Global variables cannot be used inside function of a Jenkinsfile

I have Jenkinsfile like this:

def globalVariable = "my variable"

def myFunction() {
    def myString = "my String and ${globalVariable}
    .....
}

pipeline {
    ...
    < The globalVariable can be accessed inside the pipeline >  
}

I receives a runtime error, complaining globalVariable cannot be accessed inside the myFunction.

I thought globalVariable is global and can be accessed anywhere from the file.

Is there anyway I can reference it inside function?