It is a familiar observation that political parties are coalitions containing a variety of ideologies and interest groups. It is a little less obvious that ideologies too are coalitions.
Consider the American conservative movement of the fifties and sixties. It was made up of at least three distinct groups: Traditionalist conservatives, classical liberals/libertarians, and southern conservatives, in large part populist. Where some had strong views and the others didn't--anti-communism, for instance--they tended to all go along. Where different groups had different strong views--government control over sexual behavior, for example--they agreed to disagree. The more common the latter situation was, the greater the tension within the ideology.
The modern American left is also a coalition. Someone who thinks of himself as a leftist is probably in favor of increasing environmental regulation, redistribution in favor of the poor, greater government regulation of business, gay rights, feminism, prohibition of private discrimination on any of a considerable variety of grounds. He is probably against aggressive foreign policy, anti-nuclear, suspicious of law enforcement and the criminal justice system but supportive of class actions, punitive damages, and similar features of the tort system.
In that coalition too, there are potential strains. For someone in favor of helping poor people, the economic development of China and India is arguably the best news of the past fifty years. Development was, after all, the explicit goal of foreign economic aid, development planning, a variety of programs in the post-war period that were supposed to lift the third world out of poverty--and didn't. The fact that more than two billion people are now in the process of moving from extreme poverty towards the sort of life westerners have long lived represents an enormous improvement in the condition of the world's poor.
It also represents a sharp increase in the consumption of depletable resources and production of carbon dioxide. The same changes that should be good news to the leftist qua egalitarian are very bad news to the leftist qua environmentalist. Not only are people who used to be poor consuming more and polluting more, they are cutting down rain forests in South America, threatening endangered species in Asia. They are, in other words, doing the same things our ancestors did.
Nuclear power is another obvious problem for the left. It provides a way of replacing a large fraction of fossil fuel power with an alternative that does not produce CO2, using current technology at costs not wildly above current power costs—as France has demonstrated. Arguably, it provides the only such way. With fairly modest improvements
in the technology of synthesizing liquid hydrocarbons, it could replace practically all fossil fuel use. But the left is traditionally anti-nuclear for a mix of reasons, including hostility to nuclear weapons and a more general suspicion of technology.
Biofuels present the most recent example of a conflict between environmentalism and concern for the poor. Supporters argue that they reduce dependence on foreign (and depletable) supplies of oil and reduce CO2 production—although there now seems to be evidence
that the latter may not be true. Critics point out that diverting large amounts of farm land and farm output from producing a lot of food to producing a little fuel will cause--indeed, has already caused--a steep increase in food prices, an implicit tax that falls most heavily on the poor.
All of these conflicts among left wing objectives are accidental—it just happens that the same change which helps in one direction hurts in another. There are additional problems that are more fundamental. One reason some on the left don't want a nuclear solution to global warming is that they see the threat of global warming as a useful argument for lifestyle changes—less power consumption, urban instead of suburban life styles, less consumption—that they favor for other reasons. For those in that position, a way of preventing global warming that doesn't require other people to revise their lives is a threat, not a promise.
In much the same way, the threat of nuclear winter was pushed not so much because its supporters believed in it as because its supporters were, understandably enough, looking for ways to prevent nuclear war. In both cases, the problem with such indirect motives is not that they are necessarily unjustified but that they are likely to lead to dishonest arguments. If your objective is not to prevent global warming or nuclear winter but only to use their threat to persuade other people to do things you want them to do or not to do things you don't want them to do, whether the arguments you offer are right becomes less important than whether they are persuasive.