Editors’ Vox is a blog from AGU’s Publications Department.

As elevated atmospheric CO2 concentrations continue to change the Earth’s climate, it’s important to study past climates and climate transitions as they can provide windows into future environmental responses.

A recent paper in Reviews of Geophysics explores the characteristics and climate evolution during the Late Pliocene and the Pliocene-Pleistocene transition, which could give insight into what regions might be vulnerable or resilient to future climate forcings. We asked some of the authors to give an overview of the climate during these time periods, describe how they conducted their study, and outline what questions remain.

In simple terms, why is it important to study past climates?

Past climates are a record of when, how, and why the climate changes. We have recorded large changes in the past, including a snowball earth and hot climates when palm trees were found at the poles, as well as more subtle and shorter-lived events. Past climates allow us to explore climate change at global, regional, or local scales. We can better understand the impacts of different causes of climate change, including the relative importance of changes to greenhouse gases or energy from the Sun, which can vary on different timescales.

Past climates show us the parts of the climate system that are sensitive to change, and allow us to identify regions or systems that are more resilient.

Past climates show us the parts of the climate system that are sensitive to change, and allow us to identify regions or systems that are more resilient, including the oceans, ice sheets, and ecosystems. When considering future climate projections, past climates have been critical for testing models of key climate processes and characteristics which lie outside of our very short and relatively restricted era of human measurements during the “instrumental era”.

When was the Pliocene Epoch and how is its climate characterized?

The Pliocene Epoch occurred 2.7 to 5.3 million years ago, and was primarily characterized by a warmer than “pre-industrial” climate (when we started burning fossil fuels). Global temperature was 2-3 °C warmer than pre-industrial, but significant local differences are recorded, particularly much higher temperatures in the polar regions and a more restricted sea ice cover. Higher sea levels than today show that the Antarctic and Greenland ice sheets were smaller in the Pliocene, and the vegetation zones we know today were shifted towards the poles. Early in the Pliocene there were also some differences in ocean gateways, including an open Panama gateway, deeper Indonesian gateway, and closed Bering Straits. These gateways influenced ocean circulation but shifted towards their modern layout through the early to the late Pliocene.

What are the “intensification of Northern Hemisphere Glaciation” (iNHG) and “the mid-Piacenzian warm period” (mPWP)?

The mPWP occurred about 3 million years ago and was the final interval of sustained warmth towards the end of the Pliocene. For about 200,000 years, the climate was warmer than today, with reconstructed atmospheric CO2 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.

After the mPWP, the climate began to cool and there was expansion of the Greenland and Antarctic ice sheets. A major change then occurred: new ice sheets developed and expanded across much larger areas of the northern hemisphere, including North America, Iceland, and Eurasia. These new ice sheets advanced and retreated over tens of thousands of years during the Pleistocene, sometimes referred to as the “ice ages”. The transition from the mPWP to the ice ages is called the “intensification of northern hemisphere glaciation” and took place between about 3 and 2.5 million years ago.

Why is the Pliocene-Pleistocene transition important to understand?

The mPWP and iNHG occurred during the transition from the Pliocene to Pleistocene. The primary question we aimed to answer with this review paper was whether the timing and amplitude of climate variability remained similar in space and time across this transition. This was important because knowing where the shift first occurred gives us clues about the mechanisms driving this large-scale climate transition. mPWP and iNHG differed in terms of ice sheets extents as well as oceanic circulation patterns due to changes in oceanic gateways. By contrasting the mean and amplitude of climate change of these two time intervals we could shed light on how different parts of the climate system influenced its stability and resilience, especially the ice sheets on land.

This knowledge provides a peek into the future, where extensive ice sheet melting may occur as a result of global warming under prolonged high atmospheric CO2 concentrations (pCO2), and allows us to think about how ocean and atmosphere circulation may react to those changes.

What data records or analyses did you use to answer these questions?

The study of past climate generally relies on geochemical indicators extracted from archives such as sediment cores raised from the seafloor. These indicators are also known as “paleoclimate proxies” and vary as a function of climate. We collected data sets of proxies for sea surface temperature and global ice volume recorded in seafloor sediment cores. We only considered well-dated records to ensure that the timing of climate change was determined with confidence. We then used Bayesian statistics-based algorithms to determine when climate shifts occurred. For sea surface temperature estimates, we relied on several proxies, based either on the remains of phytoplankton or zooplankton. This approach allows us to assess the confidence we can place on the results obtained, and also to make direct comparisons to climate models.

The marine sediment records collated in this study to reconstruct the climate change across the Pliocene-Pleistocene transition. Sea surface temperatures were recorded with several geochemical proxies (UK’37, Mg/Ca, TEX86) whereas global ice volume and ocean temperatures were recorded with oxygen isotopes in microfossils from the surface (planktonic d18O) or seafloor (benthic d18O). Credit: McClymont and Ho et al. [2023], Figure 2

What are the main conclusions of your review paper?

Our data analysis shows that the climate shift across the Pliocene-Pleistocene transition is not a globally synchronous event. Ocean temperature change occurred earlier in some regions, even pre-dating changes in ice sheet evolution. In fact, some oceanic regions were not at all impacted by the growing ice sheets in the northern hemisphere across this climate transition. Furthermore, the warm Pliocene is also not as stable as generally thought, especially in the southern hemisphere. This paper highlights the spatial heterogeneity of climate shifts in response to a pCO2 forcing that is similar to that projected for the end of the 21st century.

What are some of the unresolved questions where additional research, data, or modeling are needed?

There is 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.

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 analysis also reveals proxy inconsistencies in the estimates of ocean temperature and global ice volume, which calls for the generation of more systematic multi-proxy records to shed light on how secondary processes affect paleoclimate proxy records. Due to a lack of well-dated terrestrial records, this review paper is based primarily on marine proxy records. The terrestrial realm plays an important role in global climate change as it provides feedbacks to the climate system via vegetation and soils. A more comprehensive understanding of the linkage between ocean, ice and land across the Pliocene-Pleistocene transition thus awaits future integration of terrestrial and marine proxy records.  

—Sze Ling Ho (slingho@ntu.edu.tw; 0000-0002-4898-9036), National Taiwan University, Taiwan; and Erin McClymont (erin.mcclymont@durham.ac.uk; 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.

Citation: Ho, S. L., and E. McClymont (2023), Marine sediments reveal past climate responses to CO2 changes, Eos, 104, https://doi.org/10.1029/2023EO235027. Published on 21 September 2023.
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