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experts comment on two papers published in Nature on links between ancient climate shifts and ocean temperatures / greenhouse gases
Prof Carrie Lear, Professor of Past Climates and Earth System Change at Cardiff University, said:
“These papers don’t rewrite the role of CO₂, they underline how sensitive the climate system is to even small nudges. Once ice sheets cross a threshold, the whole system can shift dramatically, and that’s exactly why today’s rapid CO₂ rise is so alarming.
“The press release captures the broad findings, but a general reader might mistakenly infer that these studies overturn the role of CO₂ in climate change. This new work does not change our understanding of how strongly Earth’s climate responds to greenhouse gases.
“What these studies provide is a rare glimpse into climate transitions over the past 3 million years using exceptionally old Antarctic ice. The data are exciting, but the ice is highly compressed and discontinuous, which means the records mainly reflect long‑term trends rather than the large glacial–interglacial CO₂ swings we know occurred.
“The key message is that relatively small shifts in climate forcing can push the Earth system across thresholds, altering ice volume, ocean circulation, and ocean heat storage. That concept is not new, but the noble‑gas temperature reconstructions help reinforce it by showing that the Plio-Pleistocene climate transition was accompanied by long‑term ocean cooling rather than a large change in CO₂.
“A key takeaway is that the cryosphere (our planet’s ice) responds in non‑linear ways to climate forcing. Modern CO₂ levels are rising far faster, and to far higher values, than anything seen in these ancient records. This work reminds us that once climate thresholds are crossed, changes in ice sheets and ocean circulation can become irreversible on human timescales.
“These studies highlight the non‑linear behaviour of ice sheets and the ocean-climate system. Far from diminishing CO₂’s importance, this work reinforces concern: today’s CO₂ rise is far larger and far faster than anything seen in these ancient records. Rapid, substantial greenhouse‑gas forcing increases the risk of crossing thresholds that commit the planet to a very different climate state.”
Prof Richard Allan, climate scientist at the University of Reading, said:
“Uniquely ancient ice cores from the Allan Hills in Antarctica are like a 3 million year time machine. They provide a glimpse of a different planet Earth that was slowly cooling and only just beginning to sustain ice in the northern hemisphere, which laid the foundations for wild swings between glacial and milder inter-glacial conditions over the past million years, during which time modern humans sprung into being.
“Improved measurements and techniques show that greenhouse gas changes were not the main player in this ancient global cooling. This is an important but not altogether unexpected result since in this past climate system North and South America had only just connected so its vegetation, weather patterns and ocean circulations operated in a very different way. Once the world could support ice in the Arctic and northern continents, trapped bubbles of fossilised air in other, deeper ice cores show that greenhouse gases instead played a starring role in amplifying more recent natural climate oscillations driven by periodic changes in Earth’s orbit around the sun over tens to hundreds of thousands of years. However, present day carbon dioxide is massively above the levels seen in these natural glacial cycles over the past million years and multiple lines of evidence show it is an undisputed fact that increasing greenhouse gases are currently heating our climate up.
“This ancient view into our planet’s distant past can nevertheless offer a new angle on how planet Earth works. This new knowledge can further improve the capability to represent our dynamic climate in complex computer systems and improve future projections, which are heading for uncharted territories due to continued burning of coal, oil and natural gas.”
Prof Raymond Pierrehumbert FRS, Professor of Planetary Physics at the University of Oxford, said:
“The press release is generally accurate, and does not imply that these results in any way cast doubt on the fundamental connection between CO2 and climate. They also do not imply that anywhere in the papers. The press release says the climate transitions are ‘associated with’ a change in ocean heat content, but it’s important to note this is not proposed by the authors as a causal connection.
“Ocean heat content is itself a measure of general climate change, and the most that can be said from this data is that the general cooling trend from the Pliocene (with an unglaciated Northern Hemisphere) into the era of Pleistocene glacial-interglacial cycles is correlated with ocean heat content. A third thing could have been causing both.
“We should note first that both papers use data from a recently developed ice analysis technique. Conventional Antarctic ice cores, which give high time resolution, only go back about 800,000 years. There is older ice, but it crops out, has been highly deformed, and is thoroughly ‘messed up’. You can’t track individual layers in it, but innovative new techniques can get data from messed up ice up around 4 million years old. This data give information on CO2, and also on ocean heat content (via another innovative new technique relying on the oceans temperature-dependent uptake of atmospheric noble gases). The slogan for this project could be ‘Good data from bad ice’. The community doing this has done due diligence, and involves very talented people, but it is a new field and with new techniques there is always room for surprise as people work more with them.
“But taking the data as given note that this yields time averaged data across several glacial-interglacial cycles, and does not resolve individual cycles, unlike conventional ice cores. Thus, the CO2 trends dealt with are much smaller than the 100ppm change in the course of a single glacial-interglacial cycle (pre-industrial) The current CO2 increase driving global warming is a 150ppm increase from the pre-industrial times (following the end of the last glaciation) and still rising. We are hitting the climate much harder than anything being discussed in these papers.
“The papers deal with two important unexplained features and provide new data: (1) the Pliocene-Pleistocene transition, across which the Northern Hemisphere became glaciated and entered glacial-interglacial cycles ( Antarctica was glaciated long before that), and (2) the shift in the periodicity of ice ages, mid-Pleistocene, from 40,000 years (roughly) to 100,000 years. The latter is an important mystery to unravel, but a rather subtle effect. One of the proposed mechanisms was a long-term downward drift in CO2, which the new data appears to rule out (unless the climate is extremely, extremely sensitive to the small drop in CO2) but there have always been a zoo of other mechanisms for this transition, involving things like the change in glacial bed conditions as glaciers progressively grind up rock.
“The Pliocene-Pleistocene transition is of more broad interest. Even with the conventional (and uncertain) geological reconstructions of CO2 from the mid-Pliocene, it was hard to account for an unglaciated Northern Hemisphere with a CO2 that was only modestly above pre-industrial (and pretty much the same as we have now). There are initial condition and time scale effects at work, to be sure, but the fact that the Pliocene was so different from the Pleistocene and the interglacial we lived in up until fossil fuels (the Holocene – we are now in the Anthropocene, if we’re honest) is highly disturbing, in light of how hard we are hitting the climate now with massive rapid increases in CO2. The ice doesn’t go back all the way through the Pliocene, so it’s possible that the Pliocene-Pleistocene transition is a delayed response to an earlier drop in CO2, but regardless, what this new data underscores is that the climate has ways of undergoing massive transitions, which we do not currently understand. I think a lot of ‘tipping point’ stuff is rather poorly formulated, but the Pliocene-Pleistocene transition is one of the things that makes me worried about the unknown territory we are heading into through massive and rapid anthropogenic increases in CO2. We’d be better off if we understood the Pliocene-Pleistocene transition. The new papers do seem to show that there is more at work than just CO2 in that transition, and it should be no source of comfort regarding the future that we don’t understand what that is.”
Dr Anieke Brombacher, Research Scientist, National Oceanography Centre, said:
“These two papers show how complicated it is to grow large ice sheets. The low CO2 levels of the last 3 million years alone were not enough. You also need a change in ocean circulation that lowers global ocean temperatures, probably followed by a complex feedback system including local ice sheet dynamics and albedo. Only then can ice sheets reach the size seen in the most recent glacial periods.”
* ‘Broadly stable atmospheric CO2 and CH4 levels over the past 3 million years’ by Julia Marks-Peterson et al. and ‘Global ocean heat content over the past 3 million years’ by Sarah Shackleton et al was published in Nature at 4pm UK time Wednesday 18 March 2026.
DOIs: https://doi.org/10.1038/s41586-025-10032-y & https://doi.org/10.1038/s41586-026-10116-3
Declared interests
Richard Allan: “no conflicts of interest”
For all other experts, no reply to our request for DOIs was received.