Rise And Fall

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History books hark on about earlier ages when ancient civilizations prospered - and occasionally fought - in bright and sunny climes. The birth of civilization is thought to have happened against a balmy backdrop; it didn't arise when the world was in the middle of an ice age. It all seems to point to a nice easy relationship: warm equals good, cold is bad. It's tempting to simplify the whole messy debate about climate change along the same lines. But it's not as simple as that. Even when the world was warm, it wasn't a smooth ride for our ancestors. Climate continued to vary - albeit not as dramatically as before - and these changes wielded a big stick.

We have to be careful here. The idea that the environment - in particular climate - influences the behaviour of a society is known as environmental determinism. Over the long term, changes in climate can make the difference between a rise or fall in civilization. But some supporters of this idea seem to passionately believe that every slight change in climate has a big knock-on effect. Its opponents are equally vehement and argue that the environment has no role to play; if it gets cold, people light a fire; if it gets dry, people draw on a well. You get the idea. In the few times I get invited to a dinner party, it can be good sport to toss the phrase 'environmental determinism' into a conversation with a colleague and see what happens. Either they'll momentarily look stunned, as if you might have escaped from the local lunatic asylum, and then rant and rave about what a load of rubbish the whole idea is, or they'll nervously scan around, and conspiratori-ally whisper that they think there might be something in it. I'll be up front right now and state that I don't think that climate is omnipotent. Neither is it impotent. Climate - and the associated changes in the environment - has an impact on people. By the same token, people (as we're seeing today) influence climate. You can't separate one from the other. Over the next couple of chapters I'll try to explain why.

A great place to start is in North Africa. During the last ice age, the Sahara was a lot larger and drier than it is today; the desert expanded to cover a third of Africa's surface. Then a big change happened. If we were able to go back 10,000 years we might be surprised at what would greet us. The Saharan region wasn't nearly as dry as today. In fact, it would have been very much wetter. This might seem a bit odd because an arid Sahara seems like one of those unassailable facts; a bit like a wet British summer. So why did the sand dunes fall back and rainfall increase in the early Holocene?

Up till now we've had a good look at climate changes recorded in ice cores from Greenland and Antarctica. Although these are fantastic archives, they are not as useful as we might like for looking at climate change in areas inhabited by early civilizations and societies. In warmer climes, the ice core records can only give us an indirect idea of what happened. We need to look at other natural chronicles.

Fortunately, across North Africa there is a plethora of archives. Depressions in the ground testify to where lakes once existed. Some were small, others humungous. Today's Lake Chad is just a shadow of the MegaChad in the early Holocene period; this enormous lake covered an area of at least 330,000 square kilometers; beaches, spits and deltas litter a region where sand is now king. Ancient sediments contain pollen grains that show much of the Sahara was covered in verdant grasslands and shrubs. And if that wasn't enough, there is ample evidence of people living within a menagerie that included hippos, giraffes and elephants. It all indicated there had been lots of rain. Welcome to the African Humid Period.

We might think that now we're in a warm period we can forget about those irritating changes in the way the Earth orbits the Sun. It's tempting to think that Milankovitch's ideas can be forgotten and we can get on with looking at other causes of climate change. Although it's true that orbital changes are the driver of climate cycles over the long term, things don't just stop when we hit an interglacial. As we saw in the Arctic, changes in tilt, wobble and eccentricity all contribute to changing the way heat from the Sun is distributed on the surface. The result is that different parts of the world can feel warmer at different times. Because of the precession of the equinoxes, the northern hemisphere summer of the early Holocene found itself in that part of the Earth's orbit closest to the Sun. Summer heat gradually increased across North Africa, reaching a high point between 11,000 and 10,000 years ago. At its peak, the amount of insolation from the Sun was 8% greater than today. This might not sound an awful lot, but it had a disproportionate effect.

A clue to the changing climate of the Sahara lies in the fact that much of the rain that falls in the region over a year comes from the monsoon in summer (Figure 8.1). This is a wonderfully simple but effective way of moving lots of water from the oceans onto the land. As the land starts to heat up at the start of the summer, the air also warms up. This has a big knock-on effect: the air expands, becomes less dense and then rises. The consequence of all this is that there is a lot less air pushing down on the African landscape. Air has to come in from somewhere to replace the stuff that's rising. Fresh air sweeps in from offshore. But this air isn't dry; it contains lots of moisture from all of the evaporation that's been going on at the ocean's surface. Suddenly we start to see a cycle develop. The moist ocean air hits Africa, gets heated by the land and then also rises. As it does so, it cools and the evaporated water condenses, forming storm clouds that dump their load over Africa. The rising air eventually cools and sinks over the ocean. The whole thing continues until the Sun loses its bite and the land is no longer

hotter than the ocean. How far the ocean air can penetrate inland decides how much of the continent receives rain.

In 1981, John Kutzbach at the University of Wisconsin took what happens today and proposed that a stronger system operated in the early Holocene. Because the summer Sun was much stronger at this time, the monsoon was far more intense: the land warmed up a lot more, so more air - and moisture - was bought in from the sea. A warmer land surface meant more air penetrating deeper into the continent, bringing more rain with it. It seemed to explain why the Sahara was green. An 8% increase in summer heat seemed to result in a 40% increase in rainfall. It was an elegant solution and seemed to fit the bill. But there was just one problem. No matter how hard the scientific community tried, they just couldn't get the climate models to explain the lakes seen across the Saharan region. Forty per cent wasn't enough. Changes in the Earth's orbit alone couldn't get enough rain into northern Africa. There had to be feedbacks.

It now looks like other processes exaggerated the effect of the changing orbit. When climate does change it often has a knock-on effect. North Africa at this time was no exception. For a start, more summer heat doesn't just mean that the land is the only place warming up. The ocean would also have got warmer, meaning more evaporation, so more moisture would have been delivered into the Sahara. As high rainfall became the norm, more plants would have grown across the landscape. Soils would have developed, holding onto the moisture for longer and near the surface: rather than draining away as it does in large parts of today's Sahara. It was a recipe for recycling water back into the air and strengthening the monsoon. When all this is added back into the mix, rainfall would have been roughly twice what it is today.

It was in 1833 that a young Charles Darwin realized that the frequent haziness over the eastern tropical Atlantic was caused by dust from Africa. We now know the quantities that can be thrown up into the air are enormous; today some 400 million tonnes of the stuff comes off northwest Africa each year. If the wind is blowing in the right direction it's not uncommon to find a blanket of red dust on the bonnets of cars as far away as the UK. Importantly, the amount of dust varies depending on the size of the desert. To look at how this might have changed in the past, Peter deMenocal and colleagues at the Lamont-Doherty Earth Observatory at Columbia University probed off the west African coast and analyzed an ocean core that's inspiringly known as 658C. The site is perfectly located to work out what happened when. Unlike most ocean cores, the sediments in 658C accumulated at a lightning rate of 18 centimetres every thousand years (that's fast for the seabed) meaning that finely sampled layers could give a detailed record. It also lies right under the main plume of dust. By looking at the changing dust content down through the ocean muds it was possible to work out what was happening on land. The weaker the African summer monsoon, the larger the desert and the more dust that would have hurtled offshore. The results showed that the African monsoon story was far from simple.

You'd be forgiven for thinking that as the summer Sun got stronger, the monsoon would act in proportion: the stronger the Sun's influence, the stronger the monsoon. But this wasn't the case. It looks like the monsoon took a while to respond. As Africa came out of the last ice age, the western Sahara stayed very arid until around 15,000 years ago. Then, when the region got more than about 4% extra summer heat, the monsoon suddenly became disproportionally stronger; there was suddenly a massive collapse in the amount of dust coming offshore. The change was abrupt. Once this threshold was crossed, it looks like everything described above - the warmer sea, the vegetation and the soils -suddenly all worked in unison. The Sahara greened. There was the odd blip during the Younger Dryas but the summer Sun was strong enough to override its effects as soon as the North Atlantic chill had passed. Things then carried on happily for several more thousand years until the summer heat dropped back down to below a level of 4% greater than today. The seas, vegetation and soils all seemed to stop working together around 5,300 years ago. The monsoon became far less effective and dust levels ramped back up to where they had been in the ice age. It all seems to show that the African monsoon works at two different levels. We're in the low gear at the moment. At other times, it can green a desert.

All these changes had a big impact on people living in the region. Across the Sahara there is ample evidence that people existed in the region for thousands of years. There's a fascinating trend in the estimated 10 million paintings and engravings that grace the rocks of the Sahara. A great example is the World Heritage site Tassili n'Ajjer in Algeria which has some 15,000 pieces of art. The drawings and engravings show a clear transition from the early Holocene, when wild animals such as buffalo, giraffes and antelopes were common, to a later period when domesticated livestock like cattle, goats and sheep became the norm. There are even some fabulous scenes showing people dancing to music. It's a remarkable place. But crucially for us, the themes also show a clear progression as the environment responded to the changing monsoon.

In the east Sahara, archaeologists Rudolph Kuper and Stefan Kroplein at the University of Cologne have looked at how people responded to these changes in the monsoon. It's a great story and shows the extent to which people have successfully adapted to a changing climate over time. The results confirm that very few people were visiting the Sahara at the end of the last ice age because of the extremely arid conditions. But things changed abruptly 10,500 years ago when increased monsoonal rainfall caused a greening of the desert in the eastern Sahara. As the buffalo, giraffes and antelopes moved in, prehistoric people rushed in for the kill. It was a golden age of abundant sun, grass and animals. This rich landscape seems to have persevered for a few thousand years, but it all turned sour when the rains started to fail. Between 7,000 and 5,300 years ago, the monsoon became less effective at penetrating the eastern Sahara and the desert began to expand. Those who stayed had to keep mobile. Livestock became a popular way of tracking water in oases and more mountainous areas. There was no point in staying in one place; the environment was too fickle. Most of the desert, however, was simply evacuated and many people migrated south into northern Sudan, following the retreating monsoon. Migration was the key; the mantra was keep with the rains and you should be all right. The people adapted. Most left.

The Nile valley was one of the few refugees left beyond the retreating monsoon; the sudden settlement and development of the pharaohs' civilization along the Nile looks like it only happened when people could no longer survive the full desert conditions around 5,300 years ago. Before then the Nile had been too boggy and forested to prosper in. With the drying that followed, the Nile suddenly became one huge oasis that has continued through to today. Fascinatingly, this leads to an interesting conclusion. Nick Brooks at the University of East Anglia has suggested that a complex society like that of the Egyptians only came about because of climate change. It was when the monsoon started to weaken that people flocked to the Nile. Lots of people had to be organized if the resources were to be effectively used. A political system had to be implemented and a hierarchy established. A benign stable Holocene climate doesn't look like it was the cause of civilization. Civilization, Brooks argues, was an adaptation to increasing aridity.

Something big certainly looks like it happened around 5,300 years ago in North Africa. But is this true of everywhere? What about in the tropics? It's not just an academic question. The tropics contain half of the world's land surface. They're also home to 70% of the world's population. And if that wasn't enough, the tropics are the energy powerhouse of the world. The high latitudes might be crucial for putting the world into and out of an ice age, but it's the tropics that drive the atmospheric circulation of our planet. If we want to learn from the past it's important to see what happened in this part of the world.

Over the course of a year, the tropics receive more energy from the Sun than they lose to space. It's all down to angles. Because of the Earth's angle of tilt, the tropics are almost perpendicular to the Sun all year round. The top of the atmosphere gets the full complement of possible energy from our star with the result that the surface stays relatively warm throughout the year. In contrast, the poles only receive sunlight during the six months around summer; as the Earth rotates through the year, no sunlight falls on the North or South Pole during winter. The bigger the temperature difference between the tropics and the poles, the more heat that has to be transported to high latitudes. Huge amounts of water are evaporated from tropical waters and the energy is temporarily locked up by the moisture within the air and transported polewards. Big storms are an extremely effective way of moving this heat to higher latitudes. But as a hemisphere moves into summertime, the difference in temperature between the tropics and pole decreases. Less energy has to be moved about so the number of storms falls away.

But this isn't the full story. There can be big differences in the amount of rain falling across the tropics, and the 'normal' pattern can change at a moment's notice, influencing the rest of the planet. Probably the single greatest cause of change is El Niño, a phenomenon that disrupts the tropical Pacific once every three to eight years.

El Niño is arguably the best known but most disliked feature of our world's climate system. The name is instantly recognizable; there are few brand names that can compete with its exposure. As a result, the media love it. It's something people both fear and abhor in equal measure. If an unusual climate event has happened, El Niño can be paraded as the cause. Depending on your geographical perspective, it can cause droughts, floods or warming. It's often blamed for some truly cataclysmic events in history; a severe El Niño in the late 18th century was supposedly the cause of French riots that led to revolution; more icebergs in the North Atlantic during 1912 allegedly caused the sinking of the Titanic. In climate change, it pretty much catches all but as a result is frequently misunderstood. The name alone is enough to get some people upset, often without them realizing what it is and how it works. In 1998, there was a fantastic report of some poor guy living in Niporno, California, called Al Nino. During a particularly intense El Niño, people had apparently found Al in the telephone book and called to castigate him for the dreadful climate. Sometimes you have to wonder whether everyone's neurons are firing properly.

The name 'El Niño' comes from the Spanish for 'Christ Child' after it was noticed that big changes in the ocean's temperature seem to happen off the west coast of South America around Christmas time. These big changes were first formally described in 1816, when the Governor of St Helena suggested that droughts stretching from India to the Caribbean might have a common cause. But it took another century before anyone really came up with a mechanism to explain how such disparate areas might be linked. In 1923, Sir Gilbert Walker was in charge of the Indian Meteorological Office when he suggested how atmospheric conditions might change across the region.

Walker's insight can best be understood if we imagine looking at the tropical Pacific Ocean from above. Walker argued that changes in atmospheric pressure across the tropical Pacific drove the circulation. It's in equatorial regions that the trade winds of the tropics converge, forming a belt of low pressure known as the Intertropical Convergence Zone. A large part of what drives these winds in the Pacific is rising air in the west and sinking air in the east. It's a bit like a seesaw. When there's low pressure in the west and high pressure in the east, the trade winds blow towards Asia, a phase known as La Niña (Figure 8.2). The rising air over the west Pacific causes huge thunderstorms, resulting in rainfall that can exceed two metres a year, while in the east, the air sinks over the Pacific, off the Peruvian coast. When the difference in pressure across the Pacific weakens, the winds slacken off and can even reverse. Walker wasn't sure what drove these changes but he suspected that the oceans might be the key.

We now know that in a normal or La Niña state, the winds blowing from the east drive the South Equatorial Current along the surface of the tropical Pacific Ocean. Plotting the ocean temperatures shows that the warmest part normally lies in the west and is usually at a higher sea level; the ocean surface in the east is typically 40 centimetres lower than the west. But this water can't keep on flowing to the west indefinitely; there has to be seawater returning east to replace the stuff stacking up in the west. This balancing act is maintained 100 metres under the surface by a large return flow of water towards the west coast of South America known as the Equatorial Undercurrent. The Equatorial Undercurrent picks up nutrients on its travels so that by the time it converges on South America and wells up it can support large populations of fish that are a valuable part of the local economy.

160 ICE, MUD AND BLOOD Normal and La Niña

160 ICE, MUD AND BLOOD Normal and La Niña

East Pacific

East Pacific

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