Editing Tropopause (section)

==Tropical tropopause layer cold trap==
In 1949 [[Alan West Brewer]] proposed that tropospheric air passes through the tropopause into the stratosphere near the equator, then travels through the stratosphere to temperate and polar regions, where it sinks into the troposphere.
<ref>{{cite journal|title=Evidence for a world circulation provided by the measurements of helium and water vapor distribution in the stratosphere
|last=Brewer|first=A. W.|date=Oct 1949
|journal=Quarterly Journal of the Royal Meteorological Society|volume=75|issue=326|pages=351–363|doi=10.1002/qj.49707532603|bibcode=1949QJRMS..75..351B }}</ref>
This is now known as [[Brewer-Dobson circulation]].
Because gases primarily enter the stratosphere by passing through the tropopause in the tropics where the tropopause is coldest, water vapor is condensed out of the air that is entering the stratosphere. This ″tropical tropopause layer [[cold trap (astronomy)|cold trap]]″ theory has become widely accepted.
<ref>{{cite journal|title=Cold trap dehydration in the Tropical Tropopause Layer characterised by SOWER chilled-mirror hygrometer network data in the Tropical Pacific
|last1=Hasebe|first1=F.|last2=Inai|first2=Y.|last3=Shiotani|first3=M.|last4=Fujiwara|first4=M.|last5=Vömel|first5=H.
|last6=Nishi|first6=N.|last7=Ogino|first7=S.-Y.|last8=Shibata|first8=T.|last9=Iwasaki|first9=S.
|last10=Komala|first10=N.|last11=Peter|first11=T.|last12=Oltmans|first12=S. J.|date=Apr 2013
|journal=Atmospheric Chemistry and Physics|volume=13|issue=8|pages=4393–4411 |doi=10.5194/acp-13-4393-2013|bibcode=2013ACP....13.4393H |hdl=20.500.11850/67923|hdl-access=free |doi-access=free }}</ref>
This cold trap limits stratospheric water vapor to 3 to 4 parts per million.
<ref>{{cite book|title=Atmospheric Evolution on Inhabited and Lifeless Worlds
|last1=Catling|first1=David C.|last2=Kasting|first2=James F.|year=2017|bibcode=2017aeil.book.....C }}</ref>
Researchers at [[Harvard]] have suggested that the effects of [[Global Warming]] on air circulation patterns will weaken the tropical tropopause layer cold trap.
<ref>{{cite journal | doi=10.5194/acp-23-7447-2023 | title=Weakening of the tropical tropopause layer cold trap with global warming | date=2023 | last1=Bourguet | first1=Stephen | last2=Linz | first2=Marianna | journal=Atmospheric Chemistry and Physics | volume=23 | issue=13 | pages=7447–7460 | bibcode=2023ACP....23.7447B | s2cid=259520137 | doi-access=free }}</ref>

Water vapor that is able to make it through the cold trap eventually rises to the top of the stratosphere, where it undergoes [[photodissociation]] into [[oxygen]] and [[hydrogen]] or [[hydroxide]] ions and hydrogen.<ref>{{cite journal|title=The aeronomic dissociation of water vapor by solar H Lyman α radiation
|last1=Lewis|first1=B. R.|last2=Vardavas|first2=I. M.|last3=Carver|first3=J. H.|date=June 1983
|journal=Journal of Geophysical Research|volume=88|issue=A6|pages=4935–4940|doi=10.1029/JA088iA06p04935
|bibcode=1983JGR....88.4935L }}</ref>
<ref>{{cite journal|title=On the photodissociation of water vapour in the mesosphere|last=Nicolet|first=Marcel|date=July 1984
|journal=Planetary and Space Science|volume=32|issue=7|pages=871–880|doi=10.1016/0032-0633(84)90011-4
|bibcode=1984P&SS...32..871N }}</ref>
This hydrogen is then able to [[atmospheric escape|escape]] the atmosphere.
Thus, in some sense, the tropical tropopause layer cold trap is what prevents Earth from losing its water to space.
[[James Kasting]] has predicted that [[Future of Earth|in 1 to 2 billion years]], as the [[Sun]] increases in luminosity, the temperature of the Earth will rise enough that the cold trap will no longer be effective, and so the Earth will dry out.<ref>{{cite journal|title=The life span of the biosphere revisited|last1=Caldeira|first1=K|last2=Kasting|first2=J F
|journal=Nature | volume=360 | issue=6406 | pages=721–23 |date=December 1992 | doi=10.1038/360721a0|pmid=11536510 |bibcode=1992Natur.360..721C |s2cid=4360963 }}</ref>