56
and
over the last two decades the observed rate of increase in GMST has been at the lower end
of rates simulated by CMIP5 models.
In other words, the projections of ΔT by the CMIP5 GCMs tend to be too warm
based on comparison of observed and modeled ΔT for prior decades (Stott et al.
2013 ; Gillett et al. 2013 ). The trapezoid shown in Fig. 2.3 represents an expert
judgement of the upper and lower limits for the evolution of ΔT over the next two
decades. The vertical bar is the likely mean value of ΔT over the 2016–2035 time
period. This projection is meant to apply to all four RCPs: i.e., it considers the full
range of possible future values for CO 2 , CH 4 , and N 2 O between present and 2035.
Our analysis of the Paris Climate Agreement will be based on the CMIP5 GCM
output as well as calculations conducted using an Empirical Model of Global Climate
(EM-GC) developed by our group (Canty et al. 2013 ). The EM-GC is described in Sect.
2.2. While the EM-GC tool only calculates ΔT, this simple approach is computationally
efficient, allowing the uncertainty on ΔT of climatically important factors such as radia-
tive forcing by tropospheric aerosols and ocean heat content to be evaluated in a rigorous
manner. We then compare estimates of how much global warming over the 1979–2010
time period can truly be attributed to human activity (Sect. 2.3). Following a brief com-
ment on the so-called global warming hiatus (Sect. 2.4), we turn our attention to projec-
tions of ΔT (Sect. 2.5). The green trapezoid in Fig. 2.3 is featured prominently in Sect.
2.5: projections of ΔT found using the EM-GC approach are in remarkably good agree-
ment with this IPCC ( 2013 ) expert judgement of ΔT over the next two decades, lending
credence to the accuracy of our empirically-based projections.
2.2 Empirical Model of Global Climate
Earth’s climate is influenced by a variety of anthropogenic and natural factors.
Rising levels of greenhouse gases (GHGs) cause global warming (Lean and Rind
2008 ; Santer et al. 2013b) whereas the increased burden of tropospheric aerosols
offset a portion of the GHG-induced warming (Kiehl 2007 ; Smith and Bond 2014 ).
The most important natural drivers of climate during the past century have been the
El Niño Southern Oscillation (ENSO), the 11 year cycle in total solar irradiance
(TSI), volcanic eruptions strong enough to penetrate the tropopause as recorded by
enhanced stratospheric optical depth (SOD) (Lean and Rind 2008 ; Santer et al.
2013a), and variations in the strength of the Atlantic Meridional Overturning
Circulation (AMOC) (Andronova and Schlesinger 2000 ). Climate change is also
driven by feedbacks (changes in atmospheric water vapor, lapse rate,^8 clouds, and
the surface albedo in response to radiative forcing induced by GHGs and aerosols)
(^8) Lapse rate is a scientific term for the variation of temperature with respect to altitude. As shown
in Fig. 1.5, over the past 50 years the upper troposphere (~10 km altitude) has warmed by a larger
amount than the surface. When this type of pattern occurs, climate scientists conclude the lapse
rate feedback is negative, because Earth’s atmosphere is able to radiate heat into space more effi-
ciently. The interested reader is referred to a detailed yet accessible text entitled Atmosphere,
Clouds, and Climate (Randall 2012 ) for more information.
2 Forecasting Global Warming