Germany’s busiest airport canceled more than 100 flights as environmental activists launched a coordinated effort to disrupt air travel across Europe at the height of the summer holiday season to highlight the threat posed by climate change. Frankfurt Airport said flights were halted for safety reasons after climate activists breached security fences triggering a response from police, firefighters and airport security officers. The airport said on its website that all runways were back in operation by 7:50 a.m. local time. Climate activists staged similar actions in Finland, Norway, Switzerland and Spain on Wednesday. Environmental groups said they planned to target airports around Europe this summer to remind people about the link between fossil fuels and climate change.