At first, let me tell you a story of our closest neighbour in the solar system 'Venus'. On Venus, there were oceans and land, just like ours, here on Earth. Yes, there were occasional natural catastrophes, massive volcanic eruptions and once in a while, a meteorite would come and hit the surface to do some damage. But, for the first billion years or so, it would have seemed like a paradise. But, now, the planet Venus, which once might have seemed like a heaven, turned into a kind of hell. Venus' oceans are long gone. The surface is so hot that it can melt lead.
You might think it's because Venus is 30 per cent closer to the Sun than is the Earth, but that's not the reason. Let's look at her atmosphere; Venus is completely covered with clouds of sulfuric acid that keeps almost all the sunlight from reaching the surface. But, it should make this planet's atmosphere colder than the Earth. Then, why is the planet so hot? It's because the small amount of sunlight that trickles in through the clouds to reach the surface cannot get back out again. The flow of energy is blocked by the dense atmosphere of carbon dioxide (CO2). This CO2 absorbs all the energy as it makes a smothering blanket to keep the heat in, which is called the 'Greenhouse Effect'. And it became so hot that the oceans vaporised all its water long ago. There was no intelligent life but burning coal or driving gas-guzzlers on Venus. Nature can destroy an environment without any help of intelligent life. Venus is in the grip of a runaway greenhouse effect.
Our planet Earth and Venus started out with about the same amount of carbon, but the two planets followed completely two different paths and carbon was the decisive factor in both stories. On Venus, it's almost all in the form of gas - carbon dioxide, in the atmosphere. But, most of the carbon on Earth has been stored for eons in solid vaults of carbonate rock.
We see these carbonated rocks as a part of a chain in form of white hills and cliffs all around the world like the hills of Birishiri in Bangladesh and the celebrated White Cliffs of Denver, right on the English Chanel. But, who built these wonders?
Volcanoes supply carbon dioxide to the atmosphere, and the oceans slowly absorb it. Working over the course of millions of years, the microscopic algae harvested the carbon dioxide and turned into tiny shells and they accumulated in thick deposits of chalk or limestone on the ocean floor. Later, the restless Earth pushed up the seafloor and carved these massive cliffs. Other marine creatures took in carbon dioxide to build enormous coral reefs. And the oceans converted dissolved CO2 into limestone even without any help from life. As a result, only a trace amount of CO2 was left as gas in Earth's atmosphere.
Earth's atmosphere holds fewer than three molecules of CO2 out of every ten thousand of other particles. And yet, it makes the critical difference between a barren wasteland and a garden of life on Earth. With no carbon dioxide, the earth would be frozen and it would look like a giant snowball. Again, with twice as much CO2, we are still talking about only six molecules out of ten thousand. Even this tiny change can have drastic effects on Earth. Things would get uncomfortably hot.
But, the Earth is not a barren, forsaken land, not yet. In stunning contrast, the Earth is alive. It breathes, but very slowly. A single breath takes a whole year. The forests contain most of the Earth's life, and most forests are in Northern Hemisphere. When spring comes to the north, the forests inhale carbon dioxide from air and grow, turning the land green. The amount of carbon dioxide goes down. When fall comes and the plants drop their leaves, they decay, exhaling the carbon dioxide into the atmosphere. The same thing happens in the Southern Hemisphere at the opposite time of the year. But, the Southern hemisphere is mostly ocean. So it is the forests in the north that controls the annual changes in the global CO2. The Earth has been breathing like this for tens of millions of years. But nobody noticed until 1958 when an oceanographer named Charles David Keeling devised a way to accurately measure the amount of carbon dioxide. Mr Keeling discovered the Earth's exquisite respiration.
But he also discovered something shocking - a rapid rise, unprecedented in human history, in overall level of CO2, one that has continued ever since. It is a great leap from the carbon dioxide levels that prevailed during the rise of agriculture and civilisation. In fact, the Earth has seen nothing like it for three million years.
How can we be so sure? The evidence is written in water.
Ancient air bubbles are trapped inside Ice layers of Antarctica and in the Arctic. It gives us the unbroken record of Earth's atmosphere back over the last 80,000 years. But the amount of carbon dioxide in the air never rose above three hundredth of one per cent. But, from the 20th century the amount of carbon dioxide is steadily and rapidly increasing. It is now 40 per cent higher before the Industrial Revolution.
Now, let's go back about 350 million years ago when the dragonflies looked like giant eagles and millipedes looked like alligators. The dinosaurs were still more than 100 million years in the future. There were no birds and no flowers. It was a world of trees and insects. Why are they so big? Because, the air was different. The atmosphere had more oxygen than at any other time in Earth's history, before or since. This allowed insects to grow much larger than they are today. Trees evolved a way to defy gravity in their competition for sunlight. A plant molecule 'Lignin' made trees possible to be both strong and flexible. But lignin had a downside: it was hard to swallow. The fungi and bacteria could not digest lignin and the termites would not evolve for at least another 100 million years. When trees died they fell down and got buried by mud that built up over eons.
Eventually, there were hundreds of billions of trees entombed in the Earth, buried forests all over the Earth. These trees were buried before they could decay that means they took the carbon and stored solar energy with them and left the oxygen behind to build up the atmosphere. That's what happened around 300 million years ago. That's how the bug got so big. And, the buried carbon became coal which we use now to run our factories. We also use gas and oil. When sponges, green algae animals too small to see in the water died, they sank to the bottom and were buried in the silt. Over millions of year, their remains were converted into oil and gas.r (The last part will appear tomorrow)
Engr Mohammad Towsif Hossain is Lecturer, Department of Civil Engineering, Stamford University Bangladesh. Email: [email protected]