 David and Victoria Beckham may have been overjoyed to welcome their new daughter, Harper Seven, last week but, according to a growing group of campaigners, the birth of their fourth child make the couple bad role models and environmentally irresponsible.  as the world's population is due to hit seven billion at some point in the next few days, there is an increasing call for the UK to open a public debate about how many children people have.  Now the Green MP, Caroline Lucas, has joined other leading environmentalists in calling for the smashing of what TV zoologist Sir David attenborough has called the "absurd taboo" in discussing family size in the UK.  Lucas said: "We need to have a far greater public debate about population, whether it focuses on improving family planning or reducing global inequality - and looking again at how we address the strain on our natural resources. The absence of an open and honest discussion about this issue means most people don't give much thought to the scale of global population growth in recent years. In 1930, just one or two generations ago, the world's population stood at around two billion. Today it is around seven billion, and by 2050 it is projected to rise by a third to 9 billion.  "We live as if we have three planets instead of just one. It is interesting that public figures, environmental groups and NGOs in general have tended to steer away from population to the extent that it's become a taboo issue. The horrific consequences of China's one-child policy and of other draconian efforts to regulate procreation have, for many, rendered discussion of the subject completely unpalatable. Yet as long as an issue remains a taboo subject where no one talks about it, then there's very little chance of finding the solutions we need."  It is a view that is being pushed by the UK-based Optimum Population Trust, whose chief executive, Simon Ross, is calling for the government to tackle the UK's high rates of accidental pregnancy and to give child benefits and tax credits only for the first two children. "That would send a clear signal that the government will support sustainable families, but after that you are on your own," he said. "There is a big issue there, family planning is cheap, yet many people don't use it properly and accidental pregnancy rates are very high. We need to change the incentives to make the environmental case that one or two children are fine but three or four are just being selfish.  "The Beckhams, and others like London mayor Boris Johnson, are very bad role models with their large families. There's no point in people trying to reduce their carbon emissions and then increasing them 100% by having another child," he said. "England is one of the most densely populated countries in the world and the fastest-growing in population terms in Europe. In 15 years we'll have an extra 10 million people here."  attenborough has attacked the last two UN climate summits in Cancun and Copenhagen for ducking the population issue. Giving the President's Lecture at the Royal Society of arts in March, he made a passionate speech about how the world's baby-making was damaging the planet 