10
hence they are denoted ΔR F.^2 Red is used to represent warming (positive ΔRF); blue is
used to show cooling (negative ΔRF). These ΔRF curves are based on the GHG and
aerosol precursor abundances used to drive climate model simulations of IPCC ( 2013 ).
The RF of climate due to human release of CO 2 , CH 4 , and N 2 O is the primary focus of
this book. The historic ΔRF due to all anthropogenic GHGs (dashed red) exceeds that
of the CO 2 /CH 4 /N 2 O triplet (dotted red) by a small amount, with most of the difference
due to a class of compounds called Ozone Depleting Substances (ODS). Even though
ODS exhibit a greenhouse effect, they are generally not labeled as GHGs because their
most important detrimental effect is depletion of Earth’s ozone layer. Also, industrial
production of ODS has been successfully curtailed by the Montreal Protocol and the
effect of these compounds on climate will diminish in the future (WMO 2014 ).
The human release of pollutants that increase the burden of small particles in the
troposphere, known as aerosols, leads to a reduction in the RF of climate (blue line,
Fig. 1.3a). This occurs because many aerosols reflect sunlight. An estimate of ΔRF
due to aerosols provided by Meinshausen et al. ( 2011 ) is shown in Fig. 1.3a.^3 As
detailed below, there is considerable uncertainty in this term.
The total ΔRF due to human activity is shown by the solid red curve in Fig. 1.3a,
b. All told, human activities have increased the RF of climate by about 2.3 W m−2
between 1750 and present. Figure 1.3b shows that the time variation of total ΔRF
due to humans (red line) closely resembles the observed rise in global mean surface
temperature anomaly (black and grey lines). Below we conduct quantitative analy-
sis of these two quantities, both within this chapter as well as throughout Chap. 2.
Figure 1.4 details the change in RF of climate, between 1750 and 2011, due to
various factors.^4 Numerical values are from Chap. 8 of IPCC ( 2013 ). Error bars
denote the 5 and 95 % confidence intervals for each quantity. Processes that effect
RF of climate but are not regulated under GHG legislation, such as Stratospheric
and Tropospheric Ozone and Land Use Change, as well as minor terms such as
Contrails and Solar Irradiance, are also shown. The solid red lines of Fig. 1.3 con-
sider all of the terms shown in Fig. 1.4.
There are several aspects of Fig. 1.4 worth emphasizing. Human release of GHGs
has warmed climate, with CO 2 being the most important contributor. The combined
effect of the two other most important long-lived anthropogenic GHGs (CH 4 , N 2 O)
plus all of the ODS compounds has enhanced this CO 2 -based warming by about 60 %.
Tropospheric ozone (O 3 ), a pollutant harmful to human health and agriculture, has
warmed climate over the course of the Anthropocene by nearly as much as CH 4.
Tropospheric O 3 is regulated by air quality regulations that vary by country and focus
(^2) Delta is the first letter of the Greek word diaphorá, which means difference. Scientists often use
either Δ (capital delta) or δ (lowercase delta) to represent difference. The ΔRF data in Fig. 1.3 start
in 1765 because this is the first year for which numerical values are available (see Methods).
(^3) The blue line is the combination of the three terms: the direct radiative effect of aerosols, the
perturbation to the reflectivity of clouds induced by aerosols, and the darkening of snow caused by
the deposition of black carbon. This estimate includes the following types of aerosols: sulfate,
nitrate, mineral dust, as well as organic carbon and black carbon from both fossil fuel combustion
and biomass burning.
(^4) Figure 1.3 spans 1765–2011 whereas Fig. 1.4 tabulates ΔRF between 1750 and 2011. The differ-
ent start years are a result of how scientists who worked on IPCC ( 2013 ) handled various data
streams. This difference is inconsequential since human activity imposed very little influence on
ΔRF between 1750 and 1765.
1 Earth’s Climate System