 Imagine you're driving towards a bridge. Have you ever pulled over, taken out your phone and started browsing blogs about bridge building to decide whether the bridge was safe to cross? Of course not. You trust in the engineers who built that bridge. That's how we form our views on complicated issues. Rather than master a whole body of knowledge, we use a mental shortcut. We rely on experts and trust them to do their jobs. Or failing that, we trust that their colleagues would catch their mistakes before anything goes wrong. Now in a perfect world, everyone would be aware of all the lines of evidence for human cause global warming. We would know all the human fingerprints that are being observed across our climate. But life is busy. We have to pay the bills, get the kids or ourselves off to school and keep track of all the characters on Game of Thrones. So most of us use the mental shortcut of expert opinion. In the case of climate change, the experts are climate scientists who are actively publishing peer-reviewed climate research. Because the body of evidence is so strong, there's overwhelming agreement among climate scientists that humans are causing global warming. How do we know the level of agreement? In 2009, climate scientists Peter Doran and Maggie Kendall Zimmerman at the University of Illinois in Chicago surveyed earth scientists. The survey asked the scientists if humans are significantly changing global temperature. When the survey results came back, they found that not every group shared the same opinion. Different groups had different levels of agreement about climate change. For example, the group of scientists with the lowest agreement that humans were causing global warming were economic geologists at 47%. The next highest were meteorologists at 64%. However, what they found was that the higher the level of expertise in climate science was stronger the agreement about human cause global warming. For the most qualified group, climate scientists actively publishing climate research, there was 97.4% agreement that humans were significantly changing global temperature. A year later in 2010, another study took a completely different approach to estimating the level of consensus. William Anderegg at Princeton University and his colleagues collected a number of public statements from scientists about human cause global warming, including as many dissenting statements as they could find. Then they narrowed their focus to only the scientists who had published climate research in scientific journals. They found the same result as Doran did the previous year. Among publishing climate scientists, there was 97 to 98% agreement that humans are causing global warming. And just like Doran, Anderegg found that scientists convinced of human cause global warming had published substantially more climate research than what he termed unconvinced scientists. More recently in 2013, I led a team of researchers at Skeptical Science in conducting the most comprehensive analysis of climate research to date. We looked at climate papers from 1991 to 2011. This amounted to over 12,000 papers. We found that among papers stating a position on human cause global warming, 97.1% affirmed the consensus. So three different studies using three different methods all found overwhelming scientific agreement. But that's not the only evidence of a consensus. Virtually every scientific organization that has made a statement about climate change has endorsed the consensus. Note the social diversity in the organizations listed. They come from the fields of geophysics, chemistry, meteorology, physics, oceanography and geology. The diversity of the consensus also applies to countries. The academies of science from 80 countries have endorsed human cause global warming. Not a single Academy of Science in the world has rejected the consensus. Many lines of empirical evidence tell us that humans are causing global warming. Similarly, a number of independent sources find overwhelming scientific agreement about human cause global warming. We see it in surveys of scientists, in analyses of published research and in the diversity of scientific organizations all over the world. There is one myth about climate change which argues that there's no scientific consensus because 31,000 scientists have signed a petition rejecting the consensus position. The petition is a website called the Global Warming Petition Project. However, the only requirement to be listed in this petition is an undergraduate degree in any kind of science. According to the US Department of Education, over 10 million people signed a science degree between 1971 and 2008. So while 31,000 people signed this petition, that's only 0.3% of Americans with science degrees. And most importantly, only 0.1% of those 31,000 scientists are climate scientists. So the claim that the Global Warming Petition Project disproves a scientific consensus is a myth that uses the technique of magnified minority. This involves making the petition seem like a large number when in reality it represents a tiny percent of the scientific community. This myth also uses fake experts. This involves conveying an impression of expertise 31,000 scientists when 99.9% of the signatories aren't climate scientists. Now it's crucial to reiterate that science is decided by evidence, not by popular opinion. However, it's also important to recognize how the general public think about complex issues. They rely on the opinions of experts. So we need to be aware when fake experts are being used to confuse people about the level of agreement among real experts.