The Story of the Water :: Water Vapor

Water vapor is the most important gaseous source of infrared opacity in the atmosphere, itaccounts for about 60% of the essential greenhouse effect for the clear skies 1, and provides thelargest positive feedback in model projections of climate transfer 2. Therefore, water vaporvariability is an important issue in the word of global climate change 3 and in particularthe variability of stratospheric water vapor has important radiative and chemical consequencesthat impact the global protrude climate change 4.An increase of roughly 1% per year in stratospheric water vapor content has been observedduring the last half(a)(prenominal) of the 20th century 5, 6, with a much convincingly documented increaseduring the 1980s and most of the 1990s than earlier. However, an updated burn analysis 7of water vapor in the lower mid-latitude stratosphere from Boulder balloon measurements andfrom HALOE (Halogen occultation Experiment) 8 spaceborne observations provides trendestimates for the pe riod 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 everyplacethe industrial period, however, the observed trend in stratospheric water vapor during the lasthalf of the 20th century is too large to be attributed to methane oxidation unaccompanied 5, 9.The temperatures near the tropical tropopause should control the stratospheric water vaporcontent according to the equilibrium thermodynamics, importing more water vapor into thestratosphere when temperatures are warmer. However, tropical tropopause temperatures hand overcooled slightly over the period of the stratospheric water vapor increase 10, 11. Other mechanismshave been proposed to explain the increase of the stratospheric water vapor occurred inthe 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 witha ne ar-zero trend between 1996 and 2000 12, 13. Furthermore, at the end of 2000 there wasa dramatic drop of about 10% of stratospheric water vapor 13. The trend analysis reportedin 14 extends until spring 2008 and it shows that a minimum was approximately reachedbetween 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 haveslowed the rate of increase in global surface temperature over 2000-2009 by about 25% comparedto that which would have occurred due yet to carbon dioxide and other greenhousegases 4. On the other hand the increase in stratospheric water vapor occurred between 1980and 2000 would have enhanced the decadal rate of surface warming during the 1990s by about