The correct answer is Carbon dioxide. Explanation & Key Takeaways: • Carbon dioxide (CO2) is one of the primary greenhouse gases present in the Earth's atmosphere. • It is released naturally through processes like volcanic eruptions, respiration, and ocean-atmosphere exchange and also through human activities such as the burning of fossil fuels, deforestation, and industrial processes. • Greenhouse gases, including CO2, trap heat in the Earth's atmosphere, leading to the greenhouse effect, which is essential for maintaining the planet's temperature but can contribute to global warming when present in excessive amounts. • Carbon dioxide accounts for approximately 76% of global greenhouse gas emissions, making it the most significant contributor to human-induced climate change. • Its concentration in the atmosphere has increased from pre-industrial levels of around 280 parts per million (ppm) to over 420 ppm as of recent years due to human activities. Related Concepts: • Methane (CH4): • Methane is a potent greenhouse gas with a global warming potential approximately 25 times greater than CO2 over a 100-year period. • It is emitted through natural sources like wetlands and human activities, including livestock farming, rice cultivation, and landfills. • While methane is a significant greenhouse gas, its atmospheric concentration is much lower than that of carbon dioxide. • Water Vapour (H2O): • Water vapour is the most abundant greenhouse gas in the atmosphere, but it acts as a feedback mechanism rather than a direct cause of climate change. • Its concentration is controlled by temperature, and it amplifies the effects of other greenhouse gases like CO2 and CH4. • For example, as CO2 warms the atmosphere, more water evaporates, increasing the amount of water vapour and amplifying the warming effect. • Nitrogen Oxides (NOx): • Nitrogen oxides, particularly nitrous oxide (N2O), are potent greenhouse gases and also contribute to the depletion of the ozone layer. • They are emitted through agricultural activities, fossil fuel combustion, and industrial processes. • Although their concentration is lower compared to CO2 and CH4, they have a higher global warming potential. • Greenhouse Effect: • The greenhouse effect occurs when greenhouse gases absorb and re-radiate heat in the atmosphere, keeping the Earth's surface warmer than it would be without these gases. • While this natural process is crucial for life on Earth, human-induced increases in greenhouse gases have enhanced the effect, leading to global warming and climate change. Important Notes: • Global Warming Potential (GWP): GWP measures the heat-trapping ability of a greenhouse gas relative to CO2 over a specific time frame. For instance, methane has a GWP of 25 over 100 years, making it far more potent than CO2 in the short term. • Sources of CO2 Emissions: The primary sources include the combustion of coal, oil, and natural gas, deforestation, and changes in land use. • IPCC Reports: According to the Intergovernmental Panel on Climate Change (IPCC), reducing CO2 emissions is critical to limiting global temperature increases to 1.5°C above pre-industrial levels. • Mitigation Efforts: Measures such as renewable energy adoption, afforestation, and carbon capture technologies are vital to reducing CO2 emissions.