This question appeared on the "Official Questionnaire" for the 2012 New York Tree Survey from the National Arbor Day Foundation.
Of all the questions cited on the said questionnaire, it appears that this particular query stumped more people compared to the other items.
It provides several real important functions to choose from: providing shade; providing oxygen; being a source of beauty; absorbing carbon dioxide; filtering water; saving energy by cooling our homes and neighborhoods; providing habitats for birds and animals.
And we choose to pick "absorbing carbon dioxide". Releasing oxygen is almost as important, but oxygen can be made from other things too see Source of Half Earth's Oxygen Gets Little Credit.
Very few things absorb carbon dioxide on earth. One of the seemingly ideal and direct solutions to climate change is to efficiently vacuum up greenhouse gases straight from the atmosphere. But a new study finds that such a proposal is very far-fetched and tremendously expensive.
Howard Herzog, a senior research engineer at the Massachusetts Institute of Technology's Energy Initiative said that the most practical one is probably biological, like trees and vegetation. That's driven by solar energy. That's not inexpensive, but it's a lot more feasible. See ScientificAmerican.com for further reading.
The end products of trees are all around us. Save them by making small modifications to your daily routine. Here are 15 Ways to Save a Tree—or least part of a tree, shared by RecycleBank.com.