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So a rainforest creates a positive feedback loop—vegetation creates evapotranspiration which encourages rainfall, which in turns encourages more vegetation.

If you wanted to badly enough, could you irrigate an area of land with enough water to produce vegetation, so that the vegetation created a similar positive feedback loop? Could you reforest or de-desertify areas of Earth this way? If so, how large would the area of land need to be before it could change the climate? Would you need the land area to be in a specific location? Like, would the new forest have to be in an area where it could generate rain more readily, or could rain be encouraged wherever? Would you need to do things other than just import water to do this, and would the forest eventually no longer need irrigation?

What other repercussions would result? For example, surely if you forested too much of Earth, you would cause drastic changes to the climate.

Edit:

Main question: could you irrigate an area of land to create a forest which encouraged enough rainfall so that, eventually, you need not water it anymore, in an area where a forest did not previously exist?

I've toyed with the idea of an advanced civilization, having run out of other convenient sources of water, turned to desalinating seawater. If a civilization or nation relied on desalination for their freshwater, they would need large networks to take water from coastal areas inland. Such a network could provide the water in this question, if no other sufficient source was available.

Apologies for the broadness of this post. Many other questions are related to it, and I was seeking ideas for those related questions as well as the main one.

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  • $\begingroup$ What area are you looking at. Some places have the wrong climate. $\endgroup$ – Bellerophon Aug 18 '16 at 21:25
  • $\begingroup$ Climate is complicated. And it's not that you can simply "import water". You need to take it from somewhere. Also, i can't see two things: what exactly you are asking here? You mentioned a topic, but what;s the actual question? And how is it about worldbuilding? It looks like a real world question. $\endgroup$ – Mołot Aug 18 '16 at 21:29
  • $\begingroup$ I'd have to agree with @Mołot. This is either too broad to have a definable answer, or off topic (or both). $\endgroup$ – Fayth85 Aug 18 '16 at 21:31
  • $\begingroup$ You're talking about large scale climate change, which is very complex. Basically, you want to know if someone could change, what, a desert into a rainforest, just by importing water? What climate are you even starting with? Any mountains? How hot is it? The problem with this question is that it is not specific. It's so broad, that there could be a million different answers. $\endgroup$ – Erin Thursby Aug 18 '16 at 21:43
  • $\begingroup$ Generally, if the conditions are right for a rainforest to exist, you'll find one there already. $\endgroup$ – user2448131 Aug 18 '16 at 21:52
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It should be possible to create a microclimate in an area with the right topography and prevailing winds, but not the scale of, or typical of, a tropical rainforest -- and probably not for long.

Why? Because of those prevailing winds and the Hadley cells governing them. Tropical rainforests form at the equator because that is a convergence zone for two moisture-carrying cells (north and south of the equator). As these cells reach the warmer equatorial latitudes they rise and the water vapour they carry condenses and falls. Similarly, because there's nowhere else to go, the clouds of vapour released by tropical rainforests rises and turns to rain. (The inverse largely explains the positioning of deserts, by the way.)

Hadley cells

Tropical rainforests also differ significantly from temperate forests by receiving consistent amounts of sunlight year-round, without seasonal variation (and the plant-killing cold of winter). Something you can't induce in a self-perpetuating way.

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The question as posed is at best a slippery slope. Let me explain with examples.

Rainforests are something we know a lot about, but if they formed there because of high rainfall, or they are the cause of the high rainfall is not something you're looking at closely enough.

Desalinating water is something that is quite commonly done. In the Netherlands for example, the majority of drinkable water is purified (unsalted, if you will) via the dunes in Zuid-Holland, pumped up and further purified. I could offer sources, but the majority will come in Dutch that I can think of off the top of my head.

As well, if you were to take a large chunk of the Sahara desert (for example, but pick any desert, really) and a large chunk of Antarica and introduce a lot of water to both, does this mean you'll 'create an environment fertile enough for future rainforests'? (hint: soil composition is also a key factor in this, in addition to temperature, and existing climate conditions)

As is posed, I do not feel this is something you've really looked at the implications of your question.

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If the evaporation caused by transpiration doesn’t rain back in the same place then it will not create this new equilibrium. So as you suspect simply importing water is not sufficient to produce this cycle.

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  • $\begingroup$ Is there a mechanism that exists in a rainforest which makes it more likely that rain will fall on it? $\endgroup$ – Kronimiciad Aug 18 '16 at 21:39
  • $\begingroup$ If the humid air simply blows away to somewhere else rather than forming clouds in place. $\endgroup$ – JDługosz Aug 18 '16 at 21:45
  • $\begingroup$ The question is backwards: the conditions which cause high rainfall support the evolution of rainforests, wether in the tropics or along (for example) the West Coast of Canada and the United States. $\endgroup$ – Thucydides Aug 19 '16 at 4:08

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