How do scientists know that global temperature has been rising for the past several centuries?
Actual temperature records dating back about 250 years for many spots on Earth show a general trend upward. The temperature averaged across all locations over many years shows an increase, even though each individual location has not warmed. And even though actual temperature readings were scarce before the 1700s, nature has a way of preserving temperature records in the bodies of living things. Trees, for example, tend to grow faster in warmer weather. By looking at the growth rings for different species over time, scientists can estimate which years were cooler or warmer. Another example comes from the ocean, where tiny, shelled creatures called “forams” live. These creatures take oxygen out of the seawater and combine it with calcium and carbon to make their shells. How can the oxygen in these shells tell us about Earth’s temperatures long ago? Even though oxygen seems all the same to us, it has several different forms, or “isotopes.” Normal oxygen is called oxygen 16, but there is
