If the population continues to double every 50 years then:
P2 = P1 x 2^(# of years/50) =

Year        Pop (Billions)
2000        6
2100        24
2200        96
2300        384
2400        1,536
2500        6,144
2600        24,576
2700        98,304
2800        393,216
2900        1,572,864
3000        6,291,456

So who says we don't have a population problem?

A more sustainable population, I think, would be a maximum of 1
billion people, as existed in 1850 AD.  Thats a reduction of 5.5
billion people from the present population (WW2 took only 70
million lives), i.e. about 5 out of every 6 people have to go, or 5
out of 6 cities have to go...  Thats a bit much to ask.  The very
least we should do is to not let the population grow any more.  
Zero population growth.  A small negative growth would be
better.
World Population:   
(The world population was
reduced to 3 - 10 thousand
people 75 thousand years
ago when the Toba volcano
erupted on Sumatra.  So
essentially time zero was
75,000 years ago.)     

Year         Pop (Millions)
-10,000      1
-8000         5
-5000         5
-4000         7
-3000         14
-2000         27
-1000         50
-500         100
-400         162
-200         150
1 AD        170
200         190
400         190
500         190
600         200
700         207
800         220
900         226
1000         254
1100         301
1200         360
1250         400
1300         360
1340         360
1400         350
1500         425
1600         545
1650         470
1700         600
1750         629
1800         813
1850         1,128
1900         1,550
1910         1,750
1920         1,860
1930         2,070
1940         2,300
1950         2,400
1970         3,412
1980         4,500
1990         5,223
2000         6,035
2010         6,973
2020         8,056
2030         9,309
2037         10,300
World Population