W005 Environment Climate 080 087

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L

szl lmsy should have hadbetter things to do than pursu-ing an interest in rocks. A secret

agent of the German army, hesmuggled German spies intoEgypt. He knew the paths through thehostile sands and boulders of the south-ern Sahara better than almost any Eu-ropean. The novel The English Patientis a romantic literary memorial to him.Ultimately, however, Almsys militaryactivities were of secondary importance.From todays perspective, the rockslmsy busied himself with the cliffsof the Gilf el-Kebir plateau, dusty cavesfull of prehistoric wall paintings in thesouthwest corner of Egypt are far

more interesting.Bedouins led lmsy to the hidden

murals in the 1930s, and he made themknown to the Western world. They de-pict rhinos, hippos and tillage farmers,people of the Neolithic period on thecusp of transition from hunter-gather-ers to settled small farmers. It occurredto lmsy that the southern Saharacould not always have been so dry andhostile. There must have been timeswhen cereal crops and prairie grassswayed in the wind, where now only

dust and stones extend in all directions.In the 1930s, people laughed atlmsy and his crazy ideas about agreen Sahara, but the adventurer andself-made discoverer has been provenright. It is now known that the Saharahas turned green and dried up againseveral times throughout its history.The last wet phase began in western Af-rica about 16,000 years ago, and in east-ern Africa about 10,000 years ago. Thewhole region then dried up again some5,500 years ago.

TEXT TIM SCHRDER

ENVIRONMENT & CLIMATE_Sahara

p

p Today, plants in the Sahara grow only in oases, although the y

extended over large parts of northern Africa during the Neolithic

Age. The photo shows an oasis at Mhamid in Morocco, near

the Algerian border. In the foregr ound: the dunes of Erg Chigaga.

1 | 12 MaxPlanckResearch 81

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1 Adding green to the climate model: While analyzing the interplay between climate and vegetation, Martin Claussen discovered that

the Sahara sometimes adopts a bistable state. Depending on the initial situation, it can then turn either green or desert-like.

2 Forbidding beauty with a lively past: Sand dunes now dominate the landscape once occupied by a savannah some 6,000 years ago.

3 The rock paintings of Wadi Howar, south of Gilf el-Kebir, bear witness to the Saharas fertile past. The Wadi stretches from Chad

into Sudan and was an important tributary of the Nile during the Neolithic period.

Claussen had not yet turned his atten-tion to the Sahara. He simply wantedto know what our world would looklike today if he started a simulationwith different vegetation. The resultswere baffling: When he started out with

a densely forested Earth, plant lifearound the world developed as weknow it today except that the Sahararemained fairly green, with extensivesavannahs covering the region. Whenhe started with a bright Earth, howev-er, the Sahara remained a bright, drydesert. That was a strong indicationthat there is interplay between vegeta-tion and climate, and that they must beconsidered in combination. Vegetation

In recent decades, archaeologists havefound many indications of past civili-

zations in the once green desert earth-enware shards, pollen, seeds. Duringdry periods, people moved to the fertilelowlands of the Nile. During wettertimes, they migrated into the wideplains of the south, far from theswampy, uninhabitable Nile area. For agood ten years now, researchers havebeen trying to understand this constantinterchange of wet and dry phases, butthey are no longer limited to looking atcave paintings and pollen. On the con-trary, they now have the benefit of con-

centrated computer power. Claussenand his colleagues look into the past us-ing models and simulations in the hopethat they will also be able to predict thefuture of the desert region.

The computer systems required forthis task are huge. One of them is locat-ed in Hamburg: the mainframe of theGerman Climate Computing Center, anensemble of several dozen head-heightgrey cabinets packed into a room thesize of a gymnasium. There, millions ofelectronic calculations are processed si-multaneously. Martin Claussen is a reg-

ular user of this electronic super-brain.The meteorology professor and his col-leagues from the Max Planck Institutefor Meteorology in Hamburg feed it withinformation on the Sahara and climate.

INTERPLAY OF VEGETATION

AND CLIMATE

For Claussen, climate simulation is likea game and the climate computergives him the chance to play one of theworlds biggest computer games. Some-times he pushes the simulation to thelimits, causing rain to fall over Africa asit hasnt done for years, or deactivatesevaporation for entire continents. Thisis no arbitrary game, however. Claussenwants to understand the big picture, be-cause climate is hopelessly complex. It

is all too easy to become lost in detail,with air streams and ocean currents,the reflectivity of wet surfaces, fallingair masses and the color of topsoil hundreds of variables that together de-termine climate. A broad perspectiveprovides a first estimate.

This was the case back in the mid-1990s, when Claussen developed the Sa-hara simulation. The scientific consen-sus at the time was that climate alonedetermines which plants grow where:vegetation follows climate. However,

in the light of the destruction of tropi-cal rain forests, the idea occurred toClaussen that vegetation, in turn, af-fects climate; that each has a direct in-fluence on the other. He checked hishypothesis, initially using an extremecomparison: In one simulation, hechanged the continents to completelygreen zones with parts covered in darkforests. For the comparison, however,he made the entire Earth very bright.The computer calculated air streams,ocean currents, and the transport ofheat and moisture around the globe.

The average amount of solar energy, or to be more

accurate, solar energy flux density (measured in

watts per square meter) in the Northern Hemi-sphere in the summer months of June, July,

August has fallen gradually over the last 9,000

years (top graph). In contrast, the vegetation cover-

age in the Sahara, which was very high in the Neo-

lithic Age according to a relatively crude simulation,

falls to zero in just 1,000 years (middle graph). At

the same time, the volume of desert sand deposited

as sediment in the Atlantic has risen (measured as

a proportion of sediment and in grams per square

centimeter per thousand years; bottom graph).


470

460

450

440

W/m2

Solar Energy Flux

0.9

0.6

0.3

0.0

fraction

Vegetation Coverage

1

P h o t o : D a v i d A u s s e r h o f e r

12

10

8

69000 8000 7000 6000 5000 4000 3000 2000 1000 0

Years before Present

Terrigenous

(%)

Terr. flux

Terr. %

Dust Fluxinto the Atlantic

60

50

40 (g/cm2/ka)

82 MaxPlanckResearch 1 | 12


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(

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is a variable of the climate system, justlike temperature and air pressure, saysClaussen. An underlying reason is thatdark continents absorb more heat,

while bright surfaces reflect sunlight.This influences evaporation and airstreams, for example.

Then Claussen remembered readingthat the Sahara had been green some10,000 years ago. He reasoned that thegradual changes in the Suns radiationcould have been a major influence. Af-ter all, the Earth totters around the Sunlike a wobbly spinning top. This meansthat Earths position in relation to theSun, the length of the seasons and thetilt of the Earths axis vary almost cycli-

cally, every 20,000, 40,000 and 100,000years. Consequently, the northernhemisphere receives more sunlight in

summer and less in winter during someperiods, resulting in hotter summersand colder winters.

Claussen changed the solar radia-tion in his model, setting it as it wouldhave been on Earth 6,000 years ago.He was surprised to find that his cli-mate model yielded, not two, but justone single solution, no matter wheth-er he fed the calculation with an ini-tially green or bright, dry Sahara: the

green Sahara. His explanation? De-pending on the distribution of solarradiation, the climate and vegetationsystem moves toward just one or two

stable states. Claussen concluded thatif two solutions arise through thecourse of time, the system can jump

from one to the other. Thus, Saharanvegetation can tip in one direction orthe other aridity or a life-supportingsavannah.

THE SAHARA CAN CHANGEABRUPTLY

This juxtaposition of several stablestates is called multistability or bistabil-

ity, and is also known in other fields ofmathematics, such as macroeconomics,which is concerned with variables thatcan lead to economic crises. In biology,too, some nutrient-rich lakes remainclean and clear, while others tip in re-sponse to some trigger and are trans-formed into putrid, algae-filled watersin record time.

Bistability for the desert was a com-pletely new concept. The simulationmade it clear that a deserts state couldchange abruptly, with catastrophicconsequences for the peoples who had

settled there. The main climate factorof solar radiation has changed slowlyand constantly over the last 20,000years. Bistability, however, means that

vegetation and climate can changemuch faster than the steady pace of or-bital forcing.

With his early simulation in themid-1990s, Claussen had, for the firsttime, linked a model of the atmospherewith a vegetation model, creating asimulation in which climate and veg-etation influenced each other. Thiscaused a small sensation at the time.However, those early simulations hadthe disadvantage that they could calcu-late climate for only one target date

today or 6,000 years ago, for instance.They were unable to deliver a progres-sion through time, over millennia, to

reveal gradual climate change.Together with his Russian research

partners Victor Brovkin, Andrey Ga-nopolski and Vladimir Petoukhov,Claussen forged ahead in 1999. Thefour met at that time at the Potsdam In-stitute for Climate Impact Research,where Claussen worked for more thanten years after starting his scientific ca-reer. They developed a fast climatemodel that not only fully linked the at- G r

a p h i c : M P I f o r M e t e o r o l o g y

The Sahara can now exist in two states. Both simulations (1a,1b) of northern African vegetation

show that it can have a higher or lower degree of plant life at different times. This bistabilityis represented on the diagram at top left by the t wo troughs in which the system, represented

by a ball, lies in a stable state. Simulation 2 shows only one stable state for Neolithic times:

the green one.

right: A satellite image of the Sahara: The simulation of the desert state (1a) is a very good

reflection of actual veget ation in the Sahara.

40 N

20 N

EQ

20 W 0 E 20 E 40 E

40 N

20 N

EQ

20 W 0 E 20 E 40 E

40 N

20 N

EQ

20 W 0 E 20 E 40 E20 W 0 E 20 E 40 E

Today

Neolithic Age


1a 1b

2

Today

Ecosystem state

DisturbanceNeolithicAge



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mosphere, oceans and vegetation, butalso processed long periods of time rap-idly. They started their simulations inthe more distant past about 9,000

years ago, when lions and rhinosroamed the central Sahara, and the gi-ant Lake Mega-Chad was hundreds ofkilometers wide.

The computer was cranked up again,rearranging enormous volumes of data.The simulation program calculated theocean currents and air streams for eachday and the state of vegetation for eachyear according to the wetness or dry-ness at each stage all the way up to thepresent day. The results confirmed theirassumption that, sometime around

5,500 years ago, the Sahara began to dryup, surprisingly abruptly. Within a fewcenturies or even decades, life musthave disappeared over large expanses ofthe Sahara.

Initially, this was not much morethan mere theory, just ugly numbersgenerated by a computer in Germany,some 5,000 kilometers distant from theSahara. Then, in 2000, just one year lat-er, geologist Peter deMenocal of NewYorks Columbia University published

the results of an expedition to the west-ern African coast. He had drilled deep

into the sediment of the sea floor,through layers dating back thousands

of years.

THE REGIONS OF NORTHERNAFRICA REACT DIFFERENTLY

The analysis of the drill cores took theclimatologists completely by surprise:until about 6,000 years ago, the sedi-ment on the sea floor consisted ofcoarse, solid grains that are commonlywashed into the sea by rivers. Thismeans that there must have been abun-dant rain in the Sahara at that time.However, more recent layers of sedimenttold a different story. Starting around

5,500 years ago, only dust was found,leaving room for just one conclusion:winds must have blown dust into thesea from the fast-drying Sahara. Claussenand his colleagues were delighted, ashard geological facts now seemed to cor-roborate their calculations.

Still, Claussen was not really happywith his climate model. It was able togenerate very fast calculations, but itscontinental resolution was too low. Wewanted a new model that would look atthe whole world in detail, taking region-al differences into account. At the time,

a higher-resolution climate model suchas they were looking for was available atthe Max Planck Institute for Meteorolo-gy. For their purposes, however, Claussen

back in Hamburg once more and hiscolleagues needed to supplement it withvegetation so that the model could in-clude green growth in its calculations,

integrating the migration of prairiegrass, brushwood and trees through thecourse of time.

This comprehensive climate modelhas been running for some years anddivides the world, and the Sahara, intosmall squares of up to 120 km in length.In addition, new knowledge about thereflectivity of the soil and evaporation

in different areas is constantly fed intothe model. All this allows Claussen toobtain high temporal and spatial reso-lution. This combination would usual-ly bring a computer to its knees, andeven Hamburgs large climate comput-er needs a whole day to simulate a hun-dred years. Consequently, despite thehuge computing power available tothem, the researchers need patience fora journey 5,000 years back in time.

Still, a look at the details is worth it.We used to consider the Sahara as onebig box, says Claussen, which would



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mean that the entire desert dried out.Now he knows that its not that simple.There are, apparently, marked differ-ences between the regions. Every nowand then, greener and drier strips ofland would occur, and the western Sa-hara seemed to dry out more quicklythan the eastern part. Some areas seemto have changed their appearance quiteabruptly. Plus it seems that some re-gions pull others along with them. If

the desert expands in one part, neigh-boring areas follow suit.The findings are consistent with

studies by a Cologne-based group ledby Stefan Krpelin, who took sedimentsamples from the ancient saline LakeYoa in northern Chad a few years ago.Based on pollen and spore findings, thescientists deduced that tropical plantsand ferns suddenly vanished 5,000years ago. Yet plants native to the sa-vannahs, Mediterranean region anddesert are still found today at Lake Yoa,in the eastern Sahara. This rules out a

sudden aridification. The loss and de-struction of life was abrupt for somespecies, but insidious for others.

THE SOUTHERN SAHARA MAY

SOON BE GREEN AGAIN

The team in Cologne combined theirsediment samples with knowledgeabout prehistoric settlements, bonesand pottery shards, concluding that the

region enjoyed a warm, humid climate8,500 years ago the period whenlmsys rock paintings were made.The fertile Sahel region stretched muchfurther north. As the eastern part slow-ly dried out some 5,300 years ago, peo-ple retreated further to the south, or tothe north, where the Nile became a newlifeline. While the fertile steppes of theSahara sank into dust, the era of thePharaohs began on the Nile. Our fastclimate model treated the Sahara as auniform area, says Claussen. But wenow know that it has many facets.

The Sahara is a complex habitat thatmuch is clear. Claussen thus believesthat predicting its future will be com-plicated. Nonetheless, he has used hisclimate models to look into the futureand attempt to answer the question ofwhether climate change could causeplants to grow in the Sahara again inthe near future. If humankind contin-ues to blow carbon dioxide into the airunabated, the planet will heat up swift-

ly. Global warming will increase evap-oration like water in a saucepan, raisingprecipitation levels. This could lead togreening of particularly the southernSahara as early as this century, whiledry areas expand in other regions, suchas the Mediterranean basin at leastthat is what the simulations show.

Without human intervention, thepicture would be quite different: Withthe slow, steady changes in solar radi-ation, the Sahara would stay dry forabout another 10,000 years. Thenheavy monsoon rains would move in P h

o t o : G e r m a n C l i m a t e C o m p u t i n g C e n t e r ; g r a p h i c : M P I f o r M e t e o r o l o g y

Only a very powerful computer, such as Blizzard in the German Climate Computing Center (above), can simulate Saharan vegetation with

good spatial and temporal resolution. The green crosses symbolize the probability of short green periods: the bigger the cross, the stronger the

greening. The Sahel stretched further north 6,000 years ago, and plants repeatedly sprouted throughout the Sahara.

0 30E 60E

0.2 0.3 0.4 0.5 0.6

Today

6,000 years ago

Proportion of desert



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over Africa as they last did 6,000 yearsago. The possibility of green growthextending into the Sahara in the com-

ing decades can already be seen as atrend from satellite measurements,with increasing vegetation perceptiblein the south. However, overgrazingand poor land management couldquickly destroy that plant cover, cau-tions Claussen.

In any event, it seems that thegreening would not last long. Ournew, more complex model shows aninitial greening of the Sahara, but thatfurther warming would cause a markedreversal of that trend. Still, no one can

say with certainty just what the futureof the Sahara will be, as the results gen-erated by different climate models cur-rently vary too much.

For this very reason, Claussen goesa step further, feeding his powerful cli-mate model with more details. If trees

GLOSSARY

Bistability

Describes the circumstance that a system

can exist in two states. The state in which

the system is actually found at a given

time depends in part on the background

history. However, as soon as an external

factor changes to even a small degree, the

system can switch from one state to the

other. In the case of the Sahara, this

means that, if the world as a whole is rela-tively greener or more barren, plant life or

desert extends over northern Africa. A

slight change in solar radiation can change

the landscape abruptly, making it either

dry or fertile.

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lose their leaves, if moist humus isformed, how much soil is visible be-tween plants all this influences cli-

mate and vegetation, he explains. Heis collaborating with colleagues fromthe Max Planck Institute for Biogeo-chemistry in Jena who know biomassconversion under different climaticconditions like the back of their hands.Claussens hope is that the more close-ly he can reconstruct reality, the moreaccurate the predictions will be.

The climate computer in Hamburgneeds even longer to crack these morerealistic simulations, so Claussen oftengoes back to the fast model that can run

even on a standard PC. Its calculationsare not as detailed, but its geographicalresolution is quite acceptable. It allowsus to check major trends fast, saysClaussen. Calculating 10,000 years inone day is a great advantage here. Butultimately, the detailed simulation is

crucial for, say, an accurate predictionof the next one hundred years in theSahara. And that is of burning interest

to more than just climatologists.



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