Carbon dioxide (CO2) is the most prevalent greenhouse gas emitted by human activities. Its sources are diverse, including the combustion of fossil fuels for energy generation, transportation, and industrial processes. Deforestation also contributes significantly, as trees absorb CO2 during photosynthesis; their removal reduces this natural carbon sink. Land-use changes and cement production are other notable sources. CO2 remains in the atmosphere for a very long time, with its warming effect persisting for hundreds to thousands of years, making it a critical focus for climate mitigation efforts.

Methane (CH4) is a powerful greenhouse gas, with a global warming potential significantly higher than CO2 over a 20-year period. Its primary anthropogenic sources include leaks from natural gas and oil extraction and distribution systems, agricultural activities like enteric fermentation in livestock and rice cultivation, and the decomposition of organic matter in landfills and wastewater treatment. While methane breaks down in the atmosphere much faster than CO2 (typically within a decade), its high potency means that reducing methane emissions can lead to more immediate climate benefits.

Nitrous oxide (N2O) is another significant greenhouse gas, primarily originating from agricultural practices, particularly the use of synthetic nitrogen fertilizers. Soil management techniques, as well as livestock manure management, are major contributors. Industrial processes, such as the production of nitric acid and adipic acid, also release N2O. Fossil fuel combustion is a smaller, but still relevant, source. N2O has a long atmospheric lifetime, comparable to CO2, and a high global warming potential, making its control crucial for climate action.

The most direct impact of increased greenhouse gas concentrations is global warming, characterized by a sustained rise in the Earth's average surface temperature. This warming trend is not merely an increase in heat but also a driver of more extreme weather events, such as heatwaves, droughts, floods, and more intense storms. The uneven distribution of warming can lead to significant regional climate shifts, impacting agriculture, water resources, and human health.

Ecosystems are highly sensitive to changes in climate. As temperatures rise and weather patterns shift, many species struggle to adapt. This can lead to changes in migration patterns, breeding seasons, and the geographic distribution of plants and animals. Habitats may be degraded or destroyed, increasing the risk of species extinction. Ocean acidification, caused by the absorption of excess CO2 by seawater, poses a severe threat to marine life, particularly coral reefs and shellfish.

Global warming causes sea levels to rise through two primary mechanisms: thermal expansion of seawater and the melting of land-based ice. As ocean water warms, it expands in volume. Simultaneously, glaciers and ice sheets in polar regions and mountain ranges are melting at an accelerated rate, adding vast amounts of water to the oceans. This sea-level rise poses a significant threat to coastal populations, infrastructure, and low-lying island nations, increasing the risk of flooding, erosion, and saltwater intrusion into freshwater sources.