Avoiding dangerous climate change.pdf

7/28/2019 Avoiding dangerous climate change.pdf

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1Avoiding dangerous climate change

Avoiding dangerous

climate change


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2 Avoiding dangerous climate change

Introduction

The latest results rom the Met OfceHadley Centre add to the increasingevidence that early and rapid reductionsin emissions are required to avoidsignifcant impacts o climate change.

Here, we present the latest evidence ocontinued change and projections based oncurrent models and understanding. Finally,we highlight several emerging scienceissues that could make climate change moredangerous than previously expected.

The indications are that climateis continuing to warm due toincreasing man-made greenhousegases. We provide an update onchanges in global temperature, UK

temperature and Arctic sea ice.

Many of the dangers of climatechange are made worse byinteractions between the climate,chemistry, biological systems andfresh water. These are important

because they have impacts onwater availability, air quality and

The latest climate model human health. We highlight:projections show that the risksof dangerous climate change - Examples of these interactionsincrease rapidly as greenhouse and improvements in climategas concentrations increase. modelling to represent them.

- The impact of climate changeon air quality and health.

- The impact of land use change,in particular deforestation,on emissions targets.

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Brohan et al. 20060.2

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0.2

0.4

0.6

Temperaturedifference(C)

rela

tiveto19611990

0.02 C/decade

+0.09 C/decade

+0.17 C/decade

Avoiding dangerous climate change

The evidence is clear in the long term, globaltemperatures are rising

Some commentators have suggestedthat g obal warmng has stopped.This s not the case. The ev dences clear n the long term, globaltemperatures are r ng, and humansare largely respons ble for this r se.

Global warm ng does not mean thateach year w ll be warmer than the last natural phenomena mean that some

years w ll be much warmer and otherscooler. The El No in 1998 gave rse toa record-break ng warm year and the La

a n 2007 and 2008 led to temporarycoolng. Desp te ths, 11 of the last 13

years are the warmest ever recorded.

Average global temperatures are nowsome 0.75 C warmer than they were100 years ago. Snce the m d-1970s, thencrease in temperature has averagedmore than 0.15 C per decade. This rateof change is very unusual in the contextof past changes and much more rapthan the warm ng at the end of the lastce age. Sea-surface temperatures havewarmed slghtly less rapdly than theglobal average, whereas temperaturesover land have warmed more rap dly,at almost 0.3 C per decade.

Over the last ten years, the rate ofwarmng has been s ghtly lower than

durng prev ous decades. But thisdoes not mean that global warm nghas slowed down or even stopped.It s ent rely consstent w th ourunderstand ng of natural uctuat onsof the clmate w thin a trend ofcontnued long-term warmng.

Evidence o continuingclimate changeGlobal warming

+0.33 C/decade

1980 1990 2000 2010Year

Figure: Global average These natural uctuations include As a result of such uctuations, globaltemperature anomaly the El Nio Southern Oscillations average temperature trends calculatedfrom 19752007, relative

(ENSO) in the Pacic Ocean. In over 10-year periods have varied sinceto the 19611990average. The black El Nio years those when there isline shows the annual no cold surface water in the tropicalvalues. The red line eastern Pacic global temperatureshows the trend over the is considerably warmer than normal.

the mid-1970s, from a modest coolingof -0.02 C per decade, to a warmingrate of more than 0.3 C per decade.Model projections of future climate

full 33 years. The blue

lines show the varying A particularly strong El Nio occurred

rate of the trend over in 1998, resulting in the warmestvarious 10-year periods. year on record across the globe.

In La Nia years when more cold water rises to the surface of the Pacic Ocean temperatures can be considerably colder than normal.

change forecast that temperatures willrise by more than 2 C per century.However, even with this long-term rise,natural variations in climate can beexpected to lead to ten-year periodswith little or no war ming, both globallyand regionally, and other ten-yearperiods with very rapid warming.

Volcanic eruptions can also causetemporary drops in global temperaturesbecause of huge amounts of aerosolsthrown high into the atmospherethat reduce the amount of sunlightthat reaches the surface.

A La Nia was present throughout2007 and much of 2008, but despite this temporary cooling, 2008 iscurrently the tenth warmest yearof the global record

This complex behaviour of the climatesystem shows why we need to examinemuch longer periods than 10 yearsif we are to fully understand andquantify how the climate is changing.In its most recent assessment in 2007,the IPCC1 concluded that increasesin man-made greenhouse gasconcentrations are very likely to havecaused most of the increase in globalaverage temperatures since the mid20th century.

1 Intergovernmental Panel on Climate Change

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UK temperatures

The UK has experienced marked trends in climate in thelast 45 years. Much o the UK has warmed by more thanone degree Celsius since 1980, with 2006 being the warmestyear in the 350-year long Central England Temperature record.

The global long-term warming trend isset to continue as the concentration ofgreenhouse gases in the atmospherecontinues to increase. Inevitably, thiswill lead to further impacts on our livesand the worlds natural ecosystems.Heatwaves and droughts are likely tobecome more prevalent; snow coveris projected to continue to diminish;and sea ice to continue to shrink. O nshorter timescales, natural climatevariations and other factors, such asvolcanoes, will continue to temporarilyenhance or reduce the magnitude andimpacts of these longer-term changes.

Figure: Changes in annual average dailyminimum temperature (C) between 1961 and

2006 based on a linear trend (as developed on

a high resolution 5 km grid by the Met Ofces

National Climate Information Centre).

6 Avoiding dangerous climate change Avoiding dangerous climate change 7


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Arctic sea ice

Human activity is already contributing to reductions in Arctic sea ice.We are heading or ice-ree Arctic summers, despite an expectedshort-term recovery rom the dramatic sea-ice loss o 2007.

The extent of Arctic sea-ice cover inSeptember 2008 was the second lowestsince records began; only surpassed bythe record minimum in 2007. The recentdramatic ice loss adds to the long-termtrend observed over the last 30 years.The latest Met Ofce Hadley Centreresults strengthen the evidence thathuman activity is already contributing todecreases in sea-ice extent, and will leadto ice-free summers later this century.The dramatic losses in 2007 and 2008,

however, may well be temporary.Nevertheless, people and ecosystemsare already responding to the changes.

In September 2007, sea-ice extent wasonly around 60% of the average valuefor 19792000, and the NorthwestPassage, joining the Atlantic andPacic via the northern coast ofCanada, was ice-free for the rst timein living memory. The extent of icecoverage did not fall as low in 2008,but it was only just behind (within10% of) 2007, reinforcing the long-term decline in sea ice, at least sincesatellite records be gan in 1979.

The large sea-ice retreat in 2007occurred particularly in areas to thenorth of Siberia and Alaska. The retreatwas largely a result of unusual highpressure, leading to sunny skies whichincreased warming and melting,and southerly winds which pushed theice edge north. In contrast, the ice-coverretreat in 2008 was mainly from theSiberian coast and the Greenland Sea.It may also have been affected byweather conditions, but another factor

may have been the record low ice extentin 2007, which meant that winter icecover in 2008 was thinner than usual.

Figure: Sea ice extent for September 2007 and 2008.

19792006 Median 2007 2008

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New results from the Met Ofce HadleyCentre show that there is a detectablehuman inuence on the long-termdecrease in sea-ice extent for somemonths of the year. In particular, for thesatellite record over the last 30 years,human inuence is detectable in everymonth between June and October, andalso in the annual total. This detecti onhas only just become possible in thesatellite record, which was too shortbefore. The next stage of the work willbe to look at whether changes in icethickness can be similarly accounted for.

Met Ofce Hadley Centre modelsimulations have been used to lookat whether the record low ice extentsin 2007 and 2008 are a signal that atipping point has been reached in theArctic climate system.

Results from our model show that largeyear-to-year variations are possible withsome dramatic temporary losses overone and two-year periods which areconsistent with the observed losses ofrecent years. Thus, these rapid losses areexpected to be largely reversible over afew years. The model results are crediblebecause they are consistent with theobserved long-term trend in contrastto results from many other modelswhich underestimate the trend. Thereis no strong evidence that a thresholdor tipping point has been reached,although it is still an open questionand further research is required.

When our model is run into the futureusing one of the IPCC emissionsscenarios, it projects summers largelyfree of Arctic sea ice by the 2060s.

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The scientifc evidenceor early action on

Even with large and early cuts in emissions, temperatures are like y torise to around 2 C above pre-industrial levels by the end o the century.

Only an early and rapid decline in emissions gets anywhereclose to the target o a 50% reduction in emissions by 2050.

I we are concerned about keeping the risks o dangerous climatechange to a minimum, we should also consider the worst-case outcome.

cular targets, but they also need

f they cannot belures could

In the rst scenaro, em ss ons cont nueto r se throughout the century.In the other scenar os, emss onreduct ons have been mposed atvar ous tmes and at varous rates.

If no act on is taken to curb globalwarm ng, temperatures are lkely to

se by 5.5 C and could r se by asmuch as 7 C above pre- ndustr alva ues by the end of the century.This would lead to sgn cant rsksof severe and irrevers ble impacts.

In the most opt stc scenaro,em ss ons start to decrease n 2010 andreductons qu ckly reach 3% per year.This contrasts sharply wth currenttrends, where the worlds overallem ss ons are increasng at 1% per year.Th s is faster than even the worst casesused in the IPCC emss ons scenar os.

Emissions increases rom 1990 levelsMost likely global temperature rise relative to pre-industrial levels:


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What is very clear is that some increasen temperature nev table n thenext century and that the dec onsand act ons that the world takesnow wll have a profound mpac ton the cl mate later this century.

Even f em ss ons start to decreasethe next two years and reach a rapand susta ned rate of declne of 3%per year, temperatures are lkely to rto 1.7 C above pre- ndustr al levelsby 2050, and to around 2 C by 2100.This s because carbon dox de alreadyn the atmosphere w ll be around formany years and the cl mate takes t meto respond to these changes. Onlythe scenar th an early and rapdeclne n emss ons gets anywherenear to the 50% reduct onem ss ons by 2050, as recommendedn the latest IPCC report to avolarge increases in temperature.

Overall, a delayed and slow decline This takes a worst-case temperaturein emissions would probably lead rise of under 3 C to one just aboveto nearly 2 C more warming than 5 C by the end of this century,an early and rapid decline in global bringing with it signicant riskemissions, a total temperature rise of dangerous impacts to ourof 4 C above pre-industrial levels. environment, society and economy.

The implications of these levels A major reason for this amplicationof temperature change are very is the so-called carbon cycle effect.serious, but the central projections Today, plants, soils and oceans absorbare not the only things we should be about half of the carbon dioxideworried about. When commentators emitted by mans activities, limitinglook at these projections, they rises in atmospheric carbon dioxide

40

Business as usualEarly and rapid decline

EquivalentCO2emissions(GtC/yr)

35

30

25

20

15

10

5

0tend to concentrate on the most (CO

2) and slowing global warmi2000 2040 2080 2120 2160 2200

Year

Figure: Shows the emissions for the three major greenhouse gases (CO2, CH

4

and N2O) combined into a carbon equivalent for two emissions scenarios.

8.0

Worst case (upper 10th percentile)Most likely (50th percentile)Best case (lower 10th percentile)

ng.likely temperature rises. As temperatures increase this

absorption is very likely to decrease.However, if we are concerned For example, plant matter in the soilabout keeping the risks of dangerousclimate change to a minimum weshould also consider the worst-case outcome. This worst- case

breaks down more quickly at highertemperatures, releasing carbon morequickly and amplifying the warmingtrend. Methane and CO released

2

outcome will occur if the climate from the thawing of permafrost willturns out to be particularly sensitive also add to the warming. This carbon

Early and rapid decline

Business as usual

ons, the worst-case outcome

The consequences of a late declineThe risks of worst-case outcomes4.0 in emissions are apparent by 2050. (i.e. a 10% chance of occurring)

Delaying reduction of emissions to amplify much more quickly than3.0 2030 results in a further 0.5 C of the risks of most likely outcomes.

7.0 to increases in greenhouse gases release is currently not included inand the Earths biological systems the calculations and becomes more

6.0

Temperature

rise(C)

cannot absorb very much carbon. of a risk for larger temperature rises.Two other scenarios are also signicant.

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Hence the rsks of dangerous c matechange wll not ncre ase slowly asgreenhouse gases ncrease. Rather,the r sks w ll mult y if we do notreduce emss ons fast enough.

warming by 2050 compared with early, For an early and rapid decline in2.0if slow, reduction from 2010. By the issiemend of the century, the differencesare even greater more than 1 C.

The consequences of an early but slowdecline in emissions of 1% per yearcompared with a rapid decline appear tobe small in 2050. However, they increaseto 0.8 C by the end of the century.

is around 0.7 C higher than themost likely temperature rise. Withmuch slower action taken muchlater, the difference between themost likely and worst-case outcomeis almost twice as big, at 1.2 C.

1.0

02000 2040 2080 2120 2160 2200

Year

Figure: Shows the projected temperatures with the red line

indicating the median projection (50% chance). The grey

lines show the 10% best and worst case projections.



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Interaction betweenclimate, chemistry

and biologyWhich gases are importantor climate change?

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Greenhouse gases and aerosols bothcontr bute in dffer ent ways to globalwarm ng and pollut on generally.

We are workng on new ways ofcompar ng the effects of thesespeces. For example, we wllbe abe to compare the re atbenets of controllng blackcarbon and CO emissions.

Mitigation of climate change is likelyto require reductions in a range ofgreenhouse gas emissions, not justCO

2. The other greenhouse gases,

such as methane, nitrous oxide, ozone

and the halocarbons remain in theatmosphere for very different periodsof time and have non-radiative aswell as radiative effects on climate.They can affe ct plant biology, wateravailability and air quality, for example.

The relative importance of differentgases is usually compared with CO

2

in terms of their global warmingpotentials. This compares their effecton radiative forcing, i.e. how muchthe atmosphere is warmed directly bythem, over a specied period of time.

The Met Ofce Hadley Centre is usingthe results from complex climate modelsto develop improved methods forcomparing the impact of these gases which take account of all their effects.In particular, this will enable the effectsof short-lived gases or atmosphericaerosols (that remain in the atmosphere

for a few weeks) to be compared withthe effects of long-lived gases (thatremain for tens or hundreds of years).

For instance, it has been argued thatreducing black carbon and troposphericozone in the atmosphere could be aquick win to reduce climate changewhile developing the technologyneeded to reduce CO emissions. This

2

is only true if the black carbon andozone emission cuts are not associatedwith an increase in CO emissions. We

2

have shown how a new measure, theglobal temperature change potential,could be useful in assessing how longthe climate benet of a particularreduction in black carbon emissionslasts as a function of the associated fuelpenalty. This may help future decisions

about air quality and control of climatechange to be better integrated.

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and clmatepo es

Short- vedspeces: ozoneand aerosols

We -m xedgreenhouse

gases

Earth Systemmode n g

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mplemodels

Clmatemetrcs

1960

Carbondioxdeconcentration(ppmv)

390

380

370

360

350

340

330

320

3101 97 0 1 98 0 1 99 0 2 00 0

Figure: Schematic showing the interactions between

short-lived and long-lived atmospheric radiatively

active species. Complex modelling such as that done

at the Met Ofce Hadley Centre and other climate

centres is necessary to understand these interactions

and calibrate simpler models which can be used

as fast-response tools to inform decision making.



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Impacts on air quality

The increase in surace ozone due to climate change isalso expected to have a large impact on human health.

Actions to improve air quality and Many greenhouse gases and aerosols These issues mean that actions to )'reduce climate change need to important for climate change are also improve air quality and reduce climate (/be linked. Ozone and aerosols, in important for air quality and human change need to be linked. A further (-particular, are important for both. health. For example, ozone contributes driver for linking action to curb (+In addition, complex interactions to global warming and is a powerful different types of pollution comes ()between climate, plants and chemistry respiratory irritant at levels frequently from new scientic evidence which ('change the amounts of the important found in urban areas. Aerosols or shows much stronger interactions /gases and aerosols in the atmosphere. particulate matter contribute to both between the carbon cycle, tropospheric -These issues need to be considered warming and cooling and exacerbate ozone and atmospheric aerosols than +in decisions about climate change respiratory and cardiovascular diseases. initially thought. These mean that

)because worsening air quality is likely all important climate change agents

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to increasingly affect human health. By the 2090s a signicant fraction need to be considered simultaneously ;A= D8D AA8 JFEof the worlds population will be in determining their effects. J\Xjfeexposed to ozone levels well abovethe World Health Organization (WHO) The Met Ofce Hadley Centre hasrecommended safe-health level. Met shown that complex interactions Figure: Percentage of world population exposed to

Ofce Hadley Centre models project between climate change agents through ozone levels above the WHO recommended safe-health level of 60 parts per billion (ppb) for 1990 andthat increases will be large in all seasons, their impact on plant biology could haveserious consequences for air quality 2090, modelled using the Met Ofce global climatewith more than a threefold increase

in the percentage of population and make climate change worse. Ourmodel coupled to a chemistry model forced by a

medium to high emissions scenario (IPCC SRES A2).affected. In some regions particularly results show that increased CO reduces Changes are inuenced by ozone variations, with2industrialised developing countries the removal of ozone by plants d uring population held constant at 1990 levels. Note that

the growing season, resulting in higher these results are based on seasonal averages greatersuch as India and China increasesthan 60 ppb whereas WHO guidelines are based on

will be considerably larger. atmospheric ozone concentrations. eight hour exposure above 60 ppb. A single eight-Moreover, high levels of ozone damage hour exposure may be considerably higher than thethe plants and reduce the rate of seasonal average. However, the A2 scenario is now

thought to overestimate ozone values because itphotosynthesis. This will, in turn, reducethe absorption of CO

2by plants, leading probably underestimates local pollution controls.

to increased global warming; anotherexample of a non-radiative interaction.

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The impact o deorestationand land degradation

CO2

emissions rom deorestation are currently estimated to exceed those rom transport.

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New est mates of future deforestat on are much largerthan those used by IPCC and deforestaton is alreadya ma or contr butor to CO em ssons larger thantransport. Cl mate change puts further pressure onforests. Reductons n ra nfall and assocated ncreasesdrought and re r sk add to the destructon of forest.

(a)

80 W 70 W 60 W 50 W 40 W

10 N 10 N

The Met Ofce Hadley Centre is assessing the potential impacts of ongoing deforestation on climate change and 0 N 0 Nthe extent to which reducing deforestation could contribute to stabilising greenhouse gas concentrations. Most current

10S 10Sprojections of future climate use the SRES emissions scenarios.However, these may signicantly underestimate the amount

LandtypeDeforestedof deforestation in critical regions, such as the Amazon.Forest

20S 20SOtherlandtype

Outsidemodel

The Figure compares deforestation estimate used in the 80 W 70 W 60 W 50 W 40 WSRES A2 scenario with two more recent estimates. With (b)no controls on deforestation the area of forest lost couldbe ve times greater than in the SRES scenario. Even with

80 W 70 W 60 W 50 W 40 W

effective governance the loss could be double. Clearly, this 10 N 10 N

could have a signicant impact on car bon emissions. Forexample, global CO

2emissions from forests are currently

0 N 0 Nestimated to exceed those from transport. The next stage is to quantify the implications for stabilisation of the climate.

10S 10S

LandtypeDeforested

Forest20S 20S

Otherlandtype

Outsidemodel

80 W 70 W 60 W 50 W 40 W

(c)

Figure: Projected deforestation in 2050 for:

(a) IPCC SRES A2 scenario, total area of Amazonia4deforested 435,000 km2.

4 (a) S trengers, B. et al. (2004) The land use

projections and resulting emissions in the IPCC SRES (b) scenario based on highway paving plans, currentscenarios as simulated by the IMAGE 2.2 model. protected areas and legislation compliance, area

5GeoJournal, 61(4), 381-393. deforested 2,067,000 km2.5 (b) and (c) Soares-Filho, B.S. et al. (2006) (c) highway paving plans but greater implementation

Modelling conservation in the Amazon basin. and enforcement of protected areas,5Nature, 440, 520-523. area deforested 913,000 km2.

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In add on to man-made deforestat on, clmate changemay cause the d e-back of the Amazonan forest. However,delberate deforestaton n Amazon s lkely to have a

gger mpact n the short term. By avoidng deforestat onn the rst part of the 21st century, emss ons would bereduced by up to 27 GtC by 2050. Furthermore, preservat onof the forest would manta n a carbon s nk due to COfert saton of photosynthess worth 4 GtC by 2050.

Cl mate change and deforestat on are also lkely tonteract w th each other. In prev ous drought eventssuch as 2005, res used for forest clearance becameuncontrolled and larger areas were burnt through thre leakage. Cl mate change is l kely to reduce ranfall inthe reg on. Even f this does not dre ctly damage plantsvery much through drought, s l kely to ncrease the

sk of re leakage wh ch would magnfy the impact ofdeforestat on. The areas proected to exper ence an ncreasen re danger due to cl mate change overlap cons derably

th the areas pro ected to undergo deforestat on.

10 W

10 S

20 S

50 W

Deforestaton only

gh f re danger only

Combned deforestaton and hgh f re danger

No mpact

40 W


gure: Proected overlap of deforestat on and h gh re r sk for 2081-2090.

Red areas show where all members of the Hadley Centre cl mate model

ensemble agree on high re risk.



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Cl mate change is strongly dependenton nteract ons between clmateand many other aspects of the EarthSystem, such as pant and ocean

ology. The new Met Ofce HadleyCentre Earth System model wnclude many of these nteract ons.It w ll be used to provde better adv ceon emss ons reduct ons and the

sks of dangerous clmate change.It w ll also be used to look at thenteract ons whch could amplfycl mate change and mpac t on otheraspects of the envr onment such as

r qualty and ocean ac cat on.

The next generation of Earth Systemmodels will represent more complexfeedbacks and improve climateprojection accuracy. We are close tonalising our new Earth System model,HadGEM2-ES, which will includethe land and ocean carbon cycles,dynamic vegetation, ocean biology, andatmospheric chemistry and aerosols. It

will contribute to the forthcoming IPCCFifth Assessment Report as part of aninternational collaboration to compareinformation from different Earth Systemmodels. This will help to improve ourestimates of the maximum permissibleemissions of carbon dioxide and othergreenhouse gases to achieve a stableclimate over the next few centuries.

l

demand

Impacts

Organiccompounds

Greenhousegases

Clouds

Fi

Aerosols

i

Damming

Humanemissions

Urbanisation

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Funders and co laborators:

Water

of climate

change

Physicalclimate

Permafrost res

Dust

Chemistry

Water cycle Carbon cycle

Ecosystems

Irrgation

Deforestation

AgrcultureForestry

Improvements in decision-making tools

laboration to compare inormation rom dierent EarthSystem models will help improve our estimates o the maximumpermissible emissions o carbon dioxide or a stable climate.

Editorscky Pope, Fona Carroll, Dave Br tton, Jason Lowe

Figure: Earth System models include an increasing

number of complex feedbacks between

physical climate and biogeochemical cycles.

International col



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Avoiding dangerous climate change.pdf