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How do historical documents such as ship logs help scientists study the Little Ice Age?

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Line graph showing estimated temperature variations from 1000 to 2000 CE for the Northern Hemisphere and central England, with three lines representing different data sources. The chart highlights the medieval warm period around 1100-1300 and the Little Ice Age from about 1500 to 1850, followed by a sharp temperature rise in the late 20th century. Temperature is shown as deviations from the 1961-1990 average. — Little Ice Age Estimates of temperature variations for the Northern Hemisphere and central England from 1000 to 2000 ce.

Historical documents, such as ship logs, serve as vital, non-instrumental archives that help scientists reconstruct past climate conditions during the Little Ice Age (roughly 14th to mid-19th century).123

Because high-quality instrumental weather records are largely unavailable for this period, researchers rely on these written accounts to provide indirect, or proxy, evidence of climatic features. Ship logs are particularly valuable because they offer firsthand observations of environmental conditions encountered during maritime travel, including:13

Sea Ice: Logs can document the presence and extent of pack ice, which helps track the severity of cold conditions in the North Atlantic region.14
Weather Patterns: These records provide information on regional climatic features such as frosts, droughts, floods, and the timing of monsoons.1

By cross-referencing these historical documents with natural archives—such as ice cores, tree rings, and coral growth—scientists can better understand the variability in temperature and precipitation that characterized the Little Ice Age across different parts of the globe.123