The view from outside our hotel room on the Isla del Sol.
We entered Bolivia in the remote south-west and crossed the Salar de Uyuni; we left by the far northern route, crossing Lake Titicaca.
The Salar is the largest salt lake in the world. Not to be outdone, Titicaca is the highest navigable body of water in the world, one of the world’s deepest lakes, and the largest freshwater lake in South America.
Even now during the Bolivian winter the temperature never drops below zero at night, and during the day it was a very pleasant 18°C . The Isla del Sol - supposed birthplace of the founders of the Inca Empire and so the most famous of the 70-odd islands scattered across the lake - is green and fertile. Foreign backpackers and Bolivian daytrippers from La Paz sip coca tea and watch the sun go down from one of the many terraces built a thousand years ago, long before the Incas, to irrigate the soil.
Yet Titicaca and the Isla del Sol are two or three hundred meters higher than the Salar and its bizarre Isla Pesca, pictured at the bottom of my previous post.
In this satellite image of Bolivia that I robbed from NASA, Titicaca is the blue patch at the top and the Salar de Uyuni is the white patch at the bottom. There is a smaller salt lake above the Salar de Uyuni called Coipasa, and the light grey area above and to the right of them is the very salty Lake Poopó.
To give a sense of the scale in this picture, the Salar de Uyuni is over 10,000 square kilometers in area - about the same size as Lebanon. Neil Armstrong could see it shining from the moon.
The western half of Bolivia is dominated by two parallel ranges of the Andes. Between them lies a depression called the altiplano, the high plain where we spent almost all of our time. This corridor of land, larger in area than Ireland, slopes gently downwards from Titicaca in the north to the Salar in the south.
12,000 years ago, as the last Ice Age came to an end, the altiplano was entirely covered by a vast inland sea that geologists call Lake Ballivián. (This lake in turn may originally have been part of the Pacific Ocean, cut off by the great forces that pushed and folded the Andes into existence.) Glacial meltwaters emptying into the northern end of Ballivián gradually diluted it, carrying the salt south; rising temperatures caused it to evaporate. As the water level fell, two new, smaller lakes were formed: Titicaca in the north, and Lake Minchin in the south, connected by rivers. Finally, Minchin evaporated completely, leaving the shallow, salty Poopó - into which Titicaca still drains via the River Desaguadero - and the 10 billion tons of salt we call the Salar de Uyuni.
This is a story of climate change, and not just steady global warming following the end of the Ice Age. The desert coastal regions to the west of the altiplano get less than 100 mm of rain per annum, the Amazon rainforests to the east more than 1,500 mm; slight changes in those rates of precipitation and in the rate of evaporation have significant effects on the levels of the lakes. But the lakes themselves have a profound effect on climate. Titicaca is so deep that its temperature is near constant, and it regulates the climate of the surrounding area - which is why the Isla del Sol is fertile and never freezes and the Salar de Uyuni is a barren place where temperatures can fall to -30 °C on the same night, the question that bugged me in the first place. So climate change even in this relatively small part of the world can be extremely complex. Yet it can have drastic effects on human life. Changes in the level of Titicaca were partly or wholly responsible for the collapse of the Tiwanaku civilization, two hundred years before the Incas.
The Kalasasaya temple at Tiwanaku, already a ruin when the Spanish arrived. It used to be on the shores of Lake Titicaca, but the lake is now 20 kilometers away.
I don´t understand why some people think that science undermines our sense of awe and wonder. I don´t know of any story of gods or monsters throwing up mountains or gouging out lakes that is more extraordinary than these theories, or that carries a more potent moral - for those who take climate change for granted today.