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The greenhouse effect

Carbon dioxide is one of several greenhouse gases. The other main ones are water vapour, methane and nitrous oxide. Together, these greenhouse gases create the greenhouse effect – a natural atmospheric process that keeps the Earth’s surface at a temperature comfortable for humans and other life to exist.

More than a quarter (29 per cent) of the radiation reaching Earth from the Sun is reflected back into space by clouds, by the surrounding atmosphere and – to a small extent – by the ground. About another quarter (23 per cent) is absorbed by the Earth’s atmosphere.

This leaves only about half (48 per cent) of the Sun’s incoming energy to reach the surface and warm the ground.

The warmed ground then emits heat as infrared radiation.

Most of the gas in the air is nitrogen and oxygen, which allow the infrared radiation to pass straight into space. However, a lot of the infrared radiation is trapped in the air by the greenhouse gases, which radiate heat back to the ground.

There isn’t much carbon dioxide in the atmosphere. It makes up only 0.04 per cent of the air – just over 400 parts per million (ppm).

But the effect of this and other greenhouse gases – that is, the greenhouse effect – radiating heat back to the surface of the Earth is to keep the atmosphere at a habitable temperature.

The natural greenhouse effect keeps the surface of our planet at a habitable temperature, about 33 °C higher than it would be without an atmosphere.

Scientists use ice cores to reconstruct Earth's temperature back in time. Here Dr David Etheridge handles a newly drilled ice core in Antarctica.

An enhanced greenhouse effect

While the natural greenhouse effect is essential for life on Earth, it is the enhanced greenhouse effect that is threatening life on Earth.

The natural greenhouse effect has been enhanced over the past two centuries by an increase in activities such as the burning fossil fuels for energy and transport, and expanding agriculture and deforestation.

Global atmospheric concentrations of carbon dioxide and other greenhouse gases continue to increase, from 277 ppm in 1750 to 412 ppm in 2020.

Measurements of air trapped in Antarctic ice show that for the past 800,000 years, carbon dioxide concentrations remained below 280 ppm.

The carbon dioxide concentration today is much higher than the natural range of 172 to 300 ppm that existed for hundreds of thousands of years. In fact, carbon dioxide concentrations are now likely to be the highest they have been in at least the past 2 million years.

Other greenhouse gases also have increased due to human activities.

  • Global atmospheric methane concentrations have more than doubled and reached 1,875 parts per billion (ppb) at the end of 2019 – more than two and a half times what they were prior to the industrial revolution.
  • Nitrous oxide has steadily increased and is now more than 22 per cent higher than the industrial revolution, with an atmospheric concentration of more than 330 ppb.
  • Halocarbons (including CFCs, which also harm the ozone layer) and related compounds, which didn’t even exist naturally in the atmosphere, have increased rapidly. Although some have begun to be reduced in concentration, the concentration of synthetic gases combined is about 1.5 ppb, rising from zero several decades ago.
  • Water vapour, while the most abundant greenhouse gas in the atmosphere, is only slightly influenced by human activities (mainly through irrigation and deforestation).

The combined influence of all greenhouse gases in the air added together is expressed as an 'equivalent carbon dioxide' atmospheric concentration.

Global CO₂ equivalent reached 516 ppm in 2021.

Impact on climate

The increase of global greenhouse gas levels coincides with a period of increasing global warmth and other observed changes to the climate.

Isotopic analysis shows that the extra carbon dioxide comes from human activities.

Studies of radiative forcing and their contributions to global warming clearly show that human activity is a major cause of global warming since pre-industrial times.

The Intergovernmental Panel on Climate Change (IPCC) concluded that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

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