The basic idea
The earth’s surface is warmed by sunlight and gives heat back out as infrared radiation. Greenhouse gases are gases in the atmosphere that absorb some of that outgoing heat and radiate part of it back toward the surface. This natural effect keeps the planet considerably warmer than it would be without these gases, and it is widely regarded as necessary for life as we know it.
The concern in climate discussions is not the existence of the effect but the increase in the concentration of these gases from human activities. Scientific assessments conclude that this increase is the main driver of the warming seen since the industrial era, although projections of future change come with ranges and uncertainties.
The main gases
Water vapor. The most abundant greenhouse gas by volume and an important part of the natural effect. Its amount in the air mostly responds to temperature, rising as air warms, so it is generally treated as a feedback instead of a direct target for emissions policy.
Carbon dioxide (CO2). The largest contributor to human-driven warming because so much is emitted. It comes mainly from burning coal, oil and natural gas, from cement production, and from deforestation and other land-use change. It also persists: a significant fraction stays in the atmosphere for centuries, though it is gradually absorbed by oceans and plants.
Methane (CH4). Released by fossil-fuel production and distribution, livestock, rice farming, landfills and wetlands. Per ton, it traps much more heat than carbon dioxide, but it breaks down in the atmosphere in roughly a decade, so its effect is stronger in the short term. Leaks from natural gas systems are one reason methane gets attention in energy debates.
Nitrous oxide (N2O). Comes mostly from fertilized soils and some industrial processes, and also from fuel combustion in smaller amounts. It is strong per ton and long-lived.
Fluorinated gases. A family of synthetic gases used in refrigeration, air conditioning, insulation and some industrial processes. They exist in small quantities but can be very potent and long-lasting, which is why international agreements have targeted some of them.
Ozone and others. Ground-level ozone is also a greenhouse gas, formed by reactions among pollutants rather than emitted directly.
Where they come from
Natural sources include respiration, decomposition, volcanoes, wetlands and oceans. These flows were roughly in balance with natural removal for thousands of years before the industrial era.
Human sources are commonly grouped by sector. Estimates differ by method and year, but the broad picture is stable: energy supply and use, including electricity, heat and transport, accounts for the largest share of global emissions. Agriculture, forestry and land use, industrial processes and waste make up most of the rest. The share of each varies a great deal between countries, depending on their economies and energy sources.
How they are compared
Different gases trap different amounts of heat and last for different lengths of time. To put them on a common scale, analysts use a measure called global warming potential, which compares a gas with carbon dioxide over a chosen time horizon, commonly 100 years. Emissions are then expressed as tons of carbon dioxide equivalent.
The choice of time horizon matters. Over 20 years methane counts for noticeably more than over 100 years, because it is short-lived but strong. Neither choice is wrong, and each answers a slightly different question. This is why you may see different equivalent figures for the same methane release in different reports. Conversion values are also updated as scientific assessments are revised.
Greenhouse gases in energy contexts
Energy appears in the discussion in several ways.
- Combustion. Burning fuels releases carbon dioxide in proportion to the carbon in the fuel. Coal power typically emits more per unit of electricity than gas, though the exact figure depends on plant efficiency and fuel type.
- Upstream emissions. Extracting, processing and transporting fuels can release methane and use energy, so the footprint of a fuel includes more than the moment of burning.
- Low-emission sources. Solar, wind, hydropower and nuclear power produce little or no greenhouse gas while operating, but building the equipment, mining materials and managing reservoirs or waste all have impacts. A life cycle assessment is the usual tool for comparing such effects, and results vary with assumptions.
- Energy efficiency. Using less energy for the same service generally means fewer emissions, depending on the fuel being displaced.
- Removal and capture. Technologies such as carbon capture and natural methods like forests and soils aim to reduce what reaches the atmosphere or to take some of it back out.
Related terms worth knowing
Emissions are flows, meaning the amount released per year, while concentration is the amount present in the atmosphere. Because carbon dioxide lingers, the total concentration keeps rising as long as emissions exceed removal, even if annual emissions level off. This is the reasoning behind the idea of balancing emissions and removals, or net zero.
Keeping the picture balanced
Greenhouse gases are a physical fact, while the right response involves choices about cost, fairness, technology and timing that societies debate. Good reading habits include checking which gases a figure includes, which time horizon it uses, and whether it counts only direct emissions or upstream ones as well.