Here in America we only accept people as experts if they agree with us. And when we're easily convinced that someone is an "expert" who says things that coincide with our preconceptions, we open ourselves up to easy manipulation in the "wars of the experts" that play out in the media every day on a wide range of topics.
I've taken a lot of coursework in biology, particularly focusing on ecology. While ecology often presupposes the veracity of the climate change hypothesis, I am by no means an "expert" on climate change - the closest I come is having taken a year of general chemistry. I happen to accept the consensus that human activities are contributing to it. I've never done any research in climatology, so my task becomes to weigh the evidence and the various claims made to explain it, and determine who has the most compelling view. A little research shows an overwhelming international consensus by scientific organizations. It seems highly unreasonable to disagree with such a vast group of concurring professionals, especially since I don't claim access to some secret evidence that can disprove their claims. I really don't know anything about the models they use to predict climate change. But they're the ones doing the research, and I do trust that the checks and balances inherent refereed journal publication will weed out hypotheses that cannot be supported.
Many people find it difficult to reach the same conclusion, however, because there are "experts" on both sides. A little more searching reveals over 31,000 (over 9,000 PhD-holding) scientists who have signed a petition asserting that "there is no convincing scientific evidence" (I guess it depends on how you define "convincing") and even that "increases in atmospheric carbon dioxide produce many beneficial effects upon the natural plant and animal environments of the Earth." Many (over 40%!) of these "scientists" have only a Bachelor's degree; some have degrees in areas such as economics, some are medical doctors who treat patients and do not conduct scientific research on any topic, others are physicists who do not devote a significant part of their time to studying climate specifically but claim knowledge of "fundamental physical and molecular properties of gases, liquids, and solids." While the total number of signatories seems impressive, when you look more closely at its makeup the group hardly seem qualified to provide an opinion on this issue. In fact, only 39 (0.1% of all signers) identify themselves primarily as climatologists; to contrast, 9.7% of signers are medical doctors and a whopping 32% are engineers. Experts to be sure, but not experts on climate science.
Climate change, vaccination-induced autism, evolution, whether or not HIV causes AIDS, the effectiveness of unproven alternative medicine techniques that are taught without criticism...all debates in which people hold strong emotional attachments to their own viewpoints, and in which there are seeming "experts" arguing for the dissenting opinion. The average person is not trained to discern credibility in scientific claims and may just go with their feelings or make a decision for social, religious, political, or economic reasons. The media, by providing equal time to "both sides of the issue," tends to exacerbate the problem by creating the appearance of balance, when in reality only one side of the debate can be considered credible. A logical approach shows no reason to believe that vaccines cause autism, and studies have thoroughly debunked this idea, yet often anecdotes and a poor understanding of statistics cause parents to trust any "expert" that suggests such a link.
We live in a democratic society; the general public, not scientists, dictate policy, which makes it important to communicate and foster both a better understanding of science and more trust in scientists as people who are genuinely committed to solving important problems and are here to help. It's dangerous when emotions trump evidence in deciding truth. The scientific community struggles to really convey the strength of its positions - and "scientists" who claim expertise in a field they are not directly involved in aren't helping scientific credibility in general.
It's also important that the public learn to accept their own lack of expertise and defer decisions requiring advanced knowledge to those who have devoted themselves to the study of the particular issue. As Americans, it seems that distrust of authority is in our blood. But if we continue to hold stubbornly to emotional arguments that can be refuted by logic, and refuse to listen to those who have real knowledge, we will not be equipped to make the progress we'll need to face the big challenges of today and the future.