A greenhouse gas (GHG) is any gas in the atmosphere that traps heat by absorbing and re-emitting infrared radiation – the heat energy that the Earth’s surface radiates back toward space after being warmed by the sun.
The name comes from the analogy with a glass greenhouse. Sunlight passes through the glass and warms the inside, but the glass prevents much of that heat from escaping. Greenhouse gases do something similar in the atmosphere – they let sunlight in but trap a portion of the heat that would otherwise radiate back into space.
Without any greenhouse gases at all, the Earth’s average surface temperature would be roughly -18°C instead of the current 15°C. So the greenhouse effect itself is not a bad thing – it is what makes the planet habitable. The problem is that human activity has dramatically increased the concentration of greenhouse gases in the atmosphere, trapping more heat than the natural system can handle and driving global temperatures upward.
🔬 The Main Greenhouse Gases
There are several greenhouse gases, but four account for the vast majority of human-caused warming. Each has a different warming potency and atmospheric lifetime, which is why they are compared using a common unit called CO2 equivalent (CO2eq).
🏭 Carbon Dioxide (CO2)
CO2 is the most abundant human-produced greenhouse gas. It enters the atmosphere mainly through burning fossil fuels (coal, oil, natural gas), cement manufacturing and deforestation. Globally, atmospheric CO2 has risen from around 280 parts per million (ppm) before the Industrial Revolution to over 420 ppm today – a 50% increase.
CO2 is less potent per tonne than other greenhouse gases, but the sheer volume of it makes it the dominant driver of warming, responsible for roughly 65% of the warming effect from human activities. Once emitted, CO2 can persist in the atmosphere for hundreds to thousands of years, which means the CO2 we release today will continue to affect the climate for generations.
In Ireland, CO2 accounted for 61.1% of total greenhouse gas emissions in 2023. The main sources are transport (burning petrol and diesel), home heating (oil and gas boilers) and electricity generation (natural gas power plants).
🐄 Methane (CH4)
Methane is a far more potent greenhouse gas than CO2 – roughly 28-30 times more warming per tonne over 100 years, and around 80 times more over 20 years. However, it has a much shorter atmospheric lifetime of approximately 12 years before it breaks down into CO2 and water.
Human sources of methane include livestock farming (cattle and sheep produce methane during digestion), rice cultivation, landfills, and leaks from natural gas and oil production. In Ireland, methane is 28.9% of total greenhouse gas emissions, almost entirely from agriculture. This is an unusually high proportion compared to most EU countries and reflects the size of Ireland’s cattle herd. For more on how agricultural methane works, see the page on biogenic methane emissions.
🌾 Nitrous Oxide (N2O)
Nitrous oxide has a Global Warming Potential of 273 – meaning one tonne causes 273 times more warming than one tonne of CO2 over 100 years. It also stays in the atmosphere for over a century on average.
The main sources are agricultural soils (particularly from synthetic and organic fertiliser use), industrial processes and wastewater treatment. In Ireland, nitrous oxide accounts for 8.8% of total greenhouse gas emissions, primarily from nitrogen fertiliser applied to farmland. Reducing fertiliser use is one of the key actions in Ireland’s Climate Action Plan for cutting agricultural emissions.
❄️ Fluorinated Gases (F-gases)
Fluorinated gases – including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6) – are entirely human-made. They do not exist in nature. They are used in refrigeration, air conditioning, electrical switchgear and semiconductor manufacturing.
F-gases are produced in tiny quantities compared to CO2 or methane, but they are extraordinarily potent. Some have Global Warming Potentials in the thousands or tens of thousands. Sulphur hexafluoride, for example, has a GWP of 25,200 and can persist in the atmosphere for over 3,000 years. In Ireland, F-gases account for 1.2% of total emissions – a small share, but one that is growing as these gases replace ozone-depleting substances being phased out under the Montreal Protocol.
🇮🇪 Ireland’s Greenhouse Gas Emissions
Ireland’s total greenhouse gas emissions were 53.8 MtCO2eq in 2024, down 2% from the previous year. The breakdown across sectors in recent years shows agriculture as the dominant source (38%), followed by transport (21%), energy industries (14%) and residential heating (11%).
Ireland has the second highest greenhouse gas emissions per capita in the EU at 10.4 tonnes of CO2eq per person (2023 figures). This is 51% above the EU average, driven primarily by the large share of agricultural methane and nitrous oxide in the national total.
Under the Climate Action and Low Carbon Development Act 2021, Ireland has committed to reducing greenhouse gas emissions by 51% by 2030 compared to 2018 levels, with a target of net zero by 2050. Legally binding carbon budgets set caps on total allowable emissions over five-year periods, and sectoral emissions ceilings allocate these among electricity, transport, buildings, industry and agriculture.
🌡️ The Greenhouse Effect vs Climate Change
It is worth being clear about the distinction between the greenhouse effect and climate change. The greenhouse effect is a natural and necessary process – without it, the Earth would be frozen. Climate change is what happens when human activity amplifies the greenhouse effect by adding excessive greenhouse gases to the atmosphere.
Since the Industrial Revolution, human activities have increased atmospheric CO2 by 50%, methane by over 150% and nitrous oxide by roughly 23%. This enhanced greenhouse effect is trapping more heat than the natural system can balance, causing global average temperatures to rise. The consequences include more extreme weather events, rising sea levels, ocean acidification and disruption to ecosystems and food production.
In Ireland, the impacts are already visible. 2023 was the warmest year on record for the country, with an average temperature of 10.96°C. Three of the four warmest years since 1961 have occurred since 2021. Rainfall patterns are becoming more extreme, with increased flooding risk in many areas.
🔗 How Greenhouse Gases Connect to Other Climate Terms
- 🌿 MtCO2eq – the standard unit for measuring and comparing greenhouse gas emissions
- 🌿 Carbon footprint – a measure of total greenhouse gas emissions from an individual, household or organisation
- 🌿 Carbon budgets – caps on total allowable greenhouse gas emissions over set periods
- 🌿 Biogenic methane – methane from biological sources like livestock and wetlands
- 🌿 Scope 1, 2 and 3 emissions – how businesses categorise their greenhouse gas output
- 🌿 Net zero – reducing greenhouse gas emissions to zero or balancing them with removals
❓ Frequently Asked Questions
What is the most harmful greenhouse gas?
It depends how you define “harmful.” CO2 causes the most total warming because of the enormous volume emitted. But tonne for tonne, other gases are far more potent – methane is 28-30 times stronger than CO2 over 100 years, nitrous oxide is 273 times stronger, and some F-gases are thousands of times stronger. Reducing all greenhouse gases matters, but cutting CO2 from fossil fuels and methane from agriculture are the two most impactful actions globally.
How many greenhouse gases are there?
The seven main greenhouse gases tracked under international climate agreements (the Kyoto Protocol and Paris Agreement) are carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride. Water vapour is also a greenhouse gas – the most abundant one, in fact – but its concentration is controlled by temperature rather than direct human emissions, so it acts as a feedback mechanism that amplifies warming from other gases rather than being a primary driver.
What is Ireland’s biggest source of greenhouse gas emissions?
Agriculture, at 38% of total emissions in 2024. This is unusual for an EU country and is driven by Ireland’s large livestock sector. Transport (21%) and energy industries (14%) are the next biggest contributors. At a household level, home heating and driving are the main sources of greenhouse gas emissions that individuals can directly influence through retrofitting, switching to a heat pump, installing solar panels or moving to an electric vehicle.
Is water vapour a greenhouse gas?
Yes. Water vapour is actually the most abundant greenhouse gas in the atmosphere and contributes more to the natural greenhouse effect than CO2. However, humans do not directly control its concentration – it responds to temperature changes caused by other greenhouse gases. As the planet warms from rising CO2 and methane, more water evaporates, which traps more heat, which causes more evaporation. This feedback loop amplifies warming but is a consequence of other emissions rather than a cause we can directly address.