Rising Levels of Human-Caused Water Vapor in Troposphere will Intensify Climate Change Projections

Posted July 28th, 2014 at 7:01 pm (UTC+0)

Illustration of annual mean T2-T12 field that provides a direct measure of the upper-tropospheric water vapor. Purple = dry and Red = moist. (Eui-Seok Chung, Ph.D./University of Miami Rosenstiel School of Marine and Atmospheric Science)

Illustration of annual mean T2-T12 field that provides a direct measure of the upper-tropospheric water vapor. Purple = dry and Red = moist. (Eui-Seok Chung./University of Miami Rosenstiel School of Marine and Atmospheric Science)

When it comes to greenhouse gases that contribute most to global warming, most people think of substances such as carbon dioxide, methane or even hydrofluorocarbons.

But did you know that, for a while now, scientists have considered the vapor of the most important ingredient in sustaining life on Earth – water – as one of the most abundant greenhouse gases in the atmosphere and a key driver of global warming?

A new study led by scientists from the University of Miami Rosenstiel School of Marine and Atmospheric Science confirmed that rising levels of water vapor in the troposphere – the layer of the atmosphere closest to the surface and rises to a height of between 5 to 20 km above Earth’s surface – will increasingly play an important role in climate change projections in the coming years.

The Florida researchers said their new study is also the first to demonstrate that the increasing amount of atmospheric water vapor is being caused by human activities.

The researchers wanted to find out what was causing a 30-year trend of increasing water vapor in the upper troposphere.

So they took data collected over the years by satellites from the US National Oceanic and Atmospheric Administration (NOAA) and compared it to climate models that predicted water circulation between the ocean and the atmosphere.

This allowed the team to determine whether or not the observed changes in atmospheric water vapor were the result of natural or human-made causes.

The experiments revealed that natural causes such as volcanic activity or variations in solar activity can’t explain the growing amounts of water vapor in the upper troposphere.

But their experiments did suggest that human activity can account for the increase.

In an email to Science World, Brian Soden, a co-author of the study and professor of atmospheric sciences at the University of Miami’s Rosenstiel School pointed out that those human-caused increases in water vapor in the upper troposphere are not the result of actual man-made water vapor emissions. Instead the water vapor is created as a response to man-made warming of the atmosphere, which he said is due primarily to the increase in CO2 from the burning of fossil fuels.

A color enhanced satellite image of upper tropospheric water vapor. (NASA)

A color enhanced satellite image of upper tropospheric water vapor. (NASA)

“Because the concentration of water vapor increases as the air temperature increases, this man-made warming triggers a natural “feedback” mechanism that causes the water vapor increase,” said Soden. “Because water vapor is itself a very powerful greenhouse gas, the increase in water vapor further amplifies the initial CO2 induced warming. Our study confirms the presence of this key feedback mechanism which is a crucial component of global warming projections in water vapor can be explained by a rise in the amount of other greenhouse gases such as carbon dioxide being fed into the atmosphere.”

If water vapor is plays a key role in global warming, which increases water evaporation, which in turn leads to even more atmospheric water vapor, could this feedback mechanism spin out of control?

“Fortunately the feedback is not strong enough to push the climate into a “runaway” mode which would cause indefinite warming,” Soden explains. “However, some hypothesize that such a runaway greenhouse effect may have occurred on Venus and led to its extremely large greenhouse effect and warm temperatures.”

The researcher’s study called “Upper Tropospheric Moistening in response to Anthropogenic Warming” was published today (July 28, 2014) in the early on-line edition of the Proceedings of the National Academy of Sciences (PNAS).

6 Responses to “Rising Levels of Human-Caused Water Vapor in Troposphere will Intensify Climate Change Projections”

  1. John Fish says:

    “we can’t explain it so it must be humans”
    “lets model it”
    “wow, the model programmed with the assumption that the cause is humans confirmed that the cause is humans”
    “that’s all the proof we need”

  2. Dr.Don says:

    I’ve known for quite a number of years that H2O was the number one greenhouse gas. The problem is that it is not just a warming agent it is also a cooling mechanism. This article also relies on the assumption that CO2 levels is a direct cause of global warming. It also assumes that global warming is still happening.Both are false assumptions. CO2 levels follow temperature rises. Temperatures have been more or less steady since 2000 and CO2 levels continue to rise.

  3. Rob Sparrow says:

    As the ice sheets retreat, sea levels rise and the albedo of those ice sheets is reduced. This causes oceans to warm further and evaporate more water that indeed cause more positive feedback. However, we should also remember that for every previous ice age, sea levels dropped 400 feet. How did so much water end up as ice at the poles except by clouds. Whilst high level thin clouds may have a warming effect, low level clouds have and overall cooling effect.

  4. Anna says:

    Lots of theories are existed now, but we can’t be a hundred per cent sure about the key reason. It is clear something is happening and Man has contributed to the enviromental processes, there is no doubt

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