Winter is not caused by the Earth tilting the northern hemisphere away from the sun. Earth's axis of rotation is stable and fixed, always pointing approximately at Polaris, the north star. Looked at from the side, this axis is tilted about 23 degrees from perpendicular to the plane of Earth's orbit around the sun. As the planet swings around the sun and we (north) head toward summer, the lean of the axis points more toward the sun, which means the top half (north) progressively receives more sunlight per day. In other words, the days grow longer. A longer day means more total heat and light absorbed. This is the principal effect driving seasonal temperature changes. The same effect when we're on the opposite side of the orbit produces southern summer and northern winter.
Carbon dioxide in the air is relatively transparent to visible light, but less so to infrared light. It will let sunlight stream down to the surface, but once that sunlight strikes the surface and is converted to heat, the energy radiates back into the atmosphere as infrared, where it is either absorbed or scattered by the carbon dioxide. The net effect is that thermal energy that would otherwise have escaped into space is retained in the atmosphere, making it warmer. This is the Greenhouse Effect.
There's an easy way to tell just how shaky the science is behind various global warming hypotheses, and that's all the heated yelling going on between people who think the sky is falling and people who think there's no price to pay for human-induced changes to the atmosphere. In simplest terms, if there was good science, people wouldn't be yelling at each other until their faces turn red. It would be like screaming over the time of tomorrow's sunrise or that there are nine major planets in the solar system. It's only when there are gaps in our knowledge that we substitute political opinion for scientific analysis.
Despite what partisans on either side tell you, no one knows what to expect if global warming proceeds. The best computer models (and they are not very good at all) predict that the poles will actually grow colder, while the tropics grow warmer. Middle latitudes will see shifts in precipitation, with some regions growing wetter, and others drier. It's also thought that cyclonic systems (the swirls of high and low pressure that drive mid-latitude weather) will be more energized, leading to stronger storms and more dynamic movements of the jet stream, although on nowhere near the scale Hollywood movies depict. No one can say, even on a subcontinental level, exactly which changes will affect which regions.
Weather is extraordinarily complex, involving interactions between the ocean, atmosphere, planet motions, and solar output. Climate (weather over large amounts of time) is another order of complexity higher. It's absurd how oversimplified it's made for the purposes of argument - pop science at its worst.
Is global warming real? Absolutely. Do we need to do something about it? Again, absolutely. But what?
No one knows. The best advice science has come up with so far is: when in doubt, play it conservative. CO2 limitation is a good start. Undoubtedly, there are other things (deforestation, albedo changes, arid land irrigation, etc., etc.) involved as well; we're going to have to figure them out one variable at a time.
Interested in how weather actually works? Check out this NOVA site.
http://www.pbs.org/wgbh/nova/elnino/anatomy/ and click on "Global Weather Machine"
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