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Most of the discussed geoengineering ideas aim to change the average temperature of the earth instead of trying to control local weather.

Aerostats can be used to change the local temperature in several ways:

  • if covered with reflective material they can create shadow;
  • if covered with dark material they can significantly increase the temperature above the cloud potentially dispersing the cloud;
  • if combined into a structure similar to a solar updraft tower, they can transport dust and humidity to the height where they would not be transported by natural winds and by that either condensate the water or create new clouds.

With a large enough number of devices like this it should be possible to guide humid air to deserts, or to prevent hurricanes by removing convective available potential energy on their path.

Is there any research about the possibility of controlling the weather with devices like these, or is there some obvious flaw that would make this completely impossible? (Lack of helium is not a good argument, as these aerostats are not meant to lift any additional weight, and are meant to be as large as possible, so could use air heated by the sun).

If this is possible, what percentage of earth surface would we need to cover with these devices to counteract CO2 effects? From a rough assessment of incoming radiation vs radiation trapped by human released CO2 it seems to be much smaller than 1%.

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No, the idea is quite impractical. Lack of helium is no obstacle because unmanned balloons, whether tethered or remote controlled could be filled with hydrogen, but to affect the climate you would need millions of them, which is too expensive and impractical. Geo-engineering projects are generally the stuff of science fiction and not practical in the real world. One way climate could be modified on a local scale is to refill places like the dried up Aral Sea with water from the ocean. Apart from other beneficial effects, this would probably produce enough evaporation to affect local rainfall. Rainfall is also locally affected by the presence or absence of trees, so if you want to increase rainfall in arid areas, one way to achieve it is large scale tree planting. That might be difficult in semi-desert environments, but nevertheless it has been done. Sometimes tree roots can reach down to a water table, which is how enormous cypress trees are in some places able to grow in the sand seas of the Sahara Desert.

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  • $\begingroup$ Price is not a good argument either because various renewable and agricultural subsidies already cost lots of money. If changing weather is possible in principle, then some places like Amsterdam or Dubai would be willing to spend billions to get more sunny days or more rain. $\endgroup$ – a user Aug 16 at 15:43
  • $\begingroup$ The citizens of Holland or UAE wouldn't want their sky filled with hundreds of thousands of balloons, even if they produced the desired result, which is unlikely, and even if somebody else would pay for them, which is also unlikely. In some parts of the world there are cheaper and more effective ways to produce local climate change. In coastal Namibia, for example, moisture laden,foggy breezes blow in from the sea and condense on any object sticking out of the sand. Nobody has attempted planting trees there, but it might be worth a try. There are other, similar deserts where this might work. $\endgroup$ – Michael Walsby Aug 16 at 16:41
  • $\begingroup$ Balloons are not meant to fill the blue sky, for amsterdam the balloons will be above the clouds, in UAE they will be deployed in the days when atmosphere is too humid, making sky grayish mess. Planting trees is a good thing, but they are too unstable on their own. Creating artificial inland lakes requires huge infrastructure to pump the water up, but provides relatively little benefit (e.g. when Aral sea existed the area around it was still a desert, just a slightly milder). The balloons would help to create initial conditions for forests and lakes to form. $\endgroup$ – a user Aug 16 at 17:49
  • $\begingroup$ As for the price: my friends in Amsterdam say they'd be willing to pay 400 for 40 more sunny days in a year, which would be a good start on city level, if the scheme could work in theory. $\endgroup$ – a user Aug 16 at 17:49
  • $\begingroup$ Bad weather would play havoc with your balloons, it simply wouldn't work. Filling the Aral Sea would be a major project, but no pumping would be necessary. It is well below mean sea level, so water could be siphoned. All kinds of benefits would ensue: new fisheries, marine transport, an end to wind blown salt which pollutes farmland, and a more humid climate which might improve rainfall. $\endgroup$ – Michael Walsby Aug 16 at 18:17

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