Most people think of
terraforming as if it were something that requires
technology more advanced than we have today. Not so.
Most people also think of terraforming as necessarily being progress towards a more human-friendly planet. Not so.
We are terraforming Terra. Right now, as you read this, Homo sapiens is altering the coastlines of Earth, changing the temperature, removing parts of the upper atmosphere and adding other parts to it.
This was not an intentional choice. We didn't decide en masse that it would be peachy keen to raise the average temperature, strip away the ozone that blocks UV radiation, melt the polar icecaps, and begin to submerge low-lying land.
We are, nevertheless, learning what it means to terraform a planet. Earth has become not only our laboratory but our test subject. Let's hope we don't make any changes we might later regret.