This is a very idiomatic usage of the noun "reason". Policemen and lawyers, and judges talk about whether there is "reason to believe that....." or there is reason to be concerned about global warming. This is a formal, and highly educated way of speaking. If you were to say: Scientists have shown that usage of aerosol sprays is a reason for the development of global warming; Then it is ok to use an article in front of the word reason. Another example: The teenager did not want to go to school, and his mother asked him for one good reason why not. He responded by saying that his favorite two teachers were both absent with the flu, and that the substitute teachers just assigned exercises to keep them busy for hours. When we say there is a reason, we are being specific, and boiling things down to one thing that explains why.

Thank you for your further explanation, Richard. :)

The two expressions "There is reason to be concerned" and "There is A reason to be concerned" are slightly different. The first is more formal and implies importance to the reason (it is similar to "There is good reason to be concerned"). The second one, depending on context, covers everything from the reason being important through to the reason being so unimportant you are going to do the opposite.

Andrew's explanation is correct. To help you to accept the use of the noun reason without an article, just substitute the word "cause" or "justification" and it may be a little more acceptable: e.g., there is cause/justification to be concerned.... good question Kelvin