Jordan117 has put up a nice bit of analysis in the HBO forum alleging that various artistic and design choices in Halo 2, specifically those made in the Outskirts and Metropolis levels, indicate that Bungie is postulating increasing global warming in Earth's future.

While probably not essential to the plot, if it's true it does demonstrate the insane attention to detail that Bungie puts into its games.

Thanks Louis Wu for the heads-up at HBO.