Writing a speech is a lot like writing an essay. And as this speech is meant to be a part of a debate, you must take into account arguments from both sides and come up with some advantages and disadvantages for both.
Concerning air travel, for example, you can discuss the main environmental issue with traveling by plane: aircraft emissions, such as gas and heat. You could also try proposing a solution for the problem. On the other hand, you could argue for the fact that, in some cases, international traveling (either for business or leisure) is not possible without airplanes.
Only when you compare the pros and cons can you decide where you stand and in what direction you want to take your speech.
You also have to back up your arguments with scientific data. Luckily, as global warming has become a part of everyday conversation, there is plenty of information about it online, not just in literature. You do, however, have to make sure you fact-check all the information. Your best source of data would be various scientific research papers from recent years.