79This dependence of AAWR on OHC is illustrated by the colored symbols in Fig.
2.13, which show the best estimate (symbols) and range of AAWR (error bars) that
is found for each of the six OHC records. The three groupings of data points show
AAWR found using ΔT from CRU (Jones et al. 2012 ), GISS (Hansen et al. 2010 ),
and NCEI (Karl et al. 2015 ). Nearly identical values of AAWR are found, regardless
of which data center record is used to define ΔT. The mean value of the 18 empirical
determinations of AAWR in Fig. 2.13 is 0.109 °C/decade, with a low and high of
0.028 and 0.170 °C/decade, respectively. The notation 0.109 (0.028, 0.170) °C/
decade is used to denote the mean and range of this determination of AAWR.
Figure 2.13 also contains a graphical representation of AAWR extracted from the
41 GCMs that submitted results for RCP 4.5 to the CMIP5 archive (see Methods for
details on how AAWR from GCMs is found). The GCM values of AAWR are dis-
played using a box and whisker symbol. The middle line represents the median
value of AAWR from the GCMs; the box is bounded by the 25th and 75th percen-
tiles, whereas the whisker (vertical line) connects the maximum and minimum
values. The median value of AAWR from the CMIP5 GCMs is 0.218 °C/decade,
about twice our best estimate of the actual rate of warming caused by human activi-
ties. The 25th percentile lies at 0.183 °C/decade, which exceeds the empirically
determined upper limit for AAWR of 0.170 °C/decade over the time period 1979–
- In other words, the CMIP5 GCMs on average simulate an anthropogenically
induced rate of warming that is twice as fast as the actual climate system has warmed
a
b
Fig. 2.12 Sensitivity of Attributable Anthropogenic Warming Rate to ΔRF of aerosols. (a) 1/χ^2
from the EM-GC simulations in the lower panel; the larger the value, the better the fit; (b) values
of AAWR for 1979–2010, computed as the slope of ΔTHUMAN, for EM-GC simulations that use the
15 time series of aerosol ΔRF shown in Fig. 2.7a. AAWR is displayed as a function of aerosol ΔRF
in year 2011 (AerRF 2011 ). All calculations used the mean value of OHC computed from the six
datasets shown in Fig. 2.8. The best estimate for AAWR, found using five estimates weighted by
1/χ^2 , as well as the lower and upper estimates for AAWR, are indicated. See Methods for further
information
2.3 Attributable Anthropogenic Warming Rate