30. Second abundant greenhouse gas after CO₂is

(1) Methane (2) NO_x
(3) CFC (4) Ozone

Understanding the composition and impact of greenhouse gases is crucial in addressing climate change. While carbon dioxide (CO₂) is widely recognized as the most significant greenhouse gas resulting from human activities, the identity of the second most abundant greenhouse gas is less commonly discussed. This article explores which gas holds this position, its sources, its potency, and its role in global climate dynamics.

The Greenhouse Gas Hierarchy

Greenhouse gases (GHGs) are atmospheric components that absorb and emit infrared radiation, trapping heat and contributing to the greenhouse effect. The most abundant greenhouse gases, in order of atmospheric concentration, are:

• Water vapor (H₂O)

• Carbon dioxide (CO₂)

• Methane (CH₄)

• Nitrous oxide (N₂O)

• Halogenated gases (such as CFCs and HFCs)

However, when considering gases whose concentrations are directly influenced by human activities—often termed “anthropogenic greenhouse gases”—the order shifts. Among these, carbon dioxide (CO₂) is the most abundant, and methane (CH₄) is the second most abundant.

Why Methane Is the Second Most Abundant Greenhouse Gas

Methane (CH₄) is a hydrocarbon and a primary component of natural gas. It is the second most abundant anthropogenic greenhouse gas after carbon dioxide, accounting for about 11% of global greenhouse gas emissions from human activities. While water vapor is more abundant overall, its concentration is largely controlled by natural processes and is not typically included in discussions of anthropogenic emissions.

Key Points About Methane

• Abundance: Methane is less concentrated in the atmosphere than CO₂ but still ranks second among anthropogenic greenhouse gases.

• Potency: Methane is over 28 times more effective than CO₂ at trapping heat over a 100-year period, and even more potent over shorter timescales.

• Lifespan: Methane has a much shorter atmospheric lifetime than CO₂—about 10 years compared to hundreds of years for CO₂.

• Sources: Methane is emitted from both natural sources (wetlands, termites, natural gas seepage) and human activities (agriculture, fossil fuel extraction, landfills, and wastewater treatment).

• Impact: Despite its shorter lifespan, methane’s high heat-trapping ability makes it a critical target for climate mitigation efforts.

Comparing Other Greenhouse Gases

To clarify why methane is considered the second most abundant greenhouse gas after CO₂ (among anthropogenic sources), let’s examine the other options:

• Nitrogen oxides (NOx): While NOx gases (such as NO and NO₂) play a role in atmospheric chemistry and can influence ozone formation, they are not classified as major greenhouse gases and are not among the most abundant.

• Chlorofluorocarbons (CFCs): These halogenated gases have high global warming potential but are present in much lower concentrations than methane and are primarily regulated due to their ozone-depleting properties.

• Ozone (O₃): Surface-level ozone is a greenhouse gas and a pollutant, but its atmospheric concentration is much lower than methane. Ozone’s primary climate impact is through its role as a short-lived climate forcer rather than as a major, persistent GHG.

The Role of Water Vapor

Water vapor is the most abundant greenhouse gas in Earth’s atmosphere, but its concentration is largely controlled by natural processes and is not directly influenced by human emissions in the same way as CO₂ and methane. For this reason, water vapor is often excluded from lists of anthropogenic greenhouse gases, even though it is a critical component of the natural greenhouse effect.

Methane’s Sources and Human Impact

Human activities have significantly increased atmospheric methane concentrations. Key anthropogenic sources include:

• Agriculture: Livestock (especially cattle) produce methane during digestion (enteric fermentation). Rice paddies are another significant source.

• Fossil Fuel Extraction and Use: Methane is released during the production, processing, and transport of coal, oil, and natural gas.

• Waste Management: Landfills and wastewater treatment plants emit methane as organic waste decomposes.

• Biomass Burning: The burning of agricultural residues and other biomass releases methane.

Natural sources, such as wetlands and termites, also contribute, but the rapid increase in methane levels over the past two centuries is primarily due to human activities.

Methane’s Potency and Climate Impact

Methane’s high global warming potential means that even small increases in its atmospheric concentration can have a significant impact on climate change. Over a 20-year period, methane is more than 80 times more effective at trapping heat than CO₂. Although it remains in the atmosphere for a shorter time, its potency makes it a critical target for short-term climate mitigation.

Reducing methane emissions can have a rapid and significant effect on slowing global warming, making it a key focus of international climate policy.

The Importance of Addressing Methane Emissions

Given methane’s potency and the significant contribution of human activities to its atmospheric concentration, reducing methane emissions is a vital part of any strategy to limit global warming. Strategies include:

• Improving agricultural practices: Reducing methane emissions from livestock and rice cultivation.

• Reducing fossil fuel emissions: Capturing methane from oil and gas operations and preventing leaks.

• Enhancing waste management: Capturing methane from landfills and wastewater treatment for energy use.

Key Takeaways

• Methane (CH₄) is the second most abundant greenhouse gas after carbon dioxide (CO₂) among anthropogenic sources.

• Methane is much more potent than CO₂ at trapping heat, but it has a shorter atmospheric lifespan.

• Human activities—such as agriculture, fossil fuel extraction, and waste management—are the primary drivers of increased methane concentrations.

• Reducing methane emissions is a highly effective strategy for slowing global warming in the short term.

Summary Table

Conclusion

Methane (CH₄) is the second most abundant greenhouse gas after carbon dioxide (CO₂) among gases influenced by human activities. Its high heat-trapping ability and significant contribution to global warming make it a critical focus for climate mitigation. Reducing methane emissions can have a rapid and substantial impact on slowing climate change, underscoring the importance of targeted action to address this potent greenhouse gas.

In summary, the correct answer is:
(1) Methane.
Understanding and addressing methane’s role in climate change is essential for effective global climate policy and a sustainable future.