Does the world have a defined amount of water?

Question

I was thinking the other day, after having a very wet winter season in New Zealand, if we are getting all this rain, are other parts of the planet, getting droughts? and of course this lead to the question, does the world have a defined amount of water, or is water created when Hydrogen(H) an Oxygen(O) combine somehow?

I understand water is evaporated, and rises, then condenses and becomes clouds/rain (and of course these clouds don't just release their rain over th place that the water was evaporated from, so the water moves, around. And of course a lot of the water on the planet exists inside people/animals etc.

But is their a defined amount?

Answer 1

Yes, there is a limited amount of water on (and in, and above) Earth. There's a limited amount of everything on Earth. Most things on Earth stay on Earth due to the pull of gravity, gaseous water included* (Mars doesn't have enough gravity to keep its water though).

Chemistry

A water molecule is made up of two hydrogen atoms and one oxygen atom, giving it the formula H₂O. These form a relatively tight bond (compared with some molecules) so that they don't easily break apart, but a small proportion of them do break apart under the right conditions, such as when photosynthesis turns the combination of water and carbon dioxide into oxygen and sugar. Often water is formed again later, such as when the sugar is consumed (eaten) by animals or microbes.

Where Earth's Water is

The majority of the easily accessible water on Earth is in our oceans. Only a small proportion (3.5%) is fresh water in lakes, rivers, swamps, aquifers, and ice, and most of that is largely inaccessible. There's some excellent info here on that (see the table). Apparently only 0.04% of the fresh water is in the atmosphere (To be clear, that's one twenty-fifth of a percent of the fresh water, or one thousandth of a percent of the total water).

Why are there droughts then?

The system as a whole is in a kind of complicated balance, where warm dry air causes increased evaporation of water from oceans into the atmosphere, and as moist air cools it becomes clouds then rain. In places where there's a lack of rain, it's not caused by a shortage of water on Earth. Rather, it's caused by localised weather which is caused by shapes of land masses and their location on Earth.

What's probably most interesting is local weather. Because temperature has significant effects on weather, you're probably better to focus on climate patterns and effects of (anthropogenic) climate change rather on how much water there is in total.

*Lighter gases, Helium in particular, but water too, do tend to be lost into outer space from the outside of Earth's atmosphere. Earth has lost some already, but it's a very slow process and nothing to worry about too much for quite some time... we might need to worry about the deterioration of our sun before we lose too much water. http://www.thenakedscientists.com/HTML/questions/question/2321/

Answer 2

To a decent approximation the amount of free water (i.e. liquid and gas) is fixed. Ice sheet melting of course increases this slowly. But so much of the free water is in the oceans that the amount that's available for most human uses can still vary hugely. Sea surface temperatures significantly affect evaporation rates, and these vary e.g. El Niño.

The distribution of rainfall is predicted to be seriously affected by climate change, both in space and time. Flooding and drought could both increase.

The amount tied up in plants and animals is small; should may account for more (citation needed).

There's essentially no free hydrogen to combine with oxygen (0.5 parts per million according to Wikipedia). There also aren't any major sources of hydrogen.

Answer 3

This is a little off from your question, but I think it helps explain your experience in New Zealand. Because of global warming there is more water in the atmosphere, so, while weather patterns will still cause drought in some areas, overall we can expect more precipitation. Also, water itself is a greenhouse gas.

Increasing water vapor leads to warmer temperatures, which causes more water vapor to be absorbed into the air. Warming and water absorption increase in a spiraling cycle.

Water Vapor Confirmed as Major Player in Climate Change