In the Star Trek science fiction series, starship captains are always relieved to find a "class M" planet. "Class M's" are Earth-like -- with breathable atmospheres, abundance of water, etc. -- and are great places to repair a damaged spaceship, replenish supplies, or have an inspiring adventure.
Real-world astronomers are also attracted to Earth-like planets. They use something called the Planetary Habitability Index (PHI) to measure how hospitable a planet might be to supporting life. It uses a scale from 0 to 1 and assumes that Earth rates a 1. The problem is that Earth is not as habitable as it used to be.
What if we were to track an Earth Habitability Index (EHI)? What makes the Earth habitable? At the most basic level: land, air, and water, the plants and animals that provide food and energy, and a human population that draws on these resources at a rate that does not deplete them over time.
Let's take a look at some potential components of an Earth Habitability Index. These components are diverse. But the astrobiologists' PHI planetary index also includes diverse factors such as having stable ground, an energy source, appropriate chemistry and the potential for holding a liquid like water.
I am not a scientist, but I do have a personal interest in living on a habitable planet. You probably share this interest, as well as sharing my interest in passing a habitable planet along to our children. I encourage you to look into this and get active.
Land (earth): Roughly one-third of the world's cropland is now losing topsoil faster than it can be re-formed. This gives a land rating of 0.67 (calculated 1 minus 0.33). Another land measure could be deforestation. About half of the Earth's mature tropical forests have been destroyed since 1947. So a tropical deforestation rating would be 0.5.
Air: Many experts now say that the upper safe limit for CO2 in the air is 350 parts per million (ppm). According to NASA the current level is 393 ppm. So air gets a 0.89 (calculated: 350 divided by 393).
Water: One-fifth of the world's population lives in areas of freshwater scarcity. So we could use four-fifths (0.80) as a rating for fresh water. As for salt water, the first complete global Ocean Health Index is currently scheduled to launch June 2012. We do know that the acidity of the ocean has increased about 29 percent since pre-industrial times, reducing the ocean's ability to support marine life. So a saltwater rating would certainly be less than 1.
Plants and animals: According to a 1998 survey of biologists, nearly 70 percent believe that we are in the early stages of a human-caused extinction and agree that up to 20 percent of all living populations could become extinct within 30 years. More recently, biologist E. O. Wilson has estimated that one-half of all species of life on earth could be extinct in 100 years. The current rate of species extinctions is estimated at 1,000 to 10,000 times the "background" or average extinction rate. Using the conservative 20 percent en route to extinction estimate gives a rating for this factor of 0.8 (calculated 1 minus 0.20).
Human population: All of the problems mentioned above are driven by human population growth. The Global Footprint Network reports that today humanity uses the equivalent of 1.5 planets to provide the resources we use and absorb our waste. One divided by 1.5 would give this factor a rating of 0.67. Here's another way to look at it: the current global population of over 7 billion is much larger than the approximately 2 billion that could be sustained at a European standard of living. Two divided by 7 would give a population rating of about 0.29.
Earth Habitability Index: These individual ratings range between 0.29 and 0.89. Other ratings could be chosen and there could be many ways to integrate the numbers into a combined EHI. But clearly the result would be less than the ideal 1.0. So the Earth may not be bad as an emergency-landing planet for the starship Enterprise, but it's not nearly as good as we'd like to have for our home planet.
To make our planet habitable will take all our political commitment and a lot of money, but the money needed is only 12 percent of the world's annual military spending. (For details on the plan and to learn what you can do, see earthpolicy.org/action_center/C30.). The money needed to save civilization should become our new security budget.
Chris Hoffman, of Boulder, is an organization consultant and ecopsychologist, the author of "The Hoop and the Tree" and the creator of earth-dashboard.org. References for this article are available at ehindex.wikispaces.com.