C: Drivers of Climate Change

Natural and anthropogenic substances and processes that alter the Earth's energy budget are drivers of climate change.

Radiative forcing14 (RF) quantifies the change in energy fluxes caused by changes in these drivers for 2011 relative to 1750, unless otherwise indicated.

Positive RF leads to surface warming, negative RF leads to surface cooling.  

RF is estimated based on in-situ and remote observations, properties of greenhouse gases and aerosols, and calculations using numerical models representing observed processes. Some emitted compounds affect the atmospheric  concentration  of  other  substances.  

The  RF  can  be  reported  based  on  the concentration changes of each substance15. Alternatively, the emission-based RF of a compound can be reported, which provides a more direct link to human activities. It includes contributions from all substances affected by that emission.

The total anthropogenic RF of the two approaches are identical when considering all drivers. Though both approaches are used in this Summary, emission-based RFs are emphasized.

 

SUMMARY

Total radiative forcing is positive, and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750 (see Figure SPM.5). {3.2, Box 3.1, 8.3, 8.5}

Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climate change. Values are global average radiative forcing (RF15) partitioned according to the emitted compounds or processes that result in a combination of drivers. The best estimates of the net radiative forcing are shown as black diamonds with corresponding uncertainty intervals; the numerical values are provided on the right of the figure, together with the confidence level in the net forcing (VH – very high, H – high, M – medium, L – low, VL – very low). Albedo forcing due to black carbon on snow and ice is included in the black carbon aerosol bar. Small forcings due to contrails (0.05 W m–2, including contrail induced –2 cirrus), and HFCs, PFCs and SF6 (total 0.03 W m) are not shown. Concentration-based RFs for gases can be obtained by summing the like-coloured bars. Volcanic forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms. Total anthropogenic radiative forcing is provided for three different years relative to 1750. For further technical details, including uncertainty ranges associated with individual components and processes, see the Technical Summary Supplementary Material. {8.5; Figures 8.14–8.18; Figures TS.6 and TS.7}

 

  • The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W m−2 (see Figure SPM.5), and it has increased more rapidly since 1970 than during prior decades. The total anthropogenic RF best estimate for 2011 is 43% higher than that reported in AR4 for the year 2005.  This  is  caused  by  a  combination  of  continued  growth  in  most  greenhouse  gas concentrations and improved estimates of RF by aerosols indicating a weaker net cooling effect (negative RF). {8.5}

 

  • The RF from emissions of well-mixed greenhouse gases (CO2, CH4, N2O, and Halocarbons) for 2011 relative to 1750 is 3.00 [2.22 to 3.78] W m–2 (see Figure SPM.5). The RF from changes in concentrations in these gases is 2.83 [2.26 to 3.40] W m–2. {8.5}

 

  • Emissions of CO2 alone have caused an RF of 1.68 [1.33 to 2.03] W m–2 (see Figure SPM.5). Including emissions of other carbon-containing gases, which also contributed to the increase in CO2 concentrations, the RF of CO2 is 1.82 [1.46 to 2.18] W m–2. {8.3, 8.5}

 

  • Emissions of CH4 alone have caused an RF of 0.97 [0.74 to 1.20] W m−2 (see Figure SPM.5). This is much larger than the concentration-based estimate of 0.48 [0.38 to 0.58] Wm−2 (unchanged from AR4). This difference in estimates is caused by concentration changes in ozone and stratospheric water vapour due to CH4  emissions and other emissions indirectly affecting CH4. {8.3, 8.5}

 

  • Emissions of stratospheric ozone-depleting halocarbons have caused a net positive RF of 0.18 [0.01 to 0.35] W m−2 (see Figure SPM.5). Their own positive RF has outweighed the negative RF from the ozone depletion that they have induced. The positive RF from all halocarbons is similar to the value in AR4, with a reduced RF from CFCs but increases from many of their substitutes. {8.3, 8.5}

 

  • Emissions of short-lived gases contribute to the total anthropogenic RF. Emissions of carbon monoxide are virtually certain to have induced a positive RF, while emissions of nitrogen oxides (NOx) are likely to have induced a net negative RF (see Figure SPM.5). {8.3, 8.5}

 

  • The RF of the total aerosol effect in the atmosphere, which includes cloud adjustments due to aerosols, is –0.9 [–1.9 to −0.1] W m−2  (medium confidence), and results from a negative forcing from most aerosols and a positive contribution from black carbon absorption of solar radiation. There is high confidence that aerosols and their interactions with clouds have offset a substantial portion of global mean forcing from well-mixed greenhouse gases. They continue to contribute the largest uncertainty to the total RF estimate. {7.5, 8.3, 8.5}

 

  • The forcing from stratospheric volcanic aerosols can have a large impact on the climate for some years after volcanic eruptions. Several small eruptions have caused a RF of –0.11 [– 0.15 to –0.08] W m–2  for the years 2008–2011, which is approximately twice as strong as during the years 1999–2002. {8.4}

 

  • The RF due to changes in solar irradiance is estimated as 0.05 [0.00 to 0.10] W m−2. Satellite observations of total solar irradiance changes from 1978 to 2011 indicate that the last solar minimum was lower than the previous two. This results in a RF of –0.04 [–0.08 to 0.00] W m–2 between the most recent minimum in 2008 and the 1986 minimum. {8.4}

 

  • The total natural RF from solar irradiance changes and stratospheric volcanic aerosols made only a small contribution to the net radiative forcing throughout the last century, except for brief periods after large volcanic eruptions. {8.5}

 

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