The Washington Post recently featured new research by Emily Judd and her team, reaffirming what scientists have long understood: CO2 and temperature are closely linked. What’s new is that this study extends that relationship nearly 500 million years into Earth’s history, showing that both CO2 and temperatures were much higher in the distant past.
While this isn’t news to scientists, it’s unfamiliar territory for many non-scientists. Climate deniers have seized on this, twisting the findings to push the false narrative that climate change is a hoax. They claim life on Earth will be just fine, maybe even better, with a changing climate.
It’s a convenient story for those who don’t want to deal with the realities of science, but it’s flat-out wrong.
The article and the research it referenced were clear: while life adapted to extreme CO2 and temperature conditions, humans would not have made it. It’s a reminder that just because the Earth survived doesn’t mean we will.
Figure 1 shows that both temperatures and CO2 emissions have dropped to some of the lowest levels in geological history. At first glance, this might lead to the assumption that the current alarm over climate change is overblown, a case of exaggeration. One might even argue that life on Earth has flourished at higher temperatures and carbon levels before, so why all the concern?
But this is a superficial take. The conditions necessary for human survival are nothing like those needed by reptiles or fish. It’s highly unlikely that humans could have thrived when average global temperatures were around 24°C, as they were before 8 million years ago, compared to today’s more hospitable 14°C.
CO2 concentrations tell a similar story. Levels remained above 400 ppm until around 8 million years ago but fell to an average of 270 ppm starting about 10,000 years ago—a period coinciding with human civilization’s rise. Now, after millennia of relative stability, CO2 levels have surged to about 420 ppm following the industrial revolution. This isn’t just a return to some previous “natural state”—it’s a shift at a rate and scale that ecosystems and societies may not be prepared for.
The reality is that it was the natural decline in temperature and CO2 levels that enabled the conditions for human life to emerge. These changes weren’t just a geological footnote—they were a prerequisite for our existence.
Figure 1. High temperatures & high levels of atmospheric CO2 before about 8 million years ago were unfavorable for human life. Source: Judd et al (2024) and Labyrinth Consulting Services. Inc. Click on the image to enlarge.
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Figure 2 lays out CO2 and temperature trends, just like in Figure 1, but with a key distinction: the fall in CO2 and temperature coincided with a surge in biodiversity starting around 90 million years ago.
The takeaway here is obvious, even without factoring in climate change—lower CO2 levels were part of the special conditions that led to this explosion of life. The current ecosystem, with its complexity and variety, didn’t happen by accident. It required specific circumstances, and lower CO2 was fundamental to that process.
Figure 2. Falling CO2 and temperature levels coincided with an explosion of biodiversity on Earth beginning about 90 million years ago. PETM: Paleocene-Eocene Thermal Maximum; MMCT: Middle Miocene Climate Transition; Order-Sil: Ordovician-Silurian extinction even; L Dev: lower Devonian extinction event; Perm Trias: Permian-Triassic extinction event; KT: Cretaceous-Tertiary extinction event; EO: end Oligocene extinction event. Source: Judd et al (2024) and Labyrinth Consulting Services. Inc. Click on the image to enlarge.
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The Judd study made it clear: the link between CO2 and climate change is undeniable. It’s not up for debate—CO2 drives the climate, period.
“Carbon dioxide is really that master dial. That’s an important message…in terms of understanding why emissions from fossil fuels are a problem today.””
Jess Tierney, Climate Scientist at the University of Arizona
The science on climate change has been clear for a long time. So why does public doubt still linger?
The same week that The Washington Post published its article, my friend Nate Hagens shared a conversation with Stefan Rahmstorf on the topic. Much of what follows is based on the work he and his colleagues at the Potsdam Institute for Climate Impact Research have done.
Is Climate Science Settled or Still Uncertain?
The greenhouse effect has been understood for over a century. Joseph Fourier outlined it in 1824, and by the late 19th century, Svante Arrhenius calculated the role of CO2 in warming the planet.
The evidence is clear: temperature records, ice core data, and satellite measurements all confirm Earth is warming. The link between rising CO2 and higher temperatures is well-documented, and there’s broad consensus that fossil fuel burning is driving this.
We see the results—more heatwaves, melting glaciers, and shifting ecosystems—all matching scientific predictions. The science of climate change is settled.
What’s uncertain isn’t the past or present but the future trajectory. Climate systems are complex, and predicting specifics—like regional weather patterns or critical temperature thresholds—is harder. That’s true for any modeling of the future, not just climate.
How Can Tiny Amounts of CO2 Drive Major Temperature Changes?
CO2 levels have risen from 280 ppm in 1850 to nearly 400 ppm today, pushing the edge of what’s safe for humans. That’s just 0.04% of the air, so it’s easy to question how something so small could have such a massive effect. But it only takes a tiny shift to upset the balance, and we’re already seeing the consequences.
Take this example: just 2 nanograms of botulinum toxin is enough to kill a person—that’s only 0.00000002% of body weight. It’s not about quantity, it’s about the properties of the substance.
The same is true with CO2. While other atmospheric gases influence weather, not all are greenhouse gases. Greenhouse gases absorb and emit infrared radiation, trapping heat and preventing it from escaping into space. That’s how a small amount of CO2 can have such a big effect on the planet’s climate system.
Doesn’t a Warmer Planet and More CO2 Really Lead to Better Crop Yields?
Rahmstorf points out that the broader effects of climate change—shifting climate zones, extreme weather, and water shortages—will likely outweigh any localized benefits from higher CO2 or longer growing seasons.
Yes, some regions may see a longer growing season, but in tropical and subtropical zones, the picture isn’t so bright. More extreme heat, droughts, and unpredictable rainfall could cut into agricultural productivity.
Rising temperatures bring more severe weather: heatwaves, storms, and floods that can devastate crops and disrupt food production. And it’s not just about CO2. Water is essential for farming, and warmer temperatures mean greater water stress in many areas. Without enough water, any supposed benefit from more CO2 vanishes.
CO2 can indeed boost plant growth—it’s called the CO2 fertilization effect—but this is limited by other factors like water, nutrients, and suitable temperatures. Once those limits are hit, more CO2 won’t keep increasing yields, especially under tougher climate conditions.
Aren’t Volcanoes and Solar Activity Really Driving Climate Change More Than CO2?
No. Volcanoes, solar activity, and orbital shifts do play a role in Earth’s climate, but they don’t explain the rapid warming we’ve seen in recent decades. The evidence overwhelmingly points to increased greenhouse gases—especially CO2—as the main cause of today’s climate change.
Volcanoes do release CO2, but their contribution is minor compared to human activities. Humans emit about 100 times more CO2 than volcanoes. Plus, volcanic eruptions often cool the planet short-term by releasing particles that reflect sunlight away from Earth.
Solar activity also affects climate, but it hasn’t increased in the past 70 years. In fact, it’s slightly decreased, which would actually promote cooling, not warming. The current warming trend can’t be blamed on the sun.
Milankovitch cycles—Earth’s orbital changes—affect climate over tens of thousands of years, not decades. These cycles are far too slow to explain the rapid warming we’ve seen over the last century.
Climate models that include only natural factors like solar activity and volcanic eruptions cannot explain the observed warming. When human factors, particularly greenhouse gas emissions, are included, the models accurately reproduce the observed temperature rise.
The real driver of recent climate change is the rise in greenhouse gases, particularly CO2, from human activities like burning fossil fuels and deforestation. This is backed by extensive research and data, including direct measurements of increased heat trapped by CO2.
How Do We Know Fossil Fuels Are Causing the Current Temperature Rise?
The evidence is clear—fossil fuels are driving the temperature rise. No debate there.
Rahmsdorf explains that burning fossil hydrocarbons releases large amounts of CO2 into the atmosphere, and isotopic analysis confirms that this CO2 has a specific carbon signature tied directly to fossil fuels.
The increase in CO2 lines up almost exactly with the rise in global temperatures over the past century. Climate models that factor in greenhouse gases from fossil fuels match the warming we’ve observed, while models that leave them out don’t come close.
On top of that, Earth’s energy balance, measured by satellites and ground stations, shows that more heat is being trapped, and it’s this excess heat that’s causing the warming trend we’re experiencing today. The link to fossil fuels isn’t speculative—it’s measured.
Doesn’t the Medieval Warm Period Disprove Fossil Fuel-Driven Climate Change?
Climate skeptics often cite the Medieval Warm Period (MWP) as proof that climate change occurs without fossil fuels. However, the MWP isn’t strong evidence against fossil fuel-driven warming.
First, the MWP was regional, not global, mostly affecting Europe and parts of North America, while other areas cooled. Today’s warming is global.
Second, the MWP’s warming was modest compared to the rapid, large-scale rise since the late 19th century, which natural variability can’t explain.
Third, different drivers—solar activity, volcanic eruptions, and ocean currents—caused the MWP, not CO2 emissions.
The MWP reflects that natural variability is a factor, but it doesn’t change the fact that human activities, particularly fossil fuel burning, are driving today’s global warming.
What Are the Worst Effects of Climate Change Likely to Be?
Rahmstorf doesn’t pull any punches when it comes to how bad climate change could get. He lays it out clearly, and the picture isn’t pretty. The risks are enormous, and the consequences could be devastating if we don’t get serious about dealing with them.
Extreme weather will become the new normal, with more frequent and intense heatwaves leading to higher death tolls and widespread health issues. Droughts will hit harder, straining water supplies, agriculture, and fueling wildfires. At the same time, heavier rainfall and flooding will wreck homes, infrastructure, and farmland.
Sea levels will continue to rise, eroding coastlines, flooding low-lying areas permanently, and making storm surges even more destructive. Coastal communities and ecosystems will face enormous risks, displacing millions. Oceans will become more acidic, endangering marine life like shellfish and coral, while warmer waters create deoxygenated “dead zones” where nothing can survive.
Biodiversity will suffer as species struggle to adapt or move, leading to extinctions and ecosystem imbalances. Coral reefs will bleach and collapse, while shifting habitats disrupt the natural order.
Agriculture will take a hit as unpredictable weather reduces crop yields, and pests and diseases expand their reach in warmer climates. Food security will be at risk.
Human health will deteriorate as heatwaves trigger more heat-related illnesses and diseases like malaria and dengue spread to new areas. Water shortages will worsen with changing precipitation patterns and melting glaciers, leaving millions without reliable freshwater.
The economic fallout will be severe—extreme weather and rising seas will damage infrastructure, insurance costs will skyrocket, and some regions may become uninsurable. Resource scarcity will trigger migration and conflict.
The Science Is Settled
Stefan Rahmstorf is clear: CO2 and climate are locked together, and the evidence is undeniable. High CO2 levels in the past made life as we know it impossible, and recent research by Emily Judd stretches this link back 500 million years. The lower CO2 levels we see today were essential for human life and biodiversity to emerge.
Yet, denial remains. People twist the data, pointing to things like volcanoes and solar activity—distractions that don’t account for the rapid warming we’re seeing today. Rahmstorf cuts through the noise: more extreme weather, rising seas, and ecosystem collapse are what lie ahead. The message is simple—human activity is driving climate change, and the effects are clear, measurable, and happening now.
The science is settled. What remains unsettled is its acceptance by those who refuse to confront it. The same, tired, and debunked arguments get dragged out time and again by people who either don’t want to change their behavior or world view, or worse, by those who are paid to deny reality.
Anyone willing to look at the data objectively can see the crisis we’re in. This isn’t new science. CO2’s role in warming has been known since the 1820s with Fourier, and Arrhenius laid out the details in the 1890s. What’s unsettled is not the science but the refusal of some to accept it.
Yes, there are always dissenters when scientific paradigms shift. It was the same with plate tectonics until those voices faded away. Dissent is fine, but in climate science, it’s a rounding error in the face of overwhelming evidence. This isn’t about opinion—it’s about data. The numbers don’t lie.
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ABOUT THE AUTHOR
Art Berman is Director of Labyrinth Consulting Services, Sugar Land, Texas, and a world-renowned energy consultant with expertise based on over 40 years of experience working as a petroleum geologist. Visit his website, Shattering Energy Myths: One Fact at a Time, and learn more about Art here.
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