Partisanship, Global Warming and Immigration
One example is the way in which, if anyone posts anything critical of the conclusion that we ought to be taking strong action to slow warming, he is promptly labeled a "denier" and accused of denying that warming is happening. Those responding are lumping everyone who disagrees with them together, assuming that any argument made by anyone on the other side must be supported by everyone on the other side.
A second example is the way in which threads on climate tend to morph into disagreements on other and unrelated issues, with each side seeing the other not in terms of their view on the subject being argued about but as part of a broader enemy—very roughly speaking, "left" vs "right." In at least one case, people on both sides turned it into atheism vs Christianity, in others liberal vs conservative.
None of this is surprising, only depressing. Humans seem to have a strong, probably hard wired, tendency to see the world in terms of us and them, ingroup and outgroup.
For a still more striking example, consider arguments about immigration. One common argument for restricting it, usually coming from people who think of themselves as egalitarians, is that a flood of poor immigrants would depress the wages of the present poor. That conclusion may or may not be correct. But if it is, that means that people who think they are in favor of equality are willing to block an enormous improvement in welfare for people who currently live on less than a dollar a day—the foreign poor who would come—in order to avoid a smaller decrease in the welfare of people who currently make about eight dollars an hour. That is explainable only on the grounds that the foreign poor, being members of the national outgroup, don't really count in moral calculations.