The rise in long - lived greenhouse gases (decades to centuries) warms the atmosphere and surface, and that increases
the average amount of water vapor in the atmosphere.
That's because reorganization likely wouldn't maintain the same
average amount of water vapor in the atmosphere (it's the most important «greenhouse gas,» not carbon dioxide or methane).
Trenberth notes that global warming has already increased
the average amount of water vapor in the atmosphere by about 4 %, «extra moisture flowing into the storms that produced the heavy rains and likely contributed to the strength of the storms through added energy.»
Not exact matches
A NOAA website on atmospheric rivers contains this fascinating statistic that illustrates just how much moisture can be transported by winds
in the mid-to-upper
atmosphere: «A strong atmospheric river transports an
amount of water vapor roughly equivalent to 7.5 - 15 times the
average flow
of liquid
water at the mouth
of the Mississippi River.»
And since the 1970 ′ s on
average there's about a 4 % increase
in water vapor over the Atlantic Ocean and when that gets caught into a storm, it invigorates the storm so the storm itself changes, and that can easily double the influence
of that
water vapor and so you can get up to an 8 % increase, straight from the
amount of water vapor that's sort
of hanging around
in the
atmosphere.