One of the favorite topics of classic science fiction was terraforming. Mars, it was thought, might some day have an atmosphere and climate enough like earth's to allow humans to wander around happily without spacesuits. More grandiose space-operas envisioned this process being taken to planets scattered across our galaxy.
Meanwhile, more or less by accident, we have been reverse-terraforming earth. Generalizing the term to mean any major global changes in a planet, we have been terraforming earth.
And we don't need spacesuits. Not yet. Global temperatures have risen, but mostly that just means more t-shirt weather and the annoying expansion of insect populations.
We were once well down the path to destroying the ozone layer, but scientists convinced politicians to put a halt to that process. Unfortunately part of the terraforming process seems to be the gradual but obvious decrease in intelligence and ethical capacities of politicians. Two decades ago a politician who denied Darwin and other basic science was quaint. Now denying Darwin is a requirement to head science appropriation committees.
The great thing about terraforming is that it is a lot like home decorating: you can go a lot of ways with it. Back before ozone destruction and global warming were well-recognized designer motifs there was nuclear winter. Scientists calculated that if enough atomic bombs went off more or less at once we would not have to worry too much about death by background radiation (presuming we were among those not vaporized by the initial blasts). Instead so much dust and water vapor would be sent up into the stratosphere that the sun's radiation would be reflected back into space. The whole planet would freeze. A few people might survive for a while huddled around nuclear reactors or petroleum depots, but with no sunlight there would also be no agriculture, no food, and eventually not even a satirist left to comment on the stupidity of it all.
By way of terraforming, many plans have been suggested for keeping global warming from becoming too much of a reality. After all, if it ever hits 135 degrees in New York City and the air conditioners on Wall Street break down, the politicians are really going to get an earful. To prevent this we could put up artificial shade, like big umbrellas in orbit, or just purposefully put just the right amount of dust up into the stratosphere. Some global climate change denialists even believe that temperatures will be self correcting because more heat causes more water to vaporize, which forms more cloud cover, which in turn reflects sunlight. Mugginess is the worst case scenario if they are right.
In addition to designing a new room temperature, there are a lot of knickknacks beginning to clutter up old Terra. Canals that cut across continents and shipping both allow species mixing between formerly separate bodies of water. All kinds of cute insects and plants and such are finding new homes that are free of their traditional enemies. This can result in whole forests dying, which is the opposite of trying to get trees to grow on Mars, but which is impressive for a species that likes to think it does great things on purpose.
Asphalt too has a nice way of absorbing sunlight and baking up organic life forms in summer. Factory farming can turn whole states into monocultures just waiting for the right disease or weed to arrive to flip productive farmland into desert.
The earth's ecosystem is large and resiliant. It has survived about a million years of human pranks. Despite the carnage in species, there are still millions of species fighting it out over living space.
For most of those millennia, however, there just were not that many people around. No one was actually counting back then, but likely the human population of earth remained under a billion until around 1900. Now its seven billion or so, and everyone wants a cell phone, centralized heating and air-conditioning, and medical services guaranteed to keep them alive forever.
Maybe we really should think about terraforming Mars. I'd be willing to pay a few extra tax dollars to ship Paul Ryan there to be its first President, along with maybe 80 million Tea Party Republicans to help him get started.