In 2013, the Australian Bureau of Meteorology had to add another color to its forecast maps because meteorologists predicted that sections of the country would then experience unprecedented heat over 50 degrees Celsius, 122 degrees Fahrenheit. It's examples like this that convinced many of us that climate change is no hoax. When Hillary Clinton declared that she believed in science at the Democratic National Convention, she followed up by adding, "I believe climate change is real." To many, the most important scientific question isn't how species adapt or how the universe expands, but how climate change is going to affect us in the future? But how do scientists know that climate change is real in the first place? It starts with the utmost confidence in the existence of the greenhouse effect. A theory proposed by the physicist Joseph Fourier almost 200 years ago. The theory says that gases such as carbon dioxide and methane trap the sun's heat in the Earth's atmosphere, much like a greenhouse. Other gases in the atmosphere like nitrogen and oxygen, which make up the majority of it can't trap heat as effectively. Carbon dioxide and methane molecules capture the sun's energy by bending and stretching. Eventually they pass this energy onto other molecules as they bump around the inner atmosphere. This motion is heat at the molecular level. So as more of these gases accumulate, they can capture more of the sun's energy and heat builds up. To be clear, the greenhouse effect is mostly a good thing for us. Without it, the planet would be a frozen ball of ice and incapable of supporting life, but too much of a good thing can tip the scales in the wrong direction. We are producing a lot of carbon dioxide much more than it had been in our atmosphere for 400,000 years. You've probably heard the term global warming repeated in public countless times. The phenomenon of global warming also occasionally gets referred to as an enhanced greenhouse effect. The idea is that increased amounts of carbon dioxide and other greenhouse gases from human activities has and will lead to a warmer Earth. Now, scientists also are nearly certain that global warming is occurring because they've been collecting detailed data on the earth's temperatures and atmospheric composition since the 19th century. The data comes from ships and buoys in the ocean, land stations, and more recently from satellites. These diverse sources of data all show the same trend, a warming earth. Numerous forms of evidence also showed that global warming has altered and will continue to alter weather patterns around the globe. This evidence generally consists of simulations, observational records and theories from physics. Take days of extreme heat. The observational record suggests that there has been an upwards trends and an upwards trends also in line with theories from physics and simulations. So that leads scientists to have very high confidence when they say that global warming has led to more days of extreme heat. They're less certain, however, when they try to link forest fires to global warming. The observational record for wildfires is patchy and much shorter. Many factors also go into bringing about a wildfire, such as the density of the forest, the amount of rainfall in the region, land management practices, all of these make wildfires difficult to accurately simulate with models. Nevertheless, scientists are fairly sure that global warming is linked to increased forest fires. Now it's hard for scientists to be as precise in their predictions of the future as an analysis of the past. Why? For one thing, scientists don't know exactly how much more carbon dioxide we're going to put in the atmosphere in the coming decades. That's the kind of information they need to use to make precise and accurate predictions. But even relatively low emission scenarios don't look great for some of us. So take the region around Washington DC. Scientists say that this region will see twice as many days over 90 degrees between 2041 and 27 compared to the end of last century. That's a change from about 25 days over 90 degrees to about 65 days over 90 degrees. Scientists paint a similar picture for much of the Mid-Atlantic and Northeastern regions. It's also hard for scientists to make precise predictions about sea level rise because they don't know exactly how large influential system is like ice sheets in Antarctica and Greenland are going to react to global warming. If these ice sheets were to melt very quickly, then we'd likely see multiple feet of sea-level rise by 2100. But if they melt a bit more slowly, we might see only one foot of sea level rise by 2100. Both, of course, they're terrible scenarios. Regardless, scientists are nearly certain that sea levels will continue to rise it's just a matter of how much and by when. It's also worth mentioning that many of scientists past predictions have turned out to be accurate. For example, since 1990, the United Nations Intergovernmental Panel on Climate Change, the IPCC, has published a report every five to seven years outlining up-to-date information on predicted as well as observed changes in our planet's climate. Their predictions for today's global temperatures have turned out to be right in the ballpark. There's obviously a lot of evidence that the Earth is warming. But how do scientists know that it's error fault and not some natural phenomenon? Maybe the sun emit anymore heat or volcanoes emit large amounts of carbon dioxide when they erupt maybe it's their fault. When scientists do their calculations, they factor in these natural causes and they found that these natural causes alone can't account for all of the warming that we've seen. How sure are they that it's our fault? The latest IPCC report says that scientists are virtually certain, meaning that there are at least 99 percent sure that natural causes alone cannot account for the observed global warming since 1951. Scientists we'll never have 100 percent certainty about any of their theories. This is part of what makes science science, but we'll be talking about this more in the later weeks of the course.
