Conflicting expectations of global surface warming

106 Pages·2017·9.4 MB·English

Conflicting Expectations of Global Surface
Warming
Christopher Hedemann
Hamburg 2017
196
Berichte zur Erdsystemforschung
Reports on Earth System Science 2017

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Christian Klepp, Clotilde Dubois,
Christian Klepp, Katsumasa Tanaka
Conflicting Expectations of Global Surface
Warming
Dissertation with the aim of achieving a doctoral degree
at the Faculty of Mathematics, Informatics and Natural Sciences
Department of Earth Sciences of Universität Hamburg
submitted by
Christopher James Hedemann
Hamburg 2017
Christopher Hedemann
Max-Planck-Institut für Meteorologie
Bundesstrasse 53
20146 Hamburg
Tag der Disputation: 14.07.2017
Folgende Gutachter empfehlen die Annahme der Dissertation:
Prof. Dr. Jochem Marotzke
Prof. Dr. Johanna Baehr
Prof. Dr. Gabriele Hegerl
Berichte zur Erdsystemforschung / Max-Planck-Institut für Meteorologie 196
Reports on Earth System Science / Max Planck Institute for Meteorology 2017
ISSN 1614-1199
Typeset using the classicthesis template developed by André Miede, available at:
https://bitbucket.org/amiede/classicthesis/

For Maree
1954 2013
–

ABSTRACT
In this dissertation I examine processes that cause Earth’s surface
warming to deviate from what we might expect. Using frameworks
thatincorporateregionalandglobalenergyexchange,Iscrutinisepre-
vious theories for why these deviations occur.
1998 2012
Thefirstpartofthisthesisexaminesthe – surface-warming
hiatus, in which the surface warmed more slowly than might be ex-
pected from examining model simulations or the long-term trend in
observations. The preferred explanation for thehiatus is that internal
variability in regional ocean heat uptake caused the surface warming
to slow. However, observational analyses disagree about the ocean
basininwhichthedefinitiveheatuptakeoccurred.Energybudgeting
100
for the ocean surface layer, over a -member historical ensemble of
simulations, reveals that variability in the top-of-atmosphere balance
couldalsohavecausedthehiatus.Althoughpreviousstudieshaveat-
tributed the hiatus to fluctuations as large as 0.5 Wm−2, I show that
aslittleas0.08Wm−2 couldbenecessary.Thesensitivityoftheseflux
deviations to the observational dataset and to energy budget choices
helps explain why previous studies conflict, and suggests that the
origin of the recent hiatus may never be identified.
The second part of this thesis examines how climate sensitivity in
model simulations grows with surface warming. The ‘pattern effect’
theory attributes this phenomenon to changing spatial patterns of
warming, but previous accounts of the pattern effect disagree. I pro-
poseanewframeworktounitetheoriesabouthowregionalprocesses
1000
affect climate sensitivity, and apply the framework to -year sim-
ulations with a coupled climate model, exposed to abrupt CO in-
2
creases up to sixteen-times pre-industrial concentrations. Applying
theassumptionsofpreviousstudiestothemodeloutputleadstomis-
diagnosis of radiative forcing. Furthermore, the fact that past studies
find different critical regions for the pattern effect may result from
theirassumptionsandnotdivergentmodelbehaviour.Thepatternef-
fect in the four simulations depends partly on the time elapsed since
theforcingincrease,andnotmerelythesurfacetemperature,suggest-
ing that current observations could underestimate climate sensitivity.
Bothpartsofthisthesisrepresentareasoftensioninclimatescience
between different perspectives and tools. Normative understandings
may presume the superiority of empirical measurement over model
simulation,orthesuperiorityofthedetailedregionalperspectiveover
the general global perspective. However, the findings presented in
thisthesisservetohighlightthepitfallsofrestrictingourselvestoone
tool or view in the endeavour to understand climate.
v