What is it about?
Evaporated water becomes a gas (water vapor) in the air where it traps heat by absorbing thermal infrared radiative energy as well as sunlight. Water vapor is also the "fuel" for rain and snowfall. As the climate warms, water vapor increases in the lowest few kilometers of the atmosphere, therefore, causes greater trapping of heat but also heavier precipitation events. This study looks at how water vapor has changed since 1979 by examining satellite measurements, observations at ground level and complex computer simulations that are also used to make predictions of future climate change. We find that the total water vapor in the atmosphere is increasing by about 1% every 10 years. Changes calculated as a percentage of the initial amount are larger higher up in the atmosphere, which is consistent with simple physics. There are some differences between the observations and simulations: some simulations overestimate the observed changes and this is partly because natural fluctuations in the ocean temporarily slowed the warming over the period studied (1988–2014). It is not known for sure what other differences between observations are caused by but it seems possible that decreases in humidity in some data sets may not be real and we are confident that water vapor is increasing with warming of climate.
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Why is it important?
Water vapor is a powerful greenhouse gas and it is the fuel for heavy rainfall events. It is known that the amount of water vapor in the air increases with global warming, enhancing the greenhouse effect and making climate change larger than it would otherwise be. As the air warms it also becomes more "thirsty", so more effective at sucking water from the ground, transporting it with the winds into weather systems and monsoons where it intensifies precipitation. Therefore it is important to asses how well we understand current changes in atmospheric moisture and what are the current changes based on state of the art ground-based observations, satellite measurements and complex computer simulations that are used to project future climate change. This study confirms that atmospheric moisture is increasing around 1% every 10 years because of global warming but finds some discrepancies between observations and simulations that require further scrutiny that may improve our ability to monitor climate change.
Perspectives
I began this work while writing the Intergovernmental Panel on Climate Change in 2019 and realising there was a gap in recent studies on how atmospheric moisture was changing. Collaborating with colleagues as part of the Met Office Academic Partnership and National Centre of Earth Observation we looked at many global datasets of water vapor and made assessments of how atmospheric moisture is currently changing and why there were some discrepancies between datasets. It is always fulfilling to make progress looking at data in a way that no one else has before and to publish the results in the scientific literature for others to make use of and advance scientific understanding of climate change.
Professor Richard P Allan
University of Reading
Read the Original
This page is a summary of: Global Changes in Water Vapor 1979–2020, Journal of Geophysical Research Atmospheres, June 2022, American Geophysical Union (AGU),
DOI: 10.1029/2022jd036728.
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