There is or there are?

Does the sentence below sound odd to you?

(1) There are a cat and a dog in the field.

If it does, you are probably not alone.

A friend had asked me some time back whether it should be (2) instead:

(2) There is a cat and a dog in the field.

The short answer is no, and the long answer will take us to two differing views.

Sentences beginning with "There" are termed expletive sentences. The word "There" is called a dummy subject whose purpose is to introduce the sentence. It does not refer to anything nor does it encode any grammatical information.

Traditional grammars argue for sentence (1) because the plural are agrees in number with the plural a cat and a dog, similar to the kind of agreement obeyed in the following pairs:

(4a) Some water is in the pond.
(4b) There is some water in the pond.

(5a) Many trees are planted here.
(5b) There are many trees planted here.

(4a) and (5a) are declarative sentences while (4b) and (5b) respectively are the expletive "There" counterparts.

Notice that it's easier to see the number agreement between Some water and is and that between Many trees and are in the declarative structure than in the expletive structure, since Some water and Many trees occupy the prototypical subject position in (4a) and (5a).

The trick, therefore, is to turn sentence (1) into a declarative sentence to make this agreement obvious to see:

(6) A cat and a dog are in the field.

We now have a straightforward conjoined plural noun phrase A cat and a dog in the subject position agreeing with the plural be verb are, and this same plural be verb should be used in the "There" structure as we saw in (4b) and (5b), where the is and are respectively mirror their more prototypical counterparts.

This preference for the plural are in sentence (1) is the position held by traditional grammarians and echoed by others such as Grammar Girl.
 
In actual usage, however, there is a preference by most native speakers to choose the verb that agrees in number with the noun or noun phrase nearest to it (Marianne Celce­-Murcia et al, 1998). This preference can be explained by the Proximity Principle.

According to this principle, subject-verb agreement should occur with the subject noun nearest to the verb. Sentences (7) and (8) below illustrate this rule:

(7) Either John or his friends are attending the party.
(8) Either the students or their teacher was present.

In (7), the plural subject noun friends is nearer to verb to be than John, so the plural are is chosen. Similarly in (8), the singular subject noun teacher is nearer to the verb to be, so the singular was is preferred.

With expletive constructions, the proximity rule will give rise to the following two sentences:

(9) There is a cat and two dogs.
(10) There are two dogs and a cat.

So should we apply the proximity principle to "There" constructions? Well, no. Particularly in formal contexts, the grammatically acceptable version is the one based on the traditional prescription.