Maybe that's a viable solution but to me controlling green house gases by shedding our atmosphere sounds like a terrible idea. I'm pretty sure we would want to keep the oxygen around. Finding ways to capture CO2 from the atmosphere and sequester the carbon should hopefully also be easier than controlling the magnetic field of the earth.
I think losing our atmosphere to control the greenhouse effect would be ... a very bad idea. It would work, but we would end up wishing we hadn't done it.
E.g. could we partially cancel out earth's magnetic field in order to control atmosphere loss and thereby control the greenhouse gas effect?