Considering language use in the text, it’s correct to say:
BLACK, Richard. Arctic warmest in two millennia. Disponível em:<www.bbc.co.uk/ worldservice/learningenglish/language/wordsinthenews/2009/09/090904_witn_arctic.shtml>
BLACK, Richard. Arctic warmest in two millennia. Disponível em:<www.bbc.co.uk/ worldservice/learningenglish/language/wordsinthenews/2009/09/090904_witn_arctic.shtml>