Once the invalid data are eliminated, are global temperature data ready to use?

Not yet. In order to use temperature data effectively, we have to make adjustments to account for all the changes that have occurred over the past 100-150 years. Here are some of the reasons why:

• Changing landscapes: The landscape has changed dramatically in the last 100-150 years. Villages have become cities; dirt roads have been paved; trees have been planted and cut down.
• Observer changes: The people who record weather data have also changed. Some weather watchers may have retired, and new ones living in different parts of town or on hillsides have taken over. This can introduce variations in the data.
• Different Observation Times: The times at which weather observations are taken have shifted over time, affecting the data's consistency.
• Advances in Technology: We've moved from using old-fashioned mercury-in-glass thermometers to more modern temperature measurement systems. When old instruments break or become outdated, we replace them with new ones, and their readings may not be consistent.

These changes have nothing to do with climate, but they do leave a mark on temperature data. Some changes, like urbanization, are known to raise temperatures. Other changes, like switching thermometer types, can lower them. If we don't correct for these biases, we cannot accurately understand how the climate is changing.