Speciation and extinction
From one ancestral language (e.g. latin), several languages are born (e.g. spanish, portuguese, french, italian, romanian, ...). Languages therefore speciate. Such speciation could eventually happen via hybridization of existing languages. Languages also get extinct.
When the rate of speciation is higher than the rate of extinction, then the number of languages is increasing and when the opposite is true, then the number of languages is decreasing.
Atlas of endangered languages
UNESCO, in its Atlas of the World’s Languages in Danger (also listed on wikipedia) recognizes a large number of endangered languages. Increase world-wide communication and globalization makes me think that the number of languages should be currently decreasing.
Do we have estimates of the speciation rates and extinction rates of languages (or estimates of the rate of decrease/increase) today? A rate would be in unit of number of languages lost or gain per amount of time.