Human Alteration of the Carbon Cycle and Global Warming

A Cycle in Balance, Until We Altered It

For thousands of years, the carbon that plants took from the air through photosynthesis was almost equal to the carbon that returned through respiration, decomposition, and other processes. The CO₂ in the atmosphere stayed fairly stable.

Sinks are places that store more carbon than they release: forests, jungles, mangroves, soils, and oceans. Sources are the ones that release it. Today, human activities add a very large extra source.

Nitrogen also moves in a cycle, and people alter it. In the soil, certain bacteria carry out the fixation of nitrogen from the air (N₂) and turn it into compounds that plants can use; nitrification transforms other compounds into nitrates, which roots absorb; and denitrification returns nitrogen to the air. With too much fertilizer, nitrous oxide (N₂O) is released, another greenhouse gas. In the milpa, beans live alongside nitrogen-fixing bacteria and corn makes use of that nitrogen: this way less fertilizer is needed.

Fossil Fuels: Carbon Stored for Millions of Years

Oil, coal, and natural gas formed millions of years ago from the remains of buried living things. Their carbon was stored underground, outside the fast cycle.

When we burn gasoline in a car, LP gas in a stove, or coal in a power plant, that carbon combines with oxygen and is released as CO₂. In a few minutes, carbon that took millions of years to accumulate returns to the air.

Deforestation: Losing Sinks and Releasing Carbon

Trees store carbon in their wood. When they are cut down and burned, or when they rot, that carbon returns to the air as CO₂. In addition, once the forest or jungle disappears, there is nothing left to absorb it through photosynthesis.

That is why deforestation has a double effect: it releases carbon and removes a sink. It happens, for example, when jungle is cleared for pastures or crops.

The Greenhouse Effect and Global Warming

The Earth receives light from the Sun and warms up. Gases like $\text{CO}_2$ and methane (CH₄) let light pass through, but they trap part of the heat that the Earth sends back toward space. It is like a blanket: without it, the planet would be too cold for life.

The greenhouse effect is natural and necessary. The problem is that, with more CO₂, the "blanket" gets thicker and the average temperature rises: that is global warming.

Is Planting Trees Enough?

Forests are sinks: while they grow, they store carbon. But they are temporary sinks. When a tree dies, burns, or rots, part of its carbon returns to the atmosphere. In addition, a young tree absorbs little and takes decades to store a lot.

Reducing emissions without caring for forests is not enough either: we would keep losing sinks. The most effective approach is to combine actions:

• Reduce emissions: use public transportation (Metro, Metrobús), ride a bicycle, save electricity, and consume fewer fossil fuels.

• Protect existing forests: prevent logging and fires; they are sinks that are already large.

• Reforest with local species and follow up so that they survive.

• Responsible consumption: reduce, reuse, and recycle; use less excess fertilizer; avoid products that involve deforestation.

Back to the beginning: Diego noticed more heat because burning fuels and deforestation add CO₂ faster than sinks can remove it, and the greenhouse effect intensifies.

Summary