Hello John, the 1 billion figure is based on what population levels were supported by agriculture before the fossil fuel era has really kicked into gear (around 1800). This number could be higher, if we are lucky (i.e.: the climate allows it) AND if we are really as ingenious as we think we are (adopting permaculture, and coming up with workable low tech solutions). On the other hand it could be lower as well (0 even), if the current ecosystem collapses around us and Earth turns into a hothouse.