If a science issue has made headlines, chances are that scientists will be talking about it as the world's largest scientific society meets in St. Louis. The American Association for the Advancement of Science expects up to 9,000 scientists, journalists, policy makers and citizens to converge here and examine issues including Katrina, stem cell research, evolution and global warming. About 200 meetings and lectures are planned during the five-day meeting, which was scheduled to begin Thursday night. Dr. Gilbert Omenn, president of AAAS, said the public's understanding of science is a crucial element of heated political, social, religious and technical debates. "Scientific thinking is absolutely essential to preserving democracy," Omenn said. Some areas of science continue to be disputed and are often confusing, he said. It must be clarified that research is an ongoing process. "These are not beliefs. These are not catechisms. These are changeable facts," Omenn said. Doug King, president of the St. Louis Science Center, said that interest in science is growing and today's visitors are likely to be adults who come without children. King said they are searching for facts that could help them understand current scientific debates. "When I went to school there was no doubt that there are nine planets," King said. "Now, maybe there are eight or 10 and we've found a whole bunch around other stars." Members of the public will have a chance to participate in several free events and lectures at the science meeting and family events are scheduled for Friday and Saturday at the America's Center. The AAAS is the world's largest scientific society and publishes the journal Science. The society last held its meeting in St. Louis in 1952.