While watching yet another scary documentary, I heard that one negative of continuing global warming is thermal expansion: hotter ocean water expanding and causing weather havoc. Why doesn't hotter water evaporate, because that is all that I have ever observed it do? (What's now more scary is experience not being the best teacher after all.)

Heating water does cause it to expand. The change is small enough that you'd have a lot of trouble measuring it in a cup, but when the volume of water being expanded is the size of all the world's oceans, it might cause problems.

My apologies to Frankvan, I've accidentally deleted his post (why do I seem to be the only moderator who does that?). He gave a similar answer to the above, and also noted that 39.2 deg. F / 4 deg. C is the temperature at which ice has its maximum density so heating or cooling from that would decrease its density (expand its volume).

He said something I wanted to address:

"that's why ice floats."

Water has a density of 0.99984 g/cm³ at 0 Celsius (compared to its maximum of 0.99998 g/cm³ at 4 Celsius). Ice at 0 Celsius has a density of 0.917 g/cm³. Although freezing is obviously temperature related, it's not really right to say that ice is less dense than water for the same reason that 0 deg. C water is less dense than 4 deg. C water. Ice has a different structure from water, one with more open areas, which makes it less dense than water.

Comparatively warm water, at 8.2 C or above, it may be worth noting, is less dense than 0 deg. C water, but all water is much more dense than ice.



(All densities are from the CRC Handbook of Chemistry and Physics)

I should also note that it's really the surface water we're concerned with, not all the oceans. Most of the water is actually below 4 deg. C, so warming it would actually decrease its density. I don't want to wade too deep into matters I don't know enough about, but I believe this deep water is not affected much by global warming. The surface water is, on average, well above 4 deg. C, so warming it will increase its volume.

From The Oregonian, April 19, 2000, pA22

FIRST QUARTER IS THE WARMEST ON RECORD IN U.S.

The average temperature is 41.7 degrees, one degree higher than the
previous record set in 1990


By CURT SUPLEE, LA Times-Washington Post Service


What the NOAA data show, Baker said, is that the amount of precipitation in the largest storm systems has been growing. that is the outcome one might expect if gradual global warming evaporated greater quantities of water into the atmosphere.
"What we're seeing is that, in stronger storms, the amount of rainfall per storm has been increasing, up about 10 percent" in recent
decades, Baker said. "That's the only real measure we have at this point for increased intensity of extreme events. However, we are also seeing a slight increase in the number of heat waves and in the number of days in a row when nighttime temperatures set records."
During the first three months of 2000, every state in the continental United States was warmer than average, with Oklahoma, Iowa and Wisconsin setting records for the January-to-March period.
This year's winter warmth is likely to exacerbate the drought conditions that persisted in many parts of the United States during 1998
and 1999, produced in large part by La Nina conditions.
La Nina, the opposite of El Nino, occurs when cooler than average
sea water accumulates in the equatorial Pacific Ocean. Typically it
causes winter temperatures that are higher than average in the
Southeast, and lower than normal in the Northwest.
"We're witnessing very typical warm and dry conditions" in the middle of the country and in the southern-tier states, Baker said, "along with a wet Pacific Northwest. So there are really two-things in the current pattern - the underlying warming trend plus the overlay of
La Nina conditions."
That situation, Baker said, is likely to continue for another three to six months. During that period, southern-tier states from Arizona to Florida will be at increased risk of drought, along with parts of Nebraska, Iowa, Illinois, Indiana and Ohio.
--------
Note that this was written before the increase in severity of hurricane was noted by most weather scientists.

At first, it does seem that global warming wouldn't cause the problems that it does, but in order to understand that it does, you have to look at a larger picture with several processes involved. As an example, look at the Gulf Stream in the North Atlantic, bringing warm water north along the East coast of the US and Canada (thus making their waters warmer than the water off California, Oregon, and Washington, which is coming down from the Arctic Seas.). The Atlantic stream then turns east, towards Northern Europe. (It is now called the North Atlantic Drift.) Normally, the water is cooled only somewhat by the subduction of water from the Arctic Sea (water which is headed to the Equator, where it will be warmed, rise, and start the cycle again) before it affects the European land mass. However, with the greater amount of colder water coming in from the Arctic sea (due to ice pack melt because of global warming), the water is cooled more than before, which means that colder (or less warm) water is hitting the European land mass, not warming it as much as it previously did. The difference is only slight, but it seems to be cumulative. Thus, global warming means colder winters in Northern Europe. Naturally, there will be shorter fluctuations in temperatues that swing both warmer and colder at times, but the overall trend wil be for colder winters in Northern Europe, caused by global warming.

This would be much easier to understand if you were looking at a map of the North Atlantic and I was waving my arms around when I explained it. Really.


The bottom line, as far as warmer water evaporating more, is that more water in the atmosphere means that more water will come down from the atmosphere, but not necessarily where it always used to come down. Global warming is changing long-established patterns.

I think that I should conclude that the ocean does not evaporate, because not all of it gets heated, only its surface. Also, I will bear in mind that the ocean can do more than this. Finally, I should not compare a pot of water with an ocean and apply my experience of one to the other.