Greenhouse effect

Greenhouse gases such as carbon dioxide, methane, nitrous oxide occur naturally in the atmosphere. Greenhouse gases absorb part of the infrared radiation emitted from the Earth and then re-radiate the energy in all directions, also in the form of infrared radiation. Part of the infrared radiation goes back to the Earth, heating up the surface. This is known as the greenhouse effect.

Carbon cycle

Carbon cycle is a biogeochemical cycle in which carbon is continuously exchanged and recycled among several natural reservoirs, including atmosphere, ocean, biosphere, rocks and fossil fuels, where carbon is stored.

In an unperturbed natural carbon cycle, these exchanges between reservoirs are approximately balanced.

In the last 800,000 years before the Industrial Revolution, the atmospheric concentration of carbon dioxide fluctuated roughly between 180 and 280 ppm¹ . However, human activities have disturbed the global carbon cycle. The present-day atmospheric concentration of carbon dioxide has exceeded 400 ppm, unprecedented in the past 800,000 years. And the concentration keeps on rising.

What human activities increase greenhouse gases

Human activites emit huge amount of greenhouse gases to the atmosphere, enhancing the greenhouse effect to trap more energy on Earth and leading to global warming.

(a)Burning fossil fuels

Fossil fuels include coal, oil and natural gas. Humans have been burning massive amount of fossil fuels for many purposes since the Industrial Revolution, such as power generation, heating and powering machines and vehicles. Large amounts of carbon dioxide, methane and nitrous oxide are emitted to the atmosphere.

(b)Deforestation

To acquire land for agricultural and urban development, forests are cut down or burned on a massive scale. Carbon dioxide is released from the burning process. Moreover, plants capable of absorbing and storing carbon dioxide decrease, resulting in increasing atmospheric concentration of carbon dioxide. Data^viii show that around 140 million hectares of forest are lost globally for 1990-2015.

(c)Agricultural activities

Pastoral farming emits a lot of methane, e.g. cows ruminating, and so does rice farming. The use of large amounts of nitrogen fertilizers releases nitrous oxide. Moreover, agricultural machineries are mostly powered by fossil fuels and therefore contribute to carbon dioxide emission.

(d)Industrial activities

Industrial activities emit large amount of greenhouse gases. For example, the production of nylon releases nitrous oxide and the production of cement releases carbon dioxide. Chlorofluorocarbons (CFCs), which were developed and used as refrigerants and aerosol propellants in the past century, are also greenhouse gases.

(e)Waste treatment

With improving living standard and the rise of consumerism, humans consume resources and produce large amounts of waste as well. Wastes are treated by either incineration or landfilling. The incineration process produces carbon dioxide, methane and nitrous oxide while waste decomposition in landfills produces methane.

Since 1750, the atmospheric concentration of carbon dioxide has increased by more than 40%. Methane and nitrous oxide have increased by about 160% and 20% respectively.