Greenhouse gases and the atmosphere

We hear a lot about greenhouse gases – and the huge amounts that humans are responsible for pumping into the atmosphere each year. It might be surprising to find out that the three main greenhouse gases – carbon dioxide, methane and nitrous oxide – comprise just a tiny fraction of Earth’s atmospheric gases.

Most of the atmosphere (to a height of 25 km) is composed of just two gases: nitrogen (78%) and oxygen (21%). Carbon dioxide comprises 0.04% of the atmosphere, methane 0.00018% and nitrous oxide 0.00003%.

The greenhouse effect

Greenhouse gases, which also include water vapour and fluorinated gases, make the Earth a favourable place to live because they stop the Earth’s heat energy from escaping into space. Without these gases, the average temperature of the Earth’s surface would be -19℃. This is known as the greenhouse effect.

A balancing act

When sunlight reaches the Earth, two things can happen. The heat energy is either reflected back into space or it is absorbed by the Earth’s surface and oceans and eventually radiated back into space. Greenhouses gases in the atmosphere absorb some of this heat energy and slow or prevent its loss. A common analogy is how a blanket warms you by trapping some of your body’s heat and prevents the warmth from escaping.

This balancing act between energy entering and leaving the Earth’s systems has created a relatively stable climate for thousands of years, but this is a very delicate balance. Around 300 years ago, humans started using fossil fuels, which emit greenhouse gases and cause more heat energy to be held in the atmosphere. The greenhouse gases act like extra blankets, holding in more and more warmth.

The atmosphere does not exist in isolation. It interacts with other components of the Earth system. More than 90% of this extra atmospheric energy has been absorbed by our oceans. This additional energy is responsible for raising the water’s surface temperature, melting ice shelves and adding to sea level rise.

Comparing greenhouse gases

All greenhouse gases absorb energy, but different gases have different effects on warming. Carbon dioxide, methane and nitrous oxide remain in the atmosphere for long enough to allow them to mix together. As a result, the gas concentrations are about the same around the globe, regardless of the source or location of the emissions.

Carbon dioxide (CO₂) is the primary greenhouse gas because it is emitted in large quantities and it has a long-lasting influence. Most CO₂ emissions have come from the burning of fossil fuels, but deforestation and cement production also add CO₂. Over the last 300 years, atmospheric CO₂ concentrations have risen by about 46% and are now at a concentration last experienced over 3 million years ago. Carbon dioxide can stay in the atmosphere for hundreds to thousands of years. The CO₂ emissions we release today will continue to influence the climate for a long time to come.

Methane (CH₄) is the second most abundant greenhouse gas. Atmospheric concentrations have increased by about 150%, and methane is more abundant now than at any time in the last 800,000 years. Globally, the leading sources of human-influenced methane emissions are from agriculture – primarily from livestock and rice production. Methane also comes from energy production, wastewater treatment and landfills. Methane emissions only last for about 10–12 years, but they absorb a lot more heat energy than carbon dioxide.

Globally, about 40% of nitrous oxide (N₂O) emissions come from human activity and have increased by 15% in the last 300 years. A large proportion of these emissions come from agricultural soils as soil microbes break down urine, faeces and nitrogen fertilisers. Other sources include transportation and industry. Nitrous oxide emissions remain in the atmosphere for more than 100 years and pose a second danger – N₂O can be converted to nitrogen oxides, which damage the ozone layer.

Natural removal from the atmosphere

Atmospheric greenhouse gases are removed by natural processes. These include chemical reactions within the atmosphere and physical exchanges between the atmosphere and the oceans. Carbon dioxide can also be removed by plants as they grow and store carbon. Forests and soils are able to absorb more carbon than they release.