Answer:
What Steffen meant by "On the one hand we have the unthinkable, and on the other hand we have the unimaginable" is:
There are worst-case scenarios which are as a result of the planet depletion. These effects are known to be unthinkable. Then the alternative is the unimaginable which is something we are yet to know. The environmentalists are working towards the unimaginable but are yet to grasp it. Whereas as the planet experiences a downward trend then the unthinkable is what will happen.
Explanation:
Alex Steffen, in his TED Talk in 2015, spoke on "The Route To Sustainability".
He revealed the state of our planet and the activities being carried out that led to its current. Also, he revealed the need for green cities if sustainability must be achieved in our planet.
