Greenhouse Gases and Global Warming Explained

Global warming refers to the long-term heating of Earth's climate system observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth's atmosphere. Greenhouse gases (GHGs) trap heat and make the planet warmer. While a natural phenomenon, human activities have significantly increased the concentration of these gases.

Gas Contribution to Global Warming Analysis

The question asks us to arrange four specific greenhouse gases – Nitrous oxide, Carbon dioxide, Chlorofluorocarbons, and Methane – in decreasing order based on their total contribution to global warming. This contribution depends not only on how much heat a gas traps per molecule (its Global Warming Potential or GWP) but also on its atmospheric concentration and lifetime.

Carbon Dioxide ($CO_2$) Contribution

Carbon dioxide ($CO_2$) is the primary greenhouse gas emitted through human activities. Its largest source is the combustion of fossil fuels (coal, oil, and natural gas) for energy production, transportation, and industry. Deforestation and land-use changes also contribute significantly. Although its GWP is considered the baseline (1), its extremely high atmospheric concentration and long lifespan make it the largest contributor to current global warming.

Methane ($CH_4$) Contribution

Methane ($CH_4$) is the second-largest contributor to global warming from human activities. Major sources include agriculture (livestock digestion, rice cultivation), decomposition of waste in landfills, and the production and transport of fossil fuels. Methane traps much more heat per molecule than carbon dioxide over shorter time scales (e.g., 20 years), but its atmospheric lifetime is shorter (around 12 years), making its overall contribution significant but less than $CO_2$.

Nitrous Oxide ($N_2O$) Contribution

Nitrous oxide ($N_2O$) is released into the atmosphere through agricultural and industrial activities, combustion of fossil fuels and solid waste, as well as during treatment of wastewater. Fertilizers are a major source of $N_2O$ emissions from agriculture. It has a higher GWP than methane and a longer atmospheric lifetime (over 100 years), placing it as the third most significant contributor.

Chlorofluorocarbons (CFCs) Contribution

Chlorofluorocarbons (CFCs) are synthetic gases used in various industrial applications, such as refrigerants and aerosol propellants. Although largely phased out under the Montreal Protocol due to their ozone-depleting properties, they are extremely potent greenhouse gases with very high GWPs, thousands of times greater than $CO_2$. However, their atmospheric concentrations are much lower compared to $CO_2$, $CH_4$, and $N_2O$. Therefore, while potent, their *total contribution* to warming is less than the other three gases mentioned.

Decreasing Order of Global Warming Contribution

Considering the atmospheric concentrations, lifetimes, and GWP of these gases, the order of their contribution to total global warming, from largest to smallest, is generally accepted as:

• B. Carbon dioxide ($CO_2$): The largest contributor due to high concentration.
• D. Methane ($CH_4$): The second largest contributor.
• A. Nitrous oxide ($N_2O$): The third largest contributor.
• C. Chlorofluorocarbons (CFCs): Potent but lower concentrations lead to a smaller overall contribution compared to the others.

This corresponds to the order B, D, A, C.

The correct option reflecting this decreasing order is B, D, A, C.