Trees cause rain.
The Amazon Rainforest generates ~50% of its own rain just by having so many trees together.[1] Trees pull water from the ground and store it in themselves, which is then released into the air through the stomata of their leaves. This is evapotranspiration.[2] Past a critical density and critical size in area of trees, the humidity generated by the trees will generate rain.
Australia is mostly dry grassland and desert in its interior. Yet, 50,000 years ago the geologic record shows that Australia was covered in forests* and twice as wet.[3] What happened?
The evidence points towards people.
People arrived in Australia 50,000 years ago, and soon afterwards, the continent dried out. No other climatic effects were observed in the rest of the world. Part of this was due to a change in monsoon patterns. When the monsoons returned 14,000 years ago, Australia’s rainfall did not come back.
How did people cause this change in climate?
Two lines of thinking are people’s direct and indirect impacts. The direct impact is theorized to be that people cut down the trees and used control burns as a form of agriculture. When the trees disappeared, the interior dried out, turning inland Australia into a desert.[4]
The second theory is that people overhunted the animals that kept the forest ecosystem in equilibrium. Without the animals to maintain ecological equilibrium, the forests could not maintain themselves and the desert took over.[5]
Also worth noting is that in the desert interior of Australia is a depression that is below sea level, which occasionally becomes a lake during the intermittent desert rain. This place is known as Lake Eyre.

(Lake Eyre: Wikimedia Commons)
Since the 1800s dreamers have floated schemes of digging a canal from the sea to flood Lake Eyre with seawater, in hopes that the presence of a lake would generate rain in the parched interior. However, these dreams did not take into account the rates of evaporation in the desert, where such a canal would only result in Lake Eyre becoming a much thicker salt flat that does not cause rainfall in the desert. A sea-flooded Lake Eyre does not have sufficient size to generate rain in such a dry environment.[6]
The alternative scheme was to dig massive tunnels through the mountains of Queensland to divert their massive amounts of rainfall into the interior. But the engineering feasibility and sheer cost of the massive tunneling project has stalled that idea as well.
(The Boomerang Scheme: p. 174, Inter-basin water transfer: case studies from Australia, United States, Canada, China, and India, by Fereidoun Ghassemi and Ian White)
So, here’s another idea: replant trees in the interior of Australia.
Start on the leeward size of the mountains and slowly move towards the desert. Of course, doing so means that we need to learn from China’s Great Green Wall. China’s artificial forest, the largest in the world, is meant to stop the desertification of northern China. However, the use of poplar monocultures means that the forest is not biologically diverse and lacks a mutually-reinforcing biological feedback loop. The result has been die-offs in 1/4 of the trees planted and many trees with stunted growth.[7] As such, China’s planted forest is not as effective as it could be in preventing desertification and locking up carbon.
A planted artificial forest, if we are to do this in Australia, must consider biological diversity needed to keep such a forest sustainable. This begins with successional dynamics, where first grassland shrubs are planted, which then paves way for larger and larger trees until a climax forest is reached.
This planted forest would start on the leeward side of the mountains of Queensland and slowly grow towards the interior over a period of decades. Once this forest passes the critical size it will generate rain. Studies have shown that if the interior of Australia can return to its pre-human level of precipitation, Lake Eyre would become a permanent freshwater lake with a size larger than Taiwan (or Belgium or Maryland for our Western readers).[8]
The downsides:
• A government program to replant the Australian interior will cost a lot of money and people-power.
• Replanting will take some grazing and farming land out of productivity.
One solution to the two problems posed above is that ranchers and farmers could be assigned land stewardship jobs in taking care of the vegetation that will draw rain back to the interior. The European Union and the United States already have similar land stewardship programs.[9] This, of course, has the other downside of needing subsidies, as protecting the land instead of selling meat does not generate profits.
• Another downside is that more trees will draw down groundwater levels until the aquifers below are replenished, which could take decades to centuries as well.[10]
The positives:
• Increased rainfall – Australia is the driest continent, and water availability limits the country’s economic growth and carrying capacity. More water can help Australia increase both. This will be especially important as population in Australia increases and the country takes in people from island nations that are to be flooded by global climate change.
• Flooding Lake Eyre – This will provide Australia with a reservoir of water that would provide a new home for temperate climate creatures and people (acknowledging that creatures adapted to Lake Eyre’s current condition may be out of luck), the lake would be a driver of economic growth with lakeshore development, moderate the climate of the interior, and if it exceeds the critical size, be able to generate rain in the interior of Australia.
• Locking up carbon dioxide – along with Canada and the United States, Australia has one of the highest carbon emissions per capita in the world. With world governments hesitant to slow economic growth for the sake of the environment, more trees are needed on a massive scale to counter the massive amounts of carbon dioxide that humans are pumping into the atmosphere.
Of course, increased rainfall will occur if and only if the forest that generates the rain exceeds the critical size needed to cause rain. This means that the forest must be more or less contiguous, not carved up into separate blocks. Separate blocks of forests would dissipate the humidity generated by the trees and no rain will result.
Lessons for the Amazon
The Amazon is currently being cut into disparate blocks in the name of development, and the resulting drying out of the rainforest has already become noticeable.[11] Just as there is a tipping point for enlarging forests to increase rainfall, the Amazon rainforest is heading for the opposite tipping point where decreasing forest size will lead to a massive drying out – leading to conditions present in the desert interior of Australia today.
This is not to say that the people of Brazil and the Amazon Basin should not or cannot develop. Rather, there are ways – ways rooted in ancient, pre-Agricultural Revolution traditions – where economic development can take place by only slightly retooling the ecosystem and NOT entirely subjugating nature to the will of humans. But that is a story for another time.
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* Australia has several species of eucalyptus trees that cannot cross-breed. When Australia was one contiguous forest, eucalyptus trees had a clinal gradient where trees close to each other could cross-breed but those far away could not. Remember from biology that as long as such a reproductive gradient exists, these trees are considered to be of the same species. When the forest disappeared, the gradient also disappeared, leaving trees that were formerly of one species separated into several different, sexually incompatible species.
[1] http://rainforests.mongabay.com/amazon/rainforest_ecology.html , http://environment.nationalgeographic.com/environment/habitats/last-of-amazon.html
[2] http://en.wikipedia.org/wiki/Evapotranspiration
[3] http://www.independent.co.uk/news/science/prehistoric-polluters-484363.html
[4] Ibid. #3
[5] Diamond, Jared. The Third Chimpanzee. p. 329, 357, 364. HarperPerennial. 1992
[6] http://www.k26.com/eyre/The_Lake/Ideas/Fill_Lake_Eyre_/fill_the_lake.html
[7] http://en.wikipedia.org/wiki/Green_Wall_of_China#Measuring_success
[8] Ibid. #3
[9] http://www.npr.org/templates/story/story.php?storyId=4748066
[10] http://en.wikipedia.org/wiki/Green_Wall_of_China#Problems
[11] Lenton, T.M. et al. “Tipping elements in the Earth’s climate system.” 2008. Proc. Natl. Acad. Sci. USA 105:1786-1793.