Check your spelling. Use short sentences and simple grammar. Choose words that will be understood worldwide, not just in your own country or culture. Solicit comments on a draft before sending it out. Your message will circle the earth, so double-check.
Errors can be disastrous.
Start a movement, not a panic. Do not overstate. Do not plead. Do not rant about the urgency of telling everyone in the universe about your issue. And if the issue really is time-critical then just explain why, in sober language.