Carbon Dioxide: The Villain of Earth?

Our air is composed of 99% nitrogen and oxygen (78% and 21%, respectively). Most of the remaining 1% is an inert gas called argon. Among the trace gases, one worth mentioning is carbon dioxide. However, its amount is so small in the air (only 0.038%) that it should barely have been considered significant. Yet, interestingly, carbon dioxide has not only drawn our attention—it has become one of the most talked-about gases in the modern world.

As the name suggests, carbon dioxide contains one carbon atom and two oxygen atoms. It is a colorless and odorless gas. We exhale carbon dioxide with every breath. Though the small amount present in the air isn't harmful, it can become toxic if it increases significantly. Being heavier than air, it tends to settle in unused wells or low-lying pits, where it has suffocated unaware people who climbed down and couldn’t breathe.

This isn’t just a rare occurrence—there have been mass casualties due to carbon dioxide. In Africa, there are three lakes with a unique structure. Beneath them lie volcanic vents, continuously releasing carbon dioxide, which accumulates under the lake. Occasionally, due to natural causes, a massive amount of carbon dioxide erupts from below the lake. As mentioned, carbon dioxide is heavier than air—so just as water floods lower lands, this gas rolls over nearby lowland areas, displacing the air. People suffocate to death without warning.

A witness to such an incident was driving a truck. Suddenly, the engine stopped—since engines also need oxygen, and the carbon dioxide had displaced the oxygen, the engine stalled. When the driver stepped down to inspect, he collapsed instantly. Two people sitting on the truck survived because they hadn’t stepped into the gas cloud. Like floodwater halts at a certain height, the carbon dioxide plume stopped at a specific level. Everyone within that range died from suffocation; those above it survived. The deadliest such "carbon dioxide flood" occurred in Lake Nyos, Africa, in 1984—over a thousand people and countless animals, insects, and pests perished.

Carbon dioxide can easily be converted into solid form at -80°C, known as dry ice. Unlike regular ice that melts into liquid first, dry ice sublimates—turning directly from solid to gas.

Carbon dioxide also plays a vital role in sustaining life on Earth. Green plants absorb carbon dioxide from the air, water from the soil, and energy from sunlight to make food through a process called photosynthesis. As a byproduct, they release oxygen, which is essential for us. This is why forests and plants are so crucial for our survival.

Though carbon dioxide is present in minute amounts in the air, its level isn't constant—it fluctuates. It also dissolves in water in small quantities. The oceans hold about 50 times more dissolved carbon dioxide than the atmosphere. How much it dissolves depends on the water temperature—cooler water can hold more CO₂. We see this in soft drinks—cold ones hold more fizz (CO₂), while warm ones release bubbles.

Carbon dioxide is a greenhouse gas. Just as a greenhouse traps heat, CO₂ traps heat in our atmosphere. Sunlight penetrates the atmosphere and heats the Earth’s surface, which then radiates some of that energy back as infrared radiation. CO₂ cannot absorb visible light but can absorb infrared radiation, preventing it from escaping back into space. This trapped heat warms the atmosphere and maintains a balance in Earth's temperature between day and night. Without it, days would be unbearably hot, and nights freezing cold.

Nature has created many such mechanisms over millions of years to make Earth habitable. Unfortunately, in the 20th century, humans began disrupting this delicate balance. Since carbon dioxide is a greenhouse gas, an increase in its concentration means a rise in temperature. A warmer planet leads to melting of polar ice caps, raising sea levels and potentially submerging low-lying regions. If CO₂ levels continue rising at the current rate, a third of Bangladesh could either be submerged or become too saline for agriculture.

This rise in Earth's temperature due to CO₂ is known as global warming. Scientists have long warned about it. Naturally, CO₂ is released from sources like volcanoes, which contribute about 40% of atmospheric CO₂. But nature also has mechanisms to absorb it—such as oceans and forests. However, human activities like running factories, cars, ships, and airplanes continuously pump more carbon dioxide into the air. Today, scientists have concrete evidence that CO₂ levels are rising steadily, pushing the planet toward a climate crisis. Initially, industrialists and factory owners denied it, but eventually, even the United Nations issued a formal warning—and the world acknowledged the threat. It is now accepted that urgent action is needed.

While humans have recklessly damaged nature and the environment, they are now also working to heal it. A global “green movement” is underway, aiming to reduce carbon dioxide levels. CO₂ is now often simply referred to as "carbon", and a new generation is fighting to save the planet.

Carbon dioxide was once a harmless, essential gas. Somehow, it has now become Earth’s greatest villain.