**9. Acknowledgements**

The author thanks original authors or publishers for permission to reproduce their figures or data, particularly Dr. Josep Canadell for Figure 7. He thanks Dr. James Boyce and especially Dr. Tina Evans for comments on drafts of the manuscript; to Dr. Wes Jackson for a copy of his manuscript in advance of publication; to Dr. Richard Grossman for prompting his initial foray into this subject (White & Grossman 2010); and to Smith College for funds toward the cost of publication. He particularly thanks Dr. Angeline Schrater for astute editorial comments, careful proofreading, and patient support.
