The iris hypothesis is a hypothesis proposed by
Richard Lindzen et al. in 2001 that suggested increased
sea surface temperature
Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mas ...
in the
tropics
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referr ...
would result in reduced
cirrus cloud
Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...
s and thus more
infrared radiation
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
leakage from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's
atmosphere. His study of observed changes in cloud coverage and modeled effects on infrared radiation released to space as a result supported the hypothesis. This suggested infrared radiation leakage was hypothesized to be a
negative feedback in which an initial warming would result in an overall cooling of the surface. The consensus view is that increased sea surface temperature would result in increased cirrus clouds and reduced infrared radiation leakage and therefore a positive feedback.
Other scientists subsequently tested the hypothesis. Some concluded that there was no evidence supporting the hypothesis. Others found evidence suggesting that increased sea surface temperature in the tropics did indeed reduce
cirrus cloud
Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...
s but found that the effect was nonetheless a positive feedback rather than the negative feedback that Lindzen had hypothesized.
A later 2007 study conducted by
Roy Spencer et al. using updated satellite data potentially supported the iris hypothesis. In 2011, Lindzen published a rebuttal to the main criticisms. In 2015, a paper was published which again suggested the possibility of an "Iris Effect". It also proposed what it called a "plausible physical mechanism for an iris effect." In 2017, a paper was published which found that "tropical anvil cirrus clouds exert a negative climate feedback in strong association with precipitation efficiency".
If confirmed then that finding would be highly supportive of the existence of an "Iris Effect".
See also
*
Fixed anvil temperature hypothesis
*
Global dimming
Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...
*
Global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
References
External links
NASA summary of Global Warming and Iris Hypothesis(June 2002)
{{global warming
Climatology
Climate change feedbacks