The impact of new arrivals to the US on the economy and on government spending is already a hot topic. But now the Census Bureau is projecting that immigration will help push the US population up by half, to at least 400 million by the year 2050. That kind of forecast has many who are already here arguing that our environment can't sustain such growth, and that immigration should be curtailed. Betsy Bayha of member station KQED in San Francisco reports on this increasingly fractious debate.