The current tropospheric temperature trend from 1979-2016
is influenced by large, natural, interannual fluctuations which
if removed reveal a trend about a third less positive than is
directly measured (+0.155 down to +0.095 K dec−1
). This
underlying trend is essentially the same as calculated in CM94
(+0.09 K dec−1
) when only 15 years were available and who
determined the underlying trend at that time needed adjustment
upward, from −0.04 to +0.09 K dec−1
. We find that the influence
of the tropical oceans and mid-latitude SST indices on
the temperature trend has been essentially zero since 1979, so
that removing the cooling in the early part of the record from
the eruptions of El Chichon and Mt. Pinatubo dominates the
adjustment.
The assessment of tropospheric climate sensitivity from the
calculation of the underlying trend above requires significant
assumptions.
If we assume, among other things, that the
impact of the net of natural external and internal forcing
variations has not influenced the observed trend and that
anthropogenic forcing as depicted in the average of the IPCC
AR5 models is similar to that experienced by the Earth, then
observations suggest the tropospheric transient climate response
(TTCR) is 1.10 ± 0.26 K. This central estimate is likely less
than half that of the average of the 102 simulations of the
CMIP-5 RCP4.5 model runs also examined here (2.31 ± 0.20).
If this result is borne out, it suggests many explanations
including the possibility that that the average feedbacks of the
CMIP-5 generation of climate models are likely skewed to
favor positive over negative relative to what is present in the
actual Earth system. As noted, we cannot totally discount that
natural variability or errors in forcing might also account for
the discrepancy between modeled and observed TTCR. However,
given the facts that the processes controlling the uptake
of energy by oceans and the transfer of heat in the tropical
atmosphere are largely parameterized, it is not scientifically
justified to dismiss model error, possibly substantial, as one
source of the discrepancy.
Acknowledgements.
This research was supported under the
US Department of Energy, DE-SC0012638. We thank the
reviewers and editor for their helpful suggestions.
Edited by: Kyong-Hwan Seo