Swedish scientist Stas Barabash (of the Swedish Institute of Space Physics), believes that comparing the amount of escaping ions from Venus (which has no magnetosphere), Mars (which used to have one, but no longer) and Earth proves that our magnetic field may be accidentally helping our oxygen ions to abandon us, writes New Scientist:

Taking into account the different masses of the three planets, their atmospheric make-up and their distance from the sun, Barabash compared the rate of loss of oxygen ions from each one. He focused on oxygen ions because these are the most abundant ions in the ionospheres of all three planets. He found that Earth lost oxygen around three times as fast as the other planets.

Barabash points out that a planet's magnetosphere will always be far larger than the planet itself or its atmosphere. This, he reasons, means that a planet with a magnetic field will absorb more energy from the solar wind than it would if it didn't have one. This extra energy would be funnelled down towards the magnetic poles, so molecules in the ionosphere above these regions could be accelerated enough to escape (see diagram). Barabash presented the results this month at the International Conference on Comparative Planetology at Noordwijk in the Netherlands.

Don't get too worried about running out of air to breathe, however; Barabash estimates that we're only losing about 60,000 tonnes of gas each year from an overall total of "thousands of trillions of tonnes."

Earth's protective shield is stealing our air [New Scientist]

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.

Source: Scripps Institute of Oceanography