Last week, an item in the Guardian (UK) highlighted a surprising connection between the Amazon rainforest and the Sahara desert — how one region in Chad supplies the Amazon with half of the rainforest’s mineral nutrients.
Around 40m tons of dust is carried by prevailing winds from the Sahara to fertilise the Amazon basin each year. This is a very satisfying finding, since the extraordinary fertility of the Amazon rainforest – one of the richest and most biodiverse places on earth – has been a puzzle. Tropical rains leach nutrients from jungle soils, and the soils of the Amazon forest are notoriously poor, which is why clearance for cattle farming is such a bad idea. Biologists had calculated that the forest needed at least 50m tons of fresh mineral nutrient each year to keep its trees tall and in leaf. In 2006 an international team of researchers established that at least half of this annual mineral supply is quarried from one tiny location in the Sahara, the Bodélé depression in Chad. A combination of fortuitously placed mountain ranges that flank a basin of diatomite sands so focus the winter winds as to scour the depression and lift from it an average of 700,000 tons of dust each day, and air-freight it across the Atlantic.
So for thousands of years, and without any fuss, a tiny part of one of Africa’s poorest countries has annually subsidised the growth economy of one of the world’s most richly endowed. This discovery is yet another insight into the intricate dance performed by earth, air, fire and water in the service of life; and another reminder of the enduring intercontinental interdependence that sustains human civilisation. We should respect the IUCN‘s [International Union for Conservation of Nature] concern for the deserts. Without green things, we could not breathe. Without deserts, there might be no forests.
According to the Climate Matters blog of Columbia University’s Earth Institute, scientists from a wide range of disciplines convened earlier this summer to coordinate their research on dust, much in the way that the oceanographic community came together in the 1990s to collaborate more effectively. Why dust?
Dust is a global phenomenon. Where it comes from, where it goes, and how it impacts climate and the biogeochemistry of land and oceans are questions that span all the realms of earth science.
Dust influences the radiative balance of the planet in two different ways, either directly by scattering and absorbing incoming solar radiation, or indirectly by changing the optical properties of clouds, themselves an important player in the climate system. Dust also contains iron, a limiting nutrient in many areas of the ocean, so when dust falls onto the ocean, it can act as a fertilizer for the growth of algae, or phytoplankton, which uses CO2. Dust not only affects climate, but also is influenced by it: its production, atmospheric transport and deposition are sensitive to climatic conditions.
During Earth’s history, dust has been strongly linked with climatic conditions: Ice cores and marine sediments tell us that the ice age world was much dustier than today’s world. Thus dust is both a driver and a passive recorder of climate change under different climatic regimes of the Earth’s past. However, its exact role in past climate change remains poorly constrained. Understanding the links between dust and climate in the past will be crucial to evaluate the future impacts of dust on the Earth’s climate system in a warming world.
One could add a recent study (here and here) that suggests soot may be the second greatest contributor to global warming — behind carbon dioxide, but ahead of methane. Produced by the combustion of fossil fuels and borne around the globe by winds, soot helps to accelerate the melting of glaciers and Arctic ice by absorbing more of the sun’s radiation. Controlling soot may be the most effective short-term measure to slow warming.
On this ceaseless swirl of land, air, and water, everything is in motion. The movement patterns are part of the relative stability of the climate system around which life has evolved and upon which we rely. Climate change’s uncertain impacts on the atmospheric and oceanic flow and upon the well-timed relationships between plants and animals naturally raise great concern. Wind and water also transport pollutants across state and national boundaries and even from continent to continent. Whether they involve global warming or specific pollutants, these vast connections make all life “downstream.”