Earth was hotter in the past, but that doesn’t make humans safer from modern climate change

On 7 January, Wide Awake Media – a known promoter of climate-related disinformation that appears to be operated by one person – posted a clip on X featuring part of an interview with Patrick Moore. In the interview, Moore claims that “it is not too hot now” because temperatures and CO2 levels in Earth’s atmosphere are lower today than in much of our planet’s geological history. Wide Awake Media’s post has garnered 150,000 views at the time of publication.

Science Feedback has reviewed several of Moore’s statements, which we found to be misleading or incorrect. But Moore isn’t the only one to make this particular claim – many opponents of climate action may acknowledge that the climate is warming, but assert that Earth will still be relatively cool compared to higher temperatures in the distant past. Those who hear this may get the impression that recent climate change isn’t as severe as climate scientists have stated.

It’s a misleading impression; these claims only tell half the story. The geological record tells us that Earth was much warmer tens of millions of years ago, but it’s important to remember that human civilization didn’t exist tens of millions of years ago. Human civilization evolved on a world close to that which existed prior to our Industrial Revolution. Now, due to to greenhouse gases from human activity, the climate is changing very rapidly,

Below, we’ll take a closer look at why this is the case.

No evidence of temperatures this warm in the recent past

Our planet’s surface has warmed about 1.4°C on average since 1900, and scientists project between 2.2 and 3.7°C of total warming by the end of this century. Earth has been hotter than this in the distant past, but as we’ll show, it’s far more important to look at more recent trends – these are what affect human life and civilization. On that front, the evidence tells us that modern global warming is a very sudden change^[1].

How do we know? Reliable temperature measurements only date back to the 19th century, but scientists can estimate past climate conditions using what they call “proxies” – evidence coming from ice cores, sedimentary rocks, fossilized pollen, tree rings, and other sources. By combining these proxies, scientists can reconstruct a geological record of temperatures long before any humans were around^[1]. 

Proxies tell us that temperatures remained stable for most of the past 10,000 years (Figure 1)^[1]. These 10,000 years cover the entire history of human civilization – in sharp contrast, the global warming of the past century is a very sudden spike. Even if we look at conditions before 10,000 years ago, when Earth warmed relatively quickly after the last ice age, Earth is warming at least ten times more rapidly today.

The record suggests that the last time Earth experienced temperatures similar to those today was approximately 125,000 years ago, during a warm period between past ice ages^[1,2]. Furthermore, climate projections indicate that the planet is on course for temperatures that the planet hasn’t experienced in several million years^[3]. Human civilization didn’t exist that long ago, and it didn’t develop under those conditions.

What about CO2? The concentration of CO2 in Earth’s atmosphere has risen from around 280 parts per million before 1900 to around 420 parts per million in 2024. As with temperatures, proxies like ice cores can tell us how CO2 levels in the atmosphere have risen and fallen over time (Figure 2). Over geological timescales, CO2 levels and temperatures usually rise and fall together – as CO2 levels increase in the geological record, so do temperatures^[4]. 

The data tells us that CO2 levels remained relatively stable for most of the past million years until the Industrial Revolution kicked off the dramatic increase that we see today^[5,6]. This correlation between modern-day global warming and this abrupt rise in CO2 is another piece of evidence that today’s climate change is not caused by natural variation^[7]. 

The last time that Earth had CO2 levels comparable to today’s was about 3 million years ago, during what scientists call the Pliocene epoch^[3]. At the time, the world was about 2 to 3 °C (4 to 5 °F) warmer than in the 19th century. That meant a very different world from our own – for example, with less water frozen in ice, sea levels were about 17 meters (56 feet) higher than pre-industrial levels^[8]. Today, hundreds of millions of people live in coastal cities and densely populated coastal plains that are less than 17 meters above sea level.

Humans are not adapted for living in a warmer world

These examples illustrate part of why modern-day climate change is so worrisome. Earth’s life today – humans included – is acclimated to a planet with temperatures closer to pre-industrial times. We are not acclimated to a planet that is even several degrees warmer.

Look at long-term measurements of Earth’s climate (Figures 1, 2, and 3) and you will notice that average temperatures and CO2 levels typically rise and fall very gradually. If temperatures change naturally, without human intervention, they will change by 1°C over many thousands of years^[9,10]. In comparison, modern-day global warming has seen average temperature rise more than 1°C in around a hundred years.

The more greenhouse gas we emit into the atmosphere, the more we’ll heat up the atmosphere^[11]. A warmer world increases the risks of crop failures. In a warmer world, more humans will be exposed to heat waves, floods, and other disasters. Higher temperatures will increase the risk of tropical diseases like malaria and dengue, potentially countering programs to eradicate them. So, contrary to the claim that “we are replenishing the CO2 to a much better level” by burning fossil fuels, continuing to emit greenhouse gases will only make future humans’ lives more difficult^[7,11].

The more quickly the climate warms, the less time we’ll have to adapt. The same is true for other life on Earth. The authors of a 2013 study estimated that species of vertebrates typically adapt to 1°C of change in global temperatures per one million years^[12]. The study was preliminary, and it’s certainly possible that some species might adapt more quickly, but it is evidence that Earth’s atmosphere is warming faster than the biosphere can reasonably sustain.

Can we find similar examples in the geological record? Potentially, yes – and there is some evidence that the resulting CO2 spikes may correspond to events that wiped out a great part of life on Earth (Figure 4). More research is needed, but science suggests that at several times in Earth’s history, colossal volcanic eruptions may have spewed CO2 into the air at far higher rates than any volcano we see today^[13,14]. 

One study found that the end of the Permian period, 250 million years ago (a mass extinction that killed 96% of species) saw atmospheric CO2 levels jump six-fold^[15]. Another study finds that volcanoes may have caused CO2 to spike at the end of the Ordovician period, some 443 million years ago (killing 85% of species)^[13]. It’s clear that dramatic changes in CO2 levels aren’t beneficial for all life forms. 

Conclusion

It’s certainly true that the planet has been warmer in the past, but that statement isn’t very meaningful. What matters is how the changing climate compares to the climate in which humans evolved. Modern human civilization developed in a particular climate that remained stable for around 10,000 years. Thanks to CO2 and other greenhouse gases that we’ve put into the atmosphere, that climate is now changing at rates unprecedented in the past several million years.

The faster the climate changes – the more CO2 we put into the atmosphere, and the more the atmosphere subsequently heats up – the less time that humans will have to adapt to the changing conditions. We’re already seeing the droughts, the storms, the fires, and more that are already exacerbated by a warmer climate. This is why climate change is so alarming.

References:

1. Kaufman et al. (2020) Holocene global mean surface temperature, a multi-method reconstruction approach. Scientific Data.
2. Kaufman and McKay. (2022) Technical Note: Past and future warming – direct comparison on multi-century timescales. Climate of the Past.
3. Burke et al. (2018) Pliocene and Eocene provide best analogs for near-future climates. PNAS.
4. Harper et al. (2024) Long- and short-term coupling of sea surface temperature and atmospheric CO2 during the late Paleocene and early Eocene. PNAS.
5. Rae et al. (2021) Atmospheric CO2 over the Past 66 Million Years from Marine Archives. The Annual Review of Earth and Planetary Science.
6. Foster et al. (2017) Future climate forcing potentially without precedent in the last 420 million years. Nature Communications.
7. IPCC. (2021) Climate Change 2021: The Physical Science Basis.
8. Dumitru et al. (2019) Constraints on global mean sea level during Pliocene warmth. Nature.
9. Judd et al. (2024) A 485-million-year history of Earth’s surface temperature. Science.
10. Westerhold et al. (2020) An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science.
11. IPCC. (2022) Climate Change 2022: Impacts, Adaptation and Vulnerability.
12. Quintero and Wiens. (2013) Rates of projected climate change dramatically exceed past rates of climatic niche evolution among vertebrate species. Ecology Letters.
13. Bond and Grasby. (2020) Late Ordovician mass extinction caused by volcanism, warming, and anoxia, not cooling and glaciation. Geology.
14. Olsen et al. (2022) Arctic ice and the ecological rise of the dinosaurs. Science.
15. Wu et al. (2021) Six-fold increase of atmospheric pCO2 during the Permian–Triassic mass extinction. Nature Communications.