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I recently watched a Jeremy Rifkin video in which he stated that a 1°C increase in temperature resulted in a 7% increase in atmospheric water.
Seven percent seems like a pretty big number. If true, why wouldn't this cause the sea-level to decrease?

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    I could take a shot at it, but this would probably be better on Earth Science. – DenisS Feb 27 '18 at 19:45
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    Well, as for sea-level change, if I did my (extremely rough) calculations correctly, a 7% increase in atmospheric water vapor would take at most about a cm of sea level change, which is on the order of 3-5 years' worth of sea level rise at current levels and well within the uncertainty of any estimates and predictions, especially when taking into account the rate of temperature change as well. – Kevin Feb 27 '18 at 20:03
  • Per UW-Madison, current atmospheric water content accounts for 1" of rain. I divide .07 inches by 70% to account for landmass and I get 2.5mm. So, with compounding, a 5C increase would result in 15mm, which, as you point out, would be partly offset by decrease in water density. – John Meyer Feb 28 '18 at 0:54
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No, 7% is somewhat too high.

According to Recent Climatology, Variability, and Trends in Global Surface Humidity Journal of Climate, (2006) volume 19, pages 3589-3606:

The observed dq/dT is about 0.58, 0.38, and 0.77 g kg-1 °C-1(r2 = 0.81, 0.77, and 0.76) for annual q and T for the globe, global land, and ocean, respectively. In percentage terms, they are about 4.9%, 4.3%, and 5.7% change in q per 1°C warming, which are close to those (∼5.4%, 5.1%, and 5.5% per 1°C, respectively) suggested by the Clausius–Clapeyron equation or its empirical version [Eq. (3)] for saturation specific humidity (computed locally and then area averaged). 1

where "q" is specific humidity, "T" is temperature, and "dq/dT" is the rate of change of specific humidity with respect to temperature.

and where footnote 1 is:

The Clausius–Clapeyron equation locally gives 6.2%–6.4% change per 1°C in surface saturation specific humidity (qs) for air temperature within 15°–20°C. For regional averages, however, this percentage is lower because the area-averaged mean qs is higher than that calculated using the Clausius–Clapeyron equation and area-averaged mean air temperature and pressure.

So, in conclusion, the correct value is 4.9%.

  • This shouldn't be too controversial; if you've ever visited a tropical climate, you've probably experienced the greater capacity of the warmer air to hold moisture. Even the Wikipedia entry on relative humidity notes "A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F or 10 °C increase in temperature." Which is roughly 7% per 1 degree C. – jeffronicus Feb 27 '18 at 23:40
  • @jeffronicus There is significant ongoing research in this field. journals.ametsoc.org/doi/full/10.1175/JCLI-D-16-0351.1 The relative humidity over land (where global temperature has increased the most) is decreasing. climate.gov/news-features/understanding-climate/… Absolute humidity is increasing, but is partially mitigated by the decrease in relative humidity. – DavePhD Feb 28 '18 at 14:17
  • @davephd Your answer needs to make the difference between content and capacity more clear. – BobTheAverage Mar 1 '18 at 5:31
  • @BobTheAverage I found a great reference and will totally change my answer. journals.ametsoc.org/doi/full/10.1175/JCLI3816.1 – DavePhD Mar 1 '18 at 13:59
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I found this blog which links to this Guardian article. I can't find a source they have used, but it states at the bottom of the article that it was written in conjunction with the Met Office:

globally water vapour increases by 7% for every degree centigrade of warming

Scientific America agrees, and references the claim to Peter Stott, from the Met Office.

And for every 1-degree C (1.8 degrees Fahrenheit) rise in temperature, the amount of moisture that the atmosphere can contain rises by 7 percent

In response to your secondary question of "If true, why wouldn't this cause the sea-level to decrease?" - Climate Central says that a 1.5C rise could cause a 2.9m sea level rise. This is significantly more than the reduction caused by increased humidity. So the 7% increase would cause the sea level to reduce (slightly), but the rise from other factors (expansion of water, and melting land ice caps) would quickly outstrip it.


As a sidebar, I do not know of Jeremy Rifkin, but the little I watched of the video you linked did not impress me. I would not take what he says as gospel (for example, just after he makes the 7% claim, he seems to start cherry picking different disasters that occured recently - this is not very scientific).

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    Scientific American says "can contain", which is somewhat different. – DavePhD Feb 27 '18 at 21:09
  • Agreed about Rifkin, he is making a political case not a scientific one... – John Meyer Feb 28 '18 at 0:32
  • The capacity of air to hold water depends primarily on temperature, but simply heating air up does not add water to it. There is a difference between capacity and content. – BobTheAverage Mar 1 '18 at 5:28

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