The first thing to consider is whether overpopulation actually is a problem. This is more of a question in demography than in politics, but it's important to consider in answering this question.
There is this idea of the 'fertility rate', slightly different from how we think about the fertility of any given individual, which you can think of as a measurement of how many children a woman in a given population will have over her lifetime. The fertility rate that ensures the next generation will be the same size as the current one is slightly above 2. This is called 'replacement fertility'. This makes sense, because if your parents have 2 kids, then effectively they are 'replacing' themselves and so the next generation will have the same number of people in it.
As it turns out, the fertility rate of most developed countries (e.g. in North America and Europe) is less than replacement fertility, which means that the population of most developed countries is not really growing. There's a lot of reasons for this, that have to do with developed countries being more urbanized, less religious, more educated and having better governments. Some developing nations, particularly in Africa, have high fertility rates which means their populations are growing, but their fertility rates are dropping as people there become richer. If you're interested in learning more, the late Hans Rosling has given a number of talks on this topic.
What this means is that the world's population is not likely to explode out of control. In fact, in Western nations we're already experiencing problems with a shrinking population: Japan, for example, doesn't have enough workers. Germany also needs more immigrants.
To finally answer your question: climate change is obviously a big problem, but it's not driven primarily by increasing population, it's driven by increasing resource usage as people become wealthier. A lot of current politics is a result of population changes, but those changes are a result of immigration into aging and shrinking Western populations, not because of overpopulation. That is why the UN and world leaders are not concerned about overpopulation.