Various natural factors, including volcanic eruptions and changes in the Sun’s activity and Earth’s orbit, have altered the Earths’ past climate – but none of them can account for the warming that has occurred since about 1900. Rising greenhouse gas (GHG) concentrations from human activity do, however, explain this warming through their enhancement of the natural 'greenhouse effect’.
The Greenhouse Effect
The surface temperature of the Earth is determined by the balance between incoming solar energy and outgoing infrared energy. Incoming sunlight (shortwave radiation) is absorbed by the Earth’s surface and the heat generated is then re-emitted in the form of infra-red (longwave) radiation. This re-emission cools the surface. Some of the infrared radiation is trapped by greenhouse gases (GHGs) – such as (in order of importance) water vapour, carbon dioxide, methane, nitrous oxide and ozone – therefore warming the surface and lower atmosphere.
This natural process has operated for millions of years. Without it, the Earth’s surface would be, on average, about 33°C colder than it is now (-18°C instead of 15°C) and unsuitable for human life.
The accumulation of key GHGs (most importantly carbon dioxide, methane and nitrous oxide, but also certain man-made substances) in the atmosphere due to human activities is enhancing the natural greenhouse effect. This has disturbed the balance of incoming and outgoing energy, causing the Earth to warm up. This initial warming effect is further amplified by various ‘feedback’ processes within the climate system, such as increasing water vapour in the atmosphere and the effects of melting sea ice on surface reflectivity (albedo).
The observed spatial patterns of warming, combined with modelling results, provide strong evidence that the recent observed warming has been mostly caused by increased GHG concentrations resulting from human activities.
Greenhouse gas emissions
Carbon dioxide (CO2) levels in the atmosphere have increased by about 40% since the beginning of the industrial revolution, from around 280 to about 390 parts per million (ppm).
Records from ice cores confirm the CO2 concentration is now higher than for at least the past 800,000 years and that the extra CO2 in the air today has a chemical fingerprint that links it to fossil fuels.
Levels of other GHGs, such as methane and nitrous oxide, have also increased significantly since pre-industrial times – again driven by human emissions from fossil fuel burning, forest clearance and agriculture.
If GHG emissions continue unabated, average global temperatures may rise (relative to 1990 temperatures) by between 1.1 and 6.4°C by the end of this century1.
1 IPCC Fourth Assessment Report: the range of ‘likely’ values (all scenarios)
Global warming
As well as observed temperature increases, evidence of recent global warming is also shown by:
- continuing sea level rise
- an increase in water vapour in the atmosphere
- an increase in the heat stored in the oceans
- a major decline in Arctic sea ice volume and summer extent over recent decades.
Unless action is taken to reduce GHG emissions, there is a high risk of global warming well beyond a 2°C increase since pre-industrial times. This would have significant impact and could lead to severe, and possibly irreversible, damage to ecosystems and natural processes.
More information on climate change is available on the Met Office website.