Geoengineering is a branch of science which is focused on using technology to change the Earth's environment. Most geoengineering ideas are purely hypothetical, and many focus on mitigating the effects of global warming, which is becoming a major issue of concern for many nations around the world. Some of the schemes which have been proposed by geoengineers are quite radical, and they would require global discussion and cooperation before they could be implemented.
The idea of altering the environment of a planet to make it more habitable is actually fairly old, although no one has actually embarked on the modification of an entire planet's environment as yet. Early authors of science fiction proposed that settlers in new environments could shape them to make them more like Earth, which is widely considered to be a rather ideal planet to support humans. One of the fundamental parts of planetary engineering is terraforming, actually physically altering the surface of a planet to give it more Earth-like features. It has been suggested that the Earth's surface played a very important role in making the planet hospitable to life, so terraforming is not an entirely crazy idea.
Unlike planetary engineering, geoengineering is focused specifically on the Earth, which is facing some serious problems as a result of human activity. While global warming is among the most major of these problems, other issues may come to be addressed by geoengineering in time as well. Many members of the scientific community have their doubts about the value of geoengineering, since it is a radical branch of science which may harm as much as it helps, as the balance of life on Earth is still not perfectly understood.
Every now and then, geoengineering proposals pop up in mainstream media. One idea which is sometimes bandied about is an intentional darkening of the Earth's atmosphere, in the hopes of triggering global cooling events like those documented during periods of high volcanic activity. Other geoengineers have proposed fertilizing the world's oceans to stimulate algae production, the hopes that the algae could scrub carbon dioxide from the air, while some have suggested that carbon dioxide levels could reduced by sequestering it deep below the Earth's surface.
Deliberate alterations to the Earth's environment should not be taken lightly. This is why most geoengineering projects do not go beyond the planning stage, since most scientists would prefer to err on the side of caution. Changes to the Earth's environment would obviously impact all life on Earth, for better or for worse, and some scientists would prefer to see less drastic approaches to human-caused climate change and environmental problems.