Human activities are increasing the concentrations of greenhouse gases in the atmosphere, leading to an 'enhanced' greenhouse effect and causing surface temperatures to rise.
What is the greenhouse effect?
To maintain stable temperatures at the Earth's surface the incoming energy from the Sun must be balanced in the longer-term by an equal amount of heat radiated back to space.
Greenhouse gases in the atmosphere absorb some of this re-radiated heat, which increases the temperature of the Earth's surface, ocean and atmosphere.
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. Without any greenhouse gases, the Earth's average surface temperature would be much lower, about -18 °C rather than today’s average of about 14 °C.
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.
What is the 'enhanced' greenhouse effect?
For many centuries before the industrial era (i.e. prior to 1750), the incoming sunlight and outgoing heat were balanced, and global average temperatures were relatively steady.
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.
Increasing concentrations of greenhouse gases in the atmosphere is causing surface temperatures to increase. This is due an energy imbalance of around 0.7–0.8 Wm–2 (i.e. extra heat is building up in the Earth system) averaged over the global surface of the Earth.
Global atmospheric concentrations of carbon dioxide and other greenhouse gases continue to increase, from 277 ppm in 1750 to 412 ppm in 2020.
The atmosphere and oceans will continue to warm until enough extra heat can escape to space to allow the Earth to return to balance. Because increased levels of carbon dioxide and other long-lived greenhouse gases persist in the atmosphere for hundreds of years, further warming and sea-level rise will occur.
Which greenhouse gases contribute to the enhanced greenhouse effect?
Carbon dioxide is the main greenhouse gas emitted through human activities. However, other greenhouse gases also have increased in concentration 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.
How do we know what past greenhouse gas levels were?
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.
Scientists can tell what past greenhouse gas concentrations were by taking measurements of air trapped in Antarctic ice. These measurements show that for the past 800,000 years, carbon dioxide concentrations remained below 280 ppm.
How do we know human activity is to blame?
Scientists know that the source of CO₂ in the atmosphere is the result of human activity (e.g. burning fossil fuels to produce electricity, transport, and industrial processes). This is because they can analyse the different isotopes of carbon in atmospheric CO₂ to understand their sources.
Measurements of carbon-13 and carbon-14, relative to carbon-12, confirm that the increase in CO₂ concentration since 1800 originates principally from fossil fuel and land clearing emissions.
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.