( CNN ) -- This week the world will reach 7 billion people . Understandably that raises concern about a soaring world population . But there is a good news story from the demographic data that is not often told . We -- or rather the poor women of the world -- are defusing the population bomb . Women today are having half as many children as their mothers and grandmothers . The global average is now down to 2.5 children per woman , and it continues to fall . This is not just a rich-world phenomenon . Much of Asia now has fertility rates below two , from Japan and Korea to China , with its one-child policy , through Taiwan , Vietnam , Burma , Singapore and much of southern India and parts of the Middle East . Behind the veil , the women of Iran have cut their fertility from eight to less than two in a generation . The young people out on the streets demanding democracy during the Arab Spring are arguably far more politically active because they are not at home raising large families . Falling fertility happens faster if countries get richer and if women are better educated . Similarly urbanization helps a lot . While even young children can be an economic asset on an African peasant farm , they are an economic liability in cities , where they require education before they can get a job . The teeming megacities of the poor world may look like symbols of overpopulation , but they are part of the solution , too . But the real story is that rich or poor , Muslim or Catholic , secular or devout , socialist or capitalist , with tough government birth control policies or none , most countries tell the same story . Small families are becoming the new norm . The reason , I believe , is very simple . Women are having smaller families because for the first time in history they can . In the 20th century , the world largely eradicated the diseases that used to kill off most children . Today , most kids get to grow up . Mothers no longer need to have five or six children to ensure the next generation . Two or three is enough , and that is what they are choosing to have . There are holdouts , of course . In much of Africa , rural women still typically have five children or more . But if Africa follows Asia , then we can see an end to population growth . We are , I believe , likely to see `` peak population '' by about mid-century . Perhaps at around 9 billion people . After that , on current trends of fertility falling to below replacement levels , we will see a falling world population . And rapid aging . With longer life expectancy and fewer babies , this is all but inevitable . China will soon be aging faster than anywhere on Earth . Aging is set to be the dominant demographic phenomenon of the 21st century , just as the population boom dominated the 20th century . What does this mean for the environment ? Well , peak population is good news , of course . But do n't hang the flags . It is a pervasive myth that it is all those extra people that are wrecking the planet . That 's no longer the case . Rising consumption today is a far bigger threat to the environment than a rising head-count . And most of that extra consumption is still happening in rich countries that have long since given up growing their populations . According to Stephen Pacala , the director of the Princeton Environmental Institute , the world 's richest half billion people -- that 's about 7 % of the global population -- are responsible for half the world 's carbon dioxide emissions , the primary cause of man-made climate change . Meanwhile the poorest 50 % of the world are responsible for just 7 % of emissions . So there is no way halting population growth in the poor world today would have more than a very marginal effect on climate change . It is the world 's consumption patterns we need to fix , not its reproductive habits . Every time we talk about too many babies in Africa or India , we are denying this fact . The population bomb may be being defused by the women of the poor world . But the rich world has not even begun to defuse the consumption bomb . The opinions expressed in this commentary are solely those of Fred Pearce .
Fred Pearce says there 's good news attached to the 7 billion population milestone . He says woman are having far fewer children than their forebears ; global average is 2.5 . He says small families new norm as old reasons for having more children are gone . Pearce : Problem now is smaller population has upped its consumption , environmental damage .