Despite US opposition to the Kyoto Treaty on Climate Change, the world reached an historic agreement in Bonn, Germany on implementing the treaty. Although the major industrial countries must ratify the treaty, it looks likely that a majority of countries will do so and thus the process of controlling man-made climate change can begin. The agreement reached in Germany is a triumph for the scientific process, which should be recognized and extended to other areas. Man-made climate change is not only of tremendous importance but of tremendous complexity. The basic theory of how human society is warming the environment by burning fossil fuels was first put forward more than 100 years ago. But theoretical models of the earth’s environment are only a few decades old, and remain imperfect. At the same time, the climate itself is subject to long swings in temperature, rainfall, and other patterns, unrelated to human activity. Separating the human factor from natural factors is daunting. All the more remarkable, then, that the world reached an agreed approach to this issue. Not only is the science complex and uncertain, but individual countries have different interests. Tropical countries may be hurt by global warming, while colder countries like Canada and Russia could benefit. Coastal countries may be damaged by rising oceans, while inland countries may be relatively unaffected. Coal and oil producing countries may be hurt if the world cuts back on fossil fuels; producers of other kinds of energy, such as hydroelectric power, might benefit. In short, there are numerous interests and much uncertainty about the underlying process of climate change. How, then, did the world reach agreement, albeit on only a first step in a decades-long process of action? Diplomats deserve credit for making compromises or trade-offs. But enormous praise is also due to the scientific community, which operated with skill and objectivity despite aggressive lobbying by industries, environmental groups, and countries with different interests and points of view. Scientists organized the process of analyzing climate change in a way that put the evidence first, forcing politicians to confront reality. The details of this vast scientific effort are notable, because they provide lessons for addressing other global problems. In 1988, two UN Agencies established the Intergovernmental Panel on Climate Change (IPCC). The IPCC is a vast network of scientists devoted to assessing the scientific knowledge about climate change, and the links of climate change to human society. Hundreds of scientists participate, and every effort is made to ensure objectivity, fairness and scientific excellence in judging the evidence. Every five years, the IPCC prepares a report for the global political leadership, known as Assessment Reports. The Third Assessment Report is now being finalized. Parts of the report have been released and widely discussed, confirming the growing evidence of large man-made effects on climate. Various industry groups with vested interests in the use of coal, oil, and other fossil fuels attacked the theory of climate change, seeking to undermine the scientific credibility of the IPCC. Some scientists, sometimes from outside the field of climate studies claimed that the evidence of man-made climate change did not really exist, or was exaggerated. Others accused the IPCC of political biases. The stakes were high because the Kyoto Treaty promises to lead to significant long-term changes in the role of the energy sector and in the kinds of energy technologies adopted in the future. Without doubt there were, and remain, vast scientific uncertainties, so it was easy to make the public claim that little evidence exists upon which to take action. When President George W. Bush came into office, interest groups continued to fight the IPCC. The Bush administration initially claimed that the science of climate change was too uncertain to guide policy. To the administration’s credit, the President then asked for a special committee of America’s National Academy of Sciences to review the work of the IPCC. The Academy reported that the IPCC had fairly and accurately represented the scientific evidence on climate change as it exists. Because of the careful, thorough work of the IPCC, the world has been able to move beyond the usual name-calling and partisan debates to reach an understanding of the real stakes in man-made climate change. Even though some powerful business and regional interests may be affected adversely by global actions to limit man-made climate change, the credibility of science triumphed over vested interests. All of this depended on scientists using their talents properly, and organizing their effort in a transparent, professional manner. It is too early to declare victory in the control of man-made climate change. That will require efforts over decades. But the world has made a start. Even though the US is not yet a party to the new agreement, the weight of science will push America to play a more constructive role in the future, even if American politicians continue to fight the process. In our interconnected and technology-based global society, questions of scientific complexity will increasingly affect our lives. How should we fight AIDS? Should we pursue genetic modification of crops to improve agricultural systems? How should we manage the scarcity of fresh water in parts of the world? How can we preserve biological diversity? In each case, politicians and diplomats will be needed to bridge divergent interests in a cooperative manner. But we will also need to get the most accurate and objective scientific information to help us choose the best course of action. The IPCC demonstrates that scientists from rich and poor countries can work together in a systematic process to provide objective information, even on complex topics with widely divergent interests.