Yet the first successful rounding of the tip of Africa was a step change from this slow probing along the coast: it required a radical new approach.
In the late summer of 1487, Bartolomeu Dias set out from Lisbon, passed the Canary Islands, rounded Cape Bojador, and retraced the route along the African coastline now familiar from decades of Portuguese exploration. After four months at sea, Dias had passed the stone pillar marking the furthest point previous expeditions had reached. As he continued to follow the shoreline, he named the bays and capes that he encountered after the saints’ days: the Gulf of Santa Marta (8 December), São Tomé (21 December), Santa Vitória (23 December), and so on, like time-stamps marking his progress on the map. On Christmas Day he christened the Gulf of St Christopher, the patron saint of travellers.18
All along this coastline Dias’ ships had been pushing against adverse flows, tacking both against a steady southerly wind and a current pushing north up the coastline. Then Dias made a radical decision. He turned his ships away from land and headed out into the vast ocean, watching the comfort and safety of the coast recede over the horizon. His hope was that the same trick necessary for returning home from the North African coast against the Canary current – heading out further to sea on a looping volta do mar course to pick up westerly winds – would also work in the South Atlantic to carry them round the southern tip of Africa and find the passageway to the east.
Dias’s flash of insight paid off, and by about 38° south the hoped-for westerly winds began to pick up. The ships finally turned east with these winds, and after nearly a month in the featureless expanses of the South Atlantic, they at last made landfall. Following the shoreline they realised that the coast was now heading northeast: they had successfully rounded Africa’s southern tip and were on the far side of the vast continent. But with their onboard provisions running out, Dias was forced to erect his final marker pillar and turn back home. It was only on the way back that he actually caught sight of what he believed to be the tip of the continent. He named it the Cape of Storms, reflecting the turbulent conditions at the junction of the Atlantic and Indian Oceans. On Dias’s return home King João II renamed it the Cape of Good Hope, so as not to discourage the next waves of explorers.19 fn7
Dias’ voyage would change the course of history. First, he had confirmed that the classical geographer Ptolemy was wrong and that there was an end to Africa; thus a sea-route from Europe to the riches of the Indian Ocean circumventing the Islamic world was eminently plausible. But second, and just as importantly, he had discovered the band of westerly winds in the South Atlantic that can reliably carry mariners round the tip of the continent.21 Rather than hugging the African coastline and battling against northerly currents after crossing the equator, the solution is instead to steer a wide looping course into the mid Atlantic. The same volta do mar trick developed for returning from the Canary Islands in the North Atlantic also works in the South Atlantic – the wind bands in the Northern and Southern hemispheres are mirror images of each other. This gave European navigators the first inkling of the grand-scale patterns of circulation in the planet’s oceans and atmosphere, which they soon came to understand more deeply and began to exploit.
A NEW WORLD
While the Portuguese had been finding a route around the southern tip of Africa, a Genoese navigator was trying to raise support for a voyage sailing in the opposite direction: he believed he could reach the orient by sailing west. He finally found patronage from Queen Isabella of Castile, who in 1469 had married King Ferdinand II of Aragon to unify their realms and form Spain. He was known to his sponsors as Cristóbal Colón. In English, we call him Christopher Columbus.
Contrary to a commonly held view today, no educated person in medieval times believed the Earth to be flat. In the third century BC Eratosthenes, a Greek geographer, astronomer and mathematician working at the Library of Alexandria, understood that the world is a sphere and calculated its circumference to be 250,000 stadia, or around 44,000 kilometres – remarkably close to its real value. Indeed, the techniques of celestial navigation used by sailors to plot their latitude by the stars is predicated on the very principle that the Earth is round. Columbus was also not the first person to propose that India could be reached by sailing west from Europe: the Roman geographer Strabo had suggested the same in the first century AD. And there was evidence that something did lie over the watery horizon. Reports from the Atlantic islands described flotsam drifting in from the west: unfamiliar trees, canoes and the bodies of people neither European nor African in appearance.22
In order to secure financial support for his expedition, Columbus had to convince potential sponsors that the proposed voyage was achievable. But how do you estimate the distance travelling west from the edge of Europe to China or India before you’ve accomplished such a journey? The solution was to start with the calculated circumference of the world and then subtract the overland distance from Europe to the Orient – the approximate width of Eurasia was known from travellers along the Silk Road. The problem was that these calculations yielded a westward oceanic distance of around 19,000 kilometres, or about four months of sailing with reliable winds. Such a journey was utterly impossible at the time. Ships simply couldn’t carry enough food and clean water to keep their crews alive for this long on the open seas without making landfall for fresh supplies.
Not to be deterred, Columbus pulled the sort of sleight of hand used by any intransigent believer absorbed in the strength of their own convictions. He fudged the numbers. Columbus took the lowest calculation of the Earth’s circumference available at the time, along with the greatest estimate of the breadth of Eurasia, and arrived at a significantly shortened sea distance to the west. He used the measurements of Paolo dal Pozzo Toscanelli, a Florentine mathematician and map-maker, who not only grossly underestimated the circumference of the globe, making the world a third smaller than it really is, but also believed that Japan lay 2,400 kilometres east of China, thus offering a chance to break up the long sea voyage. Columbus argued that he would be making landfall in the islands off Japan after travelling only 3,900 kilometres from the Canary Islands. This would amount to just a month’s sailing time. Indeed, Columbus claimed that the Orient lay not far over the horizon from the position of the Azores.23 He never considered the possibility of an unknown continent lying in the way: by his calculations there simply wasn’t room for one in the western seas.
The Portuguese refused to sponsor the venture, however. King João II’s advisors regarded Columbus’ numbers as a dangerous underestimate and the proposition foolhardy. And in any case, Bartolomeu Dias had just successfully rounded the Cape of Good Hope and shown Portugal the open gate to the Indian Ocean via their African route. The Genoese, Venetians and English also declined to get involved; but at last Columbus’ repeated lobbying of the Spanish court bore fruit. Queen Isabella was advised that although the proposition might be high-risk, it also offered the possibility of enormous gains. And this is where a degree of blind historical luck played to Columbus’ fortunes.
The Treaty of Alcáçovas in 1479, which ended the War of the Castilian Succession, had handed the Canary Islands to Castile while the Portuguese kept possession of Madeira, the Azores and the Cape Verde Islands. This agreement worked clearly in favour of the Portuguese in the Atlantic, with Castilian ships forbidden to sail to these archipelagoes; indeed, the Portuguese were given exclusive rights over any lands that had been or would be discovered south of the Canaries. If Castile wanted to pursue its own territorial and trading interests the captains would have to head west. And it just so happens that the Canaries offered the ideal starting point for ships sailing in that direction across the Atlantic.
Had Columbus’ proposal been accepted by King João II he would have embarked on his audacious westward voyage from the Azores. Lying as they do about 850 kilometres further west of Madeira and the Canaries, we now know that these islands are situated about a third of the way from the edge of Europe to the Amer
ican coast. But the Azores also lie further north than the other Atlantic islands, and at this latitude the prevailing winds blow towards the east – unfavourable for any Atlantic crossing. The Canaries, however, lie within the zone of the northeasterly trade winds that blow all the way to the Caribbean. By sheer historical fluke, Isabella’s support – and the Treaty of Alcáçovas – meant that Columbus attempted his crossing from an archipelago that happens to be upwind of the Americas. If his expedition had set sail from the Azores it would likely have perished deep in the ocean.24
On 3 August 1492 Columbus’ three ships slipped their moorings at the port of Palos de la Frontera and sailed southwest to the Canary Islands. Here Columbus restocked his provisions, made some repairs, and then turned the prow of his ships in the direction of the sunset. Carried by the easterly trade winds over the featureless expanse of the Atlantic Ocean, they made landfall five weeks later in the Bahamas.fn8 Columbus then continued further southwest to explore the coastlines of Cuba and Hispaniola. Here he heard of a people inhabiting the Lesser Antilles island arc whom the Spanish named cariba or caniba, from which we get our words Caribbean and cannibal.fn9
After four months of exploring these islands, Columbus was ready to return home to receive his expected riches and glory. But how to travel back from a place that has never before been reached by sea? Columbus first tried to simply sail back the way he had come, but he soon realised that his ships would have a hard time beating a course upwind into the same easterlies that had borne them on the outward journey, and that they risked running out of provisions before reaching land. He decided to turn north instead, and in the middle latitudes, he picked up the same band of westerly winds that blow past the Azores and which carried him back to Europe. Therefore Columbus’ expedition would have been impossible without the knowledge gained by Portuguese sailors that the prevailing winds blow in opposite directions in neighbouring bands of latitude, won through their methodical efforts to push down the African coast for decades before Columbus had even been born.25 Crossing the Atlantic in midwinter exposed the weary sailors to ferocious storms, but after a month’s sailing Columbus’ ships arrived safely in the Azores,26 and from there they returned to Spain.
Columbus made a total of four voyages west, mapping out the chains of tropical islands in the Caribbean, but it wasn’t until the third expedition that he actually set foot on the American mainland, in what is today Venezuela. Yet to the end of his life, Columbus still maintained that he had reached the Orient.27
By the early 1500s, scores of tropical islands had been mapped by European sailors, as well as the long South American coastline continuing past the equator with its big rivers, suggesting that they drained an expansive area inland. Other explorers also reported large land masses far to the north. Alarmed by Spain’s supposed new route to Asia along the latitude of the Canary Islands, England’s king Henry VII sent the Venetian navigator Giovanni Caboto (or John Cabot) on an expedition to find an alternative route through the North Atlantic, which reached Newfoundland.
It became clear that Columbus had not reached the Orient – but what exactly had been discovered? It began to dawn on Europeans that perhaps the lands to the west were all one continuous coastline, that they had stumbled across not a series of new islands, but an entire continent – a whole New World.
THE GLOBAL WIND MACHINE
The Portuguese had spent the best part of a century inching their way down the coast of Africa before they finally found its southern tip and the gateway into the Indian Ocean. Now, within a generation of the discovery of the Americas in 1492, European sailors ventured across all the world’s oceans and completed the first circumnavigation of the Earth. This was a revolution that heralded the birth of today’s global economy.
All this was only possible because mariners had come to understand the patterns of reliable winds and currents around the globe, which now determined the trade routes that brought great riches to Europe. But what causes these alternating bands of prevailing winds around the world, which in turn drive the great wheeling currents in the oceans?
The warmest part of the Earth is the equator, which receives the most direct sunlight over the year. The air near the equatorial surface heats up and rises, but as it ascends it cools, the moisture condenses into clouds and then falls as rain. At high altitude, the cooling air mass diverges and splits to the north and south, like a T-junction high in the atmosphere. Each of these arms travels about 3,000 kilometres before descending to the ground again, now very dry, at around 30° latitude – roughly one-third of the way between the equator and the pole – in both the Northern and Southern hemispheres. These two bands around the Earth are called the subtropical highs because the air crushing down here creates slightly higher pressure. The warm air rising from the equator, on the other hand, leaves behind a low pressure region.
From the subtropical highs at 30° latitude the air then travels back towards the equator as surface winds to complete this great vertical circuit. This zone of reliable winds, which was so important for the European crossing to the Americas, is a manifestation of the same atmospheric circulation pattern that produces the world’s great bands of tropical rainforests and mid-latitude deserts which we discussed in the last chapter. These two immense atmospheric rolling patterns, convection currents just like those around your radiator at home, are known as Hadley cells, and they operate like paired cogs, separated by the equator and rotating in opposite directions. The movement of the Hadley cells, driven by equatorial warming, is a great heat engine – no different in principle from a steam engine or the internal combustion engine in your car, albeit one with a power rating of about 200 million million watts – ten times greater than the entire power use of global human civilisation today.28
But there’s another important aspect of our planet that influences the winds on Earth. Our planet, and its atmosphere, is rotating. Because the Earth is a solid sphere, this means that the surface at the equator is moving faster than that at higher latitudes. And so as the air returns from the subtropical highs towards the equator the ground beneath turns eastwards faster and faster. There’s a small amount of friction between ground and atmosphere that starts to drag the air along with the surface, but the air is unable to pick up speed sideways fast enough as it moves, and so the winds blowing towards the equator get left behind by the rotating surface. The upshot is that they are effectively deflected in a smoothly curving path towards the west. This is known as the Coriolis effect, and it influences anything moving over the surface of a rotating sphere, for instance the trajectory of ballistic missiles. Or to put it another way, if you imagine yourself rocking about on a ship in tropical waters, the prevailing winds appear to blow from the east, but a more accurate description would be that you and the Earth’s surface are spinning quickly through the atmosphere, and the easterlies are like the wind in your hair when you’re driving fast in your car with the top down.
Any winds blowing in the Northern Hemisphere are deflected by the Coriolis effect to their right, and those in the Southern to their left. Thus between the latitude of 30° north and the equator the prevailing winds follow a curved path towards the southwest, and so by the nomenclature for winds are called northeasterlies. And the same is true in the Southern Hemisphere: the air returning north along the surface to the equator is again deflected towards the west to produce prevailing southeasterly winds. These easterlies are known as the trade winds, and as reliable winds blowing through the tropics they have been absolutely crucial for mariners.fn10
The band where the returning northeasterly and southeasterly trade winds meet each other around the equator is called by modern atmospheric scientists the Intertropical Convergence Zone (ITCZ). But to sailors it’s known as the doldrums. This is the region of low-pressure air, characterised by light winds or periods of dead calm, that was first encountered by Portuguese sailors crossing the equator on their journey down the African coast in the late fifteenth century. The region can prove disast
rous for ships as they wait for the winds to pick up again or an ocean current to carry them out. They can find themselves becalmed and stuck in the doldrums for weeks, and in this equatorial region of hot and muggy climate it may mean not only a delay in delivering your cargo back to port but it can also spell death as your onboard fresh water supplies run out. Samuel Taylor Coleridge evoked the desperation of sailors becalmed in the Pacific doldrums in ‘The Rime of the Ancient Mariner’:
Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.
The location of the ITCZ is determined by rising air warmed by the sun, and so it shifts north and south of the geometric line of the equator with the seasons. And because land warms up more quickly than the ocean in summer, the ITCZ band is pulled further away from the equator by the continents. It therefore follows a decidedly sinuous, snakelike path around the waistline of the world. This makes the exact location and width of the ITCZ hard to predict, and increases the risk of sailors getting caught out in the doldrums.
Origins Page 21