The Story of the Water

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Water vapor is the most important gaseous source of infrared opacity in the atmosphere, it

accounts for about 60% of the natural greenhouse effect for the clear skies [1], and provides the

largest positive feedback in model projections of climate change [2]. Therefore, water vapor

variability is an important issue in the discussion of global climate change [3] and in particular

the variability of stratospheric water vapor has important radiative and chemical consequences

that impact the global surface climate change [4].

An increase of roughly 1% per year in stratospheric water vapor content has been observed

during the last half of the 20th century [5, 6], with a more convincingly documented increase

during the 1980s and most of the 1990s than earlier. However, an updated trend analysis [7]

of water vapor in the lower mid-latitude stratosphere from Boulder balloon measurements and

from HALOE (Halogen Occultation Experiment) [8] spaceborne observations provides trend

estimates for the period 1980-2000 that are up to 40% lower than previously reported.

Methane oxidation is a major source of water in stratosphere, and has been increasing over

the industrial period, however, the observed trend in stratospheric water vapor during the last

half of the 20th century is too large to be attributed to methane oxidation alone [5, 9].

The temperatures near the tropical tropopause should control the stratospheric water vapor

content according to the equilibrium thermodynamics, importing more water vapor into the

stratosphere when temperatures are warmer. However, tropical tropopause temperatures have

cooled slightly over the period of the stratospheric water vapor increase [10, 11]. Other mechanisms

have been proposed to explain the increase of the stratospheric water vapor occurred in

the second half of 20th century, but so far the driving causes of this increase are unknown.

The upward trend of stratospheric water vapor decreased in the last half of the 1990s with

a near-zero trend between 1996 and 2000 [12, 13]. Furthermore, at the end of 2000 there was

a dramatic drop of about 10% of stratospheric water vapor [13]. The trend analysis reported

in [14] extends until spring 2008 and it shows that a minimum was approximately reached

between 2004 and 2006 and an increase is observed afterwards.

The drop in stratospheric water vapor that occurred at the end of 2000 is thought to have

slowed the rate of increase in global surface temperature over 2000-2009 by about 25% compared

to that which would have occurred due only to carbon dioxide and other greenhouse

gases [4]. On the other hand the increase in stratospheric water vapor occurred between 1980

and 2000 would have enhanced the decadal rate of surface warming during the 1990s by about

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