 The Earth is a pretty big place, so why are we so sure that the planet is warming? Well, weather stations around the world are monitoring surface temperatures and seeing them rise. We also monitor ocean temperatures using buoys and measurements from ships and large networks of floats. The ocean's surface is warming and heat is building up below the surface. We can also observe warming using weather balloons and satellites in the lower atmosphere. Ocean water expands as it heats up, and land ice melts and runs off into the ocean. This causes sea levels to rise, which is happening rapidly. Glaciers around the world are shrinking. Warmer air holds more moisture, and over both land and sea, moisture has been increasing in the air, just like we'd expect in a warming world. These and many other indicators tell us that the planet is unquestionably heating up. Based on all of these different lines of evidence, everyone can agree that the globe is warming, right? Unfortunately not. For example, a United States vice presidential candidate wrote that leading climate experts deliberately manipulated data to hide the decline in global temperatures. Similarly, a past chairman of the United States Senate Committee on the Environment claimed that, stolen emails show one scientist wrote of a trick he employed to hide the decline in global temperature trends. These claims are classic examples of conspiracy theory. In order to fake an increase in global temperature, you'd have to fake all of the indicators of warming we talked about. Surface thermometers, ocean buoys, weather balloons, satellites, tide gauges, glacier lengths and more. It would require the greatest conspiracy in the history of the world to pull such a deception off. And an extraordinary claim requires extraordinary evidence. But this evidence is just another tactic common in denialism, quote mining. When climate science denialists seek to refute a mainstream position, they often take phrases out of their original context and claim they mean something they don't. Sometimes this is deliberate or other times it happens because of a lack of understanding of the science that they're attacking. In the cases I mentioned, the quote mining was being performed on emails that had been stolen from a university email server back in 2009. Given that warming of the planet is unequivocal, what decline was being referred to in the stolen emails? The word decline was actually referring to a change in the behavior of a subset of climate proxies. So let me explain what I mean by that. Proxies are indirect ways of estimating something that you're interested in when you don't have direct measurements of it or when you want an additional line of evidence to solve a problem. So imagine you're walking along a beach and it's low tide. You want to know where you can put your stuff down so that it'll be safe when the water reaches high tide again. You see a line of seaweed, bits of shell, driftwood, other debris. You place your blanket and food a few feet up shore from this, thinking that this was the point of the previous high tide. You're using the debris as a proxy for the tide line. You're inferring it's a location even though you didn't observe it directly. When scientists study past climate, they have to use proxies because we can't go back in time to get those true measurements. For example, we can use records of tree ring growth to estimate how hot summers have been over the entire lifetime of a tree. Around 1960, however, some temperature proxies from tree growth stopped tracking temperature. The global temperatures went up, as evidenced by the thermometers, the weather balloons, satellites, and all those other indicators we talked about. But some tree proxies went down. And this is the decline mentioned in the email. A decline in the tree proxy data, not in the actual temperatures. This divergence problem is an area of active research in the paleoclimate community. We have good ideas about why these proxies stop tracking temperature, but we're still working on it. Many other proxies are not affected by the divergence problem. Glacier length, chemical ratios and cave deposits, borehole temperatures, none of these show a decline. So we can use these other proxies to make sure that the tree proxies showing the divergence now were still tracking temperature far back in the past. And this is similar to how we treat sensors that malfunction on modern satellite platforms. We identify the problematic data. We cross-check the earlier data against overlapping records. And we stop using data from the malfunctioning sensor. So scientists do run into problems with data. Sometimes, thermometers fail or proxies stop following reality. Still, all major lines of evidence from satellites in space to sensors in the deep ocean show that the planet is heating up. Climate science analysts resort to conspiracy theory and quote mining to claim the opposite.