Celebrating the milestone of the world reaching 7 billion on Monday, UN Secretary-General Ban Ki-moon, Nassir Abdulaziz Al-Nasser, president of the UN General Assembly (UNGA), and other senior UN officials made loud and clear calls for the international community to come together in solidarity to step up efforts in tackling a multitude of crises. The UN leaders said that the milestone represents both opportunity and challenge for the human being. \"Today the world\'s population reached seven billion,\" Ban said at a press conference at the UN Headquarters in New York to mark the milestone. \"The world\'s population reached six billion in 1998, 13 years ago. It is expected to grow to nine billion by the middle of this century, or even a few years earlier -- by 2043.\" \"This is a day about our entire human family,\" said Ban. With the world representing \"terrible contradictions,\" Ban said that there is \"plenty of food but one billion people go hungry ... lavish lifestyles for a few, but poverty for too many others ... huge advances in medicine while mothers die everyday in childbirth, and children die every day from drinking dirty water.\" Ahead of the G20 summit in Cannes, France, Ban said his message will be \"loud and clear -- think about our children ... Think about the future, with vision and foresight.\" \"Yes, we face a serious economic crisis,\" Ban said. \"For much of the world, fiscal austerity is the new order of the day. Yet even in these difficult times, we cannot afford to cut loose those who are hardest hit.\" Ban said the G20 summit must deal with all these issues \"squarely and directly.\" \"Today, we welcome baby seven billion,\" Ban said. \"In doing so we must recognize our moral and pragmatic obligation to do the right thing for him, or for her.\" Also speaking at the press conference, Al-Nasser said it was also a \"stark reminder of how many among us -- the bottom billion - - are rendered vulnerable with little or no access to basic needs. \" \"Seven billion people face, almost on a daily basis -- with varying degrees of severity -- the consequences of environmental challenges, increasing poverty, inequality, wars and economic instability,\" Al-Nasser said. But each challenge brings an opportunity, Al-Nasser said, urging the international community to come together to reach the UN Millennium Development Goals (MDGs) -- a set of eight anti- poverty targets to be reached by 2015. He said it was critical to rethink our approach to sustainable development, in the face of more pressure on the world\'s communities and on the environment. \"We know our world is a changing place,\" Al-Nasser said. \"The reality of seven billion people on our planet underscores our collective obligation to secure fundamental justice, equity and dignity for all.\" Meanwhile, Babatunde Osotimehin, executive director of the UN Population Fund (UNFPA), outlined challenges to meet the fast growing population, including in promoting the rights and health of seven billion women, men and children. \"We must ensure that, in areas of the world where population is growing fast, we raise the status of women and young girls to be able to access education and make choices for themselves,\" he said. \"We also owe it to the 250 million women worldwide who require family planning and are not getting it to make it available.\" Last week, the United Nations Development Fund (UNFPA) released a report titled \"The State of World Population 2011,\" saying that as the population grows, so do challenges facing humanity. According to the report, the population record is considered a success, because it means that people throughout the world can live longer. However, it is a challenge to all. With the current growth, the world will have 78 million more people each year, leading to increasing demands on natural resources and big pressure on the planet.