Book review: Under a Green Sky

Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future, by Peter Ward. Collins, 2007.

Under a Green Sky tells three stories: A story of Earth’s past climate, a story of how scientists have figured out what we know about Earth’s history, and a story of what our future climate might be like. It’s a scary book, but also a call to action.

Peter Ward, a paleontologist, opens with a recent bit of the scientific-discovery story: how we found out that the mass extinction that happened 65 million years ago, the dinosaur-killing one at the end of the Cretaceous, was caused by an asteroid impact. He starts there because, despite the initial controversy over this discovery, cosmic impact became for awhile the dominant model for how most if not all mass extinctions happen. Ward then goes on to describe, with plenty of interesting details about geological field trips and the back-and-forth of scientific discovery, the way this dominant model was nudged aside by the gradual realization that most of the mass extinctions we know about were actually caused by climate change.

After sharing this insider’s view of how science is done, Ward describes how climate change has caused past mass extinctions. Greenhouse gases get into the atmosphere one way or another (e.g., from volcanic eruptions, or more recently of course from human activity), and if the world gets warm enough, the oceans begin to die. Right now, differences in temperature and salinity drive a conveyer system of currents that keep the present-day oceans oxygenated and alive. (The Gulf Stream, which you’ve probably heard mentioned in connection with global warming, is part of this system. Ward describes the movie The Day After Tomorrow, by the way, as a fable that trivializes the ominous possibility of rapid climate change. It sounds like a fast freeze is not the most likely, or even the most scary, possible outcome of the Gulf Stream’s disruption.)

Greenhouse warming disrupts the conveyor system, rendering the ocean depths warmer and less oxygenated and killing the creatures that live there. One of the few things to thrive in these oceans is a type of bacterium that produces toxic (to everything else) hydrogen sulfide. In some cases, carbon dioxide and methane can arise in bubbles from the ocean and enter the atmosphere, along with deadly hydrogen sulfide gas, which is not only noxious–it also breaks down the ozone layer, increasing the amount of ultraviolet radiation reaching the planet’s surface. All these factors contribute to mass extinction. It’s a sobering scenario to contemplate in an era of global warming.

When Ward focuses on the story of Earth’s climate, you get an idea of how unusual the time we’re living in really is. We think of Earth as being like the middle bear’s porridge, just right for life, and we wonder how many other planets are out there that might also be just right. But in fact the Earth has been intermittently all wrong, deeply frozen or stifingly hot with toxic oceans, and it might be so again. A few years ago Ward co-wrote a book with Donald Brownlee about the eventual fate of the Earth; toward the end of it they talked about their belief that interstellar travel is not likely to deliver us from future events on Earth, and this world is likely all we’ll have to depend on. They note:

“We live in a glorious summer of beauty, diversity, and resources. It will not always be so.”


“…this moment on this Earth truly is a precious gift, to be savored and appreciated. If we heedlessly destroy this world, it is unlikely we will find another to replace it. Or be able to get to any refuge, even if we could find it.

Another obvious lesson is that we tinker with our atmosphere and oceans at grave risk.”

Under a Green Sky gives me more context about past conditions on Earth and how unusual the long interglacial period in which humans have had the chance to flourish really is.

This of course is important for the third story Ward tells, that of present-day climate change and where it might lead. Based on what has happened in past conditions of high CO2 levels, he describes possible futures for Earth. It’s in some sense a write-your-own-ending book. Ward closes by outlining three possible future worlds, each with a different atmospheric concentration of carbon dioxide.

Even the best-case scenario is far from ideal, and it would require us to keep the level of CO2 below 450 parts per million (ppm). (It’s currently at 380 ppm and rising.) An intermediate view shows the world at 700 ppm by the year 2100, a discouraging sight, and the worst-case scenario looks at what would happen if the levels hit 1,100 ppm by 2100–the beginning of a return to the hellish conditions of the ultrawarm Eocene epoch, dead oceans and all. He says that we can keep CO2 levels down in the range where we wouldn’t have to face anything worse than the first scenario, but we must act immediately. (It’s outside the scope of the book to go into any detail about how, but check out the action items at for some ideas about what you can do.)

Coincidentally I recently saw the last two episodes of Carl Sagan’s Cosmos series, in which he talks about SETI and about the perilous choices facing humankind, both topics that have links to what I’ve just been reading in the Ward book.

The Drake equation, which multiplies together factors affecting the odds of contacting intelligent life elsewhere in the cosmos, includes a factor sometimes called L, the average lifetime of an advanced civilization. Of course any number you plug in for L is just a guess. Somewhere years ago I saw a little jingle that went:

Of all the sad stories that SETI could tell,
The saddest would be a small value for L.

I wonder whether the odds of our own survival look subjectively better or worse than they did when Cosmos was made. Nuclear war doesn’t seem as pressing a threat, but climate change certainly seems more urgent. (It was just as urgent then but, sadly, many of us were not paying enough attention.)

In the final episode of the show, Sagan talks about the dangers of nuclear war and other hazards we face, including climate change. He adapted a quote from Deuteronomy to the effect that we have the choice of life and death before us, and we must “therefore choose life”. I hope we have the collective intelligence and will to do so.