Earth's global mean temperature would be -18C were it not for Earth's atmosphere blanketing and warming the planet to +15C, creating conditions in which humans have thrived over the last 10,000 years. But an overabundance of greenhouse gases emitted from industrial activities has increased the atmosphere's global warming effect, affecting climate, biodiversity and human progress.
4 min read | Last updated 5 March 2021
Over the last 10,000 years, Earth has been a pleasant and productive planet for humans, thanks to a global mean temperature that suits our species. Global mean temperature is modulated by Earth’s atmosphere.
Climate scientists use the term ‘radiative forcing’ to describe the difference between Earth’s incoming solar radiation and its outgoing terrestrial infrared radiation (the heat that Earth emits back into space). When these opposite energy flows are in balance, Earth is said to be in thermal equilibrium and its global mean temperature remains constant.
Earth’s global mean temperature would be about -18C were it not for Earth’s atmosphere. It allows solar radiation to pass through, but it absorbs some of the infrared energy being emitted by Earth and then it emits some of that infrared energy back to Earth again. This process makes a +33c difference, maintaining the planet at about 15C, supporting the climate and biogeochemical cycles that modern humans live within.
Earth has been warming due to changes in radiative forcing. Changes in radiative forcing and global mean temperature are measured relative to pre industrial values, circa 1750, which are set at zero. Radiative forcing has increased by 2.29 Watt per metres squared (Wm-2), resulting in a 1.2C increase (or anomaly) in global mean temperature.
The atmosphere is a mix of gases, comprising 78.09% nitrogen, 20.95% oxygen and 0.93% argon, none of which affect radiative forcing. The other 0.03% is a brew of many gases, including the so-called greenhouse gases (GHGs), which do affect radiative forcing. When the concentration of GHGs in the atmosphere stays stable, the atmosphere’s warming effect stays stable. A change in the mix or concentration of GHGs causes a change in radiative forcing (NASA, 2019).
GHGs include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and many kinds of fluorinated gas. These long-lived gases are well mixed, which means they are spread evenly in the atmosphere around the world and don’t sit in pockets above the area of the globe from where they are emitted. Water vapour is also a GHG – in fact, it is the largest GHG by volume – but it is very short lived, dissipating in hours.
GHGs are naturally part of the biogeochemical cycles in the Earth system, by which chemical elements are moved between the lithosphere (rock and soil), the cryosphere (ice), the oceans, the biosphere, the technosphere (human systems) and the atmosphere. The carbon, methane and nitrogen cycles are described in the images below, showing typical sources of CO2, CH4 and N2O emissions into the atmosphere.
Each GHG’s contribution to radiative forcing depends on three attributes: persistence, global warming potential and atmospheric abundance. A molecule of CO2 can persist in the atmosphere for 1000 years, CH4 persists for 12 years and N2O persists for 114 years. GHGs absorb terrestrial infrared energy that would otherwise be emitted out to space, trapping it in the atmosphere. Global warming potential (GWP) is a measure of how much energy one tonne of a gas will absorb over a given period of time, relative to one tonne of CO2. Relative values vary with time horizon. CO2 is used as a reference value with a GWP of 1 over any time horizon. For a 20-year time horizon, the GWP of CH4 is 72 and that of N2O is 289, which means that, in the atmosphere, CH4 absorbs 72 times more energy while N2O absorbs 289 times more energy than the same weight of CO2 (IPCC, 2018 (pdf)). The abundance of GHGs in the atmosphere has increased since preindustrial times (see table below). For instance, there is 2.5 times as much CH4 now as there was 200 years ago (European Environment Agency, 2019).
Abundances of GHGs in Earth’s Atmosphere
|GHG||yr 1800||yr 2020||% increase|
Source: Global Monitoring Laboratory
The last time the concentration of atmospheric CO2 was more than 400ppm was more than 3 million years ago, when Earth’s global mean temperature was 2C-3C higher than the preindustrial era and sea level was 15m–25m higher than today (Climate.gov, 2020).
Factors that change radiative forcing can cause warming or cooling and can come from natural or anthropogenic sources. Natural factors include solar irradiance (warming) and volcanic aerosols (cooling), while anthropogenic factors include GHG emissions (warming) and changes in land use that make the surface of Earth more reflective (cooling) or less reflective (warming).
The combined radiative forcing of anthropogenic factors since 1750 is estimated to be +2.29 Wm-2 (brown bar in the image below), being emissions of well-mixed GHGs, short-lived gases and aerosols and other factors, such as land use change, aircraft contrails and various fluorocarbons. The only significant natural forcing factor since 1750 has been a small quantity of solar irradiance, amounting to +0.05 Wm-2.
Scientists are very confident about these numbers, to a 95% probability (IPCC, 2014 (pdf)). They are also confident that the increase in CO2 in the atmosphere is not from natural sources, but is due to carbon produced by burning fossil fuels, which has a specific (therefore, traceable) ratio of heavy-to-light carbon atoms.
The result is that Earth has warmed 1.1C since the 1850-1900 period, and since 1980 has been warming at a rate of 0.19C per decade. At this rate, excluding the effects of reaching tipping points, the temperature anomaly will reach 2C by 2063 (Berkeley Earth, 2019).