Science The composition of marine sediments offers insight into historical reactions of climate to alterations in CO2 levels. Bella Brown September 21, 2023 The Publications Department at AGU has a blog called Editors’ Vox. 2 concentrations are a fundamental cause of global climate change, accurate measurements of CO2 are crucial for understanding and mitigating its impact. Measurement of CO2 is essential in comprehending and reducing its impact, as increased levels of atmospheric CO2 are a primary factor in global climate change.2 As the Earth’s climate is influenced by changing concentrations, it is crucial to examine previous climates and transitions in order to gain insight into potential environmental reactions in the future. A recent article published in Reviews of Geophysics examines the features and climate changes that occurred during the Late Pliocene and the Pliocene-Pleistocene transition. This research could offer insight into which areas may be more susceptible or resistant to potential future climate influences. We requested that the authors provide a summary of the climate conditions during these periods, explain their methodology, and discuss any lingering inquiries. Why is it crucial to examine previous climates? Historical climates document the timing, manner, and reasons for shifts in the climate. Significant shifts, such as a completely frozen earth and tropical climates with palm trees at the poles, have been documented in the past, along with smaller and temporary occurrences. By examining past climates, we can investigate climate change on a worldwide, regional, or local level. This provides insight into the effects of various drivers of climate change, such as the significance of changes in greenhouse gas levels or solar energy, which can fluctuate over different time periods. Examining past climates reveals which aspects of the climate are prone to change and helps us pinpoint areas or systems that are more resistant. Previous climates give us insight into the elements of the climate that can be easily affected, and help us pinpoint areas or systems that are more able to withstand changes, such as the oceans, ice sheets, and ecosystems. When examining predictions for future climate, past climates have played a crucial role in verifying models of important climate processes and features that fall outside of the limited time period and scope of human observations during the “instrumental era”. In what year did the Pliocene Epoch occur and how would you describe its climate? During the Pliocene Epoch, which took place 2.7 to 5.3 million years ago, the climate was significantly warmer than it was before the industrial era when fossil fuels began being used. The global temperature was about 2-3 °C higher than pre-industrial levels, but there were notable variations in different areas, such as much higher temperatures in the polar regions and a smaller amount of sea ice coverage. The sea levels were also higher during this time, indicating that the Antarctic and Greenland ice sheets were smaller. The distribution of vegetation zones was also different, with them being shifted towards the poles. In the early Pliocene, there were several changes in ocean gateways, including an open Panama gateway, a deeper Indonesian gateway, and a closed Bering Straits. These gateways played a role in ocean circulation, but eventually shifted to their modern layout throughout the Pliocene period. What do the terms “iNHG” and “mPWP” refer to: the intensification of glaciation in the Northern Hemisphere and the warm period in the mid-Piacenzian era? 2 concentrations of 400–450 ppm The mid-Pliocene Warm Period (mPWP) took place approximately 3 million years ago and marked the last prolonged period of elevated temperatures during the end of the Pliocene era. This warm period lasted for approximately 200,000 years and was characterized by higher temperatures compared to the present day. Atmospheric CO2 levels during this time were estimated to be between 400-450 ppm based on reconstructions.2 concentrations similar to today and predicted for the near future. The mPWP is a key target for understanding how the climate system will respond this century and beyond, because of these similarities to today’s world. Following the mPWP, the Earth’s climate began to decrease in temperature and the ice sheets in Greenland and Antarctica grew larger. A significant shift then happened, where new ice sheets formed and spread across a bigger portion of the northern hemisphere, including North America, Iceland, and Eurasia. These ice sheets would advance and retreat over thousands of years during the Pleistocene period, which is often referred to as the “ice ages”. This transition from the mPWP to the ice ages is known as the “intensification of northern hemisphere glaciation” and occurred roughly 3 to 2.5 million years ago. What is the significance of understanding the transition from the Pliocene to the Pleistocene? During the period of transition from the Pliocene to the Pleistocene, there were two significant events known as mPWP and iNHG. The main focus of our review paper was to determine if there were any similarities in timing and magnitude of climate variability during this transition. This was crucial because identifying the initial occurrence of this shift can provide insight into the underlying mechanisms driving this major climate change. The mPWP and iNHG varied in terms of ice sheet coverage and oceanic circulation patterns due to alterations in oceanic gateways. By comparing the average and magnitude of climate change during these two time periods, we were able to gain a better understanding of how different components of the climate system played a role in its stability and resilience, particularly the land-based ice sheets. 2 levels This information offers a glimpse into the potential future, where increased levels of atmospheric CO2 due to global warming could lead to significant melting of ice sheets.2 concentrations (pCO2 This helps us consider the potential response of ocean and atmosphere circulation to these alterations. What information did you utilize to address these inquiries? The examination of previous weather patterns typically depends on geochemical markers taken from archives such as sediment cores retrieved from the ocean floor. These markers, also known as “paleoclimate proxies,” vary according to climate conditions. We gathered data sets of proxies for sea surface temperature and global ice volume from seafloor sediment cores. Only records with reliable dates were considered to ensure accurate determination of climate change timing. Bayesian statistical algorithms were then used to identify shifts in climate. Various proxies based on phytoplankton or zooplankton remains were used to estimate sea surface temperature. This method allows for an assessment of the credibility of the results and enables direct comparisons to be made with climate models. In this study, marine sediment data was gathered to analyze climate change during the Pliocene-Pleistocene transition. Various geochemical proxies, including UK’37, Mg/Ca, and TEX86, were used to measure sea surface temperatures. Changes in global ice volume and ocean temperatures were also tracked through oxygen isotope analysis of microfossils from the surface. 18O) or seafloor (benthic d18O). Credit: McClymont and Ho et al. , Figure 2 What are the key findings of your research paper? 2 decrease Our analysis of the data indicates that the change in climate during the transition from the Pliocene to the Pleistocene was not a simultaneous occurrence globally. In certain regions, changes in ocean temperature occurred earlier, even before changes in the formation of ice sheets. Additionally, some areas of the ocean were not affected by the expansion of ice sheets in the northern hemisphere during this climate transition. Furthermore, the warm Pliocene period was not as stable as previously believed, particularly in the southern hemisphere. This study emphasizes the varying impacts of climate shifts in different regions in response to a decrease in pCO2.2 Imposing a similar level of pressure as predicted for the conclusion of the 21st century. What are some of the lingering inquiries that require further investigation, analysis, or modeling? It is crucial to collect missing data in order to achieve a more accurate understanding of the ocean and climate during previous periods of warmth on a global and regional scale. Our work highlights the limited spatial coverage of proxy records, despite having compiled all records published in the past few decades that meet our criteria. There is therefore an urgent need to fill in the data gaps to obtain a more reliable global and regional view of the ocean and climate during past warm climates. This is vital not just for proxy-based reconstructions but also for data integration and assimilation with model output. To this end, results from recent cruises under the auspices of the International Ocean Discovery Program (IODP) will prove vital. Our study also uncovers discrepancies in the data gathered from proxies measuring ocean temperature and global ice volume. This highlights the need for more thorough and consistent use of multi-proxy records to better understand how secondary factors impact proxy measurements of past climates. As there is a lack of accurately dated terrestrial records, this review primarily focuses on data collected from marine proxies. However, it is important to note that the terrestrial environment plays a significant role in global climate change through its interactions with vegetation and soil. To gain a more comprehensive understanding of the connection between the ocean, ice, and land during the Pliocene-Pleistocene transition, it is necessary to integrate terrestrial and marine proxy records in future research. —Sze Ling Ho ([email protected]; 0000-0002-4898-9036), National Taiwan University, Taiwan; and Erin McClymont ([email protected]; 0000-0003-1562-8768), Durham University, United Kingdom Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox. 2, Reference: Ho, S. L. and E. McClymont (2023), CO2’s Influence on Past Climate Uncovered Through Marine Sediment Analysis2 Modifications, Eos, 104, https://doi.org/10.1029/2023EO235027. Released on September 21, 2023. This article does not reflect the views of AGU, Eos, or any associated organizations. The opinions expressed are solely those of the author(s). Text © 2023. The authors. 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