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To compare present-day temperatures with temperatures during historical periods, scientists use indirect (or proxy) methods to determine temperatures in the past. • Proxy methods include ice cores, tree rings, coral layers, ocean sediment, and boreholes. • The northern hemisphere is warming faster than the southern hemisphere. • The heat island effect causes locally higher temperatures but is not mistaken for and is not a cause of global warming. • Satellites have determined that the troposphere is warming slightly more rapidly than at the surface, which is consistent with the concept of greenhouse gas absorption of heat.

5 percent salt—mostly sodium chloride, with some other “salts” such as magnesium sulfate mixed in. The salt content of the oceans is a good chemical signature that can identify flow patterns such as ocean currents. The boundaries of the Gulf Stream can be distinguished by abrupt changes in salinity. A lower salt concentration can mean that freshwater such as from glacial melt or increased precipitation is coming into seawater. A higher salt concentration can mean that above-average evaporation is removing water and leaving a more salty mix behind.

Scientists need to work more like crime scene detectives to discover what the earth was like in the past. Today, scientists carefully search for obscure pieces of evidence left buried in the snows of Alaska, among centuries-old debris on the ocean floor, or within multiple layers of coral. As part of this forensic effort, scientists carefully compare the different types of atoms that may have been present in a geologic sample thousands of years old. What they are finding is that the most recent 50-year period in the northern hemisphere is warmer than it has been for the previous 1000 years (Figure 2-5).