Policy brief on implications of the project results on the climate decision making process. 54 month

A. P. Siebesma

B. Stevens

S. Bony

CiteSeerX

 
 
 
 
 
EUCLIPSE 
 
EU Cloud Intercomparison, Process Study & Evaluation Project 
 
Grant agreement no. 244067 
 
 
 
 
 
Deliverable D0.15:    Policy brief on implications of the project results 
on the climate decision making process.   
 
Delivery date:     54 months 
 
Responsible partner:  KNMI 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                   EUCLIPSE Policy Brief
A. P. Siebesma, S. Bony, and B. Stevens
July 17, 2014
Synopsis: The FP7 European Union Cloud Intercomparison, Process Study and Evalua-
tion Project (EUCLIPSE) helped propel European climate science to the forefront of the eld
internationally. EUCLIPSE germinated one of the most important new international cli-
mate science initiatives, the World Climate Research Programme's grand science challenge
on clouds, circulation and climate sensitivity, and made major scientic contributions which
underpinned, and in some cases have already eclipsed, the fth assessment report of the IPCC
(AR5). As a result of EUCLIPSE it is now understood why climate sensitivity, a simple
measure of the expected average warming of the Earth from a doubling of atmospheric CO2,
remains the most vital issue in climate science. Cloud feedbacks have ceased to be a riddle
rapped in a mystery inside an enigma, and by linking cloud processes to the structure of the
atmospheric circulation and its propensity to change, a foundation has been laid for under-
standing regional climate changes. Building on the legacy of earlier EU projects (EUClouds
and EUROCS) EUCLIPSE has bridged the gaps between distinct cultures, irreversibly linking
experts in atmospheric processes to broader questions of climate change, in so doing it has
helped develop a uniquely European expertise on some of the most critical questions in climate
science, by organically linking activities and interests of Europe's leading climate research lab-
oratories with one another, and with broader expertise within the university community.
Scientic Advances
Climate sensitivity is still the most vital question in climate science
It is easy to think that the question of the equilibrium climate sensitivity, that is the globally
averaged warming Earth's surface required to balance the radiative forcing from a doubling
of the concentration of atmospheric CO2; is pass e, an academic abstraction largely irrelevant
to policy makers, planners, or the broader society. EUCLIPSE has made important contribu-
tions to helping us understand that this is far from the case. Many of the more ne-grained
aspects of the climate system appear to be directly related to the equilibrium climate sensi-
tivity. For instance, projections of changes in European temperatures, particularly during the
summer, are directly proportional to the climate sensitivity. More broadly, although dierent
models predict dierent patterns of changes, the amplitude of a particular models projected
climate change pattern scales remarkably well with the change in the globally averaged sur-
face temperature. Finally uncertainty in changes in globally averaged precipitation have been
shown to be dominated by uncertainty in globally averaged surface temperature changes, i.e.,
the equilibrium climate sensitivity. This work, through EUCLIPSE, contributes to work by
others in the broader community, which shows that the climate sensitivity is associated with
1physiological limits to adaptation, and that the transient climate response is both propor-
tional to and controlled by the same factors as that of the equilibrium climate sensitivity.
Hence narrowing climate sensitivity remains the single most vital question of climate science,
as no other single number conditions so many aspects of climate change. [1, 2, 3]
Clouds feedbacks are no longer a riddle wrapped in a mystery inside an
enigma
Of the major feedbacks on radiative forcing, most (like the surface albedo or water vapor
feedback) were identied more than a century ago, when scientists rst begin thinking about
climate change. The idea that clouds might be an important in determining the equilibrium
climate sensitivity really only was raised a few decades ago, and even then there were no
clear ideas why clouds should change with a changing climate { a point that gains emphasis
when it is realized that then, and now, it has been widely assumed that relative humidity
will not change markedly with warming. EUCLIPSE is responsible for the most important
new ideas in decades as to why and how climatologically important low-clouds might change
in a changing climate, even if the processes that regulate the large-scale patterns of relative
humidity change little. EUCLIPSE investigators showed how water vapor uctuations, an
important driver of turbulent mixing in the lower troposphere, increase with warming if
the relative humidity stays constant, and that this implies more ecient lower tropospheric
mixing which would tend to deepen and dry the lower layers of the troposphere, reducing
cloudiness over the ocean, and amplifying warming. Such a situation is also thought to be
energetically favored because a constant relative humidity implies a larger vertical gradient
in absolute humidity, and so less radiative cooling is required by clouds to balance the energy
budget of the troposphere. These ideas have proven powerful in understanding the response
of climate models to forcing, and helped break a long-standing deadlock in our understanding
of the climate system, namely whether or not clouds are contributing to or mitigating against
forced changes in the climate system. Because of work in EUCLIPSE it is now thought that
clouds act to amplify warming, although to what degree remains open. Before EUCLIPSE it
was not understood why cloudiness might depend on Earth's surface temperature, let alone
in which sense. Clouds, once a riddle wrapped inside a mystery inside and enigma, are no
longer enigmatic, and while riddles remain, even some of the mystery has been removed.
[4, 5, 6, 7, 8]
Clouds matter for more than warming
The overall distribution of dry and wet areas over the globe is primarily related to the large-
scale atmospheric circulation. In the tropics for instance, wet areas are located in the moist,
ascending branches of the tropics (associated with what are known as the Hadley and Walker
Circulations) where moisture converges and rises, giving rise to precipitation, while dry areas
are located in the descending branches of this circulation. A reason why climate models exhibit
great diculties in predicting the regional distribution of tropical rainfall over a large range
of timescales is that they fail to represent this large-scale circulation properly. EUCLIPSE
helped understand why this is so, and proposed ways to improve models. The long-standing
tendency of models to produce a double ascending branch of the tropical circulation over
equatorial oceans instead of a single one as shown by observations (sometimes called the
'double-ITCZ' problem) could be alleviated by increasing the amount of turbulent mixing
2between clouds and the environmental air around them, and better capturing their coupling
to patterns of surface evaporation. These studies, and others supported by EUCLIPSE, also
showed that a more realistic representation of the tropical convergence zones also improved
the representation of important modes of tropical variability, including phenomena such as the
Madden-Julian Oscillation and ENSO, both of which are critical to patterns of weather well
away from the tropics. EUCLIPSE studies also showed how low-cloud radiative eects play an
important role in determining the aridity of the arid regions of the tropics, and the strength
of the tropical circulation and its response to external forcing, such as might accompany an
increase in aerosols in the northern hemisphere as a result of human activity. Few aspects of
the climate system matter as much for humankind as the large-scale distribution, amount and
variability of rainfall over the globe, EUCLIPSE has greatly contributed to understanding how
and why the representation of cloud and cloud-radiative processes in models is so critical for
predicting these aspects of the large-scale circulation, and hence why models have struggled
to represent such changes more robustly. [9, 10, 11, 12, 13, 14]
CO2 also directly controls rainfall
For decades, CO2 has been thought to inuence the climate system mostly through its impact
on surface temperatures. An important contribution from EUCLIPSE has been to show that
CO2 can also inuence climate independently of its inuence on surface temperature. In
particular, EUCLIPSE demonstrated that higher CO2 amounts in the atmosphere weaken
the strength of large-scale rising and sinking motions of the atmosphere, hence aecting
the amount and distribution of rainfall at the regional scale, even in the absence of global-
mean surface temperature changes. This \direct" eect of CO2 on the atmosphere, which is
reproduced by most models and can be physically understood from basic physics principles,
occurs very fast (within just a few days after an increase in CO2) but contributes signicantly
to the long-term change in circulation and rainfall. An understanding of this direct eect has
changed the way we now interpret regional precipitation changes, their dependence on the
nature of anthropogenic forcing, and the reasons why some aspects of regional projections
are robust while others dier across models. This work shows that not only changes in global
precipitation, but also changes in patterns of precipitation, will be a residue of increasing
concentrations of CO2: As is the case for ocean acidication, changes in precipitation will
remain, even if eorts to mitigate against surface warming are successful but fail to address
rising concentrations of atmospheric CO2: [3, 15, 16]
Legacy
The Grand Science Challenge on Clouds Circulation and Climate Sensitivity To
guide its eorts over the coming decade the World Climate Research Programme launched an
initiative of six Grand Science Challenges. One of these, on Clouds Circulation and Sensitivity
germinated from the EUCLIPSE project, as its two lead coordinators are EUCLIPSE work-
package leaders, and of the twelve coordinators, ve are from Europe, and of these four formed
the core team within EUCLIPSE. This particular Grand Science Challenge will be the focus
of WCRP attempts understand the physical climate system, as such it is helping prioritize
new satellite missions, experimental strategies within projects such as CMIP (the coupled
model inter-comparison project) and activities within the core projects of WCRP, through its
3leadership of the Grand Science Challenge EUCLIPSE has helped give European scientists a
leading role within the initiative.
Guidance for parameterization: EUCLIPSE developed new parameterizations, new ap-
proaches for parameterizations, and a deeper understanding of some of the critical issues facing
parameterization. For future parameterization EUCLIPSE demonstrated how important the
coupling's among parameterizations are, whether it be between circulation and convection,
convection and clouds, turbulence and convection, or clouds and radiation. As a rule, the
couplings among the parameterizations were shown to plays as much, or more, of a role than
the specics of any particular parameterization. EUCLIPSE demonstrated the power of nu-
merical weather prediction techniques, single column model approaches, and high-resolution
process modeling for advancing parameterization.[17, 18, 13]
New Frameworks for studying climate change EUCLIPSE advanced the integration
of existing frameworks, and the development of new frameworks, to study climate change.
The use of a hierarchy of models, from single column models, to high-resolution process
models, to short time integrations of advanced climate models EUCLIPSE has shown how
this hierarchy of models must be used in combination to solve important problems in climate
science. EUCLIPSE advanced the use of new diagnostic techniques to better link observations
to modelling, both through the development of simulators that are now routinely imbedded
in climate models, but also through the development of high frequency grid-point output
for comparison with ground stations. Finally EUCLIPSE advanced the model hierarchy
by showing the power of new idealizations, ranging from three dimensional calculations of
radiative convective equilibrium using Earth System Models, to the applicability of reduced
frameworks like aqua planets for studying climate change. [13, 19, 20, 21, 22, 23]
Training of a new generation of climate scientists In addition to training a new
generation of scientists, EUCLIPSE has helped advance the question of the role of clouds,
circulation and climate more broadly { both through a book, and through a summer school.
The book, entitled Clouds and Climate will be published by Cambridge University Press in
2016, consists of thirteen chapters authored by EUCLIPSE investigators, and will help dene
the background and challenges for the next generation of climate scientists. The summer
school was designed to help develop the chapters within the book and attracted fty-ve
advanced PhD students and young postdocs for a two week period in the French Alps in the
summer of 2013. Through these activities, and through its contribution to Chapter Seven
in the AR5, EUCLIPSE has helped dene a new, and increasingly recognized, sub-discipline
within climate science more broadly.
Contribution to the IPCC Fifth Assessment Report (AR5) EUCLIPSE PIs and
science featured very prominently in the IPCC report. The EUCLIPSE community was
fundamental in establishing clouds and aerosols as one of the new process chapters to be
included in the AR5. EUCLIPSE contributions featured prominently in this report, with
gures developed through EUCLIPSE projects featuring directly in the report, and some of
the major ndings, i.e., mechanisms for positive low cloud feedbacks which supported the
IPCC assessment of a likely positive cloud feedback, or the eect of CO2 on regional patterns
of precipitation change, arising directly from research within EUCLIPSE.
4References
[1] Cattiaux, J., Douville, H. & Peings, Y. European temperatures in CMIP5: origins of
present-day biases and future uncertainties. Climate Dynamics 41, 2889{2907 (2013).
[2] Stevens, B. et al. Atmospheric component of the MPI-M Earth System Model: ECHAM6.
J. Adv. Model. Earth Syst. 5, 146{172 (2013).
[3] Popke, D., Popke, Mauritsen, T. & Stevens, B. Inter-model spread in hydrological
sensitivity is small. J. Adv. Model. Earth Syst. In preparation (2014).
[4] Brient, F. & Bony, S. Interpretation of the positive low-cloud feedback predicted by a
climate model under global warming. Climate Dynamics 40, 2415{2431 (2013).
[5] Rieck, M., Nuijens, L. & Stevens, B. Marine Boundary Layer Cloud Feedbacks in a
Constant Relative Humidity Atmosphere. J. Atmos. Sci 69, 2538{2550 (2012).
[6] Webb, M. J. & Lock, A. P. Coupling between subtropical cloud feedback and the local
hydrological cycle in a climate model. Climate Dynamics 41, 1923{1939 (2012).
[7] Sherwood, S. C., Bony, S. & Dufresne, J.-L. Spread in model climate sensitivity traced
to atmospheric convective mixing. Nature 505, 37{42 (2014).
[8] Gesso, S. D., Siebesma, A. P., de Roode, S. R. & van Wessem, J. M. A
mixed-layer model perspective on stratocumulus steady states in a perturbed cli-
mate. Quarterly Journal of the Royal Meteorological Society n/a{n/a (2014). URL
http://dx.doi.org/10.1002/qj.2282.
[9] Oueslati, B. & Bellon, G. Tropical precipitation regimes and mechanisms of regime
transitions: contrasting two aquaplanet general circulation models. Climate Dynamics
40, 2345{2358 (2012).
[10] M obis, B. & Stevens, B. Factors controlling the position of the Intertropical Convergence
Zone on an aquaplanet. J. Adv. Model. Earth Syst. 4, 1{16 (2012).
[11] Oueslati, B. & Bellon, G. Convective Entrainment and Large-Scale Organization of
Tropical Precipitation: Sensitivity of the CNRM-CM5 Hierarchy of Models. J. Climate
26, 2931{2946 (2013).
[12] Crueger, T., Stevens, B. & Brokopf, R. The Madden{Julian Oscillation in ECHAM6 and
the Introduction of an Objective MJO Metric. J. Climate 26, 3241{3257 (2013).
[13] Fermepin, S. & Bony, S. Inuence of low-cloud radiative eects on tropical circulaton
and precipitation. J. Adv. Model. Earth Syst. 1{34{ in press (2014).
[14] Voigt, A., Bony, S., Dufresne, J.-L. & Stevens, B. The radiative impact of clouds on the
shift of the inter-tropical convergence zone. Geophys. Res. Lett. 1{27{ in press (2014).
[15] Bony, S. et al. Robust direct eect of carbon dioxide on tropical circulation and regional
precipitation. Nature Geoscience 6, 447{451 (2013).
[16] Sherwood, S. et al. Adjustments in the forcing-feedback framework for understanding
climate change. Bull. Amer. Meteorol. Soc 1{41 submitted (2014).
5[17] Neggers, R. A. J. & Siebesma, A. P. Constraining a system of interacting parameteriza-
tions through multiple-parameter evaluation. J. Climate 26, 6698{6715 (2013).
[18] Brient, F., Bony, S., Schneider, T. & Tan, Z. On the antagonist role of convection and
turbulence on trpical-low-cloud feedback in climate models. Climate Dynamics in press.
[19] Bodas-Salcedo, A., Williams, K. D. & Ringer, M. A. Origins of the Solar Radiation
Biases over the Southern Ocean in CFMIP2 Models*. Journal of ... (2014).
[20] Webb, M. J. et al. The diurnal cycle of marine cloud feedback in climate models . Climate
Dynamics 1{37{ in press (2014).
[21] Popke, D., Stevens, B. & Voigt, A. Climate and climate change in a radiative-convective
equilibrium version of ECHAM6. J. Adv. Model. Earth Syst. 5, 1{14 (2013).
[22] Nuijens, L., Medeiros, B., Sandu, I. & Almgren, A. S. The behavior of trade-wind
cloudiness in observations and models. Part I: the major cloud components and their
variability. J Geophys Res-Atmos to be submitted. (2014).
[23] Medeiros, B., Stevens, B. & Bony, S. Using aquaplanets to understand the robust re-
sponses of comprehensive climate models to forcing. Climate Dynamics (2014).
6

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