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Have you ever wondered how scientists test and develop their climate models of what the future may be like? The answer lies in the past! Climate models are tested against reconstructions of past climate conditions. Those reconstructions are pieced together using proxy evidence buried in sediments and preserved in fossils.

Many US coastal communities, and their local economies, are very dependent on healthy marine ecosystems. But climate change, including warming ocean waters and increasing acidity, are changing marine ecosystems already, and there will continue to be additional future changes. As a result, it is important to understand as best we can right now, what future marine ecosystems will look like.  One way to track changes in marine ecosystem patterns through time is by studying fossil planktic foraminifera, which is exactly what USGS scientists are doing. 

“Forams” are single-celled protists that float on the ocean surface, with groups of species, or species assemblages, that are closely tied to the surface temperature of the ocean. The relationship is remarkably consistent, and as a result, scientists have been able to define species-temperature zones from the equator to the poles; in the tropics, subtropics, polar, subpolar, and transitional water through the last few million years.

Two women looking through microscopes in a lab.
Scientists examining sampled foraminifera under the microscope.
many tiny white foraminifera clustered together under a microscope
Numerous planktic foraminifera as seen under a microscope.

The USGS PRISM5 project has amassed a global distribution of species assemblage data from the Late Pliocene (3.6 to 2.58 million years ago) by collecting thousands of samples from more than 100 sediment cores that contain material dating back millions of years.  This time period is important because conditions in terms of atmospheric carbon dioxide levels and the positions of the continents, which control the pattern of ocean circulation, were similar to today. Climate transitions during the Late Pliocene are characterized by changing species assemblages across latitudes, showing heat distribution changes in response to warming or cooling ocean surface temperatures. As a result, it is a useful window of time to study and compare to what we are seeing in Earth’s changing oceans today.

But beyond simply understanding what changes happened in the past, these species assemblage data have an additional very important purpose. Past data can be used to improve climate models that simulate what future climate change will look like. In other words, modelers can use “hindcasting” where they attempt to model the past to see how well the models function and to improve their accuracy, before attempting to “forecast” future climate conditions. The data from the PRISM5 project can help modelers hindcast and then forecast the patterns of heat on the ocean’s surface. Climate model simulations are tested against the paleoclimate data collected by the PRISM5 project to see if they can simulate past heat patterns on the ocean surface accurately and look for discrepancies. Identifying discrepancies is important, as this can indicate key processes missing from climate models that need to be incorporated for accurate predictions of future climate change. 

Traditionally, data-model comparisons have focused on point-by-point comparisons of surface ocean temperature values, looking for a problem with either the paleotemperature estimate or the model performance. However, focusing on how the ocean’s surface temperature changes between the equator and the poles offers an alternate way to assess the spatial temperature pattern, one that reflects ecosystem patterns. In this way, the PRISM5 project is both learning how ecosystems migrate in a warmer world and also enabling an additional way to test and improve climate models.

Additionally, USGS scientists are quantitatively analyzing assemblage data using multiple diversity metrics. Biodiversity richness (the number of different species) is one way to measure the health of marine ecosystems. Richly biodiverse ecosystems provide food, health benefits, and economic support to coastal economies and are often associated with upwelling zones in the ocean. Knowing where these productive ecosystems will be found in the future is paramount to many coastal industries.

Thus, studying the diversity of planktic foraminiferal species assemblages is a valuable tool in paleoceanographic and paleoecologic studies and integral to improving future climate model simulations. Tracking past latitudinal species changes can help us predict where there may be changes in ocean circulation and upwelling and species changes which in turn have important economic consequences. This information can help inform marine conservation and planning efforts and help us better understand what changes are likely with continued ocean climate change.

A recent USGS article in Nature Geoscience News & Views titled, "Plankton reveal past climate" explores this topic more!

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