The making of the Thar desert

As we drove through the Thar desert, I wondered about the mysterious landscape around me. India receives so much rainfall from the monsoon that it is hard to believe that it contains a desert. The popular belief that the monsoon winds are “depleted” of moisture by the time they reach the desert is false. The distance to the sea is rather small, and the air is generally moist. In just three winter months, when the rest of India is dry, the amount of water deposited by dew in the desert is between 30% and 40% of the total rainfall. Something prevents the western branch of the monsoon from penetrating into Gujarat and Rajasthan.

It is believed that this is the immense high-pressure anti-cyclone that exists over the Sahara and West-Asian desert. In fact, the claim is that the monsoon winds prevent the spread of this pattern to the rest of India. Evidence for competition between the two systems is what happens when a third factor occasionally intrudes. In the infrequent episodes when a Western disturbance hits India during the monsoon, causing immediate heavy rainfall and flooding over a wide area of the desert. This means that the monsoon winds are not depleted of moisture in this region, but are obstructed from flowing by the anti-cyclone. If this conjecture is true, then a weakening of the monsoon due to climate change could allow the anti-cyclone to extend across India, making it an arid habitat.

Our jeep had long wandered off the road. The flat landscape that I’d grown used to gradually gave way to undulations. There was still a strong wind blowing dust and sand into my face, so I had my mask on. But when I looked around, the ground had changed. Until a minute ago we were driving through hard packed sand (see the featured photo). Now, all around us I could see something different poking through the sand. The photo above shows the surface. The broken flat stones are made of a material called calcrete. This is formed by the action of water and atmospheric carbon dioxide on wet dust and sand (the word calcrete seems to be a portmanteau of calcium and concrete). The presence of calcrete means that wet lands turned to desert at some time in the past. Dating of calcrete in the Thar desert shows repeated cycles of wetness and desert starting from 1.5 million (15 lakh) years ago to 250,000 years ago.

I walked across this landscape in search of a wheatear, but my attention was on the landscape. A short walk through sand brought me to something that looked like a dry riverbed into which sand had trickled: calcrete was overlain with sand, as you can see in the photo above. Aerial and satellite photography shows evidence for a well-organized drainage system of interlinked rivers in the far past, disrupted by climate change. Successive surveys and dating showed brief re-emergence of local drainage systems over a period of a million years, which were disrupted within a few tens of thousands of years.

The wheatear perched briefly on a bush and I managed to take a photo. After it had flown off, I looked at the land behind the bush (photo above) and it became obvious that it had been shaped by flowing water. The surface was covered in calcrete, which it why it had not been reshaped by the wind. Studies of pollen around Didwana lake in the eastern part of the desert have established that about 6500 years ago the area was wet enough to support a large variety of grasses and flowering herbs and shrubs, of the kind which are usually pollinated by butterflies. This dried up around 4000 years ago. When these studies were first published, there was speculation in the press about the connection with the mythical river called Saraswati. More studies revealed a fluctuating boundary of the desert over human history and pre-history. Palaeolithic tools and sites have been found buried intact in the sand, showing that the desert shifts fairly abruptly. The gradual abandonment of Harappan cities could also have been partly due to such climate fluctuations.

The bird gone, I climbed a little bluff to join The Family, and saw immediately clear evidence that I had walked across the bottom of an ancient river. The photo above makes it very clear. How old was this? I’m sure it has been mapped out. The geological and climatic history of the Thar desert has been examined in great detail in the last 60 years by a large number of scientists working in India. There is a group in IIT Mumbai who examined the age of sand dunes, and found some as old as 160,000 years, and others as young as 11,000 years. In several of the photos above, you can see distant dunes. It gives me shivers to realize that they were being laid down at the same time as glaciers advanced across the north of Europe, Asia and the Americas. As water got locked up in ice, this part of the world turned dry.

Geology and climate sculpt the land over periods longer than our lives, even longer than our civilizations. As a result, it has been hard to see the forces which shape our societies. Walking across that river bed I thought that I had a tiny view of these forces. In the past our actions have been too weak to shape the planet. No longer. We need to understand the Thar desert to plan a way ahead as the climate changes.