Introduction to paleoclimatology
Paleoclimatology is the study of Earth’s past climate history. By studying climate variability of the past, it is possible to learn about patterns and trends of the Earth’s climate system. Research goals range from seeking to define average conditions over decadal to millennial intervals, abrupt changes, gradual trends, or changes in the frequency of occurrence of past extreme events that can impact human society, such as droughts and wildfires. We can also learn about physical and biological processes on our planet, ecosystem functions, and how the climate responds to abrupt disturbances, such as a volcanic eruption. By learning more about the past of our home planet and its sensitivity to variations in climate, we can gain confidence in our projections of how climate will be in the future, induced by human activity or other forcings. Our ability to unlock the secrets of our past is also the key to our future.
This perspective is especially important for understanding extreme climate events like droughts or strong pluvial episodes. By inferring past climate conditions using paleo-records we can improve our estimates of the range of natural variability in the climate system and understand how this variability has changed over time. This may better constrain the risks and return periods of extreme events, place recent observed changes in a longer-term context, and link climate and historical events, which may provide a basis for counteracting the recent political and fiscal reluctance to mitigate projected climate change.
How do we look into the past?
Satellite observations of the Earth’s climate have only existed since the 1970s, and weather station observations rarely existed before the late 1800s. To peer further back in time, it is necessary to distill observations from the great wealth of archives in the natural world and carefully interpret historical documents. Within these archives, we call a specific source of information that can be linked to changes in the environment a “proxy.” Past environmental changes leave evidence in biotic and abiotic archives that contain these proxies, and from these proxies we can reconstruct past environmental change. These proxies are key parameters (such as chemical signatures, physiological characteristics, ecological, written observations, etc.) that represent changes in past climatic variables of interest, such as temperature or precipitation.
However, the Earth’s climate system varies in both space and time, and can exhibit change on the order of years, decades, centuries, or millennia. Due to the complexity of the climate, the type of archive selected for a paleoclimate study depends on a variety of factors, including the research question, the location of study, or the time period/frequency of interest. For examples of archives, proxies, and their interpretation, see Table 1.
Table 1 Caption: The extent to which natural climate archives can record past environmental changes differs by source. This figure shows the maximum amount of time different archives can extend our knowledge of climate back in time. This ranges from instrumental data that only extends back to 150 years before present to ice cores that can record climatic changes from one million years ago! Extracted from Tejedor, 2017.
How do we look into the past?
Picture: Heidi Roop.Proxy data derived from a given archive can be compiled into a record and used to estimate historic environmental variability. The interpretation of these proxy records relies fundamentally on the principle of uniformitarianism: i.e. the assumption that the physical processes determining the relationship between proxies and climate parameters operate the same way today and in the past. Using this principle, proxy data can be used to reconstruct climate by calibrating proxy records against observational data, verifying the calibrated relationship against an independent verification interval from the observational data, and extending the relationship back in time when no observational data exist.
However, even if there is no observational data that overlaps the time period of paleoclimatic data, proxy records can still be used to learn about Earth’s climate. The climatic secrets within each type of archive have inspired researchers from different disciplines to develop methods and tools to help in the interpretation of these records. Within the context of PIRE-CREATE research activities, dendrochronology and speleothem science are two key avenues of paleoclimate research. More information on these areas of study and their application for our work can be found below.
