The Invention of Nature

On 7 February 1800, Humboldt, Bonpland and José, their servant from Cumaná, departed from Caracas on four mules, leaving behind most of their luggage and collections. To reach the Orinoco, they would have to head south on an almost exactly straight line through the huge emptiness of the Llanos – vast plains the size of France. The plan was to go to the Rio Apure, a tributary of the Orinoco about 200 miles to the south of Caracas. There they would procure a boat and provisions for their expedition at San Fernando de Apure, a Capuchin mission. First, though, they would go west, on a 100-mile detour to see the lush valleys of Aragua, one of the wealthiest agricultural regions in the colonies.

With the rainy season over, it was hot and much of the land through which they rode was arid. They crossed mountains and valleys, and after seven exhausting days, they finally saw the ‘smiling valleys of Aragua’. Stretching west were endless neat rows of corn, sugarcane and indigo. In between they could see small groves of trees, little villages, farmhouses and gardens. The farms were connected by paths lined with flowering shrubs and the houses shaded by large trees – tall ceibas clothed in thick yellow blossoms with their branches plaited into the flamboyant orange blooms of coral trees.

In the midst of the valley and surrounded by mountains was Lake Valencia. About a dozen rocky islands dotted the lake, some large enough to pasture goats and to farm. At sunset thousands of herons, flamingos and wild ducks brought the sky alive as they flew across the lake to roost on the islands. It looked idyllic but, as the locals told Humboldt, the lake’s water levels were falling rapidly. Vast swathes of land that only two decades earlier had been under water were now densely cultivated fields. What had once been islands were now hillocks on dry land as the shoreline continued to recede. Lake Valencia also had a unique ecosystem: with no outflow to the ocean and only small brooks running in, its water levels were regulated by evaporation alone. The locals believed that an underground outlet drained the lake, but Humboldt had other ideas.

Lake Valencia in the Aragua Valley (Illustration Credit 4.3)

He measured, examined and questioned. When he found fine sands on the higher levels of the islands, he realized that they had once been submerged. He also compared the annual average evaporation of rivers and lakes across the world, from southern France to the West Indies. As he investigated, he concluded that the clearing of the surrounding forests, as well as the diversion of water for irrigation, had caused the falling water levels. As agriculture had flourished in the valley, planters had drained and diverted some of the brooks that fed into the lake to irrigate their fields. They had felled trees to clear land, and with it the forest’s undergrowth – moss, brushwood and root systems – had disappeared, leaving the soils beneath exposed to the elements and incapable of water retention. Just outside Cumaná, locals had already told him that the dryness of the land had increased in tandem with the clearing of ancient groves. And on the way from Caracas to the Aragua Valley, Humboldt had noted the dry soils and bemoaned that the first colonists had ‘imprudently destroyed the forest’. As the soils had become depleted and fields had yielded less, the planters had moved west along a path of destruction. ‘Forest very decimated,’ Humboldt scribbled in his diary.

Just a few decades previously, the mountains and foothills that surrounded the Aragua Valley and Lake Valencia had been forested. Now, with the trees felled, heavy rains had washed away the soil. All this was ‘closely connected’, Humboldt concluded, because in the past the forests had shielded the soil from the sun and thereby diminished the evaporation of the moisture.

It was here, at Lake Valencia, that Humboldt developed his idea of human-induced climate change. When he published his observations, he left no doubt what he thought:

When forests are destroyed, as they are everywhere in America by the European planters, with an imprudent precipitation, the springs are entirely dried up, or become less abundant. The beds of the rivers, remaining dry during a part of the year, are converted into torrents, whenever great rains fall on the heights. The sward and moss disappearing with the brush-wood from the sides of the mountains, the waters falling in rain are no longer impeded in their course: and instead of slowly augmenting the level of the rivers by progressive filtrations, they furrow during heavy showers the sides of the hills, bear down the loosened soil, and form those sudden inundations, that devastate the country.