Final answer:
To study past Pacific Decadal Oscillations, scientists analyzed various proxies including ocean sediments, ice cores, tree rings, and glacier sizes which all provide indirect measures of past climates.
Step-by-step explanation:
To determine the existence and patterns of Pacific Decadal Oscillations in the past, scientists examined various proxies such as ice cores, tree rings, glacier size, pollen counts, and especially ocean sediments. These methods allow for the interpretation of paleoclimate by providing indirect measures of past climates due to the lack of direct observational data from those time periods.
Ocean sediments, for example, contain microfossils like foraminifera with different shell coiling which can indicate past seawater temperatures, thereby inferring climate conditions during the times these organisms lived. Likewise, ice cores from places like Antarctica can reveal hydrogen isotope ratios which align with climate periods over nearly a million years, illustrating climatic variations.