Consider this tale of two planets. Earth and Venus are almost exactly the same size, and have almost exactly the same amount of carbon. The difference is that most of the carbon on Earth is in the ground — having been deposited there by various forms of life over the last 600 million years — and most of the carbon on Venus is in the atmosphere.
As a result, while the average temperature on Earth is a pleasant 59 degrees, the average temperature on Venus is 867 degrees. True, Venus is closer to the Sun than we are, but the fault is not in our star; Venus is three times hotter on average than Mercury, which is right next to the Sun. It’s the carbon dioxide.
Okay, I'm all about saving the Earth and reducing the amount of CO2 we humans disgorge into the atmosphere, but comparing the Earth and Venus to make your point? The effects of global warming are going to require an awful lot of adjustment, and, should we handle those impacts poorly, will very probably lead to much war, famine, and generally bad behavior on the part of us humans. Are we going to turn into Venus? No. Definitely not.
So, what are the expected impacts of anthropogenic global warming, you may ask. The impact I'm most interested in is the impact on aridity. It's expected that the southwestern US will become more dry under a warmer climate regime. Droughts will be longer and more severe, and the "average" moisture state will be closer to that experienced during the dust bowl in the 1930's than it will be to today.
Temperatures will increase more in the northern latitudes than nearer the equator, meaning the effects of global warming will be more pronounced as one moves toward the poles. Some species will undoubtedly go extinct as their habitats become inhospitable (like the pika), but probably fewer than if the climate cool precipitously (at least, if previous abrupt warming periods are any indication). More likely, though, the individuals in some species will grow smaller, and others (though again, fewer than if climate were cooling) will grow larger. We humans have carved up the landscape with our agriculture and cities, so it'll be harder for species to move the way they normally would, and that will likely tax some species into extinction that would otherwise have made it. It's likely the growing ranges of many food species we rely on will shift, and probably not to wider ranges in the US. Growing season temperatures on average won't shift up that much, but the number and intensity of high heat stress days is likely to increase--something every bit as important for determining whether a given crop year is successful and productive or not.
We'll loose a lot of coastal areas, but that's happened before, quite a few times, actually, and eventually won't be much of an issue. The main problem with that is there are a lot of people who will likely be displaced initially, and they will likely, even in the best of reactions and circumstances, be forced to live in even closer proximity than they currently do. This proximity will probably be great for the diseases that rely on proximity for communication from person to person, but not really for the people forced to live together. Speaking of diseases, many of the diseases and vectors that are most pernicious to us humans are winter-killed in the north. Warmer winter temperatures will encourage diseases like malaria to make a reappearance in the states, and we're already seeing infestations of wasps that successfully overwinter across the south. Eventually a new equilibrium will be reached between vector and host (us), but in the mean time, a lot of people will probably die.
Personally, I think the real life consequences of global warming are sufficiently scary without invoking Venus, Mr. Gore.