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Clouds Hearing - Testimony Alan K. Betts (Notes and Notes to Tony)
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Clouds Hearing - Testimony Alan K. Betts (Notes and Notes to Tony)
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FOIA Number: 2012-0769-F
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This is not a textual record. This is used as an
administrative marker by the William J. Clinton
Presidential Library Staff.
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Clinton Presidential Records
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Council on Environmental Quality
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Kathleen (Katie) McGinty
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2896
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Clouds Hearing - Testimony Alan K. Betts (Notes and Notes to Tony)
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61
6
3
2
SENATE COMMERCE COMMITTEE MEETING
ON ROLE OF CLOUDS IN CLIMATE CHANGE
October 7, 1991
Statement by:
ALAN K. BETTS
545-2481
Aunospheric Research
RD2, Box 3300, Middlebury, VT 05753
Subject:
GLOBAL WARMING AND THE TROPICAL WATER BUDGET
1.
Background
I was born and educated in England. with a BA (and MA) in Natural Sciences (Theoretical
Physics) from the University of Cambridge in 1967, and a PhD in Meteorology from Imperial
College, of the University of London, in 1970. I came to the Department of Atmospheric
Science at Colorado State University in 1970 to work on the analysis of a tropical field
experiment in Venezuela (VIMHEX: the Venezuelan International Meteorological and
Hydrological Experiment), in which I had participated in 1969. I remained on the academic
faculty at CSU until 1978, when I moved to Vermont to continue my research work
independently. For the past 13 years, I have lived and worked in Vermont, supported by
direct grants and contracts from the National Science Foundation and the National Aeronautics
and Space Administration.
My research has dealt with atmospheric convection and clouds; their description by
observations and theory, and the representation of their energy and water transports in global
models, both numerical weather prediction and climate models. In the 1970s my research
concentrated on convection in the tropics, and I helped plan and execute the Global
Atmospheric Research Program Atlantic Tropical Experiment in 1974 (GATE) (Betts, 1974,
Houze and Betts, 1978), as well as a second VIMHEX Experiment in 1972. For this research,
I was awarded the Richardson Prize of the Royal Meteorological Society in 1974, the
Meisinger Award in 1977 and a Special Award in 1978 by the American Meteorological
Society. I co-ordinated the convection research for GATE for the National Academy of
Science panel. In the past decade my research has broadened to cover as well the analysis of
clouds and convective boundary layer processes over land and the oceans, both in the tropics
and (mid-latitudes, and the representation of convective processes in global models. I am
currently a member of the National Academy panel for the International Satellite Cloud
Climatology Project.
2.
Water vapour and the climate system
The earth's climate system, which involves not only the atmosphere, but the ocean circulations
and the biosphere, is exceedingly complex. As a result of theoretical studies, a global
observing system, and the development of computers fast enough to model the global
atmosphere at sufficiently high resolution, we have made remarkable progress in our
understanding in the past 20 years. Yet our present models are still extremely simple, when
compared with the complexity of the real climate system of the earth. As a result our global
climate models are only as good as the conceptual components of which they are made.
I wish to discuss the links in the chain between the known global increase of the gases such
as CO2, CH4 and the CFC's, which are due to anthropogenic activity, and the changes of
temperature, water vapour and clouds in the tropics; which in turn primarily control the
surface equilibrium temperature of the earth. I will focus my discussion on water vapour:
others here will discuss the radiative effects of the clouds themselves. Water vapour is the
major absorber in the infrared contributing to the greenhouse effect. Unlike the other
greenhouse gases, whose global concentrations are slowly increasing because of anthropogenic
activity, the concentration of water vapour is controlled by the transport and precipitation
processes in clouds. Generally the water vapour content increases with temperature, because
the saturation vapour pressure increases with temperature. In the tropics, water evaporates
primarily from the oceans. which are heated directly by incoming solar energy, rises in
cumulonimbus clouds (thunderstorms), where it condenses. releasing the latent heat of
condensation and freezing. This latent heat release both drives the vertical circulation and
balances the net infrared cooling of the atmosphere. The air which flows out of these storms
in the upper troposphere is full of ice. which forms the extensive picturesque anvils so
characteristic of thunderstorms. In the tropics, clusters of thunderstorms produce extensive
anvil clouds, which are readily visible on satellite pictures. These anvils, which are often 400
mb thick. last for many hours, sometimes even a day, since they are maintained by dynamical.
convective and radiative processes. As they decay and their ice crystals evaporate, they are
responsible for most of the water vapour input to the upper troposphere. Estimates from
GATE (Gamache and Houze, 1983) have suggested that up to 10% of the water which is
evaporated at the ocean surface is injected into these thick anvils. Away from convective
disturbances where air sinks as it slowly cools radiatively, the relative humidity in the tropical
upper troposphere is around 30 - 40% up to about 350 mb, where routine rawinsonde
350ml
measurements become unreliable. At higher levels. the data from the Stratospheric Aerosol
and Gas Experiment (SAGE-II) suggest that the relative humidity (away from deep
convection) is about 20-25% at 200 mb. The annual variation is small, with higher values on
the summer side of the equator.
ahove hove SAGE have upto have raw.nsal
3.
Lindzen's argument
Lindzen (1990) has argued that there is a possibility that deep convection might dry the upper
troposphere. This is important because absolute decreases of water vapour at high levels can
offset the effect of increases at low levels in their greenhouse effect.
His argument is as follows. Boundary layer temperature and moisture increase with
increasing sea surface temperature, so that the tops of the deep convective clouds in the
tropics will go higher. This is correct. If the temperature at the tropopause falls, then the
highest outflows from these clouds will be at colder temperatures. Since the saturation mixing
ratio over ice decreases with temperature, Lindzen argues that the cumulonimbus outflow will
therefore inject drier air at the highest outflow level; and then the subsidence of this dry air
between clouds will produce a drier upper troposphere.
This
idea is wrong (Berts, 1990), because deep convective clouds inject water (as ice and
vapour) into the upper troposphere at all levels above their freezing level (this is near the
middle troposphere in the tropics: about 550 mb). not just at their tops. Near their highest
ourflow levels. most of this water is in the form of ice (the anvil clouds). so that the
comparatively small value of the saturation mixing ratio plays a relatively insignificant role
in the water budget at the highest levels. If Lindzen's model were right. the present upper
troposphere would be very dry. In fact, the relative humidity near the freezing level in the
deep tropics is about 40%, This is consistent with a subsidence of order 20 mb/day (which
is roughly in thermal balance with the radiative cooling rate), and the injection of 10% of the
surface evaporation as ice into the upper troposphere (mentioned above).
Lindzen's model is implausible on more general grounds. If the sea surface temperature rises
in the tropics, the troposphere as a whole is warmed to a higher temperature, so that, at any
given pressure, the saturation mixing ratio increases significantly. For the absolute mixing
ratio to decrease at any level, the relative humidity must therefore decrease significantly (by
12. - 15% for a 2°C rise in sea surface temperature). The theoretical evidence is to the
contrary. If the temperature warms (towards a warmer "moist adiabat") the stability of the
atmosphere increases proportionally more than the increase in the radiative cooling rate, so that
the mean sinking motion between the convective systems decreases (Betts and Ridgway,
1989). This reduced subsidence would tend to increase (not decrease) the relative humidity
at any level (Betts and Albrecht, 1987). In this simple coupled model of Betts and Ridgway
(1989), doubling CO2 increased the tropical surface temperature by 2 to 3°C, consistent with
the increase shown by global circulation models.
Clearly, we need better observations of water vapour in the tropical upper troposphere, but the
evidence we have is consistent with a relatively tight coupling of water vapour to temperature
in which the relative humidity changes rather little.
4.
Conclusion
I have concentrated on the role of water vapour in the tropics on the greenhouse effect. It is
probable that water vapour, because it is coupled thermodynamically to the temperature, will
increase as the temperature of the troposphere increases. This coupling leads to an important
amplification of the greenhouse effect, as is shown by both global climate models (e.g. Cess
et al.,1990) energy balance models (e.g. Betts and Ridgway, 1989) and observational studies
(e.g. Raval and Ramanathan, 1989). The radiative role of clouds is more complex, and others
here will discuss this in more detail.
As a society we have many choices, but one basic dilemma concerning climate change. The
waste products of our civilizarion are significantly modifying the atmosphere, making it more
opaque to infrared radiation. The coupling of temperature and water vapour amplify this
effect, while the effects of changes in cloudiness are more complex. Modelling studies
suggest that a warming of the atmosphere of several degrees will result if present trends
continue. Because of the natural fluctuations of the climate system. it may be a decade or
more before we can have confidence that the recent climate warming reflects a long-term
trend. Because of the complexity of the earth system, our relatively simple models might
mislead us, but again the improvement of our models is a steady incremental process which
also takes decades.
*
If we wait a decade or two for increased certainty, the climate impact will be much larger,
harder to reverse, and the impact on the earth's natural ecosystem may be severe. A wise
society would take action now to reduce emissions to the aumosphere, in order to limit and
delay the long-term impact on our climate. Much can be done at little incremental cost. The
scientific community can present the evidence, but political resolve and global co-operation
are clearly needed. Changing the direction of our industrial society towards one which is
more respectful of its impact on our global environment will take time, probably decades, even
if we begin now.
References:
Betts, A.K., 1974: The scientific basis and objectives of the US Convection Subprogram for
the GATE. Bull Amer. Meteor. Soc., 55, 304-313.
Betts. A.K., 1990: Greenhouse warming and the tropical water budget. Bull. Amer. Meteor.
Soc., 71, 1464-1465.
Betts, A.K., and B.A. Albrecht, 1987: Conserved variable analysis of boundary layer
thermodynamic structure over the tropical occans. J.Atmos. Sci. 44, 83-99.
Betts, A.K., and Ridgway, 1989: Climatic equilibrium of the atmospheric convective boundary
layer over a tropical ocean. J. Atmos. Sci. 46, 2621-2641.
Cess et al, 1990: Intercomparison and interpretation of climate feedback processes in 19
atmospheric general circulation models. J. Geophys. Res., 95, 16601-16615.
(32 authors).
Gamache, J.F., and R.A. Houze, 1983: Water budget of a mesoscale convective system in the
tropics. J. Atmos. Sci. 40, 1835-1850.
Houze, R. A., and A.K. Betts. 1981: Convection in GATE. Rev. Geophys. and Space Phys.
19, 541-576.
Lindzen, R.S., 1990: Some coolness concerning global warming. Bull. Amer. Meteor. Soc.
71, 288-299.
Raval, A., and V. Ramanathan, 1989: Observational determination of the greenhouse effect.
Nature, 342, 758-761.
3
Anthony Socci
US Senate Commerce Committee
427 Hart Bldg.
Washington DC 20510
To give you an indication of Dr. Betts competence and
reputation within the meteorological community, I have extracted
the following from peer reviews of a recent proposal (now funded)
by Dr. Betts, to continue his research into the coupling of the
climate with the atmospheric boundary layer.
- In my opinion this work is quite important and should be
funded. Betts is well known as a bright and innovative scientist
and he is one of the few theoreticians/modelers making
contributions in this area.
- The PI has considerably advanced our understanding of the
boundary layer and of the coupling of the boundary layer to the
upper atmosphere, and his work had given rise to a great deal of
the research done by others on these systems.
- The accomplishments of the PI under this grant have been
phenomenal. More than 10 papers, covering a wide range of topics
from atmospheric convection to coupled oceanic and coupled land
processes, were published in the past three year period under this
grant. These papers have a major impact in enhancing our
fundamental understanding of atmospheric convective processes as
well as parameterization in GCMs.
- Dr. Betts is an extremely competent scientist and a world
expert on tropical convection. He is an unique individual in his
approach to science and management. He interacts widely with a
large number of scientists in different research institutions and
universities and has been a source of valuable scientific advice
and inspiration to all his co-workers.
John Ball