2.2 Drivers of climate change

Changes in the atmospheric concentrations of GHGs and aerosols, land cover and solar radiation alter the energy balance of the climate system and are drivers of climate change. They affect the absorption, scattering and emission of radiation within the atmosphere and at the Earth’s surface. The resulting positive or negative changes in energy balance due to these factors are expressed as radiative forcing^[4], which is used to compare warming or cooling influences on global climate. {WGI TS.2}

Human activities result in emissions of four long-lived GHGs: CO₂, methane (CH₄), nitrous oxide (N₂O) and halocarbons (a group of gases containing fluorine, chlorine or bromine). Atmospheric concentrations of GHGs increase when emissions are larger than removal processes.

Global atmospheric concentrations of CO₂, CH₄ and N₂O have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years (Figure 2.3). The atmospheric concentrations of CO₂ and CH₄ in 2005 exceed by far the natural range over the last 650,000 years. Global increases in CO₂ concentrations are due primarily to fossil fuel use, with land-use change providing another significant but smaller contribution. It is very likely that the observed increase in CH₄ concentration is predominantly due to agriculture and fossil fuel use. The increase in N₂O concentration is primarily due to agriculture. {WGI 2.3, 7.3, SPM}

The global atmospheric concentration of CO₂ increased from a pre-industrial value of about 280ppm to 379ppm in 2005. The annual CO₂ concentration growth rate was larger during the last 10 years (1995-2005 average: 1.9ppm per year) than it has been since the beginning of continuous direct atmospheric measurements (1960-2005 average: 1.4ppm per year), although there is year-to-year variability in growth rates. {WGI 2.3, 7.3, SPM; WGIII 1.3}

The global atmospheric concentration of CH₄ has increased from a pre-industrial value of about 715ppb to 1732ppb in the early 1990s, and was 1774ppb in 2005. Growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. {WGI 2.3, 7.4, SPM}

The global atmospheric N₂O concentration increased from a pre-industrial value of about 270ppb to 319ppb in 2005. {WGI 2.3, 7.4, SPM}

Many halocarbons (including hydrofluorocarbons) have increased from a near-zero pre-industrial background concentration, primarily due to human activities. {WGI 2.3, SPM; SROC SPM}

There is very high confidence that the global average net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W/m² (Figure 2.4). {WGI 2.3, 6.5, 2.9, SPM}

The combined radiative forcing due to increases in CO₂, CH₄ and N₂O is +2.3 [+2.1 to +2.5] W/m², and its rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years (Figures 2.3 and 2.4). The CO₂ radiative forcing increased by 20% from 1995 to 2005, the largest change for any decade in at least the last 200 years. {WGI 2.3, 6.4, SPM}

Anthropogenic contributions to aerosols (primarily sulphate, organic carbon, black carbon, nitrate and dust) together produce a cooling effect, with a total direct radiative forcing of -0.5 [-0.9 to -0.1] W/m² and an indirect cloud albedo forcing of 0.7 [-1.8 to -0.3] W/m². Aerosols also influence precipitation. {WGI 2.4, 2.9, 7.5, SPM}

In comparison, changes in solar irradiance since 1750 are estimated to have caused a small radiative forcing of +0.12 [+0.06 to +0.30] W/m², which is less than half the estimate given in the TAR. {WGI 2.7, SPM}

Changes in GHGs from ice core and modern data

Figure 2.3. Atmospheric concentrations of CO₂, CH₄ and N₂O over the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). The corresponding radiative forcings relative to 1750 are shown on the right hand axes of the large panels. {WGI Figure SPM.1}

Radiative forcing components

Figure 2.4. Global average radiative forcing (RF) in 2005 (best estimates and 5 to 95% uncertainty ranges) with respect to 1750 for CO₂, CH₄, N₂O and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). Aerosols from explosive volcanic eruptions contribute an additional episodic cooling term for a few years following an eruption. The range for linear contrails does not include other possible effects of aviation on cloudiness. {WGI Figure SPM.2}