Global warming can be seen as an economic issue in two ways. It can be seen as economic in its origins and as economic in its effects.
Global warming is caused in part by economic reasons. Our economy runs on energy. The energy source that is most plentiful, most convenient to use, and cheapest is, at this point, fossil fuels. Sadly, it is fossil fuel use that causes global warming. Economic factors are a major reason why we are not doing anything about global warming at the moment. Solutions to global warming are all expensive at this point and could harm our economies. Therefore, we prefer not to take those steps because we fear the economic harm that could be done.
Global warming will also have economic effects. For example, if sea levels rise and low-lying areas flood, we will have to replace the economic activities that went on in those areas. As another example, if the Great Barrier Reef were to be killed by rising and warmer sea water, that area of Australia would be impacted by the loss of tourism. Finally, if weather patterns shift, areas that are now good for agriculture might stop being good. This would have economic consequences as well.
In these ways, global warming is an economic issue.
Global warming will be a huge economic issue once the green tax is thrust upon us. Based mostly on junk science and sleight of hand, the world has been gradually moving toward the green tax and most are begging for it after decades of indoctrination... But I digress.