Water Vapor and Lapse Rate
Feedbacks
Neil Gordon
ESP Seminar
April 14, 2006
Climate Change – Feedback Mechanisms
Feedback: A sequence of interactions that determines the
response of a system to an initial perturbation
source: AMS Glossary
• Some major climate feedbacks relating to climate:
– Ice-Albedo (positive feedback)
Higher surface temperatures Less ice and snow cover
Lower albedo More surface absorption higher surface temperatures
– Water Vapor (positive feedback)
More GHGs Higher surface temperatures More evaporation
More water vapor = More GHGs
– Clouds (net negative feedback)
Higher surface temperatures More evaporation Higher specific humidity
More clouds Higher albedo Lower surface temperatures
More longwave absorption Higher surface temperatures
Structure of the Troposphere
• Temperature decays with height at a
rate of ~6-7 K/km over the troposphere
(lowest 10-12 km of the atmosphere)
• Water vapor is mostly concentrated in
the lowest 1.5 km of the troposphere
Temperature Profile
from http://www.atmosphere.mpg.de/media/archive/
More Detail on GHG Forcing
• We can think of the surface atmosphere
system radiating as a whole to space
• The top-of-atmopshere (TOA) incoming
radiation is primarily constant but how the
Earth balances that is not
• As GHG concentrations increase in the
atmosphere, the effective height of emission
increases, thus reducing the outgoing
longwave radiation (OLR)
• So, more radiation is entering the climate
system and the surface and atmosphere have
to warm to compensate
Climate Energy Balance
from Soden and Held (2000)
Lapse Rate Feedback
• The rate of temperature decay with height, or
the slope of the temperature profile (lapse
rate), is controlled by radiation, large-scale
dynamics and convection
• If the lapse rate were to decrease, then the
temperature of the effective level of emission
would warm (negative feedback)
• This is a proposed negative feedback in the
tropics, but it is thought to be relatively small
(Zhang et al, 1994)
Water Vapor Feedback
• As atmospheric temperature increases,
the ability of that air to hold more water
vapor increases
• So, if relative humidity is held constant
as temperature increases, the total
moisture in the air increases
• Water vapor is opaque to IR radiation,
making it a greenhouse gas
Relative Humidity (350-500mb)
from Lindzen et al. (2001)
Water Vapor Feedback
• The increase in total moisture in the lower
troposphere as temperature increases is well-
observed (Wentz and Schabel, 2000)
• In order for water vapor to change the
radiation balance, it must increase in the free
troposphere
• So, to change the free tropospheric water
vapor, there must be a mehcanism to
transport the water aloft
• Held and Soden (2000) found that for fixed
RH, the total water vapour feedback
contributed by increases in WV below 850mb
was only 10% of the total response
Mt. Pinatubo Experiment
• Soden et al. (2002) used the global
cooling (and drying) resulting from the
eruption of Mt. Pinatubo to test the
water vapor feedback hypothesis
• Global climate models were only able to
reproduce the observed cooling if the
water vapor feedback was included
Adaptive Iris Hypothesis
• Lindzen et al. (2001) propose a
mechanism whereby increased surface
temperature leads to more vigorous
tropical convection
• The enhanced convection increases the
ability of the cloud to precipitate
• The total water then transported to the
upper atmosphere is reduced
Tropical Convection
from http://www.divinewindbook.com/figures/images/