Why deserts will inherit the Earth
Few places on Earth are less hospitable, less suited to human
life than the Sahara desert. Yet as global warming accelerates and the prospect
of profound climate change looms large, we must face the fact that vast areas of
our planet will be rendered equally barren. In his book, Fred
Pearce explains how nature can turn paradise into wilderness.
If there was a golden age for humans on the Earth - a Garden of Eden that flowed
with milk and honey - it was the high point of the Holocene, the era that
followed the end of the last ice age. From around 8,000 to 5,500 years ago, the
world was as warm as it is today, but there appear to have been few strong
hurricanes and few disruptive El Niños; and it was a world in which the regions
occupied today by great deserts in Asia, the Americas and Africa were much
wetter than they are now.
Optimists suggest that such conditions might be what awaits us in a greenhouse
world. There are celestial reasons why that might not happen, but that era, and
its abrupt ending, may still offer important lessons about our future climate in
the 21st century.
No place on Earth exemplifies the fall from this climatically blessed state
better than the Sahara. The world's largest desert was not always so arid. Where
seas of sand now shimmer, there were once vast lakes, swamps and rivers. Lake
Chad, which today covers a paltry few hundred square kilometres, was then a vast
inland sea, dubbed by scientists Lake Megachad. It was the size of France,
Spain, Germany and the UK put together. Today the lake evaporates in the desert
sun, but then it overflowed its inland basin and, at different times, drained
south via Nigeria into the Atlantic or east down a vast wadi to the Nile.
The difference is that back then the Sahara had assured rains. The whole of
North Africa was watered by a monsoon system rather like the one that keeps much
of Asia wet today. Rain-bearing winds penetrated deep into the interior. From
Senegal on the shores of the Atlantic to the Horn of Africa in the east, and
from the shores of the Mediterranean in the north to the threshold of the
central African rainforest, vast rivers flowed for thousands of kilometres.
Along their banks were swamps and forests.
Beneath the Algerian desert, archaeologists have found the remains of wadis that
once drained 1,000 kilometres from the Ahaggar Mountains into the Mediterranean.
And in waterless southern Libya, archaeologists are finding the bones of
crocodiles and hippos, elephants and antelope. If there was a vestige of true
desert at the heart of North Africa, it was very much smaller than the desert is
today.
And there were people: shepherds and fishers and hunters, and some of the
earliest known fields of grains. Archaeologists digging in the sands of northern
Chad, the dustiest place on Earth, have found human settlements around the shore
of the ancient Lake Megachad. Paintings in caves deep in the desert depict the
lives of the inhabitants of the verdant Sahara of the Holocene.
There are other, more practical remains. Rocks beneath the Sahara contain the
largest underground reservoir of fresh water in the world. They were mostly
filled from leaking wadis in the early Holocene. Some desert settlements today
tap these waters at oases. Colonel Gaddafi has constructed pumps and a huge
pipeline network to take this water from beneath southern Libya to his coastal
farmers. He calls the network his Great Man-made River, though it is a feeble
imitation of the real rivers that once ran here.
The wet Sahara and the era known more generally as the African Humid Period
began around 13,000 years ago as the ice age abated, and, except for the Younger
Dryas hiatus, lasted to the end of the "golden age" of the African
Humid Period. It coincided with a time when the Earth's tilt known as the
precession ensured that the sun was blazing down on the Sahara with full
intensity in summer. The land cooked and convective air currents were strong. As
the warm air rose, wet air was drawn in from over the Atlantic to replace it.
The process was the same one that creates today's monsoon rain system in Asia.
Meanwhile, the monsoon rains were recycled by the rich vegetation across North
Africa. Rather as in the Amazon today, the rain nurtured lush vegetation that
ensured that much of it evaporated back into the air. The continually moistened
winds took rain to the heart of the Sahara.
But the African Humid Period came to an end - known as the Fall - suddenly. In
the space of a century, the Saharan rivers emptied, swamps dried up and the
monsoon rain clouds were replaced by clouds of wind-blown sand. The climate
system had crossed a threshold that triggered massive change. Why?
First, the sun moved. Or rather, the precession continued its stately progress
and gradually took away the extremely favourable conditions for Saharan rains.
And as summer solar heating lessened, the warm air rose a little less and the
monsoon winds from the ocean didn't penetrate as far inland some years. The
process went on without any appreciable effect on rainfall in most of the Sahara
for more than 3,000 years. The vegetation feedback ensured that, most years, the
rain kept falling.
But at some point, the feedback began to falter. Perhaps there was a chance
variation in rainfall that dried out the bush for a year or two. The sun was no
longer strong enough to make good and revive the rains. Suddenly, what had been
a feedback that kept the Sahara watered became a feedback that dried it out. The
system had passed a threshold, and it never recovered. The green Sahara had
become a brown Sahara. The North African monsoon rains had died.
Not everybody agrees that the vegetation feedback was the only trigger for the
drying of the Sahara. There may have been some influence from one of the
geologist Gerard Bond's solar pulses. Bond, formerly of the Lamont-Doherty Earth
Observatory at Columbia University, New York, argued that regular pulses in
solar activity drive cycles of climate change. But climate models show that in
all probability this flip in the Saharan climate was extremely sudden.
Martin Claussen at the Potsdam Institute for Climate Impact Research in Germany
has played out this tragedy in detail in his model. He turns time forwards and
backwards, recreates the subtle orbital changes and fine-tunes the vegetation
feedbacks. More or less whatever he does to mimic the conditions of 5,500 years
ago, the result is the same. The system flips abruptly, turning bush to desert
and seas of water to seas of sand.
Other researchers have replicated his findings. Peter deMenocal at
Lamont-Doherty calculates that the system flipped when solar radiation in the
Sahara crossed a threshold of 470 watts per square metre. Jon Foley of the
University of Wisconsin found that a reduction in Holocene summer sun sufficient
to reduce temperatures by just 0.4C would have cut rainfall across the Sahara by
a quarter, and by much more in the furthest interior. He says that once a region
such as the Sahara becomes dry and brown it requires exceptional rains to
trigger a regreening. Beyond a certain point - such as that reached 5,500 years
ago - virtually no amount of extra rain is likely to be enough. Lack of
vegetation "acts to lock in and reinforce the drought".
The people of the Sahara couldn't have known if the droughts were permanent. But
as the desert asserted control, and waterways dried up, they had to leave.
Lakeside settlements near the Sudanese border in Egypt were all abandoned at
about the same time.
One was Nabta, famous as the site of the world's earliest known stone structures
with an astronomical purpose. They predate Stonehenge by 1,000 years. The key
stones point to where the sun would have set at the summer solstice 6,000 years
ago. Nobody can be sure what the structures' precise purpose was, but it is
intriguing to suppose that they were used in an attempt to track the celestial
changes that were disrupting the rains.
It may have been from such places that the myths of past golden ages, and of the
Garden of Eden, emerged. The people who departed from the Sahara would have
taken their memories of a golden past. Biblical scholars have calculated
mankind's expulsion from Eden at around 6,000 years ago, when kingdoms across
the Sahara would have been collapsing.
But the Eden need not have been in the Sahara: similar stories were played out
elsewhere. Arabia dried out at the same time, leaving behind a huge underground
reservoir of water not much smaller than that beneath the Sahara. Claussen
calculates that the desertification of Arabia could have been caused by the same
combination of gradual orbital change and a dramatic vegetation feedback.
The evidence is as yet sketchy, but dramatic drying of the Sahara and Arabia
appears to coincide with other climate changes round the world. In the Pacific
Ocean, El Niño appeared to switch into a more active mode at around this time.
There were cold periods from the Andes to the European Alps. In both cases,
glaciers advanced strongly; often they are only returning to their former
positions today.
In the Austrian Tyrol, one victim of the advance was the "ice man"
Otzi, whose freeze-dried remains emerged in 1991. In Ireland, a 7,000-year
temperature record held in tree rings shows cold times that included the coldest
summer in the entire record.
All this is intriguing because, unlike previous great climatic events of the ice
ages, there is little evidence that the primary action had much to do with the
polar regions. It seems to have been an abrupt change formed in the tropics and
with its major impacts there, and only ripples beyond.
What does this say about the future of the Sahara? Could warming in the 21st
century trigger a greener, wetter Sahara? The idea has with plenty of adherents.
Reindert Haarsma, a climate modeller at the Royal Netherlands Meteorological
Institute, says the Sahara could be destined for a 50 per cent increase in
rainfall - enough to trigger a "golden age" in which crocodiles float
through swamps where today locusts swarm.
Claussen, whose model first stimulated the idea, is more sceptical. He points
out that the orbital situation today is very different, so summer solar
radiation is not great enough to create a revived African monsoon. DeMenocal
says solar radiation is 4 per cent lower in the Sahara than it was when the
Holocene flip occurred. But on the other hand, he admits, much higher levels of
CO2 in the air might compensate for this by stimulating an earlier recovery of
Saharan vegetation.
Optimists point out that there is currently something of a revival going on in
Saharan rains - albeit from the depths of the droughts that afflicted the region
in the 1970s and 1980s. It hasn't happened everywhere, and some places have
since slipped back.
But, according to Chris Reij of the Free University in Amsterdam, improved
farming methods, such as digging terraces and holding water on the land, may
have encouraged a modest greening of parts of the Sahara, and the resulting
vegetation feedback could be one reason for the revived rains. But it would be a
big step to predict from that a reversion to "Eden" days.
While some in the Sahara may conceivably be able to look forward to greener,
wetter times, the prognosis for many other arid regions round the world is not
so good. The big fear from the American West to northern China and Southern
Africa to the Mediterranean is of a 21st century dominated by longer and fiercer
droughts.
Again, history is the first guide. DeMenocal has been looking at the history of
droughts and civilisation in the Americas, and finds strong evidence of droughts
much longer than any known in modern times. "Vast regions of North America
witnessed several such periods during the last millennium, with devastating
cultural consequences," he says. "These mega-droughts can persist for
a century or more."
The six-year Dust Bowl of the 1930s, which caused mass migrations westwards, was
"pale by comparison" with its predecessors. Droughts in the 19th
century devastated many Native Americans as well as their bison. At the end of
the 16th century, a 22-year drought destroyed an early English colony at Roanoke
in Virginia. It became known as the Lost Colony after all its inhabitants
disappeared between their arrival in 1587 and the return of a supply ship four
years later. And tree rings show there was near permanent drought from AD 900 to
1300 west of the Mississippi and through Central America that destroyed the
Mayan and Anasazi civilisations.
DeMenocal concludes that complex, organised societies can get by in short
droughts. They have stocks of food and water and know how to trade. But few can
deal with megadroughts. If hunger doesn't get them, the strife caused by trying
to survive does.
The signs are that worsening droughts are again becoming the norm in regions
that have suffered megadroughts in the past. In the American West, the biggest
river, the Colorado, is a shadow of its former self. Early in the 20th century,
the average flow was 16 cubic kilometres a year. In 2002, it fell to just 3.7
cubic kilometres - worse even than the Dust Bowl years of the mid-1930s.
In Central Asia, the Afghan war of 2002 was fought against a backdrop of drought
as debilitating as any Taliban tyranny. The Hamoun wetland covering 4,000 sq km
on Afghanistan's border with Iran has for millennia been a place of refuge for
people from both countries. But that year it turned to salt flats.
Richard Seager at Lamont-Doherty says that there is a long-standing correlation
between drought in the western US and in South America, parts of Europe and
Central Asia. And that is a pattern we see reasserting itself in the 21st
century as the Arizona desert creeps north, Southern Europe increasingly
resembles North Africa and Central Asia takes on the appearance of Iraq or the
Arabian peninsula.
Kevin Trenberth of the National Center for Atmospheric Research in Boulder,
Colorado, reports that the percentage of the Earth's land area stricken by
serious drought has more than doubled in 30 years. In the 1970s, less than 15
per cent of the land was drought-stricken, but by the first years of 21st
century it had risen to 30 per cent.
That seems to be a common view. Mark Cane, a specialist in Pacific weather at
Lamont-Doherty, says: "The medieval warm period a thousand years ago was a
very small forcing compared with what is going on with global warming now. But
it was still strong enough to cause a 300- to 400-year drought in the western
US. That could be an analogue for what will happening under anthropogenic
warming. If the mechanisms we think work hold true, then we'll get big droughts
in the West again."
Many believe that El Niño and the pattern of ocean temperatures in the Pacific
are heavily implicated in the historic megadroughts, perhaps as part of a global
reorganisation of climate systems linked to Gerard Bond's pulses. And this
should set modern alarm bells ringing, says Ed Cook, a leading tree-ring expert
from Lamont-Doherty.
"Any trend towards warmer temperatures [over the tropical Pacific] could
lead to a serious long-term increase in aridity over western North
America." Martin Hoerling of the National Oceanic and Atmospheric
Administration in Boulder, Colorado thinks that such a process is already
underway. He blames the increasingly drought-prone nature of the tropics on a
persistent ocean warming in the Pacific. The pattern of dryness is beginning to
look less like a local, short-term aberration and more like a long-term trend,
he says, and predicts that global warming "may be a harbinger of future
severe and extensive droughts".
It won't happen everywhere, of course. Climate models predict that a warmer
world will, on average, have more moisture in the atmosphere and that, in
general, the wet places will get wetter and the dry places drier. They predict
that areas of uplift, where rising air will trigger storm clouds and abundant
rain, will see the uplift become more intense. But the areas of sinking air,
which are the traditional deserts of the world, will see more intense sinking
and drying.
In many parts of the world, this "hyperweather" is likely to set
competing forces against each other. Stronger storms will blow off the oceans
and, in places, monsoon-type rains may begin to restart. But the rain-bearing
winds will often by confronted by intensifying arid zones of descending air in
the continental interiors. It is not obvious which force will win, and where.
Will the Sahara desert expand and intensify as the drought theorists argue? Or
will North Africa be reclaimed by a revived African monsoon? Megadrought or
Garden of Eden? Nobody can answer that question yet. Perhaps the greatest
likelihood is that in many places, from the Sahara to the American West and
Arabia, there will be more and longer droughts, interspersed with brief but
devastating floods.
SEE-SAW ACROSS THE OCEAN - how the Sahara desert greens the Amazon
Two of the world's largest and most fragile ecosystems face each other across
the Atlantic. On one side is the Amazon rainforest; on the other the Sahara. The
Sahara is rainless and largely empty of vegetation. The Amazon is one of the
wettest places on Earth, and its most biologically diverse. But these two
opposites are not so far apart. For one thing, the physical gap is surprisingly
small. Near the equator, the two are less than half as far apart as London and
New York.
Many believe the two areas have a surprising symbiosis. Their fates may be
intertwined in a rather unexpected way - and one that could have important
consequences in the near future.
The key to the symbiosis lies in a region called Bodele in northern Chad. Few
people go here. It is littered with unexploded bombs and land mines left behind
during Libya's invasion during the 1980s. And it is by some way the dustiest
place on Earth. Satellite images show year-round dust storms raging across
Bodele and entering the atmospheric circulation. According to Richard Washington
of Oxford University, two-fifths of the dust in the atmosphere comes from the
Sahara, and of that half comes from Bodele.
Some of this dust stays local. But much of it is carried west across the desert
wastes of Niger, Mali and Mauritania before heading out over the Atlantic. The
red dust clouds can grow three kilometres high as they approach America. They
cause spectacular sunrises over Miami, before falling in the rains of the
Caribbean and the Amazon. And there have been a lot of good sunrises in recent
decades. The amount of dust crossing the Atlantic grew five-fold between the wet
1960s and the dry 1980s.
The Sahara dust has a series of unexpected effects on the Americas. According to
hurricane forecasters in Florida, during dry, dusty years in the Sahara there
are fewer hurricanes on the other side of the Atlantic. It seems that dust in
the air interrupts the updraughts that fuel the storms. Equally surprisingly,
desert bacteria caught up in the winds are being blamed for bringing new
diseases to Caribbean coral reefs and triggering asthma among Caribbean
children. And there is a third important link. Saharan dust storms carry huge
amounts of minerals and organic matter that enrich soils widely in the Americas.
Bodele dust seems especially valuable. Its dunes are the dried-out remains of
the bed of the vast Lake Megachad, which covered central Sahara until its abrupt
demise 5,500 years ago.
Most of the dunes are not made of sand or broken rock. They are the remains of
trillions of diatomites, microscopic freshwater creatures that once lived in the
lake. These fragments are light enough to blow freely in the wind. And they make
great fertiliser.
If Bodele had any rain, the diatomites would make rich farmland. Instead, Chad's
loss is the Americas' gain, says Hans Joachim Schellnhuber, a German physicist
turned Earth system scientist, who as director of Britain's Tyndall Climate
Centre in Norwich has made a study of the unlikely connection. "The Sahara
fertilises the Amazon rainforest. This process has been going on for thousands
of years."
The two habitats are on a kind of see-saw, he says. When the Sahara is dry, as
it has been for much of the past 25 years, its dust crosses the Atlantic in huge
quantities and fertilises the Amazon. When the Sahara is wet, the dust storms
subside and the Amazon goes hungry. That the Sahara seems to have only two basic
modes, wet and dry, suggests that there may be two distinct modes in the Amazon,
too. The last big change in the Sahara came 5,500 years ago when the region
lurched from wet to dry, probably within decades. As yet we know little about
how the Amazon changed at that time. But if Schellnhuber is right, the Sahara's
loss at that time may have been the Amazon's gain.
In the 21st century, the see-saw could be on the move again. There are hints
that the Sahara may become wetter. And if the wetting turns to greening, and the
vegetation feedback kicks in, the whole of North Africa could change
dramatically. That would be good news for the Sahara. But perhaps bad news for
the Amazon, which seems to be close to its own tipping point as the climate
dries and rainforests give up their carbon. Could a wetter Sahara be the final
nail in the Amazon coffin? Schellnhuber believes so.
These are edited extracts from The Last Generation by Fred Pearce, published by
Transworld at £12.99. To order it for the special price of £11.50, including
p&p, call Independent Books Direct on 08700 798 897
If there was a golden age for humans on the Earth - a Garden of Eden that flowed
with milk and honey - it was the high point of the Holocene, the era that
followed the end of the last ice age. From around 8,000 to 5,500 years ago, the
world was as warm as it is today, but there appear to have been few strong
hurricanes and few disruptive El Niños; and it was a world in which the regions
occupied today by great deserts in Asia, the Americas and Africa were much
wetter than they are now.
Optimists suggest that such conditions might be what awaits us in a greenhouse
world. There are celestial reasons why that might not happen, but that era, and
its abrupt ending, may still offer important lessons about our future climate in
the 21st century.
No place on Earth exemplifies the fall from this climatically blessed state
better than the Sahara. The world's largest desert was not always so arid. Where
seas of sand now shimmer, there were once vast lakes, swamps and rivers. Lake
Chad, which today covers a paltry few hundred square kilometres, was then a vast
inland sea, dubbed by scientists Lake Megachad. It was the size of France,
Spain, Germany and the UK put together. Today the lake evaporates in the desert
sun, but then it overflowed its inland basin and, at different times, drained
south via Nigeria into the Atlantic or east down a vast wadi to the Nile.
The difference is that back then the Sahara had assured rains. The whole of
North Africa was watered by a monsoon system rather like the one that keeps much
of Asia wet today. Rain-bearing winds penetrated deep into the interior. From
Senegal on the shores of the Atlantic to the Horn of Africa in the east, and
from the shores of the Mediterranean in the north to the threshold of the
central African rainforest, vast rivers flowed for thousands of kilometres.
Along their banks were swamps and forests.
Beneath the Algerian desert, archaeologists have found the remains of wadis that
once drained 1,000 kilometres from the Ahaggar Mountains into the Mediterranean.
And in waterless southern Libya, archaeologists are finding the bones of
crocodiles and hippos, elephants and antelope. If there was a vestige of true
desert at the heart of North Africa, it was very much smaller than the desert is
today.
And there were people: shepherds and fishers and hunters, and some of the
earliest known fields of grains. Archaeologists digging in the sands of northern
Chad, the dustiest place on Earth, have found human settlements around the shore
of the ancient Lake Megachad. Paintings in caves deep in the desert depict the
lives of the inhabitants of the verdant Sahara of the Holocene.
There are other, more practical remains. Rocks beneath the Sahara contain the
largest underground reservoir of fresh water in the world. They were mostly
filled from leaking wadis in the early Holocene. Some desert settlements today
tap these waters at oases. Colonel Gaddafi has constructed pumps and a huge
pipeline network to take this water from beneath southern Libya to his coastal
farmers. He calls the network his Great Man-made River, though it is a feeble
imitation of the real rivers that once ran here.
The wet Sahara and the era known more generally as the African Humid Period
began around 13,000 years ago as the ice age abated, and, except for the Younger
Dryas hiatus, lasted to the end of the "golden age" of the African
Humid Period. It coincided with a time when the Earth's tilt known as the
precession ensured that the sun was blazing down on the Sahara with full
intensity in summer. The land cooked and convective air currents were strong. As
the warm air rose, wet air was drawn in from over the Atlantic to replace it.
The process was the same one that creates today's monsoon rain system in Asia.
Meanwhile, the monsoon rains were recycled by the rich vegetation across North
Africa. Rather as in the Amazon today, the rain nurtured lush vegetation that
ensured that much of it evaporated back into the air. The continually moistened
winds took rain to the heart of the Sahara.
But the African Humid Period came to an end - known as the Fall - suddenly. In
the space of a century, the Saharan rivers emptied, swamps dried up and the
monsoon rain clouds were replaced by clouds of wind-blown sand. The climate
system had crossed a threshold that triggered massive change. Why?
First, the sun moved. Or rather, the precession continued its stately progress
and gradually took away the extremely favourable conditions for Saharan rains.
And as summer solar heating lessened, the warm air rose a little less and the
monsoon winds from the ocean didn't penetrate as far inland some years. The
process went on without any appreciable effect on rainfall in most of the Sahara
for more than 3,000 years. The vegetation feedback ensured that, most years, the
rain kept falling.
But at some point, the feedback began to falter. Perhaps there was a chance
variation in rainfall that dried out the bush for a year or two. The sun was no
longer strong enough to make good and revive the rains. Suddenly, what had been
a feedback that kept the Sahara watered became a feedback that dried it out. The
system had passed a threshold, and it never recovered. The green Sahara had
become a brown Sahara. The North African monsoon rains had died.
Not everybody agrees that the vegetation feedback was the only trigger for the
drying of the Sahara. There may have been some influence from one of the
geologist Gerard Bond's solar pulses. Bond, formerly of the Lamont-Doherty Earth
Observatory at Columbia University, New York, argued that regular pulses in
solar activity drive cycles of climate change. But climate models show that in
all probability this flip in the Saharan climate was extremely sudden.
Martin Claussen at the Potsdam Institute for Climate Impact Research in Germany
has played out this tragedy in detail in his model. He turns time forwards and
backwards, recreates the subtle orbital changes and fine-tunes the vegetation
feedbacks. More or less whatever he does to mimic the conditions of 5,500 years
ago, the result is the same. The system flips abruptly, turning bush to desert
and seas of water to seas of sand.
Other researchers have replicated his findings. Peter deMenocal at
Lamont-Doherty calculates that the system flipped when solar radiation in the
Sahara crossed a threshold of 470 watts per square metre. Jon Foley of the
University of Wisconsin found that a reduction in Holocene summer sun sufficient
to reduce temperatures by just 0.4C would have cut rainfall across the Sahara by
a quarter, and by much more in the furthest interior. He says that once a region
such as the Sahara becomes dry and brown it requires exceptional rains to
trigger a regreening. Beyond a certain point - such as that reached 5,500 years
ago - virtually no amount of extra rain is likely to be enough. Lack of
vegetation "acts to lock in and reinforce the drought".
The people of the Sahara couldn't have known if the droughts were permanent. But
as the desert asserted control, and waterways dried up, they had to leave.
Lakeside settlements near the Sudanese border in Egypt were all abandoned at
about the same time.
One was Nabta, famous as the site of the world's earliest known stone structures
with an astronomical purpose. They predate Stonehenge by 1,000 years. The key
stones point to where the sun would have set at the summer solstice 6,000 years
ago. Nobody can be sure what the structures' precise purpose was, but it is
intriguing to suppose that they were used in an attempt to track the celestial
changes that were disrupting the rains.
It may have been from such places that the myths of past golden ages, and of the
Garden of Eden, emerged. The people who departed from the Sahara would have
taken their memories of a golden past. Biblical scholars have calculated
mankind's expulsion from Eden at around 6,000 years ago, when kingdoms across
the Sahara would have been collapsing.
But the Eden need not have been in the Sahara: similar stories were played out
elsewhere. Arabia dried out at the same time, leaving behind a huge underground
reservoir of water not much smaller than that beneath the Sahara. Claussen
calculates that the desertification of Arabia could have been caused by the same
combination of gradual orbital change and a dramatic vegetation feedback.
The evidence is as yet sketchy, but dramatic drying of the Sahara and Arabia
appears to coincide with other climate changes round the world. In the Pacific
Ocean, El Niño appeared to switch into a more active mode at around this time.
There were cold periods from the Andes to the European Alps. In both cases,
glaciers advanced strongly; often they are only returning to their former
positions today.
In the Austrian Tyrol, one victim of the advance was the "ice man"
Otzi, whose freeze-dried remains emerged in 1991. In Ireland, a 7,000-year
temperature record held in tree rings shows cold times that included the coldest
summer in the entire record.
All this is intriguing because, unlike previous great climatic events of the ice
ages, there is little evidence that the primary action had much to do with the
polar regions. It seems to have been an abrupt change formed in the tropics and
with its major impacts there, and only ripples beyond.
What does this say about the future of the Sahara? Could warming in the 21st
century trigger a greener, wetter Sahara? The idea has with plenty of adherents.
Reindert Haarsma, a climate modeller at the Royal Netherlands Meteorological
Institute, says the Sahara could be destined for a 50 per cent increase in
rainfall - enough to trigger a "golden age" in which crocodiles float
through swamps where today locusts swarm.
Claussen, whose model first stimulated the idea, is more sceptical. He points
out that the orbital situation today is very different, so summer solar
radiation is not great enough to create a revived African monsoon. DeMenocal
says solar radiation is 4 per cent lower in the Sahara than it was when the
Holocene flip occurred. But on the other hand, he admits, much higher levels of
CO2 in the air might compensate for this by stimulating an earlier recovery of
Saharan vegetation.
Optimists point out that there is currently something of a revival going on in
Saharan rains - albeit from the depths of the droughts that afflicted the region
in the 1970s and 1980s. It hasn't happened everywhere, and some places have
since slipped back.
But, according to Chris Reij of the Free University in Amsterdam, improved
farming methods, such as digging terraces and holding water on the land, may
have encouraged a modest greening of parts of the Sahara, and the resulting
vegetation feedback could be one reason for the revived rains. But it would be a
big step to predict from that a reversion to "Eden" days.
While some in the Sahara may conceivably be able to look forward to greener,
wetter times, the prognosis for many other arid regions round the world is not
so good. The big fear from the American West to northern China and Southern
Africa to the Mediterranean is of a 21st century dominated by longer and fiercer
droughts.
Again, history is the first guide. DeMenocal has been looking at the history of
droughts and civilisation in the Americas, and finds strong evidence of droughts
much longer than any known in modern times. "Vast regions of North America
witnessed several such periods during the last millennium, with devastating
cultural consequences," he says. "These mega-droughts can persist for
a century or more."
The six-year Dust Bowl of the 1930s, which caused mass migrations westwards, was
"pale by comparison" with its predecessors. Droughts in the 19th
century devastated many Native Americans as well as their bison. At the end of
the 16th century, a 22-year drought destroyed an early English colony at Roanoke
in Virginia. It became known as the Lost Colony after all its inhabitants
disappeared between their arrival in 1587 and the return of a supply ship four
years later. And tree rings show there was near permanent drought from AD 900 to
1300 west of the Mississippi and through Central America that destroyed the
Mayan and Anasazi civilisations.
DeMenocal concludes that complex, organised societies can get by in short
droughts. They have stocks of food and water and know how to trade. But few can
deal with megadroughts. If hunger doesn't get them, the strife caused by trying
to survive does.
The signs are that worsening droughts are again becoming the norm in regions
that have suffered megadroughts in the past. In the American West, the biggest
river, the Colorado, is a shadow of its former self. Early in the 20th century,
the average flow was 16 cubic kilometres a year. In 2002, it fell to just 3.7
cubic kilometres - worse even than the Dust Bowl years of the mid-1930s.
In Central Asia, the Afghan war of 2002 was fought against a backdrop of drought
as debilitating as any Taliban tyranny. The Hamoun wetland covering 4,000 sq km
on Afghanistan's border with Iran has for millennia been a place of refuge for
people from both countries. But that year it turned to salt flats.
Richard Seager at Lamont-Doherty says that there is a long-standing correlation
between drought in the western US and in South America, parts of Europe and
Central Asia. And that is a pattern we see reasserting itself in the 21st
century as the Arizona desert creeps north, Southern Europe increasingly
resembles North Africa and Central Asia takes on the appearance of Iraq or the
Arabian peninsula.
Kevin Trenberth of the National Center for Atmospheric Research in Boulder,
Colorado, reports that the percentage of the Earth's land area stricken by
serious drought has more than doubled in 30 years. In the 1970s, less than 15
per cent of the land was drought-stricken, but by the first years of 21st
century it had risen to 30 per cent.
That seems to be a common view. Mark Cane, a specialist in Pacific weather at
Lamont-Doherty, says: "The medieval warm period a thousand years ago was a
very small forcing compared with what is going on with global warming now. But
it was still strong enough to cause a 300- to 400-year drought in the western
US. That could be an analogue for what will happening under anthropogenic
warming. If the mechanisms we think work hold true, then we'll get big droughts
in the West again."
Many believe that El Niño and the pattern of ocean temperatures in the Pacific
are heavily implicated in the historic megadroughts, perhaps as part of a global
reorganisation of climate systems linked to Gerard Bond's pulses. And this
should set modern alarm bells ringing, says Ed Cook, a leading tree-ring expert
from Lamont-Doherty.
"Any trend towards warmer temperatures [over the tropical Pacific] could
lead to a serious long-term increase in aridity over western North
America." Martin Hoerling of the National Oceanic and Atmospheric
Administration in Boulder, Colorado thinks that such a process is already
underway. He blames the increasingly drought-prone nature of the tropics on a
persistent ocean warming in the Pacific. The pattern of dryness is beginning to
look less like a local, short-term aberration and more like a long-term trend,
he says, and predicts that global warming "may be a harbinger of future
severe and extensive droughts".
It won't happen everywhere, of course. Climate models predict that a warmer
world will, on average, have more moisture in the atmosphere and that, in
general, the wet places will get wetter and the dry places drier. They predict
that areas of uplift, where rising air will trigger storm clouds and abundant
rain, will see the uplift become more intense. But the areas of sinking air,
which are the traditional deserts of the world, will see more intense sinking
and drying.
In many parts of the world, this "hyperweather" is likely to set
competing forces against each other. Stronger storms will blow off the oceans
and, in places, monsoon-type rains may begin to restart. But the rain-bearing
winds will often by confronted by intensifying arid zones of descending air in
the continental interiors. It is not obvious which force will win, and where.
Will the Sahara desert expand and intensify as the drought theorists argue? Or
will North Africa be reclaimed by a revived African monsoon? Megadrought or
Garden of Eden? Nobody can answer that question yet. Perhaps the greatest
likelihood is that in many places, from the Sahara to the American West and
Arabia, there will be more and longer droughts, interspersed with brief but
devastating floods.
In the 21st century, the see-saw could be on the move again. There are hints
that the Sahara may become wetter. And if the wetting turns to greening, and the
vegetation feedback kicks in, the whole of North Africa could change
dramatically. That would be good news for the Sahara. But perhaps bad news for
the Amazon, which seems to be close to its own tipping point as the climate
dries and rainforests give up their carbon. Could a wetter Sahara be the final
nail in the Amazon coffin? Schellnhuber believes so.
These are edited extracts from The Last Generation by Fred Pearce, published by Transworld