Forget the ozone layer, global warming, and all of the other things environmentalists whine about: the one thing holding life together here on Earth is its powerful magnetic field. And for the past 150 years that humans have been measuring it, our only line of defense against deadly cosmic and solar radiation has been mysteriously weakening. Now, new research says the situation is even more dire than we thought. Looking back 2,000 years into the past, geophysicists have calculated that the field's been weakening the entire time, and that we've got about 500 years to go before it's gone entirely.
The Sun is obviously the biggest reason we're alive today — without it Earth would be a lifeless, frozen lump of rock at best. The same is probably true of the oceans, Earth's distance from the Sun, and so on. But Earth's magnetic field doesn't get enough credit (apart from a few terrible movies like "The Core") as being just as important as any of those ingredients for keeping life on Earth. Without it, highly energetic particles from the Sun would fry life, shatter life-giving molecules floating in the air and water, and strip away most of our atmosphere (witness Mars, whose thin atmosphere has been ravaged by solar winds).
In just a few centuries that may be a reality. Even if the field doesn't disappear entirely, in a weakened state it could let enough radiation in to cook the vast communications networks and power girds that have sprung up around the planet in the last century. But searching through ancient copper mines in Israel and Jordan has turned up some interesting new evidence. By looking at layers of metal slag that aligned themselves based on the magnetic field that was present as they cooled thousands of years ago, scientists at Scripps Institute of Oceanography and UC San Diego have managed to reconstruct the field's strength. What they found was startling: about 2,000 years ago Earth's magnetic field peaked in strength, and it's been weakening ever since.
The field itself isn't going away any time soon — it's powered by oceans of molten metal churning at the center of the planet — but for reasons we don't quite understand, every quarter million years or so it reverses polarity. Each time it does this, there's a period of a few days to a few hundred years where the field becomes so weak that it's almost non-existent, and that's what we seem to be heading for.
What does this mean for life on Earth? Bottom line is we don't know. Some scientists have argued that mass extinctions line up with field reversals in Earth's past, while others say that when the field flips it flips too fast — maybe over the course of a week or less — to do anything more than cause a glitch in your cell phone reception.
The one thing we can take comfort in is that the decline has so far been slow and steady, so humans alive today probably won't have to worry much.
But our fuzzy understanding from the geologic past suggests that as the field weakens further, it's polarity can wander all over the place, flopping back and forth like a fish out of water. If that's true, in a couple of generations global warming from CO2 in the atmosphere might be the least of our worries.