Even half a generation ago, economists interested in global warming like Thomas Schelling saw natural justice to this argument, and expected the North Atlantic to finance the carbon-neutral industrialization of the Global South--an environmental Marshall Plan for the 21st century. Global politics has not been kind to such dreams. I do not know of a way to represent a number as small as the probability that the next generation will see large-scale transfers from the North Atlantic to the Global South to fund its carbon-neutral industrialization.
But it may well be that continued inaction on dealing with global warming itself has mammoth antiegalitarian distributional consequences for the world. There are 2 billion peasants in the great river valleys of Asia from the Yellow to the Indus. Few of them have the human capital or the relationship chains needed to do well in the cities of Asia. For most of them, the economically valuable asset that they have the land that they farm--the land that requires for its economic value that there be enough melting snow off the Tibetan plateau but that the snow not melt too quickly. Global warming means either more or less snow on the Tibetan plateau. Why the governments of East Asia and South Asia are not now screaming about the consequences for their 2 billion peasants of global warming seems to me explicable only as the result of a strange kind of political myopia. To wait for large-scale transfers from the North Atlantic to begin carbon neutral industrialization in East Asia and South Asia seems to me likely to be viewed by historians a century from now as a classic dog-in-the-manger story.I think that there are two levels that I really like this argument. One, probably not surprising, is the public health argument. The biosphere is under a lot of pressure -- pushing on it hard right now is likely to result in all sorts of adverse public health outcomes as things like water supplies become more constrained. I think that the economic value argument (the melting snow makes the land economically valuable) is less salient than the potential for human suffering if there is a loss of food and clean water in this area. DeLong doesn't address this directly, but I presume it is subsumed into the consequences of the economic changes.
The second, though, is to think about the net present value of improved renewable energy sources. Even with a modest discount rate, the development of improved solar, wind, or water energy production is huge because it extends the likely window of exploitation far into the future. It is plausible that there might not be petroleum used as fuel in the future; the horizon over which the sun will stop providing energy likely exceeds the expected lifespan of the human race. Heck, even Nuclear, with known disadvantages, has a much longer time horizon for viability. This creates a real value to investment in these technologies as small improvement in efficiency could well pay large dividends.
Of course, Mark would point out that existing technology is also potentially helpful as well.