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# Predictable ice ages on a chaotic planet

Didier Paillard

► To cite this version:

Didier Paillard. Predictable ice ages on a chaotic planet. Nature, 2017, 542 (7642), pp.419 - 420.
10.1038/542419a. cea-01887624

HAL Id: cea-01887624

https://cea.hal.science/cea-01887624v1

Submitted on 3 Oct 2025

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Distributed under a Creative Commons Attribution 4.0 International License
wide-field cells use GABA molecules. Franke and colleagues' second major finding was that small- and wide-field cells have different roles in mediating bipolar-cell responses. By pharmacologically manipulating these neurotransmitters, the authors showed that the major driver of bipolar-cell behavioural diversity is the GABA-releasing wide-field cells, but they are in turn controlled by the glycine-releasing small-field cells (Fig. 1). Moreover, different bipolar-cell types receive inputs from different sets of GABA-releasing cells, which in turn receive input from different sets of small-field cells. Thus, the diversity of bipolar-cell outputs comes from circuits of amacrine cells, not individual amacrine-cell types, as was often imagined.

Finally, Franke et al. began the process of refining the definitions of bipolar-cell types. For example, they show that all 'OFF' bipolar-cell types (those whose activity is inhibited by light) make brief, paradoxical 'ON' responses at times when they should be inhibited. These are not trivial differences — they reflect meaningful diversity in signals sent by the bipolar cell to the ganglion cell.

A major advantage of the new methods is that they allow huge samples of cells to be studied side-by-side, and the detailed study of individual subtypes to become practical. A next step will be to understand in more detail the responses of the different types of bipolar cell to the changing scenes observed by freely roaming mice. This will not be easy.

The challenge is how to choose test stimuli, and how to interpret the bipolar-cell responses to them. It is unlikely that natural-istic responses can be achieved using spots of light, striped patterns or even the 'chirp' stimuli used by Franke and colleagues, which change in intensity or flicker rate over time. It is widely thought that visual scenes similar to those that would be encountered in the wild would be appropriate. But there is no agreement on how to meaningfully analyse responses to such stimuli. Where in a natural scene does the stimulus to an individual retinal cell start, and where does it end? For once, it seems that our methods have outrun our ideas for how to use them. Think hard, people. ■

**Richard H. Masland** is in the Departments of Neurobiology and Ophthalmology, Harvard Medical School, Boston, Massachusetts 02115, USA.
e-mail: richard masland@meei.harvard.edu

1. Franke, K. et al. *Nature* **542**, 439–444 (2017).

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3. Vaney, D. I., Sivyer, B. & Taylor, W. R. *Nature Rev. Neurosci.* **13**, 194–208 (2012).

4. Borghuis, B. G., Marvin, J. S., Looger, L. L. & Demb, J. B. *J. Neurosci.* **33**, 10972–10985 (2013).

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This article was published online on 8 February 2017.

# Predictable ice ages on a chaotic planet

Statistical analysis has revealed a simple rule for the occurrence of warm periods during the Quaternary, whereas on much longer timescales geological data have confirmed that the Solar System is chaotic. SEE ARTICLE p.427 & LETTER p.468

DIDIER PAILLARD

Ice ages are paced by astronomical changes. However, a persistent difficulty has been to explain the dominant 100,000-year climate cycle observed¹ during the past million years. On page 427, Tzedakis et al.² report a model that links variations in Earth's orbit to the observed sequence of interglacials. On page 468, Ma et al.³ demonstrate that astronomical imprints on Earth's past climate can be used to probe the long-term evolution of the Solar System.

In 1941, the Serbian physicist Milutin Milanković proposed⁴ that small changes in Earth's orbit and rotational axis alter the distribution of solar energy on the planet's surface, affecting its climate — an effect known as astronomical forcing. Milanković's theory predicts that ice sheets in the Northern Hemisphere should grow and melt every 41,000 years, following changes in Earth's obliquity, which is defined as the angle between the planet's rotational axis and orbital plane. But in 1976, observations revealed¹ a dominant 100,000-year climate cycle during the later part of the Quaternary — the period

that started about 2.6 million years ago — that seems to be driven by variations in the eccentricity of Earth's orbit.

Many different ideas have been put forward over the past four decades⁵ to explain the apparent link between Earth's orbital eccentricity and climate during the Quaternary. The concept of stochastic resonance was invented⁶ to explain how small external oscillations (such as those associated with orbital eccentricity) could be amplified by a noisy process (such as climate) to eventually become the dominant periodicity of a system. It was even suggested⁷ that there might be no link between eccentricity and ice ages at all — instead, the observations could be explained by statistical fluctuations around multiples of obliquity cycles.

There have been only about ten ice-age cycles over the past million years. Each of these varied in duration, severity or structure, and the astronomical forcing was never quite the same. Consequently, the relative role of simple deterministic processes versus more unpredictable, or even random, fluctuations is difficult to ascertain.

Although it is generally assumed that large

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**Figure 1 | Melting an ice sheet.** Tzedakis et al.² report a simple rule to explain how changes in Earth's orbit controlled the timing of interglacials during the past million years, whereas Ma et al.³ confirm that these orbital mechanics were chaotic before 50 million years ago. Tzedakis and colleagues' rule suggests that, the larger the ice sheet, the more easily it can be melted. Shown here are three possible explanations for this finding. First, a larger ice sheet might develop a warmer base than a smaller one, inducing a faster rate of ice flow (blue arrow), and potentially causing instabilities and enhanced iceberg calving¹¹. Second, the surface of a larger ice sheet could experience a colder and drier climate, accumulating less snow, but more dust. Less sunlight would be reflected (pale yellow arrows), resulting in increased ice-sheet melting¹². Finally, the Antarctic Ice Sheet at its maximum extent could alter bottom-water formation, break down ocean stratification — the vertical gradient of water density — and release CO₂ into the atmosphere (red arrows), favouring ice-sheet retreat⁵.
ice sheets vary slowly, filtering out any short-term climate fluctuations, there is evidence for abrupt changes not only in climate or ocean circulation, but also in the ice sheets themselves. Furthermore, during the most recent deglaciation, atmospheric CO₂ levels and global temperatures increased a few millennia before the melting of the northern ice sheets. This effect could be linked to abrupt ocean-circulation changes, suggesting that such abrupt variability had a role in the deglaciation process⁸. Therefore, although astronomical forcing played a crucial part in the evolution of the Quaternary ice ages, this is not the whole story. Without a full mechanistic understanding of ice-sheet-climate interactions, it might be difficult to determine whether or not ice ages are entirely predictable⁹.

Tzedakis and colleagues have made progress in this direction by discovering a rule that links astronomical forcing to the timing of the Quaternary interglacials. In the earlier part of the Quaternary, interglacials occurred when the amount of summer solar radiation was above a given threshold; in the later part, this threshold was a linearly decreasing function of the time elapsed since the previous interglacial. The authors' idea is not completely new¹⁰, but their demonstration is enlightening because it focuses on the key difficulty — the occurrence of interglacials. Their statistical analysis shows that the rule is robust, and that the observed sequence of interglacials is the most probable one, among a small set of possible histories. This is a strong indication that ice ages are indeed predictable, and that the underlying mechanisms behind the 100,000-year climate cycle are probably quite simple.

The time elapsed between two interglacials — the ice-age duration — is linked to the size of the ice sheets. Therefore, Tzedakis and colleagues' rule suggests that, above some critical size, the larger the ice sheet, the more easily it can be melted⁵. So far, at least three mechanisms have been proposed to explain this apparent paradox (Fig. 1).

The first explanation involves instabilities in large ice sheets: for instance, the bases of such ice sheets might become warm, inducing faster ice flow and increased iceberg calving¹¹. A second possibility is associated with ice-sheet albedo (reflectance) in a cold and dry glacial climate: for large ice sheets, less snow and more dust increases the surface absorption of solar heat and enhances ice-sheet melting¹². Finally, the Antarctic Ice Sheet at its maximum extent would affect bottom-water formation and ocean carbon storage⁵, which might be directly linked to the rise in atmospheric CO₂ that precedes deglaciations⁸. It is also possible that all three of these mechanisms act together to ultimately lead to the simple dynamics outlined in Tzedakis and colleagues' paper.

Unravelling connections between astronomical changes and Earth's climate on much longer timescales has far-reaching implications for our

knowledge of the Solar System itself. Indeed, it has been shown¹³ from celestial mechanics that our Solar System is chaotic, which prevents the precise computation of planetary motions before about 50 million years ago. However, geological data could help us to extend this horizon. Ma and colleagues have used sedimentological data to show that, between 85 million and 87 million years ago, a switch occurred from 1.2-million-year to 2.4-million-year climate cycles. The authors find that this transition was caused by a shift in the secular resonance (the synchronization of the precession) between the orbits of Earth and Mars.

Such a constraint on the dynamics of the Solar System could pave the way for an improved chronology of Earth's geological past. It is somewhat ironic that climate and sedimentological processes seem to respond to astronomical forcing in a rather simple way, but that the planets are not so predictable after all. ■

Didier Paillard is in the Laboratory of Climate and Environmental Sciences, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette, France. e-mail: didier.paillard@lsce.ipsl.fr

1. Hays, J., Imbrie, J. & Shackleton, N. *Science* **194**, 1121–1132 (1976).

2. Tzedakis, P. C, Crucifix, M., Mitsui, T. & Wolff, E. W. *Nature* **542**, 427–432 (2017).

3. Ma, C., Meyers, S. R. & Sageman, B. B. *Nature* **542**, 468–470 (2017).

4. Milanković, M. M. *Canon of Insolation and the Ice-Age Problem* (R. Serb. Acad., 1941).

5. Paillard, D. *Quat. Sci. Rev.* **107**, 11–24 (2015).

6. Benzi, R., Parisi, G., Sutera, A. & Vulpiani A. *Tellus* **34**, 10–16 (1982).

7. Huybers, P. & Wunsch, C. *Nature* **434**, 491–494 (2005).

8. Shakun, J. D et al. *Nature* **484**, 49–54 (2012).

9. Crucifix, M. *Clim. Past* **9**, 2253–2267 (2013).

10. Paillard, D. *Nature* **391**, 378–381 (1998).

11. Oerlemans, J. *Tellus A* **35**, 81–87 (1983).

12. Ganopolski, A. & Calov, R. *Clim. Past* **7**, 1415–1425 (2011).

13. Laskar, J. *Nature* **338**, 237–238 (1989).

MOLECULAR BIOLOGY

# RNA editing packs a one-two punch

**Optimal protein synthesis requires bases in transfer RNAs to be modified. A key modification has been shown to involve an unusual two-step mechanism that entails the sequential activities of two enzymes. SEE LETTER P.494**

WILLIAM T. YEWDELL
& JAYANTA CHAUDHURI

A central dogma of molecular biology is that DNA and the corresponding RNA are complementary. But this complementarity is rewired by a process called RNA editing, which recodes genomic information by modifying, deleting or inserting nucleotides in RNA transcripts. RNA editing has wide-ranging roles in various cellular processes and has clear implications in human disease¹. The molecular and biochemical mechanisms underlying RNA editing have proved a challenge to scientists since its discovery 30 years ago in trypanosomes, a type of protozoan². On page 494, Rubio *et al.*³ now provide intriguing evidence that editing of transfer RNAs is an unconventional two-step process, thus reasserting a valuable mechanistic concept for the field⁴.

An evolutionarily well-conserved editing process called base deamination converts RNA components known as nucleosides into other nucleosides: more specifically, it converts cytidine (C) into uridine (U), and adenosine (A) into inosine (I). In tRNAs, such conversions generate ‘wobble’ base pairs that are crucial for proper translation of the genetic code, and

that maintain tertiary structure⁵⁻⁷. The enzyme that edits tRNA in archaea⁷ (which, along with bacteria, constitute the microorganisms known as prokaryotes) has been identified, but its counterpart in eukaryotes (organisms that include plants, animals, fungi and protozoa) has remained elusive.

Previous studies⁸ demonstrated that down-regulation *in vivo* of the gene that encodes TbADAT2/3 — an enzyme that catalyses the deamination of A, converting it to I, in tRNAs in *Trypanosoma brucei* — leads to decreased C-to-U conversion in the tRNA (tRNA*Thr*) that carries the amino acid threonine to nascent proteins during protein synthesis. This suggests that the deaminase TbADAT2/3 acts on C as well as A. However, purified TbADAT2/3 does not convert C to U in tRNA*Thr* *in vitro*⁸.

On the basis of the observation that tRNA*Thr* nucleoside residues can be methylated in various eukaryotes⁹, Rubio *et al.* posited that TbADAT2/3 might deaminate a methylated C (m³C) in tRNA*Thr*. This would generate methylated uridine (m³U), and explain why TbADAT2/3 is unable to deaminate unmethylated tRNA*Thr* (ref. 8). Indeed, the authors observed that purified tRNA*Thr* from *T. brucei* contains both the m³C and m³U modifications.

Rubio and colleagues went on to identify