Policymakers Summary. Prepared by IPCC Working Group I

Transcription

1 Plicymakers Summary Prepared by IPCC Wrking Grup I

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3 CONTENTS Executive Summary Intrductin: what is the issue? What factrs determine glbal climate? What natural factrs are imprtant 1? Hw d we knw that the natural greenhuse effect is real 9 Hw might human activities change glbal climate'' What are the greenhuse gases and why are they increasing? Cncentratins, lifetimes and stabilisatin f the gases Hw will greenhuse gas abundances change in the future 9 Greenhuse gas feedbacks xi xm xm xm xiv xiv xv xvn xvn xvm a If emissins fllw a Business-as-Usual pattern b If emissins ai e subject t cntils What will be the patterns f climate change by Hw will climate extremes and extreme events change 9 Will strms increase in a wanner wrld 9 Climate change in the lnger term Other factrs which culd influence future climate Hw much cnfidence d we have in ur predictins? Will the climate f the future be very different? Has man already begun t change the glbal climate? Hw much will sea level rise? xxn xxn xxm xxm xxv xxvi xxvu xxvu xxvu xxvm xxix Which gases are the mst imprtant? Hw can we evaluate the effect f different greenhuse gases 9 Hw much d we expect climate t change? Hw quickly will glbal climate change 9 xvm xix xx xxn What will be the effect f climate change n ecsystems? xxx What shuld be dne t reduce uncertainties, and hw lng will this take? Annex xxxi xxxiv

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5 EXECUTIVE SUMMARY We are certain f the fllwing: there is a natural greenhuse effect which already keeps the Earth wanner than it wuld therwise be emissins resulting frm human activities are substantially increasing the atmspheric cncen tratins f the greenhuse gases carbn dixide, methane, chlrflurcarbns (CFCs) and nitrus xide These increases will enhance the greenhuse effect, resulting n average in an additinal warming f the Earth's surface The main greenhuse gas, water vapur, will increase in respnse t glbal warming and further enhance it We calculate with cnfidence that: sme gases are ptentially mre effective than thers at changing climate, and their relative effectiveness can be estimated Carbn dixide has been respnsible fr ver half the enhanced greenhuse effect in the past, and is likely t remain s in the future atmspheric cncentratins f the lng-lived gases (carbn dixide, nitrus xide and the CFCs) adjust nly slwly t changes in emissins Cntinued emissins f these gases at present rates wuld cmmit us t increased cncentratins lr centuries ahead The lnger emissins cntinue t increase at present day rates, the greater reductins wuld have t be fr cncentratins t stabilise at a given level the lng-lived gases wuld require immediate reductins in emissins frm human activities f ver 60% t stabilise their cncentratins at tday's levels, methane wuld require a 15-20% reductin Based n current mdel results, we predict: under the IPCC Business-as-Usual (Scenari A) emissins f greenhuse gases, a rate f increase f glbal mean temperature during the next century f abut 0 3 C per decade (with an uncertainty range f 0 2 C t 0 5 C per decade), this is greater than that seen ver the past 10,000 years This will result in a likely increase in glbal mean temperature f abut 1 C abve the present value by 2025 and VC befre the end f the next century The rise will nt be steady because f the influence f ther factrs under the ther IPCC emissin scenaris which assume prgressively increasing levels f cntrls rates f increase in glbal mean temperature f abut 0 2 C per decade (Scenari B), just abve 0 1 C per decade (Scenari C) and abut 0 1 C per decade (Scenari D) that land surfaces warm mre rapidly than the cean and high nrthern latitudes warm mre than the glbal mean in winter reginal climate changes different frm the glbal mean, althugh ur cnfidence in the predictin f the detail f reginal changes is lw Fr example, temperature increases in Suthern Eurpe and central Nrth America are predicted t be higher than the glbal mean accmpanied n average by reduced summer precipitatin and sil mistuic There are less cnsistent predictins lr the trpics and the Suthern Hemisphere under the IPCC Business as Usual emissins scenari, an average rate f glbal mean sea level rise f abut 6cm per decade ver the next century (with an uncertainty range f 3-10cm per decade) mainly due t thermal expansin l the ceans and the melting f sme land ice The predicted rise is abut 20cm in glbal mean sea level by 2(H0, and 65cm by the end f the next century There will be significant reginal variatins

6 Ml Plicymakers Summaiy There are many uncertainties in ur predictins particularly with regard t the timing, magnitude and reginal patterns f climate change, due t ur incmplete understanding f: surces and sinks f greenhuse gases, which affect predictins f future cncentratins cluds, which strngly influence the magnitude f climate change ceans, which influence the timing and patterns f climate change plar ice sheets which affect predictins f sea level rise These prcesses are already partially understd, and we are cnfident that the uncertainties can be reduced by further research Hwever, the cmplexity f the system means that we cannt rule ut surprises Our judgement is that: Glbal - mean surface air temperature has increased by 0 3 C t 0 6 C ver the last 100 years, with the five glbal-average warmest years being in the 1980s Over the same perid glbal sea level has increased by 10-20cm These increases have nt been smth with time, nr unifrm ver the glbe The size f this warming is bradly cnsistent with predictins f climate mdels, but it is als f the same magnitude as natural climate variability Thus the bserved increase culd be largely due t this natural variability, alternatively this variability and ther human factrs culd have ffset a still larger human-induced greenhuse warming The uneq- uivcal detectin f the enhanced greenhuse effect frm bservatins is nt likely fr a decade r mre There is n firm evidence that climate has becme mre variable ver the last few decades Hwever, with an increase in the mean temperature, episdes f high temperatures will mst likely becme mre frequent in the future, and cld episdes less frequent Ecsystems affect climate, and will be affected by a changing climate and by increasing carbn dixide cncentratins Rapid changes in climate will change the cmpsitin f ecsystems, sme species will benefit while thers will be unable t migrate r adapt fast enugh and may becme extinct Enhanced levels f carbn dixide may increase prductivity and efficiency f water use f vegetatin The effect f warming n bilgical prcesses, althugh prly understd, may increase the atmspheric cncentratins f natural greenhuse gases T imprve ur predictive capability, we need: t understand better the varius climate-related prcesses, particularly thse assciated with cluds, ceans and the carbn cycle t imprve the systematic bservatin f climaterelated variables n a glbal basis, and further investigate changes which tk place in the past t develp imprved mdels f the Earth's climate system t increase supprt fr natinal and internatinal climate research activities, especially in develping cuntries t facilitate internatinal exchange f climate data

7 Plicymaket s Summary \m Intrductin: what is the issue? There is cncern that human activities may be inadvertently changing the climate f the glbe thrugh the enhanced greenhuse effect, by past and cntinuing emissins f carbn dixide and ther gases which will cause the temperature f the Earth's surface t increase - ppularly termed the "glbal warming' If this ccurs, cnsequent changes may have a significant impact n sciety The purpse f the Wrking Grup I reprt, as determined by the first meeting f IPCC, is t prvide a scientific assessment f 1) the factrs which may affect climate change during the next century, especially thse which are due t human activity 2) the respnses f the atmsphere - cean - land - ice system 3) current capabilities f mdelling glbal and reginal climate changes and their predictability 4) the past climate recrd and presently bserved climate anmalies On the basis f this assessment, the reprt presents current knwledge regarding predictins f climate change (including sea level rise and the effects n ecsystems) ver the next century, the timing f changes tgether with an assessment f the uncertainties assciated with these predictins This Plicymakers Summary aims t bring ut thse elements f the main reprt which have the greatest relevance t plicy frmulatin, in answering the fllwing questins What factrs determine glbal climate 7 What are the greenhuse gases, and hw and why are they increasing 9 Which gases are the mst imprtant 9 Hw much d we expect the climate t change 9 Hw much cnfidence d we have in ur predictins 9 Will the climate f the future be very different 9 Have human activities already begun t change glbal climate 9 Hw much will sea level rise 9 What will be the effects n ecsystems 9 What shuld be dne t reduce uncertainties, and hw lng will this take 9 This reprt is intended t respnd t the practical needs f the plicymaker It is neither an academic review, nr a plan fr a new research prgramme Uncertainties attach t almst every aspect f the issue, yet plicymakers are lking fr clear guidance frm scientists, hence authrs have been asked t prvide their best-estimates wherever pssible, tgether with an assessment f the uncertainties This reprt is a summary f ur understanding in 1990 Althugh cntinuing research will deepen this understanding and require the reprt t be updated at frequent intervals, basic cnclusins cncerning the reality f the enhanced greenhuse effect and its ptential t alter glbal climate are unlikely t change significantly Nevertheless, the cmplexity f the system may give rise t surprises What factrs determine glbal climate? There are many factrs, bth natural and f human rigin, that determine the climate f the earth We lk first at thse which are natural, and then see hw human activities might cntribute What natural factrs are imprtant? The driving energy fr weather and climate cmes frm the Sun The Earth intercepts slar radiatin (including that in the shrt-wave, visible, part f the spectrum), abut a third f it is reflected, the rest is absrbed by the different cmpnents (atmsphere, cean, ice, land and bita) f the climate system The energy absrbed frm slar radiatin is balanced (in the lng term) by utging radiatin frm the Earth and atmsphere, this terrestrial radiatin takes the frm f lng-wave invisible infrared energy, and its magnitude is determined by the temperature f the Earthatmsphere system There are several natural factrs which can change the balance between the energy absrbed by the Earth and that emitted by it in the frm f lngwave infrared radiatin these factrs cause the radiative frcing n climate The mst bvius f these is a change in the utput f energy frm the Sun There is direct evidence f such variability ver the 11-year slar cycle, and lnger perid changes may als ccur Slw variatins in the Earth s rbit affect the seasnal and latitudinal distributin f slar radiatin these were prbably respnsible fr initiating the ice ages One f the mst imprtant factrs is the greenhuse effect, a simplified explanatin f which is as fllws Shrt-wave slar radiatin can pass thrugh the clear atmsphere relatively unimpeded But lng-wave terrestrial radiatin emitted by the warm surface f the Earth is partially absrbed and then re-emitted by a number f trace gases in the cler atmsphere abve Since, n average, the utging lng wave radiatin balances the incming slar radiatin bth the atmsphere and the surface will be warmer than they wuld be withut the greenhuse gases

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9 Phcymakei s Summaiy \\ 1990 LEVEL OFCCV AGE (THOUSAND S BEFORE PRESENT) Figure 2: Analysis f air trapped in Antarctic ice cres shws that methane and carbn dixide cncentratins weie clsely crrelated with the lcal temperature ver the lasi years Present day cncentratins l carbn dixide are indicated There are ther human activities which have the ptential t affect climate A change in the albed (icflectivity) f the land, brught abut by desertificatin r defrestatin affects the amunt f slar energy absrbed at the Earth s surface Human-made aersls, fim sulphur emitted largely in fssil fuel cmbustin can mdify cluds and this may act t lwer tempeiatures Lastly, changes in zne in the stratsphere due t CFCs may als influence climate What are the greenhuse gases and why are they increasing? We aie certain that the cncentiatins f gieenhuse gases in the atmsphere have changed natuially n ice-age timescales and have been increasing since pre-industrial times due t human activities Table l suminanzes the present and pie-industnal abundances, cm rent rates f change and piesent atmspheric lifetimes l gieenhuse gases influenced by human activities Caibn dixide methane and nitrus xide all have significant natural and human surces, while the chlrflurcarbns are nly prduced industrially Tw imprtant greenhuse gases, water vapui and zne, are nt included in this table Water vapur has the largest greenhuse effect, but its cncentratin in the trpsphere is determined internally within the climate system, and, n a glbal scale, is nt affected by human surces and sinks Water vapur will increase in respnse t glbal warming and further enhance it, this prcess is included in climate mdels The cncentratin f zne is changing bth in the stratsphere and the trpsphere due t human activities, but it is difficult t quantify the changes frm present bservatins Fr a thusand years prir t the industrial revlutin, abundances f the greenhuse gases weie relatively cnstant Hwever, as the wrlds ppulatin incieased as the wrld became mre industrialized and as agriculture develped, the abundances f the greenhuse gases increased markedly Figure 3 illustrates this lr carbn dixide, methane, nitrus xide and CFC 11 Since the industiial revlutin the cmbustin f fssil fuels and defrestatin have led t an increase l 26% in carbn dixide cncentiatin in the atmspheie We knw the magnitude f the present day fssil-fuel surce, but the input frm defrestatin cannt be estimated accurately In additin, althugh abut half f the emitted carbn dixide stays in the atmsphere, we d nt knw well hw much f the remainder is absrbed by the ceans and hw much by terrestrial bita Emissins l chliflurcaibns used as aersl prpellants, slvents refngeiants and lam blwing agents, are als well knwn, they were nt present in the atmsphere befre their inventin in the I9^0s The surces f methane and nitrus xide aie less well knwn Methane cncentratins have mie than dubled because f rice prductin cattle reanng bimass burning cal mining and ventilatin l natuial gas als lssil fuel cmbustin may have als cnlnbuted thiugh chemical reactins in the atmsphere which ieduce the iate l remval f methane Nitrus xide has incieased by abut 89f since pie-industnal times piesumabl) due t human activities, we arc unable t specify the surces but it is likely that agriculture plays a pan The effect f zne n climate is strngest in the upper trpsphere and lwer stiatsphere Mdel calculatins indicate that zne in the upper tipspheie shuld have increased due t human-made emissins f nitrgen xides, hydrcarbns and carbn mnxide While at giund level zne has increased in the Nrthern Hemisphere in respnse t these emissins bservatins are insufficient t cntnm the expected inciease in (he uppei tipspheie The lack f adequate bseivatins pi events us frm accuiatcly quantifying the climatic etfect l changes in tipsphcnc zne

10 XVI Plicymakei s Summaiy Table 1: Summaiy f Key Gi eenhuse Gases Affected by Human Activities Carbn Methane CFC-11 CFC-12 Nitrus Dixide Oxide Atmspheric cncentratin ppmv ppmv pptv pptv ppbv Pre-industnal ( ) Present day (1990) Current rate f change per year 1 8 (0.5%) 0015 (0 9%) 9.5 (4%) 17 (4%) 0.8 (0.25%) Atmspheric lifetime (years) (50-200)t ppmv = parts per millin by vlume, ppbv = parts per billin (thusand millin) by vlume, pptv = parts per trillin (millin millin) by vlume t The way in which C0 2 is absrbed by the ceans and bisphere is nt simple and a single value cannt be given, refer t the main reprt fr further discussin 360 CARBON DIOXIDE Q. - METHANE T NITROUS OXIDE ~ T Figure 3: Cncentratins f carbn dixide and methane after remaining relatively cnstant up t the 18th century, have risen sharply since then due t man's activities Cncentratins f nitrus xide have increased since the mid-18th century, especially in the last few decades CFCs were nt present in the atmsphere befre the 1930s

11 Plicymakers Summary > E a S z < a z CM %1990 EMISSIONS n % pa DECREASE. FROM % pa DECREASE FROM 1990 XMl (d) ~ (c) -i r Figure 4: The relatinship between hypthetical fssil fuel emissins f carbn dixide and its cncentratin in the atmsphere is shwn in the case where (a) emissins cntinue at 1990 levels, (b) emissins are reduced by 50% in 1990 and cntinue at that level, (c) emissins are reduced by 2% pa frm 1990, and (d) emissins, after increasing by 2% pa until 2010, are then reduced by 2% pa thereafter In the lwer stratsphere at high suthern latitudes zne has decreased cnsiderably due t the effects f CFCs, and there are indicatins f a glbal-scale decrease which, while nt understd, may als be due t CFCs These bserved decreases shuld act t cl the earth's surface, thus prviding a small ffset t the predicted warming prduced by the ther greenhuse gases Further reductins in lwer stratspheric zne are pssible during the next few decades as the atmspheric abundances f CFCs cntinue t increase Cncentratins, lifetimes and stabilisatin f the gases In rder t calculate the atmspheric cncentratins f carbn dixide which will result frm human-made emissins we use cmputer mdels which incrprate details f the emissins and which include representatins f the transfer f carbn dixide between the atmsphere, ceans and terrestrial bisphere Fr the ther greenhuse gases, mdels which incrprate the effects f chemical reactins in the atmsphere are emplyed The atmspheric lifetimes f the gases are determined by their surces and sinks in the ceans, atmsphere and bisphere Carbn dixide, chlrflurcarbns and nitrus xide are remved nly slwly frm the atmsphere and hence, fllwing a change in emissins, their atmspheric cncentratins take decades t centuries t adjust fully Even if all human-made emissins f carbn dixide were halted in the year 1990, abut half f the increase in carbn dixide cncentratin caused by human activities wuld still be evident by the year 2100 In cntrast, sme f the CFC substitutes and methane have relatively shrt atmspheric lifetimes s that their atmspheric cncentratins respnd fully t emissin changes within a few decades T illustrate the emissin-cncentratin relatinship clearly, the effect f hypthetical changes in carbn dixide fssil fuel emissins is shwn in Figure 4 (a) cntinuing glbal emissins at 1990 levels, (b) halving f emissins in 1990, (c) reductins in emissins f 2% per year (pa) frm 1990 and (d) a 2% pa increase frm fllwed by a 2% pa decrease frm 2010 Cntinuatin f present day emissins are cmmitting us t increased future cncentratins, and the lnger emissins cntinue t increase, the greater wuld reductins have t be t stabilise at a given level If there are critical cncentratin levels that shuld nt be exceeded, then the earlier emissin reductins are made the mre effective they are The term "atmspheric stabilisatin" is ften used t describe the limiting f the cncentratin f the greenhuse gases at a certain level The amunt by which human-made emissins f a greenhuse gas must be reduced in rder t stabilise at present day cncentratins, fr example, is shwn in Table 2 Fr mst gases the reductins wuld have t be substantial Hw will greenhuse gas abundances change in the future? We need t knw future greenhuse gas cncentratins in rder t estimate future climate change As already mentined, these cncentratins depend upn the magnitude f human-made emissins and n hw changes in climate and ther envirnmental cnditins may influence the bisphenc prcesses that cntrl the exchange f natural greenhuse gases, including carbn dixide and methane between the atmsphere, ceans and terrestrial bisphere the greenhuse gas 'feedbacks

12 \w// Plicymakers Summaiy Table 2: Stabilisatin f Atmspheiit Cncentiatwns Reductins in the human-made emissins f gieenhuse gases i ec/iiu ed t stabilise c ne enti at ins at pi esent day le\ els Greenhuse Gas Reductin Required Carbn Dixide >60% Methane 15-20% Nitrus Oxide 70 80% CFC II 70-75% CFC % HCFC % Nte that the stabilisatin f each f these gases wuld have different effects n climate, as explained in the next sectin Fur scenaris f future human-made emissins were develped by Wrking Grup III The first f these assumes that few r n steps are taken t limit greenhuse gas emissins and this is therefre termed Business-as- Usual (BaU) (It shuld be nted that an aggregatin f natinal frecasts l emissins f carbn dixide and methane t the year 2025 undertaken by Wrking Grup III resulted in glbal emissins 10-20% higher than in the BaU scenan) The ther thiee scenaris assume that prgressively increasing levels f cntrls reduce the giwth l emissins these aie refened t as scenaris B, C, and D They are hi icily descnbcd in the Annex t this summary Future cncentiatins l sme f the greenhuse gases which wuld arise lim these emissins are shwn in Figure 5 Greenhuse gas feedbacks Sme l the pssible feedbacks which culd significantly mdify luture greenhuse gas cncentratins in a warmer wild aie discussed in the fllwing paragraphs The net emissins l carbn dixide frm terrestrial ecsystems will be elevated it higher temperatures increase lespiratin at a faster rate than phtsynthesis r if plant ppulatins particularly large frests, cannt adjust rapidly enugh t changes in climate A net flux f carbn dixide t the atmsphere may be paiticularly evident in warmer cnditins in tundra and bieal legins wheie there are large stres f carbn The ppsite is true if higher abundances f carbn dixide in the atmsphere enhance the prductivity f natural ecsystems r if there is an increase in sil misture which can be expected t stimulate plant grwth in dry ecsystems and t increase the strage f carbn in tundra peat The extent t which ecsystems can sequester incieasing atmsphei IC carbn dixide remains t be quantified It the ceans becme wanner, their net uptake f carbn dixide may decrease because f changes in (i) the chemistry f carbn dixide in seawater, (n) bilgical activity in surface waters, and (in) the rate f exchange f carbn dixide between the surface layers and the deep cean This last depends upn the rate f frmatin f deep water in the cean which, in the Nrth Atlantic fr example, might decrease if the salinity decreases as a result f a change in climate Methane emissins frm natural wetlands and rice paddies are particularly sensitive t temperature and sil misture Emissins are significantly larger at higher temperatuies and with increased sil misture, cnversely, a decrease in sil misture wuld lesult in smaller emissins Higher temperatures culd increase the emissins f methane at high nrthern latitudes frm decmpsable rganic matter trapped in permafrst and methane hydrates As illustrated earlier, ice cre iccrds shw that methane and carbn dixide cncentratins changed in a similar sense t temperature between ice ages and interglacials Althugh many f these feedback prcesses are prly understd, it seems likely that, verall, they will act t increase, rather than decrease, greenhuse gas cn centratins in a warmei wrld Which gases are the mst imprtant? We are certain that increased greenhuse gas cncentratins increase radiative lrcing We can calculate the frcing with much mre cnfidence than the climate change that results because the frmer avids the need t evaluate a number l prly understd atmspheric respnses Wc then have a base frm which t calculate the ielativc effect n climate f an increase in cncentratin f each gas in the piesent-day atmsphere bth in abslute teims and ielativc t caibn dixide These ielativc ellccts span a wide lange methane is abut 21 times mre effective mlecule li-mlecule, than carbn dixide, and CFC 11 abut 12,000 times mre effective On a kilgram

13 Phcymakei s Summmy E Q. Q. z I- < LU z CM 300 BUSINESS- AS-USUAL SCENARD D " T <~ hw this quantity has changed in the past (based n bservatins f greenhuse gases) and hw it might change in the future (based n the fur IPCC emissins scenaris) Fr simplicity, we can express ttal frcing in terms l the amunt f carbn dixide which wuld give that frcing, this is termed the equivalent carbn dixide cncentratin Greenhuse gases have increased since premdustnal times (the mid-18th century) by an amunt that is radiatively equivalent t abut a 50% increase in carbn dixide, althugh carbn dixide itself has risen by nly 26%, ther gases have made up the rest The cntributins f the varius gases t the ttal increase in climate frcing during the 1980s is shwn as a pie diagram in Figure 7, carbn dixide is respnsible fr abut half the decadal increase (Ozne, the effects l which may be significant, is nt included) \i\ > a Z O H < OC K z u z u,- u i r BUSINESS- AS-USUAL 100 SCENARIOS CandD 0 -i r Figure 5: Atmspheric cncentratins f carbn dixide methane and CFC-11 resulting frm the tur IPCC emissins scenaris per-kilgram basis, the equivalent values are 58 fr methane and abut 4,000 fr CFC-11, bth relative t carbn dixide Values fr ther greenhuse gases are t be lund in Sectin 2 The ttal ladiative ficing at any time is the sum f thse frm the individual greenhuse gases We shw in Figuie 6 Hw can we evaluate the effect f different greenhuse gases? T evaluate pssible plicy ptins, it is useful t knw the relative radiative effect (and, hence, ptential climate effect) f equal emissins f each f the greenhuse gases The cncept f relative Glbal Warming Ptentials (GWP) has been develped t take int accunt the differing times that gases remain in the atmsphere This index defines the time-integrated warming effect due t an instantaneus release f unit mass (1 kg) f a given greenhuse gas in tday's atmsphere, relative t that f carbn dixide The relative imprtances will change in the future as atmspheric cmpsitin changes because althugh radiative frcing increases in direct prprtin t the cncentratin f CFCs, changes in the ther greenhuse gases (particularly carbn dixide) have an el feet n frcing which is much less than prprtinal The GWPs in Table 3 are shwn fr three time hrizns reflecting the need t cnsider the cumulative elfects n climate ver varius time scales The lnger time hrizn is apprpriate fr the cumulative effect the shrter timescale will indicate the respnse t emissin changes in the shrt term There are a number f practical difficulties in devising and calculating the values f the GWPs and the values given here shuld be cnsidered as preliminary In additin t these direct elfects, there are indirect effects f human-made emissins arising frm chemical reactins between the varius cnstituents The indirect effects n stratspheric water vapur, carbn dixide and trp sphenc zne have been included in these estimates Table 1 indicates, fr example that the effectiveness f methane in influencing climate will be greatei in the first few decades aftei lelease wheieas emissin f the lngci lived nitius xide will affect climate fi a much lngci time The lifetimes f the pipsed CFC replacements lange frm 1 t 40 years the lngci lived leplacements are

14 Plicymakers Summary CM O H Z _l < > 3 > E Q. a. z < Z z Figure 6: Increase in radiative frcing since the mid-18th century, and predicted t result frm the fur IPCC emissins scenaris, als expressed as equivalent carbn dixide cncentratins. CARBON DIOXIDE CFCs 11 and 12 OTHER CFCs NITROUS OXIDE f greenhuse gases in 1990 are shwn relative t carbn dixide. This is illustrative; t cmpare the effect f different emissin prjectins we have t sum the effect f emissins made in future years. There are ther technical criteria which may help plicymakers t decide, in the event f emissins reductins being deemed necessary, which gases shuld be cnsidered. Des the gas cntribute in a majr way t current, and future, climate frcing? Des it have a lng lifetime, s earlier reductins in emissins wuld be mre effective than thse made later? And are its surces and sinks well enugh knwn t decide which culd be cntrlled in practice? Table 5 illustrates these factrs. METHANE Figure 7: The cntributin frm each f the human-made greenhuse gases t the change in radiative frcing frm 1980 t The cntributin frm zne may als be significant, but cannt be quantified at present. still ptentially effective as agents f climate change. One example f this, HCFC-22 (with a 15 year lifetime), has a similar effect (when released in the same amunt) as CFC- 11 n a 20 year time-scale; but less ver a 500 year timescale. Table 3 shws carbn dixide t be the least effective greenhuse gas per kilgramme emitted, but its cntributin t glbal warming, which depends n the prduct f the GWP and the amunt emitted, is largest. In the example in Table 4, the effect ver 100 years f emissins Hw much d we expect climate t change? It is relatively easy t determine the direct effect f the increased radiative frcing due t increases in greenhuse gases. Hwever, as climate begins t warm, varius prcesses act t amplify (thrugh psitive feedbacks) r reduce (thrugh negative feedbacks) the warming. The main feedbacks which have been identified are due t changes in water vapur, sea-ice, cluds and the ceans. The best tls we have which take the abve feedbacks int accunt (but d nt include greenhuse gas feedbacks) are three-dimensinal mathematical mdels f the climate system (atmspherc-cean-ice-land), knwn as General Circulatin Mdels (GCMs). They synthesise ur knwledge f Ihe physical and dynamical prcesses in the verall system and allw fr the cmplex interactins between the varius cmpnents. Hwever, in their current state f develpment, the descriptins f many f the prcesses invlved are cmparatively crude. Because f this, cnsiderable uncertainty is attached t these

15 Plicymakers Summary xxi Table 3 Glbal Warming Ptentials The warming effect f an emissin f 1kg f each gas relative t that fc02 These figures are best estimates calculated n the basis f the pi esent day atmspheric cmpsitin TIME HORIZON 20 yr 100 yr 500 yr Carbn dixide Methane (including indirect) Nitrus xide CFC-11 CFC-12 HCFC Glbal Warming Ptentials fr a range f CFCs and ptential replacements are given in the full text. Table 4 The Relative Cumulative Climate Effect f 1990 Man-Made Emissins GWP (looyr hrizn) 1990 emissins (Tg) Relative cntributin ver looyr Carbn dixide Methane* Nitrus xide CFCs HCFC-22 Others* Varius 1500 Varius 26000t % 15% 4% 11% 0.5% 8.5% * These values include the indirect effect f these emissins n ther greenhuse gases via chemical reactins in the atmsphere Such estimates are highly mdel dependent and shuld be cnsidered preliminary and subject t change The estimated effect f zne is included under "thers" The gases included under "thers" are given in the full reprt. t Tg (teragrams) f carbn dixide = Tg (=7 Gt) f carbn Table 5 Characteristics f Greenhuse Gases GAS MAJOR CONTRIBUTOR? LONG LIFETIME? SOURCES KNOWN? Carbn dixide Methane Nitrus xide CFCs HCFCs, etc Ozne yes yes nt at present yes nt at present pssibly yes n yes yes mainly n n yes semi-quantitatively qualitatively yes yes qualitatively

16 Wll predictins f climate change, which is reflected in the range f values given, further details are given in a late* sectin The estimates f climate change presented here are based n 1) the "best-estimate" f equilibrium climate sensitivity (1 e the equilibrium temperature change due t a dubling f carbn dixide in the atmsphere) btained frm mdel simulatins, feedback analyses and bservatinal cnsideratins (see later bx "What tls d we use 9 ") n) a "bx-diffusin-upwelhng" cean-atmsphere climate mdel which translates the greenhuse frcing int the evlutin f the temperature respnse fr the prescribed climate sensitivity (This simple mdel has been calibrated against mre cmplex atm-spherecean cupled GCMs fr situatins where the mre cmplex mdels have been run) Hw quickly will glbal climate change? a If emissins fllw a Business-as-U sual pattern Under the IPCC Business-as-Usual (Scenari A) emissins f greenhuse gases, the average rate f increase f glbal mean temperature during the next century is estimated t be abut 0 3 C per decade (with an uncertainty range f 0 2 C t 0 5 C) This will result in a likely increase in glbal mean temperature f abut 1 C abve the present value (abut 2 C abve that in the pre-industrial perid) by 2025 and 3 C abve tday's (abut 4 C abve pre-industrial) befre the end f the next century Phcymakeis Summaiy The prjected temperature rise ut t the year 2100, with high, lw and best-estimate climate respnses, is shwn in Figure 8 Because f ther factrs which influence climate, we wuld nt expect the rise t be a steady ne The temperature rises shwn abve are realised temperatures, at any time we wuld als be cmmitted t a further temperature rise tward the equilibrium temperature (see bx "Equilibrium and Realised Climate Change") Fr the Ball "best-estimate" case in the year 2030, fr example, a further 0 9 C rise wuld be expected, abut 0 2 C f which wuld be realised by 2050 (in additin t changes due t further greenhuse gas increases), the rest wuld becme apparent in decades r centuries Even if we were able t stabilise emissins f each f the greenhuse gases at present day levels frm nw n, the temperature is predicted t rise by abut 0 2 C per decade fr the first few decades The glbal warming will als lead t increased glbal average precipitatin and evapratin f a few percent by 2030 Areas f sea-ice and snw are expected t diminish b If emissins at e subjec t t c nti ls Under the ther IPCC emissin scenaris which assume prgressively increasing levels f cntrls, average rates f increase in glbal mean temperature ver the next century are estimated t be abut 0 2 C per decade (Scenari B), just abve 0 1 C per decade (Scenari C) and abut 0.1 C per decade (Scenari D) The results are illustrated in Figure 9, with the Business-as-Usual case shwn fr cmparisn Only the best-estimate f the temperature rise is shwn in each case LU CO cr 3 1 it < 2~ ui in Q_ CO 5 Ui H * *,% CO CO _l < cc ? 1 BUSINESS h AS-USUAL 0 ii HIGH ESTIMATE BEST ESTIMATE LOW ESTIMATE Figure 8: Simulatin f the increase in glbal mean temperature frm due t bserved increases in greenhuse gases, and predictins f the rise between 1990 and 2100 resulting frm the Business-as-Usual emissins

17 Phcymakei s Summmy xxiu BUSINESS AS-USUAL SCENARIO B SCENARIO C SCENARIO D T 1 ' r Figure 9: Simulatins f the increase in glbal mean temperature frm due t bserved increases in greenhuse gases, and predictins f the rise between 1990 and 2100 resulting frm the IPCC Scenari B,C and D emissins, with the Busmess-as Usual case fr cmparisn The indicated range f uncertainty in glbal temperature rise given abve reflects a subjective assessment f uncertainties in the calculatin f climate respnse, but des nt include thse due t the transfrmatin f emissins t cncentratins, nr the effects f greenhuse gas feedbacks What will be the patterns f climate change by 2030? Knwledge f the glbal mean warming and change in precipitatin is f limited use in determining the impacts f climate change, fr instance n agriculture Fr this we need t knw changes reginally and seasnally Mdels predict that surface air will warm faster ver land than ver ceans, and a minimum f warming will ccur arund Antarctica and in the nrthern Nrth Atlantic regin There are sme cntinental-scale changes which are cnsistently predicted by the highest reslutin mdels and fr which we understand the physical reasns The warming is predicted t be % greater than the glbal mean in high nrthern latitudes in winter, and substantially smaller than the glbal mean in regins f sea-ice in summer Precipitatin is predicted t increase n average in middle and high latitude cntinents in winter (by sme 5-10% ver N) Five regins, each a few millin square kilmetres in area and representative f different chmatlgical regimes, were selected by IPCC fr particular study (see Figure 10) In the bx (ver page) are given the changes in temperature, precipitatin and sil misture, which are predicted t ccur by 2030 n the Business-as-Usual scenari, as an average ver each f the five regins There may be cnsiderable variatins within the regins In general, cnfidence in these reginal estimates is lw, especially fr the changes in precipitatin and sil misture, but they are examples f ur best estimates We cannt yet give reliable reginal predictins at the smaller scales demanded fr impacts assessments Hw will climate extremes and extreme events change? Changes in the variability f weather and the frequency f extremes will generally have mre impact than changes in the mean climate at a particular lcatin With the pssible exceptin t an increase in the number t intense shwers there is n clear evidence that weather variability will change in the future In the case f temperatures, assuming n change in variability, but with a mdest increase in the mean, the number t days with temperatures abve a given value at the high end f the distributin will increase substantially On the same assumptins, there will be a decrease in days with temperatures at the lw end t the distributin S the number f very ht days r frsty nights can be substantially changed withut any change in the variability f the weather The number f days with a minimum threshld amunt f sil misture (fr viability f a certain crp, fr example) wuld be even mre sensitive t changes in average precipitatin and evapratin If the large-scale weather regimes, fr instance depressin tracks r anticyclnes, shift their psitin this wuld effect the vai lability and extremes f weather at a particular lcatin, and culd have a majr effect Hwever, we d nt knw if, r in what way, this will happen

18 XXIV Plicymakers Summary ESTIMATES FOR CHANGES BY 2030 (IPCC Business-as-Usual scenari; changes frm pre-industrial) The numbers given belw are based n high reslutin mdels, scaled t be cnsistent with ur best estimate f glbal mean warming f 1.8 C by Fr values cnsistent with ther estimates f glbal temperature rise, the numbers belw shuld be reduced by 30% fr the lw estimate r increased by 50% fr the high estimate. Precipitatin estimates are als scaled in a similar way. Cnfidence in these reginal estimates is lw Central Nrth America (35 '-50 W W) The warming varies frm 2 t 4 C in winter and 2 t 3 C in summer. Precipitatin increases range frm 0 t 15% in winter whereas there are decreases f 5 t 10% in summer. Sil misture decreases in summer by 15 t 20%. Suthern Asia (5-30 W E) The warming varies frm 1 t 2 C thrughut the year. Precipitatin changes little in winter and generally increases thrughut the regin by 5 t 15% in summer. Summer sil misture increases by 5 t 10%. Sahel (10-20 N 20 W-40 E) The warming ranges frm 1 t 3 C. Area mean precipitatin increases and area mean sil misture decreases marginally in summer. Hwever, thrughut the regin, there are areas f bth increase and decrease in bth parameters thrughut the regin. Suthern Eurpe (35-50 N 10 W- 45 E) The warming is abut 2 C in winter and varies frm 2 t 3 C in summer. There is sme indicatin f increased precipitatin in winter, but summer precipitatin decreases by 5 t 15%, and summer sil misture by 15 t 25%. Australia (12-45 S E) The warming ranges frm 1 t 2 C in summer and is abut 2 C in winter. Summer precipitatin increases by arund 10%, but the mdels d nt prduce cnsistent estimates f the changes in sil misture. The area averages hide large variatins at the sub-cntinental level. Figure 10: Map shwing the lcatins and extents f the five areas selected by IPCC

19 Plicymakers Summary xxv WHAT TOOLS DO WE USE TO PREDICT FUTURE CLIMATE, AND HOW DO WE USE THEM? The mst highly develped tl which we have t predict future climate is knwn as a general circulatin mdel r GCM. These mdels are based n the laws f physics and use descriptins in simplified physical terms (called parameterisatins) f the smaller-scale prcesses such as thse due t cluds and deep mixing in the cean. In a climate mdel an atmspheric cmpnent, essentially the same as a weather predictin mdel, is cupled t a mdel f the cean, which can be equally cmplex. Climate frecasts are derived in a different way frm weather frecasts. A weather predictin mdel gives a descriptin f the atmsphere's state up t 10 days r s ahead, starting frm a detailed descriptin f an initial state f the atmsphere at a given time. Such frecasts describe the mvement and develpment f large weather systems, thugh they cannt represent very small scale phenmena; fr example, individual shwer cluds. T make a climate frecast, the climate mdel is first run fr a few (simulated) decades. The statistics f the mdel's utput is a descriptin f the mdel's simulated climate which, if the mdel is a gd ne, will bear a clse resemblance t the climate f the real atmsphere and cean. The abve exercise is then repeated with increasing cncentratins f the greenhuse gases in the mdel. The differences between the statistics f the tw simulatins (fr example in mean temperature and interannual variability) prvide an estimate f the accmpanying climate change. The lng term change in surface air temperature fllwing a dubling f carbn dixide (referred t as the climate sensitivity) is generally used as a benchmark t cmpare mdels. The range f results frm mdel studies is 1.9 t 5.2 C. Mst results are clse t 4.0 C but recent studies using a mre detailed but nt necessarily mre accurate representatin f clud prcesses give results in the lwer half f this range. Hence the mdels results d nt justify altering the previusly accepted range f 1.5 t 4.5 C. Althugh scientists are reluctant t give a single best estimate in this range, it is necessary fr the presentatin f climate predictins fr a chice f best estimate t be made. Taking int accunt the mdel results, tgether with bservatinal evidence ver the last century which is suggestive f the climate sensitivity being in the lwer half f the range, (see sectin: "Has man already begun t change glbal climate?") a value f climate sensitivity f 2.5 C has been chsen as the best estimate. Further details are given in Sectin 5 f the reprt. In this Assessment, we have als used much simpler mdels, which simulate the behaviur f GCMs, t make predictins f the evlutin with time f glbal temperature frm a number f emissin scenaris. These s-called bx-diffusin mdels cntain highly simplified physics but give similar results t GCMs when glbally averaged. A cmpletely different, and ptentially useful, way f predicting patterns f future climate is t search fr perids in the past when the glbal mean temperatures were similar t thse we expect in future, and then use the past spatial patterns as analgues f thse which will arise in the future. Fr a gd analgue, it is als necessary fr the frcing factrs (fr example, greenhuse gases, rbital variatins) and ther cnditins (fr example, ice cver, tpgraphy, etc.) t be similar; direct cmparisns with climate situatins fr which these cnditins d nt apply cannt be easily interpreted. Analgues f future greenhuse-gas-changed climates have nt been fund. We cannt therefre advcate the use f palae-climates as predictins f reginal climate change due t future increases in greenhuse gases. Hwever, palae-climatlgical infrmatin can prvide useful insights int climate prcesses, and can assist in the validatin f climate mdels. Will strms increase in a warmer wrld? Strms can have a majr impact n sciety. Will their frequency, intensity r lcatin increase in a warmer wrld? Trpical strms, such as typhns and hurricanes, nly develp at present ver seas that are warmer than abut 26 C. Althugh the area f sea having temperatures ver this critical value will increase as the glbe warms, the critical temperature itself may increase in a warmer wrld. Althugh the theretical maximum intensity is expected t increase with temperature, climate mdels give n cnsistent indicatin whether trpical strms will increase r decrease in frequency r intensity as climate changes; neither is there any evidence that this has ccurred ver the past few decades. Mid-latitude strms, such as thse which track acrss the Nrth Atlantic and Nrth Pacific, are driven by the equatr-t-ple temperature cntrast. As this cntrast will

20 uw Plu \makei s Summai \ EQUILIBRIUM AND REALISED CLIMATE CHANGE When the radiative frcing n the earth-atmsphere system is changed, fr example by increasing greenhuse gas cncentratins, the atmsphere will try t respnd (b> wanning) immediately But the atmsphere is clsely cupled t the ceans, s in rder tr the air t be wanned by the greenhuse el feet, the ceans als have t be warmed, because f their thermal capacity this takes decades r centuries This exchange f heat between atmsphere and cean will act t slw dwn the temperature rise frced by the gieenhuse effect In a hypthetical example where the cncentratin f greenhuse gases in the atmsphere, fllwing a perid f cnstancy, rises suddenly t a new level and remains there, the radiative frcing wuld als rise rapidly t a new level This increased radiative frcing wuld cause the atmsphere and ceans t warm, and eventually cme t a new, stable, temperature A cmmitment t this equilibrium temperature rise is incurred as sn as the greenhuse gas cncentratin changes But at any time befre equilibrium is reached, the actual temperature will have risen by nly part f the equilibrium temperature change, knwn as the realised temperature change Mdels predict that, fr the present day case f an increase in radiative frcing which is apprximately steady, the realised temperature rise at any time is abut 50% f the cmmitted temperature rise if the climate sensitivity (the respnse t a dubling f carbn dixide) is 4 5 C and abut 80% if the climate sensitivity is 1 5 C If the frcing were then held cnstant, temperatures wuld cntinue t rise slwly, but it is nt certain whether it wuld take decades r centuries fr mst f the remaining rise t equilibrium t ccur FORCING STABILIZATION S S 300 LU CO rr LU DC D < DC LU Q. EQUILIBRIUM TEMPERATURE REALIZED TEMPERATURE EQUILIBRIUM LU I S S 300 prbably be weakened in a warmer wrld (at least in the Nrthern Hemisphere), it might be argued that mid-latitude strms will als weaken r change their tracks, and there is sme indicatin f a general reductin in day-t-day variability in the mid-latitude strm tracks in winter in mdel simulatins, thugh the pattern f changes vary frm mdel t mdel Present mdels d nt reslve smallerscale disturbances, s it will nt be pssible t assess changes in strminess until results frm higher reslutin mdels becme available in the next few years Climate change in the lnger term The freging calculatins have fcussed n the perid up t the year 2100, it is clearly mre difficult t make calculatins fr years beynd 2100 Hwever, while the timing f a predicted increase in glbal temperatures has

21 Phcymakeis Summaiy substantial uncertainties, the predictin that an increase will eventually ccur is mre certain Furthermre, sme mdel calculatins that have been extended beynd 100 years suggest that, with cntinued increases in greenhuse climate frcing, there culd be significant changes in the cean circulatin, including a decrease in Nrth Atlantic deep water frmatin Other factrs which culd influence future climate Variatins in the utput f slar energy may als affect climate On a decadal time-scale slar variability and changes in greenhuse gas cncentratin culd give changes f similar magnitudes Hwever the variatin in slar intensity changes sign s that ver lnger time-scales the increases in greenhuse gases are likely t be mre imprtant Aersls as a result f vlcanic eruptins can lead t a cling at the surface which may ppse the greenhuse warming fr a few years fllwing an eruptin Again, ver lnger perids the greenhuse warming is likely t dminate Human activity is leading t an increase in aersls in the lwer atmsphere, mainly frm sulphur emissins These have tw effects, bth f which are difficult t quantify but which may be significant particularly at the reginal level The first is the direct effect f the aersls n the radiatin scattered and absrbed by the atmsphere The secnd is an indirect effect whereby the aersls affect the micrphysics f cluds leading t an increased clud reflectivity Bth these effects might lead t a significant reginal cling, a decrease in emissins f sulphur might be expected t increase glbal temperatures Because f lng-perid cuplings between different cmpnents f the climate system, fr example between cean and atmsphere, the Earth's climate wuld still vary withut being perturbed by any external influences This natural variability culd act t add t, r subtract frm, any human-made warming, n a century time-scale this wuld be less than changes expected frm greenhuse gas increases Hw much cnfidence d we have in ur predictins? Uncertainties in the abve climate predictins arise frm ur imperfect knwledge f future rates f human-made emissins hw these will change the atmspheric cncentratins f greenhuse gases the respnse f climate t these changed cncentratins Firstly, it is bvius that the extent t which climate will change depends n the rate at which greenhuse gases (and ther gases which affect their cncentratins) are emitted This in turn will be determined by varius cmplex \\\u ecnmic and scilgical factrs Scenaris f future emissins were generated within IPCC WGIII and are described in the Annex t this Summary Secndly, because we d nt fully understand the surces and sinks f the greenhuse gases, there are uncertainties in ur calculatins f future cncentratins arising frm a given emissins scenari We have used a number f mdels t calculate cncentratins and chsen a best estimate fr each gas In the case f carbn dixide, fr example, the cncentratin increase between 1990 and 2070 due t the Business-as-Usual emissins scenari spanned almst a factr f tw between the highest and lwest mdel result (crrespnding t a range in radiative frcing change f abut 50%) Furthermre, because natural surces and sinks f greenhuse gases are sensitive t a change in climate, they may substantially mdify future cncentratins (see earlier sectin "Greenhuse gas feedbacks") It appears that, as climate warms, these feedbacks will lead t an verall increase, rather than decrease, in natural greenhuse gas abundances Fr this reasn, climate change is likely t be greater than the estimates we have given Thirdly, climate mdels are nly as gd as ur understanding f the prcesses which they describe, and this is far frm perfect The ranges in the climate predictins given abve reflect the uncertainties due t mdel imperfectins, the largest f these is clud feedback (thse factrs affecting the clud amunt and distributin and the interactin f cluds with slar and terrestrial radiatin), which leads t a factr f tw uncertainty in the size f the warming Others arise frm the transfer f energy between the atmsphere and cean, the atmsphere and land surfaces, and between the upper and deep layers f the cean The treatment f sea-ice and cnvectin in the mdels is als crude Nevertheless, fr reasns given in the bx verleaf, we have substantial cnfidence that mdels can predict at least the brad scale features f climate change Furthermre, we must recgnise that ur imperfect understanding f climate prcesses (and crrespnding ability t mdel them) culd make us vulnerable t surprises, just as the human made zne hle ver Antarctica was entirely unpredicted In particular, the cean circulatin, changes in which are thught t have led t perids f cmparatively rapid climate change at the end f the last ice age, is nt well bserved, understd r mdelled Will the climate f the future be very different? When cnsidering future climate change, it is clearly essential t lk at the recrd f climate variatin in the past Frm it we can learn abut the range f natural climate variability, t see hw it cmpares with what we

22 \X\lll Plicymakeis Summaiy CONFIDENCE IN PREDICTIONS FROM CLIMATE MODELS What cnfidence can we have that climate change due t increasing greenhuse gases will lk anything like the mdel predictins 7 Weather frecasts can be cmpared with the actual weather the next day and their skill assessed, we cannt d that with climate predictins Hwever, there are several indicatrs that give us sme cnfidence in the predictins frm climate mdels When the latest atmspheric mdels are run with the present atmspheric cncentratins f greenhuse gases and bserved bundary cnditins their simulatin f present climate is generally realistic n large scales, capturing the majr features such as the wet trpical cnvergence znes and mid-latitude depressin belts, as well as the cntrasts between summer and winter circulatins The mdels als simulate the bserved variability, fr example, the large day-t-day pressure variatins in the middle latitude depressin belts and the maxima in interannual variability respnsible fr the very different character f ne winter frm anther bth being represented Hwever, n reginal scales (2,000km r less), there are significant errrs in all mdels Overall cnfidence is increased by atmspheric mdels' generally satisfactry prtrayal f aspects f variability f the atmsphere, fr instance thse assciated with variatins in sea surface temperature There has been sme success in simulating the general circulatin f the cean, including the patterns (thugh nt always the intensities) f the principal currents, and the distributins f tracers added t the cean Atmspheric mdels have been cupled with simple mdels f the cean t predict the equilibrium respnse t greenhuse gases, under the assumptin that the mdel errrs are the same in a changed climate The ability f such mdels t simulate imprtant aspects f the climate f the last ice age generates cnfidence in their usefulness Atmspheric mdels have als been cupled with multi-layer cean mdels (t give cupled cean-atmsphere GCMs) which predict the gradual respnse t increasing greenhuse gases Althugh the mdels s far are f relatively carse reslutin, the large scale structures f the cean and the atmsphere can be simulated with sme skill Hwever, the cupling f cean and atmsphere mdels reveals a strng sensitivity t small-scale errrs which leads t a drift away frm the bserved climate As yet, these errrs must be remved by adjustments t the exchange f heat between cean and atmsphere There are similarities between results frm the cupled mdels using simple representatins f the cean and thse using mre sphisticated descriptins, and ur understanding f such differences as d ccur gives us sme cnfidence in the results expect in the future, and als lk fr evidence f recent climate change due t man's activities Climate varies naturally n all time-scales frm hundreds f millins f years dwn t the year-t-year Prminent in the Earth's histry have been the 100,000 year glacialmterglacial cycles when climate was mstly cler than at present Glbal surface temperatures have typically varied by 5-7 C thrugh these cycles, with large changes in ice vlume and sea level, and temperature changes as great as C in sme middle and high latitude regins f the Nrthern Hemisphere Since the end f the last ice age, abut 10,000 years ag, glbal surface temperatures have prbably fluctuated by little mre than 1 C Sme fluctuatins have lasted several centuries, including the Little Ice Age which ended in the nineteenth century and which appears t have been glbal in extent The changes predicted t ccur by abut the middle f the next century due t increases in greenhuse gas cncentratins frm the Business-as-Usual emissins will make glbal mean temperatures higher than they have been in the last 1 ^0.000 years The rate f change f glbal temperatures predicted fr Business-as-Usual emissins will be greater than thse which have ccured naturally n Earth ver the last 10,000 years, and the rise in sea level will be abut three t six times faster than that seen ver the last 100 years r s Has man already begun t change the glbal climate? The instrumental recrd f surface temperature is fragmentary until the mid-nineteenth century, after which it slwly imprves Because f different methds f measurement, histrical recrds have t be harmnised with mdern bservatins, intrducing sme uncertainty Despite these prblems we believe that a real warming f the glbe f 0 3 C C has taken place ver the last century, any bias due t urbanisatin is likely t be less than 0 05 C Mrever since 1900 similar temperature increases are seen in three independent data sets ne cllected ver land and tw ver the ceans Figure 11 shws current estimates f smthed glbal-mean surface temperature ver land and cean since 1860 Cnfidence in the recrd has been increased by their similarity t recent satellite measurements f mid-trpsphenc temperatures