answer to the more dffcult queston: Why s t changng?, whch s addressed n later Sectons. Ths Secton provdes an updated summary of the observatons that delneate how the clmate system has changed n the past. Many of the varables of the clmate system have been measured drectly,.e., the nstrumental record. For example, wdespread drect measurements of surface temperature began around the mddle of the 19th century. Near global observatons of other surface weather varables, such as precptaton and wnds, have been made for about a hundred years. Sea level measurements have been made for over 100 years n some places, but the network of tde gauges wth long records provdes only lmted global coverage. Upper ar observatons have been made systematcally only snce the late 1940s. There are also long records of surface oceanc observatons made from shps snce the md-19th century and by dedcated buoys snce about the late 1970s. Sub-surface oceanc temperature measurements wth near global coverage are now avalable from the late 1940s. Snce the late 1970s, other data from Earth-observaton satelltes have been used to provde a wde range of global observatons of varous components of the clmate system. In addton, a growng set of palaeoclmatc data, e.g., from trees, corals, sedments, and ce, are gvng nformaton about the Earth s clmate of centures and mllenna before the present. Ths Secton places partcular emphass on current knowledge of past changes n key clmate varables: temperature, precptaton and atmospherc mosture, snow cover, extent of land and sea ce, sea level, patterns n atmospherc and oceanc crculaton, extreme weather and clmate events, and overall features of the clmate varablty. The concludng part of ths Secton compares the observed trends n these varous clmate ndcators to see f a collectve pcture emerges. The degree of ths nternal consstency s a crtcal factor n assessng the level of confdence n the current understandng of the clmate system. 25

6 B.1 Observed Changes n Temperature Temperatures n the nstrumental record for land and oceans The global average surface temperature has ncreased by 0.6 ± 0.2 C 3 snce the late 19th century. It s very lkely that the 1990s was the warmest decade and 1998 the warmest year n the nstrumental record snce 1861 (see Fgure 2). The man cause of the ncreased estmate of global warmng of 0.15 C snce the SAR s related to the record warmth of the addtonal sx years (1995 to 2000) of data. A secondary reason s related to mproved methods of estmatng change. The current, slghtly larger uncertanty range (±0.2 C, 95% confdence nterval) s also more objectvely based. Further, the scentfc bass for confdence n the estmates of the ncrease n global Departures n temperature ( C) from the 1961 to 1990 average GLOBAL Year Fgure 2: Combned annual land-surface ar and sea surface temperature anomales ( C) 1861 to 2000, relatve to 1961 to Two standard error uncertantes are shown as bars on the annual number. [Based on Fgure 2.7c] temperature snce the late 19th century has been strengthened snce the SAR. Ths s due to the mprovements derved from several new studes. These nclude an ndependent test of the correctons used for tme-dependent bases n the sea surface temperature data and new analyses of the effect of urban heat sland nfluences on global land-temperature trends. As ndcated n Fgure 2, most of the ncrease n global temperature snce the late 19th century has occurred n two dstnct perods: 1910 to 1945 and snce The rate of ncrease of temperature for both perods s about 0.15 C/decade. Recent warmng has been greater over land compared to oceans; the ncrease n sea surface temperature over the perod 1950 to 1993 s about half that of the mean land-surface ar temperature. The hgh global temperature assocated wth the 1997 to 1998 El Nño event stands out as an extreme event, even takng nto account the recent rate of warmng. The regonal patterns of the warmng that occurred n the early part of the 20th century were dfferent than those that occurred n the latter part. Fgure 3 shows the regonal patterns of the warmng that have occurred over the full 20th century, as well as for three component tme perods. The most recent perod of warmng (1976 to 1999) has been almost global, but the largest ncreases n temperature have occurred over the md- and hgh lattudes of the contnents n the Northern Hemsphere. Year-round coolng s evdent n the northwestern North Atlantc and the central North Pacfc Oceans, but the North Atlantc coolng trend has recently reversed. The recent regonal patterns of temperature change have been shown to be related, n part, to varous phases of atmospherc-oceanc Data from thermometers. oscllatons, such as the North Atlantc-Arctc Oscllaton and possbly the Pacfc Decadal Oscllaton. Therefore, regonal temperature trends over a few decades can be strongly nfluenced by regonal varablty n the clmate system and can depart apprecably from a global average. The 1910 to 1945 warmng was ntally concentrated n the North Atlantc. By contrast, the perod 1946 to 1975 showed sgnfcant coolng n the North Atlantc, as well as much of the Northern Hemsphere, and warmng n much of the Southern Hemsphere. New analyses ndcate that global ocean heat content has ncreased sgnfcantly snce the late 1950s. More than half of the ncrease n heat content has occurred n the upper 300 m 3 Generally, temperature trends are rounded to the nearest 0.05 C per unt of tme, the perods often beng lmted by data avalablty. 26

7 (a) Annual temperature trends, 1901 to 2000 (b) Annual temperature trends, 1910 to 1945 (c) Annual temperature trends, 1946 to 1975 (d) Annual temperature trends, 1976 to Trend ( C/decade) Fgure 3: Annual temperature trends for the perods 1901 to 1999, 1910 to 1945, 1946 to 1975 and 1976 to 1999 respectvely. Trends are represented by the area of the crcle wth red representng ncreases, blue representng decreases, and green lttle or no change. Trends were calculated from annually averaged grdded anomales wth the requrement that the calculaton of annual anomales nclude a mnmum of 10 months of data. For the perod 1901 to 1999, trends were calculated only for those grd boxes contanng annual anomales n at least 66 of the 100 years. The mnmum number of years requred for the shorter tme perods (1910 to 1945, 1946 to 1975, and 1976 to 1999) was 24, 20, and 16 years respectvely. [Based on Fgure 2.9] of the ocean, equvalent to a rate of temperature ncrease n ths layer of about 0.04 C/decade. New analyses of daly maxmum and mnmum land-surface temperatures for 1950 to 1993 contnue to show that ths measure of durnal temperature range s decreasng very wdely, although not everywhere. On average, mnmum temperatures are ncreasng at about twce the rate of maxmum temperatures (0.2 versus 0.1 C/decade). Temperatures above the surface layer from satellte and weather balloon records Surface, balloon and satellte temperature measurements show that the troposphere and Earth s surface have warmed and that the stratosphere has cooled. Over the shorter tme perod for whch there have been both satellte and weather balloon data (snce 1979), the balloon and satellte records show sgnfcantly less lower-tropospherc warmng than observed at the surface. Analyses of temperature trends snce 1958 for the lowest 8 km of the atmosphere and at the surface are n 27

8 good agreement, as shown n Fgure 4a, wth a warmng of about 0.1 C per decade. However, snce the begnnng of the satellte record n 1979, the temperature data from both satelltes and weather balloons show a warmng n the global mddle-to-lower troposphere at a rate of approxmately 0.05 ± 0.10 C per decade. The global average surface temperature has ncreased sgnfcantly by 0.15 ± 0.05 C/decade. The dfference n the warmng rates s statstcally sgnfcant. By contrast, durng the perod 1958 to 1978, surface temperature trends were near zero, whle trends for the lowest 8 km of the atmosphere were near 0.2 C/decade. About half of the observed dfference n warmng snce 1979 s lkely 4 to be due to the combnaton of the dfferences n spatal coverage of the surface and tropospherc observatons and the physcal effects of the sequence of volcanc eruptons and a substantal El Nño (see Box 4 for a general descrpton of ENSO) that occurred wthn ths perod. The remanng dfference s very lkely real and not an observng bas. It arses prmarly due to dfferences n the rate of temperature change over the tropcal and sub-tropcal regons, whch were faster n the lowest 8 km of the atmosphere before about 1979, but whch have been slower snce then. There are no sgnfcant dfferences n warmng rates over md-lattude contnental regons n the Northern Hemsphere. In the upper troposphere, no sgnfcant global temperature trends have been detected snce the early 1960s. In the stratosphere, as shown n Fgure 4b, both satelltes and balloons show substantal coolng, punctuated by sharp warmng epsodes of one to two years long that are due to volcanc eruptons. Surface temperatures durng the prenstrumental perod from the proxy record It s lkely that the rate and duraton of the warmng of the 20th century s larger than any other tme durng the last 1,000 years. The 1990s are lkely to have been the warmest decade of the mllennum n the Northern Hemsphere, and 1998 s lkely to have been the warmest year. There has been a consderable advance n understandng of temperature change that occurred over the last mllennum, especally from the synthess of ndvdual temperature reconstructons. Ths new detaled temperature record for the Northern Hemsphere s shown n Anomaly ( C) Anomaly ( C) a) b) Agung Satelltes El Chchon Pnatubo Balloons Balloons Satelltes Surface Year Year Fgure 4: (a) Tme-seres of seasonal temperature anomales of the troposphere based on balloons and satelltes n addton to the surface. (b) Tme-seres of seasonal temperature anomales of the lower stratosphere from balloons and satelltes. [Based on Fgure 2.12] Fgure 5. The data show a relatvely warm perod assocated wth the 11th to 14th centures and a relatvely cool perod assocated wth the 15th to 19th centures n the Northern Hemsphere. However, evdence does not support these Medeval Warm Perod and Lttle Ice Age perods, respectvely, as beng globally synchronous. As Fgure 5 ndcates, the rate and duraton of warmng of the Northern Hemsphere n the 20th century appears to have been unprecedented durng the mllennum, and t cannot smply be consdered as a recovery from the Lttle Ice Age of the 15th to 19th centures. These analyses are complemented by senstvty analyss of the spatal representatveness of avalable palaeoclmatc data, ndcatng that the warmth of the recent decade s outsde the 95% confdence nterval of temperature uncertanty, even durng the warmest perods of the last mllennum. Moreover, several dfferent analyses have now been completed, each suggestng 4 In ths Techncal Summary and n the Summary for Polcymakers, the followng words have been used to ndcate approxmate judgmental estmates of confdence: vrtually certan (greater than 99% chance that a result s true); very lkely (90 99% chance); lkely (66 90% chance); medum lkelhood (33 66% chance); unlkely (10 33% chance); very unlkely (1 10% chance); exceptonally unlkely (less than 1% chance). The reader s referred to ndvdual chapters for more detals. 28

9 1.0 Instrumental data (AD 1902 to 1999) Reconstructon (AD 1000 to 1980) Reconstructon (40 year smoothed) 1998 nstrumental value Northern Hemsphere anomaly ( C) relatve to 1961 to Year Fgure 5: Mllennal Northern Hemsphere (NH) temperature reconstructon (blue tree rngs, corals, ce cores, and hstorcal records) and nstrumental data (red) from AD 1000 to Smoother verson of NH seres (black), and two standard error lmts (gray shaded) are shown. [Based on Fgure 2.20] that the Northern Hemsphere temperatures of the past decade have been warmer than any other tme n the past sx to ten centures. Ths s the tme-span over whch temperatures wth annual resoluton can be calculated usng hemspherc-wde tree-rng, ce-cores, corals, and and other annually-resolved proxy data. Because less data are avalable, less s known about annual averages pror to 1,000 years before the present and for condtons prevalng n most of the Southern Hemsphere pror to It s lkely that large rapd decadal temperature changes occurred durng the last glacal and ts deglacaton (between about 100,000 and 10,000 years ago), partcularly n hgh lattudes of the Northern Hemsphere. In a few places durng the deglacaton, local ncreases n temperature of 5 to 10 C are lkely to have occurred over perods as short as a few decades. Durng the last 10,000 years, there s emergng evdence of sgnfcant rapd regonal temperature changes, whch are part of the natural varablty of clmate. 29

10 B.2 Observed Changes n Precptaton and Atmospherc Mosture Snce the tme of the SAR, annual land precptaton has contnued to ncrease n the mddle and hgh lattudes of the Northern Hemsphere (very lkely to be 0.5 to 1%/decade), except over Eastern Asa. Over the sub-tropcs (10 N to 30 N), landsurface ranfall has decreased on average (lkely to be about 0.3%/decade), although ths has shown sgns of recovery n recent years. Tropcal land-surface precptaton measurements ndcate that precptaton lkely has ncreased by about 0.2 to 0.3%/ decade over the 20th century, but ncreases are not evdent over the past few decades and the amount of tropcal land (versus ocean) area for the lattudes 10 N to 10 S s relatvely small. Nonetheless, drect measurements of precptaton and model reanalyses of nferred precptaton ndcate that ranfall has also ncreased over large parts of the tropcal oceans. Where and when avalable, changes n annual streamflow often relate well to changes n total precptaton. The ncreases n precptaton over Northern Hemsphere md- and hgh lattude land areas have a strong correlaton to long-term ncreases n total cloud amount. In contrast to the Northern Hemsphere, no comparable systematc changes n precptaton have been detected n broad lattudnal averages over the Southern Hemsphere. It s lkely that total atmospherc water vapour has ncreased several per cent per decade over many regons of the Northern Hemsphere. Changes n water vapour over approxmately the past 25 years have been analysed for selected regons usng n stu surface observatons, as well as lower-tropospherc measurements from satelltes and weather balloons. A pattern of overall surface and lower-tropospherc water vapour ncreases over the past few decades s emergng from the most relable data sets, although there are lkely to be tme-dependent bases n these data and regonal varatons n the trends. Water vapour n the lower stratosphere s also lkely to have ncreased by about 10% per decade snce the begnnng of the observatonal record (1980). Changes n total cloud amounts over Northern Hemsphere md- and hgh lattude contnental regons ndcate a lkely ncrease n cloud cover of about 2% snce the begnnng of the 20th century, whch has now been shown to be postvely correlated wth decreases n the durnal temperature range. Smlar changes have been shown over Australa, the only Southern Hemsphere contnent where such an analyss has been completed. Changes n total cloud amount are uncertan both over sub-tropcal and tropcal land areas, as well as over the oceans. B.3 Observed Changes n Snow Cover and Land- and Sea-Ice Extent Decreasng snow cover and land-ce extent contnue to be postvely correlated wth ncreasng land-surface temperatures. Satellte data show that there are very lkely to have been decreases of about 10% n the extent of snow cover snce the late 1960s. There s a hghly sgnfcant correlaton between ncreases n Northern Hemsphere land temperatures and the decreases. There s now ample evdence to support a major retreat of alpne and contnental glacers n response to 20th century warmng. In a few martme regons, ncreases n precptaton due to regonal atmospherc crculaton changes have overshadowed ncreases n temperature n the past two decades, and glacers have re-advanced. Over the past 100 to 150 years, ground-based observatons show that there s very lkely to have been a reducton of about two weeks n the annual duraton of lake and rver ce n the md- to hgh lattudes of the Northern Hemsphere. Northern Hemsphere sea-ce amounts are decreasng, but no sgnfcant trends n Antarctc sea-ce extent are apparent. A retreat of sea-ce extent n the Arctc sprng and summer of 10 to 15% snce the 1950s s consstent wth an ncrease n sprng temperatures and, to a lesser extent, summer temperatures n the hgh lattudes. There s lttle ndcaton of reduced Arctc sea-ce extent durng wnter when temperatures have ncreased n the surroundng regon. By contrast, there s no readly apparent relatonshp between decadal changes of Antarctc temperatures and sea-ce extent snce After an ntal decrease n the md-1970s, Antarctc sea-ce extent has remaned stable, or even slghtly ncreased. New data ndcate that there lkely has been an approxmately 40% declne n Arctc sea-ce thckness n late summer to early autumn between the perod of 1958 to 1976 and the md-1990s, and a substantally smaller declne n wnter. The relatvely short record length and ncomplete samplng lmt the nterpretaton of these data. Interannual varablty and nter-decadal varablty could be nfluencng these changes. 30

11 B.4 Observed Changes n Sea Level Changes durng the nstrumental record Based on tde gauge data, the rate of global mean sea level rse durng the 20th century s n the range 1.0 to 2.0 mm/yr, wth a central value of 1.5 mm/yr (the central value should not be nterpreted as a best estmate). (See Box 2 for the factors that nfluence sea level.) As Fgure 6 ndcates, the longest nstrumental records (two or three centures at most) of local sea level come from tde gauges. Based on the very few long tde-gauge records, the average rate of sea level rse has been larger durng the 20th century than durng the 19th century. No sgnfcant acceleraton n the rate of sea level rse durng the 20th century has been detected. Ths s not nconsstent wth model results due to the possblty of compensatng factors and the lmted data. Changes durng the pre-nstrumental record Snce the last glacal maxmum about 20,000 years ago, the sea level n locatons far from present and former ce sheets has rsen by over 120 m as a result of loss of mass from these ce sheets. Vertcal land movements, both upward and downward, are stll occurrng n response to these large transfers of mass from ce sheets to oceans. The most rapd rse n global sea level was between 15,000 and 6,000 years Box 2: What causes sea level to change? The level of the sea at the shorelne s determned by many factors n the global envronment that operate on a great range of tme-scales, from hours (tdal) to mllons of years (ocean basn changes due to tectoncs and sedmentaton). On the tme-scale of decades to centures, some of the largest nfluences on the average levels of the sea are lnked to clmate and clmate change processes. Frstly, as ocean water warms, t expands. On the bass of observatons of ocean temperatures and model results, thermal expanson s beleved to be one of the major contrbutors to hstorcal sea level changes. Further, thermal expanson s expected to contrbute the largest component to sea level rse over the next hundred years. Deep ocean temperatures change only slowly; therefore, thermal expanson would contnue for many centures even f the atmospherc concentratons of greenhouse gases were to stablse. The amount of warmng and the depth of water affected vary wth locaton. In addton, warmer water expands more than colder water for a gven change n temperature. The geographcal dstrbuton of sea level change results from the geographcal varaton of thermal expanson, changes n salnty, wnds, and ocean crculaton. The range of regonal varaton s substantal compared wth the global average sea level rse. Sea level also changes when the mass of water n the ocean ncreases or decreases. Ths occurs when ocean water s exchanged wth the water stored on land. The major land store s the water frozen n glacers or ce sheets. Indeed, the man reason for the lower sea level durng the last glacal perod was the amount of water stored n the large extenson of the ce sheets on the contnents of the Northern Hemsphere. After thermal expanson, the meltng of mountan glacers and ce caps s expected to make the largest contrbuton to the rse of sea level over the next hundred years. These glacers and ce caps make up only a few per cent of the world s land-ce area, but they are more senstve to clmate change than the larger ce sheets n Greenland and Antarctca, because the ce sheets are n colder clmates wth low precptaton and low meltng rates. Consequently, the large ce sheets are expected to make only a small net contrbuton to sea level change n the comng decades. Sea level s also nfluenced by processes that are not explctly related to clmate change. Terrestral water storage (and hence, sea level) can be altered by extracton of ground water, buldng of reservors, changes n surface runoff, and seepage nto deep aqufers from reservors and rrgaton. These factors may be offsettng a sgnfcant fracton of the expected acceleraton n sea level rse from thermal expanson and glacal meltng. In addton, coastal subsdence n rver delta regons can also nfluence local sea level. Vertcal land movements caused by natural geologcal processes, such as slow movements n the Earth s mantle and tectonc dsplacements of the crust, can have effects on local sea level that are comparable to clmate-related mpacts. Lastly, on seasonal, nterannual, and decadal tme-scales, sea level responds to changes n atmospherc and ocean dynamcs, wth the most strkng example occurrng durng El Nño events. 31

12 ago, wth an average rate of about 10 mm/yr. Based on geologcal data, eustatc sea level (.e., correspondng to a change n ocean volume) may have rsen at an average rate of 0.5 mm/yr over the past 6,000 years and at an average rate of 0.1 to 0.2 mm/yr over the last 3,000 years. Ths rate s about one tenth of that occurrng durng the 20th century. Over the past 3,000 to 5,000 years, oscllatons n global sea level on tme-scales of 100 to 1,000 years are unlkely to have exceeded 0.3 to 0.5 m. B.5 Observed Changes n Atmospherc and Oceanc Crculaton Patterns The behavour of ENSO (see Box 4 for a general descrpton), has been unusual snce the md-1970s compared wth the prevous 100 years, wth warm phase ENSO epsodes beng relatvely more frequent, persstent, and ntense than the opposte cool phase. Ths recent behavour of ENSO s reflected n varatons n precptaton and temperature over much of the global tropcs and sub-tropcs. The overall effect Fgure 6: Tme-seres of relatve sea level for the past 300 years from Northern Europe: Amsterdam, Netherlands; Brest, France; Sheerness, UK; Stockholm, Sweden (detrended over the perod 1774 to 1873 to remove to frst order the contrbuton of post-glacal rebound); Swnoujsce, Poland (formerly Swnemunde, Germany); and Lverpool, UK. Data for the latter are of Adjusted Mean Hgh Water rather than Mean Sea Level and nclude a nodal (18.6 year) term. The scale bar ndcates ±100 mm. [Based on Fgure 11.7] 32

13 s lkely to have been a small contrbuton to the ncrease n global temperatures durng the last few decades. The Inter-decadal Pacfc Oscllaton and the Pacfc Decadal Oscllaton are assocated wth decadal to multdecadal clmate varablty over the Pacfc basn. It s lkely that these oscllatons modulate ENSO-related clmate varablty. Other mportant crculaton features that affect the clmate n large regons of the globe are beng charactersed. The North Atlantc Oscllaton (NAO) s lnked to the strength of the westerles over the Atlantc and extra-tropcal Eurasa. Durng wnter the NAO dsplays rregular oscllatons on nterannual to mult-decadal tme-scales. Snce the 1970s, the wnter NAO has often been n a phase that contrbutes to stronger westerles, whch correlate wth cold season warmng over Eurasa. New evdence ndcates that the NAO and changes n Arctc sea ce are lkely to be closely coupled. The NAO s now beleved to be part of a wder scale atmospherc Arctc Oscllaton that affects much of the extratropcal Northern Hemsphere. A smlar Antarctc Oscllaton has been n an enhanced postve phase durng the last 15 years, wth stronger westerles over the Southern Oceans. B.6 Observed Changes n Clmate Varablty and Extreme Weather and Clmate Events New analyses show that n regons where total precptaton has ncreased, t s very lkely that there have been even more pronounced ncreases n heavy and extreme precptaton events. The converse s also true. In some regons, however, heavy and extreme events (.e., defned to be wthn the upper or lower ten percentles) have ncreased despte the fact that total precptaton has decreased or remaned constant. Ths s attrbuted to a decrease n the frequency of precptaton events. Overall, t s lkely that for many mdand hgh lattude areas, prmarly n the Northern Hemsphere, statstcally sgnfcant ncreases have occurred n the proporton of total annual precptaton derved from heavy and extreme precptaton events; t s lkely that there has been a 2 to 4% ncrease n the frequency of heavy precptaton events over the latter half of the 20th century. Over the 20th century (1900 to 1995), there were relatvely small ncreases n global land areas experencng severe drought or severe wetness. In some regons, such as parts of Asa and Afrca, the frequency and ntensty of drought have been observed to ncrease n recent decades. In many regons, these changes are domnated by nter-decadal and mult-decadal clmate varablty, such as the shft n ENSO towards more warm events. In many regons, nter-daly temperature varablty has decreased, and ncreases n the daly mnmum temperature are lengthenng the freeze-free perod n most md- and hgh lattude regons. Snce 1950 t s very lkely that there has been a sgnfcant reducton n the frequency of much-below-normal seasonal mean temperatures across much of the globe, but there has been a smaller ncrease n the frequency of much-above-normal seasonal temperatures. There s no compellng evdence to ndcate that the characterstcs of tropcal and extratropcal storms have changed. Changes n tropcal storm ntensty and frequency are domnated by nterdecadal to multdecadal varatons, whch may be substantal, e.g., n the tropcal North Atlantc. Owng to ncomplete data and lmted and conflctng analyses, t s uncertan as to whether there have been any long-term and large-scale ncreases n the ntensty and frequency of extra-tropcal cyclones n the Northern Hemsphere. Regonal ncreases have been dentfed n the North Pacfc, parts of North Amerca, and Europe over the past several decades. In the Southern Hemsphere, fewer analyses have been completed, but they suggest a decrease n extra-tropcal cyclone actvty snce the 1970s. Recent analyses of changes n severe local weather (e.g., tornadoes, thunderstorm days, and hal) n a few selected regons do not provde compellng evdence to suggest long-term changes. In general, trends n severe weather events are notorously dffcult to detect because of ther relatvely rare occurrence and large spatal varablty. B.7 The Collectve Pcture: A Warmng World and Other Changes n the Clmate System As summarsed above, a sute of clmate changes s now well-documented, partcularly over the recent decades to century tme perod, wth ts growng set of drect measurements. Fgure 7 llustrates these trends n temperature ndcators (Fgure 7a) and hydrologcal and storm-related ndcators (Fgure 7b), as well as also provdng an ndcaton of certanty about the changes. 33

14 Taken together, these trends llustrate a collectve pcture of a warmng world: Surface temperature measurements over the land and oceans (wth two separate estmates over the latter) have been measured and adjusted ndependently. All data sets show qute smlar upward trends globally, wth two major warmng perods globally: 1910 to 1945 and snce There s an emergng tendency for global land-surface ar temperatures to warm faster than the global ocean-surface temperatures. Weather balloon measurements show that lower-tropospherc temperatures have been ncreasng snce 1958, though only slghtly snce Snce 1979, satellte data are avalable and show smlar trends to balloon data. The decrease n the contnental durnal temperature range concdes wth ncreases n cloud amount, precptaton, and ncreases n total water vapour. The nearly worldwde decrease n mountan glacer extent and ce mass s consstent wth worldwde surface temperature ncreases. A few recent exceptons n coastal regons are consstent wth atmospherc crculaton varatons and related precptaton ncreases. The decreases n snow cover and the shortenng seasons of lake and rver ce relate well to ncreases n Northern Hemspherc land-surface temperatures. The systematc decrease n sprng and summer sea-ce extent and thckness n the Arctc s consstent wth ncreases n temperature over most of the adjacent land and ocean. Ocean heat content has ncreased, and global average sea level has rsen. The ncreases n total tropospherc water vapour n the last 25 years are qualtatvely consstent wth ncreases n tropospherc temperatures and an enhanced hydrologc cycle, resultng n more extreme and heaver precptaton events n many areas wth ncreasng precptaton, e.g., mddle and hgh lattudes of the Northern Hemsphere. Some mportant aspects of clmate appear not to have changed. A few areas of the globe have not warmed n recent decades, manly over some parts of the Southern Hemsphere oceans and parts of Antarctca. No sgnfcant trends n Antarctc sea-ce extent are apparent over the perod of systematc satellte measurements (snce 1978). Based on lmted data, the observed varatons n the ntensty and frequency of tropcal and extra-tropcal cyclones and severe local storms show no clear trends n the last half of the 20th century, although mult-decadal fluctuatons are sometmes apparent. The varatons and trends n the examned ndcators mply that t s vrtually certan that there has been a generally ncreasng trend n global surface temperature over the 20th century, although short-term and regonal devatons from ths trend occur. 34

15 (a) Temperature Indcators O C E A N L A N D O C E A N Fgure 7a: Schematc of observed varatons of the temperature ndcators. [Based on Fgure 2.39a] LOWER STRATOSPHERE ** lower stratosphere: 0.5 to 2.5 o C decrease snce 1979 TROPOSPHERE Upper * Lttle or no change snce 1979 Low- to Md- ** 0.0 to 0.2 o C ncrease snce satelltes & balloons * 0.2 to 0.4 o C ncrease snce ~1960 NEAR-SURFACE * 1990s warmest decade of the mllennum and 1998 warmest year for at least the N.H. ** N.H. Sprng snow cover extent: snce 1987, 10% below mean ** marne ar temperature: 0.4 to 0.7 o C ncrease snce late-19th century *** sea surface temperature: 0.4 to 0.8 o C ncrease snce the late 19th century. * global ocean (to 300m depth) heat content ncrease snce 1950s o equal to 0.04 C / decade *** massve retreat of mountan glacers durng 20th century * land nght tme ar temperature ncreasng at twce the rate of daytme temperatures snce 1950 ** lake and rver ce retreat at md and hgh lattudes snce the late 19th century (2 week decrease n ce duraton) *** land ar temperatures: 0.4 to 0.8 o C ncrease snce late 19th century * Arctc sea ce: summer thckness decrease of 40% and 10 to 15% decrease n extent durng sprng and summer snce 1950s? Antarctc sea ce: no sgnfcant change snce 1978 Lkelhood: *** Vrtually certan (probablty > 99%) ** Very lkely (probablty _ > 90% but _ < 99%) * Lkely (probablty > 66% but < 90%)? Medum lkelhood (probablty > 33% but _ < 66%) (b) Hydrologcal and Storm related Indcators O C E A N L A N D O C E A N LOWER STRATOSPHERE * 20% water vapour ncrease snce 1980 (above 18 km) Fgure 7b: Schematc of observed varatons of the hydrologcal and storm-related ndcators. [Based on Fgure 2.39b] TROPOSPHERE Water vapour upper troposphere: *no sgnfcant global trends snce 1980; 15% ncrease n tropcs (10 N to 10 S) troposphere: *many regons wth ncreases snce about 1960? 2% ncrease n total cloud amount over the ocean snce 1952 * 2% ncrease n total cloud amount over land durng the 20th century NEAR-SURFACE * 2 to 3% decrease n sub-tropcs * 2 to 3% ncrease n tropcs? no systematc large-scale change n tornadoes, thunder-days, hal 20th century land surface ranfall? no consstent 20th century change n extra-tropcal storm frequency / ntensty ** no wdespread changes n tropcal storm frequency / ntensty durng the 20th century ** 5 to10% ncrease n N. Hemsphere md-to-hgh lattude precptaton snce 1900, wth much of t due to heavy / extreme events * wdespread sgnfcant ncreases n surface water vapour n the N. Hemsphere, 1975 to 1995 Lkelhood: *** Vrtually certan (probablty > 99%) ** Very lkely (probablty > _ 90% but < _ 99%) * Lkely (probablty > 66% but < 90%)? Medum lkelhood (probablty > 33% but < _ 66%) 35

16 C. The Forcng Agents That Cause Clmate Change In addton to the past varatons and changes n the Earth s clmate, observatons have also documented the changes that have occurred n agents that can cause clmate change. Most notable among these are ncreases n the atmospherc concentratons of greenhouse gases and aerosols (mcroscopc arborne partcles or droplets) and varatons n solar actvty, both of whch can alter the Earth s radaton budget and hence clmate. These observatonal records of clmate-forcng agents are part of the nput needed to understand the past clmate changes noted n the precedng Secton and, very mportantly, to predct what clmate changes could le ahead (see Secton F). Lke the record of past clmate changes, the data sets for forcng agents are of varyng length and qualty. Drect measurements of solar rradance exst for only about two decades. The sustaned drect montorng of the atmospherc concentratons of carbon doxde (CO 2 ) began about the mddle of the 20th century and, n later years, for other long-lved, well-mxed gases such as methane. Palaeo-atmospherc data from ce cores reveal the concentraton changes occurrng n earler mllenna for some greenhouse gases. In contrast, the tme-seres measurements for the forcng agents that have relatvely short resdence tmes n the atmosphere (e.g., aerosols) are more recent and are far less complete, because they are harder to measure and are spatally heterogeneous. Current data sets show the human nfluence on atmospherc concentratons of both the long-lved greenhouse gases and short-lved forcng agents durng the last part of the past mllennum. Fgure 8 llustrates the effects of the large growth over the Industral Era n the anthropogenc emssons of greenhouse gases and sulphur doxde, the latter beng a precursor of aerosols. A change n the energy avalable to the global Earth-atmosphere system due to changes n these forcng agents s termed radatve forcng (Wm 2 ) of the clmate system (see Box 1). Defned n ths manner, radatve forcng of clmate change consttutes an ndex of the relatve global mean mpacts on the surface-troposphere system due to dfferent natural and anthropogenc causes. Ths Secton updates the knowledge of the radatve forcng of clmate change that has occurred from pre-ndustral tmes to the present. Fgure 9 shows the estmated radatve forcngs from the begnnng of the Industral Era (1750) to 1999 for the quantfable natural and anthropogenc Atmospherc concentraton CO 2 (ppm) CH 4 (ppb) N 2 O (ppb) (a) Carbon Doxde Methane Ntrous Oxde Year (b) Sulphate concentraton (mg SO 4 2 per tonne of ce) Sulphur Year Radatve forcng (Wm 2 ) SO 2 emssons (Mllons of tonnes sulphur per year) Fgure 8: Records of changes n atmospherc composton. (a) Atmospherc concentratons of CO 2, CH 4 and N 2 O over the past 1,000 years. Ice core and frn data for several stes n Antarctca and Greenland (shown by dfferent symbols) are supplemented wth the data from drect atmospherc samples over the past few decades (shown by the lne for CO 2 and ncorporated n the curve representng the global average of CH 4 ). The estmated radatve forcng from these gases s ndcated on the rght-hand scale. (b) Sulphate concentraton n several Greenland ce cores wth the epsodc effects of volcanc eruptons removed (lnes) and total SO 2 emssons from sources n the US and Europe (crosses). [Based on (a) Fgure 3.2b (CO 2 ), Fgure 4.1a and b (CH 4 ) and Fgure 4.2 (N 2 O) and (b) Fgure 5.4a] 36