Scientists have cast doubt on the existence of super-hot oceans on the ancient Earth

Scientists analysed the chemical composition of more than 18,000 sedimentary rock samples and combined this data with computer models of the ancient climate. The results show that over the last 539 million years, the Earth’s average surface temperature has probably remained largely within a range of approximately 10 to 30 °C. The early Palaeozoic was warm, but not necessarily much hotter than later warm periods.

However, this does not mean that we need not fear modern climate change. The natural mechanisms that have stabilised the Earth’s temperature operate over hundreds of thousands and millions of years — much more slowly than the current rate of warming.

Why scientists expected to find near-boiling seas

Climate history cannot be reconstructed using ordinary thermometers. Researchers therefore look for chemical signatures in ancient rocks and fossils that vary with temperature.

For a long time, one of the main such indicators was the ratio of oxygen isotopes in minerals and the fossilised remains of marine organisms. Some reconstructions based on this data suggested that during the Cambrian and early Ordovician periods, tropical seawater could have heated to over 40 °C, and at times to nearly 50 °C.

This posed a serious puzzle. It was precisely during the Early Palaeozoic that marine life rapidly became more complex and diverse. Yet the prolonged survival of animals in such hot water, especially with relatively low oxygen levels, seemed unlikely.

Furthermore, the isotopic composition of ancient seawater could have changed of its own accord. In that case, part of the signal that had been interpreted as indicating very high temperatures might have been linked to changes in ocean chemistry rather than to extreme heat.

What the researchers gleaned from the ancient rocks

In their new study, the scientists applied an independent measure — the Chemical Alteration Index (CIA). This reflects the extent to which the original minerals were broken down by the effects of water and climate.

In warm and humid climates, chemical weathering is usually more intense. Mobile elements, including calcium, sodium and potassium, are gradually leached out, whilst the less mobile aluminium remains. The ratio of these elements allows for an approximate assessment of the conditions under which the sedimentary material was formed.

The researchers compiled 18,368 geochemical records covering almost the entire Phanerozoic — the last 539 million years. They then reconstructed the ancient locations of the samples on the moving continents and compared them with the results of global climate models. The modelling covered 109 time intervals and took into account ancient geography, carbon dioxide concentrations and variations in the Sun’s brightness.

How hot was the ancient Earth?

The resulting reconstruction does not support the view of the early Palaeozoic as an exceptional era of almost unbearable heat.

According to the authors’ main estimate, the average surface temperature of the planet throughout the Phanerozoic remained roughly within the range of 10–30 °C. The temperature of the Palaeozoic oceans was comparable to that of the warm periods of the Mesozoic and Cenozoic, rather than significantly exceeding them.

This does not mean that the ancient Earth was cool. During certain warm periods, the average global temperature may have been much higher than today’s. However, this new study casts doubt on the most extreme estimates, particularly the notion of tropical seas persisting for long periods at temperatures of around 50 °C.

The authors emphasise that even the ‘hottest’ of the reconstruction scenarios they tested failed to reproduce the extreme Cambrian temperatures suggested by some isotopic data.

How the Earth maintained its temperature

The results support the idea that the Earth possesses a slow, natural ‘thermostat’.

One of its main mechanisms is the weathering of silicate rocks. When the climate becomes warmer and wetter, rocks break down more quickly. Through associated chemical reactions, carbon dioxide is gradually removed from the atmosphere, after which it becomes locked away in minerals and marine sediments.

As CO₂ traps heat, a reduction in its concentration over time weakens the greenhouse effect. If the planet cools, weathering slows down, carbon dioxide begins to accumulate more rapidly, and the climate may once again become warmer.

Such negative feedback loops may have kept temperatures within a range that allowed the biosphere to exist and evolve over hundreds of millions of years.

Why the natural ‘thermostat’ will not halt the current warming

The study does not suggest that the climate system will automatically rectify the current situation.

Geological weathering is an extremely slow process. It takes hundreds of thousands or millions of years, whereas humans are sharply increasing greenhouse gas concentrations over the course of decades and centuries.

Therefore, ancient warm periods cannot be regarded as direct evidence that modern ecosystems will survive a rapid rise in temperature unscathed. In the past, climatic conditions changed gradually, giving organisms considerably more time to migrate and adapt evolutionarily.

The pace of modern change is fundamentally different. Moreover, if the new reconstruction is correct and the ancient planet did indeed rarely reach extreme temperatures, this makes the potential for significant anthropogenic warming no less, and potentially even more, alarming.

What this changes in the history of the first animals

The more moderate temperature of the Early Palaeozoic seas helps to explain how complex multicellular organisms were able to spread within them.

The Phanerozoic began around 539 million years ago and encompasses the Cambrian explosion of animal diversity, the emergence of plants onto land, the evolution of vertebrates and the emergence of modern ecosystems. Temperature was one of the factors determining which organisms could exist and how much oxygen was available in the water.

If the early tropical oceans were not constantly heated to 40–50 °C, conditions for the development of animals may have been considerably more favourable. This new reconstruction thus reduces the discrepancy between previous temperature estimates and the fossil evidence for the spread of life.

Limitations of the study

The rock alteration index is not a direct thermometer. It is influenced not only by temperature, but also by precipitation, erosion rates, water movement, the original composition of the rock, particle size, vegetation and changes in minerals following burial.

The authors have attempted to correct for some of these biases and have separately tested the influence of particle size and sampling locations. Nevertheless, they acknowledge significant uncertainty, particularly for periods where fewer suitable rocks have been preserved. The Silurian, Triassic and Jurassic periods are less well represented than some other time intervals.

Therefore, this new study does not settle the debate on ancient climate. It offers an independent reconstruction that needs to be compared with isotopic data, fossil organisms and other climate indicators.

Why this is important

The Earth’s temperature history helps us understand not only the past, but also the possible limits of future climate.

The study shows that the planet possessed effective mechanisms for long-term stabilisation. At the same time, it serves as a reminder of their main limitation: they are too slow to offset rapid carbon dioxide emissions.

A cooler past does not make current warming any less dangerous. On the contrary, it may mean that humanity is capable of pushing the climate towards conditions that were extremely rare even in the planet’s deep history.

Background

Reconstructions of ancient climates are regularly revised as new methods emerge. A single chemical indicator rarely provides a definitive answer: each is sensitive not only to temperature but also to other natural processes.

Palaeoclimatologists therefore combine several independent sources — isotopes, sediment composition, fossilised organisms, ancient soils and computer models.

This new study is significant precisely because it uses an indicator that does not depend directly on oxygen isotopes. Its findings do not deny the existence of very warm periods, but they do cast doubt on the most extreme version of the early Earth’s climate history.

Source

Dongyu Zheng et al., “Tight regulation of Earth’s long-term temperature over Phanerozoic time”, Nature Communications, 2026.