It seems, however, that a competing effect has dominated the situation since 1940. This is the reduced transparency of the atmosphere to incoming light as a result of urban air pollution (smoke, aerosols), agricultural air pollution (dust), and volcanic ash. This screening phenomenon is said to be responsible for the present world cooling trenda total of about .2°C in the world mean surface temperature over the past quarter century. This number seems small until it is realized that a decrease of only 4°C would probably be sufficient to start another ice age. Moreover, other effects besides simple screening by air pollution threaten to move us in the same direction. In particular, a mere one percent increase in low cloud cover would decrease the surface temperature by .8°C. We may be in the process of providing just such a cloud increase, and more, by adding man-made condensation nuclei to the atmosphere in the form of jet exhausts and other suitable pollutants. A final push in the cooling direction comes from man-made changes in the direct reflectivity of the earths surface (albedo) through urbanization, deforestation, and the enlargement of deserts. The effects of a new ice age on agriculture and the supportability of large human populations scarcely need elaboration here. Even more dramatic results are possible, however; for instance, a sudden outward slumping in the Antarctic ice cap, induced by added weight, could generate a tidal wave of proportions unprecedented in recorded history. John Holdren Global Ecology: Readings Toward a Rational Strategy for Man John P. Holdren is advisor to President Barack Obama for Science and Technology, Director of the White House Office of Science and Technology Policy, and Co-Chair of the Presidents Council of Advisors on Science and Technology (PCAST) Holdren was previously the Teresa and John Heinz Professor of Environmental Policy at the Kennedy School of Government at Harvard University, director of the Science, Technology, and Public Policy Program at the School's Belfer Center for Science and International Affairs, and Director of the Woods Hole Research Center John Holdren - Wikipedia, the free encyclopedia One way to carry out this disapproval might be to insist that all illegitimate babies be put up for adoption- especially those born to minors, who generally are not capable of caring properly for a child alone. If a single mother really wished to keep her baby, she might be obliged to go through adoption proceedings and demonstrate her ability to support and care for it. Adoption proceedings probably should remain more difficult for single people than for married couples, in recognition of the relative difficulty of raising children alone. It would even be possible to require pregnant single women to marry or have abortions, perhaps as an alternative to placement or adoption, depending on the society. [emphasis mine] Evolution News & Views: In John Holdren's Own Words: the Inconvenient Truth About Population Control John Holdren, the presidents top science adviser, is playing a key role in shaping the Obama administrations strategy to combat global warming. In an interview with Yale Environment 360, Holdren discusses the prospects for achieving key breakthroughs on climate change, both in Congress and at upcoming talks in Copenhagen. Obamaâs Science Adviser Urges Leadership on Climate by Elizabeth Kolbert: Yale Environment 360 While it may seem I am picking on this particular person, it does appear that when you have a history of advocating a "Global Ice Age" then "Global Warming" not to mention somewhat troubling statements on population control, it tends to call into question the kind of advice that the President is getting from these types of individuals.