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Radiative forcing (RF) is an imbalance between the energy received by the earth and the energy that is radiated back to space. It is usually expressed as an amount of energy per surface area, in watts per square meter (W/m2). A positive forcing represents a situation where there is more energy coming in than there is going out, which leads to a warming of the system, and negative forcing leads to cooling.
Human activities have changed and continue to change the Earth’s surface and atmospheric composition. Some of these changes have a direct or indirect impact on the energy balance of the Earth and are thus drivers of radiative forcing. More...
Over the period of the industrial era, since 1750, solar and volcanic forcings are the two dominant natural contributors to global climate change.
Solar forcing: Satellite observations of total solar irradiance (TSI) changes since 1978 show quasi-periodic cyclical variation with a period of roughly 11 years. Longer-term forcing can be estimated by comparing solar minima (during which variability is least). This gives a slightly negative forcing change between the 2008 minimum and the 1986 minimum, and a slightly positive forcing since 1750. There is a high confidence that solar forcing is much smaller than the forcing caused by greenhouse gasses even if current abilities to project solar irradiance are extremely limited so that there is very low confidence concerning future solar forcing.
It has been hypothesized that changes in the cosmic rays that are associated to changes in solar activity affect climate through changes in clouds dynamics. While cosmic rays do enhance aerosol nucleation and may affect cloud condensation, the effect is considered too small to have a climate influence over the course of a solar cycle.
Volcanic aerosols: The forcing due to stratospheric volcanic aerosols is now well understood and there is a large negative forcing for a few years after major volcanic eruptions. For instance, the eruption of Mount Pinatubo, in 1991, caused a one-year negative forcing of about –3.7 W/m2. More...
3.3.1 Human activity leads to change in the atmospheric composition either directly (via emissions of gases or particles) or indirectly (via atmospheric chemistry).
Anthropogenic emissions have driven the changes in greenhouse gas concentrations during the industrial era (so since 1750). As the historical evolution of the concentrations of these gases is well known, and since their greenhouse properties are also well known and defined, calculating the radiative forcing (RF) due to greenhouse gases gives well defined values with a very high confidence. (Figure TS.6). Based on concentration changes, the RF of all greenhouse gases that are well-mixed into the atmosphere was in 2011 at 2.83 W/m2 [2.54 to 3.12]. Over the last 15 years, CO2 has been the dominant contributor to the increase in radiative forcing from greenhouse gases while methane and nitrogen oxide also are important contributors. Halocarbons, such as chloro-fluoro-carbons (CFCs) used in the past in cooling systems, among other things, are very powerful greenhouse gases, and despite being present in relatively very small amounts compared to CO2, also contribute to radiative forcing. The growth of forcing from all greenhouse gases is slower now than it was in the 1970s and 1980s because emissions from greenhouse gasses other than CO2 have been increasing more slowly5
Some greenhouse gases, such as ozone and water vapor 6, also contribute to anthropogenic forcing. In the lower atmosphere, ozone leads to a positive forcing, while in the upper atmosphere, the depletion of the ozone layer, induced in particular by chlorine atoms radicals produced from the decomposition of halomethanes, has led to negative forcing. Ozone is not emitted directly into the atmosphere; instead it is formed in the high atmosphere when oxygen reacts with ultraviolet light, and in the lower atmosphere when nitrogen oxide or hydrocarbons react with light. There is strong evidence that ozone in the lower atmosphere affects plants, and reduces their CO2 uptake, in turn contributing to the increase of CO2 in the atmosphere.
The impact of greenhouse gasses can be estimated in a number of ways, but the two main ways are in terms of Global Warming Potential (GWP) or in terms of Global Temperature change Potential (GTP). GWP expresses the radiative forcing brought by a particular greenhouse gas in reference to the warming potential of carbon dioxide, whereas GTP expresses the temperature change brought by this same gas, and it takes into account the response of the climate system. More...
5 In particular halocarbon gases production and uses have been regulated and some like CFCs progressively banned.
6 It should be noted that the main greenhouse gas present in the atmosphere is water vapor but its contribution to radiative forcing is considered constant
3.3.2 Aerosols are tiny liquid droplets or particles (such as dust from volcanos) that are in suspension in the atmosphere. They contribute to the energy balance by interacting with clouds formation, or directly by reflecting or absorbing light. There has been progress in the last few years on the understanding of the properties and distribution of aerosols but significant uncertainties remain due to difficulty of observation and their large variability. The overall radiative forcing produced by aerosols is negative (so they cause a cooling of the atmosphere), but with a large range of uncertainty.
The forcing from black carbon particles (BC) on snow and ice is assessed but with a low confidence to be slightly positive. It represents a global mean surface temperature change per unit forcing larger than from CO2 primarily because all of the energy is deposited directly into the cryosphere. Ice sheets, glaciers and sea ice reflects solar energy back into space (this reflectivity of the earth surface is called “albedo”) , and the dark deposition on ice has a feedback impact on climate that can be significant in the polar and other snow or ice covered regions.
Despite the large uncertainty ranges on the importance of aerosol forcing, there is a high confidence that aerosols have offset a substantial portion of the forcing due to greenhouse gases. Aerosol-cloud interactions can also influence the character of individual storms, but evidence for a systematic aerosol effect on storm or precipitation intensity is more limited and ambiguous. More...
3.3.3 There is robust evidence that anthropogenic land use changes, such as deforestation, have increased the land surface albedo ( which is different in a darker green forest than in a paler field, for instance). There are still uncertainties in the evaluation of the albedo of natural and managed surfaces (such as croplands, pastures) and the influence of the changes in land use over the last centuries is still debated. Changes in land use also causes other modifications such as changes in surface roughness and in river runoff that also have an impact on local temperatures and are difficult to quantify.
Persistent contrails from aviation is another contribution to a positive radiative forcing. This forcing can be much larger regionally but it does not seem to produce observable regional effects on either the mean or diurnal range of surface temperature. More...
Feedback mechanisms also play an important role in determining (future) climate change. Indeed, climate change may induce modification in natural cycles which may reinforce or dampen the expected temperature increase.
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