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Water in the Atmosphere
Water in the Atmosphere
Jonathon S. Wright
Jackson School of Geosciences
University of Texas
October 23, 2011
Water in the Atmosphere
The Importance of Atmospheric Water
The Water Cycle
Water in the Atmosphere
Overview
1
Water provides an efficient mechanisms for moving energy
within the atmosphere. This energy transport plays a large
role in determining weather and climate.
2
Water in the atmosphere accounts for approximately
three-quarters of the present-day greenhouse effect (50%
water vapor and 25% clouds): without atmospheric water, the
average temperature of Earth would be much colder (40◦ F).
3
Water vapor amounts are temperature-dependent and feed
back to temperature changes: water vapor roughly doubles the
climate sensitivity to changes in atmospheric carbon dioxide.
4
Clouds also feed back to temperature changes. The sign and
magnitude of this feedback are not yet known.
5
The water cycle plays a large role in pollutant transport within
the atmosphere, particularly vertically.
Water in the Atmosphere
Energy and Radiation
Energy Transport
Water Vapor: Fluid Sunshine
Sunlight evaporates water from the ocean surface, storing the
energy in the form of latent heat in atmospheric water vapor.
Water in the Atmosphere
Energy and Radiation
Energy Transport
Water Vapor: Fluid Sunshine
In this form, the energy can be transported efficiently to distant
places, where the heat is released by condensation.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Water Vapor: The Dominant Greenhouse Gas
Water vapor accounts for approximately half of the present-day
greenhouse effect.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Forcing: Climate Changes
Radiative climate forcings, such as increases in carbon dioxide,
change the temperature structure of the atmosphere.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Feedback: Water Vapor Responds
Water vapor is temperature dependent, and may therefore
represent a positive feedback to temperature changes.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Models: Constant Relative Humidity
Climate models suggest that the magnitude of the water vapor
feedback can be estimated by assuming that global-mean relative
humidity remains constant.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Observations of Constant Relative Humidity
This constancy can be directly observed in the water vapor
response to other climate forcings, such as volcanic eruptions.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Observations of Constant Relative Humidity
This constancy can be directly observed in the water vapor
response to other climate forcings, such as volcanic eruptions.
The Magnitude
The water vapor feedback roughly doubles climate sensitivity.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Why the Upper Troposphere?
Greenhouse gas forcings are logarithmic in concentration.
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
Stratospheric Water Vapor
An increase of water vapor in the stratosphere:
1
warms the surface
2
cools the stratosphere
3
enhances stratospheric ozone destruction
Water in the Atmosphere
Energy and Radiation
The Water Vapor Feedback
The Stratospheric Tape Recorder
Stratospheric water bears the imprint of the seasonal cycle of
temperatures at the tropical tropopause. Current work in the
Climate Group focuses on better constraining additional factors.
Water in the Atmosphere
Energy and Radiation
The Cloud Feedback
Clouds: A Balancing Act
Like water vapor, clouds are strong greenhouse absorbers, but they
also cool the climate by reflecting sunlight.
Water in the Atmosphere
Energy and Radiation
The Cloud Feedback
Low Cloud Forcing
Low clouds reflect sunlight, and their temperatures are similar to
surface temperatures. Increases in low clouds cool the climate.
Water in the Atmosphere
Energy and Radiation
The Cloud Feedback
High Cloud Forcing
Cirrus clouds are less reflective, with temperatures much colder
than the surface. Increases in high clouds warm the climate.
Water in the Atmosphere
Energy and Radiation
The Cloud Feedback
Water in the Atmosphere
Energy and Radiation
The Cloud Feedback
The Global Cloud Feedback
Observations of cloud feedback to temperature changes associated
with El Niño variability indicate that this feedback is positive but
small. El Niño is problematic as a global warming analogue.
Water in the Atmosphere
Pollutant Transport
Fire Products
Convective Transport of Fire Products
Deep convection in the tropics can transport carbon monoxide and
particles from fires at the surface to the upper troposphere and
lower stratosphere, where they can impact chemistry and the water
budget. This is an area of active research in the Climate Group at
the Jackson School of Geosciences.
Water in the Atmosphere
Pollutant Transport
Carbon Monoxide
Water in the Atmosphere
Summary
1
Water provides an efficient mechanisms for moving energy
within the atmosphere. This energy transport plays a large
role in determining weather and climate.
2
Water in the atmosphere accounts for approximately
three-quarters of the present-day greenhouse effect (50%
water vapor and 25% clouds): without atmospheric water, the
average temperature of Earth would be much colder (40◦ F).
3
Water vapor amounts are temperature-dependent and feed
back to temperature changes: water vapor roughly doubles the
climate sensitivity to changes in atmospheric carbon dioxide.
4
Clouds also feed back to temperature changes. The sign and
magnitude of this feedback are not yet known.
5
The water cycle plays a large role in pollutant transport within
the atmosphere, particularly vertically.