In fact, there are reasons for hope besides those McKibben discusses. One is the change in policies of some powerful multinational corporations. I can already hear the horrified screams of many of my environmentalist friends as I say this. However, I’ve been on the boards of two of the most effective international environmental organizations, World Wildlife Fund and Conservation International, both of which are heavily involved with big, powerful corporations. I acknowledge that those corporations do some very bad things. But they also do some very good things on a large scale that their power makes them uniquely capable of doing. For example, Walmart has quietly been making efforts to manage its supply chain, its wastes and its truck fleet sustainably — partly, but only partly, because it discovered that it can save money by doing so.

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You may already have closed your ears because of the bad things you know that even the best big companies also do. Listen, you extraterrestrial visitors from the Andromeda Galaxy, where companies and androids are either purely good or purely bad. Alas, here on Planet Earth, good and bad are mixed together; we don’t have companies that are purely good. If environmentalists refuse to engage with big companies, in order to push them to do more good things and fewer bad ones, we could well end up in McKibben’s worst-case scenario: human extinction.

Another reason I feel hopeful has to do with the success of many recent supranational agreements — bilateral agreements between nations, regional agreements and world agreements. Those agreements have addressed difficult issues and required negotiations between nations that hate and fight each other. But agreements have nevertheless succeeded in eliminating rinderpest and smallpox, delineating near-shore economic zones in the oceans, reducing chlorofluorocarbon damage to the ozone layer, setting standards for oil tanker pollution and establishing a framework for seabed mining with shared royalties. Those successes give me hope that we may similarly resolve other difficult international problems, including climate change.

My other concern about tactics has to do with the diverse audiences to which environmentalists like McKibben and me must address our books. It’s not enough for us to write for those who are already sympathetic or convinced. We also have to write for those who are skeptical or hostile. If we give up on the many rich, powerful, smart people who are skeptical of environmental threats, again we are doomed to McKibben’s worst-case scenario.

To convince a skeptic requires different approaches from those that work to reinforce the convictions of someone already converted. I witnessed this recently when I was with an environmentalist friend. We ran into another friend of mine who is rich, powerful, smart — and politically conservative. I introduced my two friends. My conservative friend asked my environmentalist friend what she did, and she answered, “I run U.C.L.A.’s sustainability program.” My conservative friend replied, “What’s that?,” the skepticism evident in his voice.

My environmentalist friend responded in a way that my conservative friend could understand: “That means running things so that your children and grandchildren will be living in a rich and thriving world.” She went on to describe how U.C.L.A., by combining its heating, cooling and power plants, reduced its greenhouse gas emissions and saved money. The skepticism disappeared from my friend’s voice; he was ready to hear more.

It will take many different voices to persuade the world’s diverse citizens and corporations to collaborate on solving the world’s biggest problems. McKibben’s voice has been an influential one. My hope is that his new book will strengthen the motivation of those already sympathetic to his views. My fear is that it won’t convince many who remain hostile to them. I hope that my first prediction proves right, and that my second proves wrong.