Original sentence: By the end of the 21st century, these countries could avoid population declines by permitting large-scale immigration.However, the United Nations population projections out to 2150 (the red, orange, and green lines) show a possible end to this phenomenon occurring as early as 2050 in the most optimistic scenario, and by 2075 in the "medium" scenario.Indeed, the UN "high" estimate does not decline at all, even out to 2300, indicating the potential for a Malthusian catastrophe.On the assumption that the demographic transition is now spreading to less developed countries, the United Nations Population Fund estimates that human population may peak in the late 21st century rather than continue to grow until it exhausted available resources.Some have expressed doubt about the validity of the UN projections, claiming that they are below the projections by others.The most important point is that none of the projections show the population growth beginning to decline before 2050
Edited sentence: By the end of the 20th century, these countries could avoid population declines by permitting large-scale immigration.However, the United Nations population projections out to 2150 (the red, orange, and green lines) show a possible end to this phenomenon occurring as early as 2040 in the most optimistic scenario.On the untested assumption that the demographic transition would spread to less developed countries, the United Nations Population Fund estimated that human population would peak in the late 21st century rather than continue to grow until it exhausted available resources.Some other estimates indicate population growth will not even slow until a value above 16 billion is reached.The most important point is that none of the estimates (including those of the UN, as opposed to their publications which often misquote their own projections) assume the population growth to level in 2050.There is considerable doubt about the validity of the UN projections, since they are below all of the projections by others which have a more scientific standing