A Deep Time Perspective on Climate Change

Earth’s history over the last 4.5 billion years—from volcanic upheavals and drifting continents to the evolution of climate—has been shaped by a simple thermodynamic reality: Earth is hot and space is cold. Like every hot object, our planet continuously loses heat as it moves toward equilibrium. The atmosphere, oceans, continents and carbon cycle are not separate stories but different expressions of that long process.

Earth’s core is nearly 6,000°C while the background temperature of space is only a few degrees above absolute zero. That enormous thermal gradient powers mantle convection, which drives plate tectonics. Plate tectonics, in turn, reshapes continents and ocean basins, influences volcanism and weathering, and regulates the long-term carbon cycle. Ocean and atmospheric circulation redistribute heat around the globe. Greenhouse gases affect how efficiently heat escapes to space. Water vapor responds to temperature. Methane responds to ecosystems that themselves respond to climate. Milankovitch cycles change the geographic and seasonal distribution of incoming solar energy. Each operates differently and on different timescales, but all participate in the movement and transformation of energy through the Earth system.

Climate is the evolving expression of these interacting processes. Most climate stories begin in the present and project forward to a future of increasing risk. That’s an important story, but it’s like beginning a book with the second-to-last chapter. I’d rather start at the beginning.

Figure 1 shows a reconstruction of global mean surface temperature over the last 485 million years—nearly the entire history of complex animal life. The most striking observation is that Earth has spent far more time in climates warmer than today’s than in climates resembling the present. The only comparably cold interval occurred more than 275 million years ago.

The Phanerozoic temperature record is not a story of progressive cooling. It includes at least two major cooling episodes separated by a prolonged return to very warm conditions. Earth cooled through the late Paleozoic, reaching cold conditions during the Carboniferous and Permian, before warming again through much of the Mesozoic. Temperature eventually reached a maximum during the mid-Cretaceous about 94 million years ago. A second great cooling trend followed, extending toward the present but interrupted by episodes of abrupt warming, including the Paleocene-Eocene Thermal Maximum about 56 million years ago.

Ancient life evolved across this extraordinary range of temperatures. Humans arrived only near the extreme right edge of Figure 1, during the cool and highly variable Pleistocene. The final point in the reconstruction is about 5,800 years ago, approximately coincident with the emergence of the earliest civilizations. The dashed line marks the pre-industrial average temperature around which civilization subsequently developed and expanded. Human civilization, in other words, arose during an unusually cool chapter of Earth’s history.

Broadly speaking, the Phanerozoic alternated among three climate regimes: a warm or hothouse Earth, a cold or icehouse Earth, and intermediate conditions. Hothouse worlds lacked permanent polar ice and had much higher global temperatures. Icehouse worlds supported large continental ice sheets and, within them, repeated glacial and interglacial cycles. Intermediate climates occupied the territory between those states.

The differences were enormous. Hothouse conditions averaged more than 25°C globally, compared with about 14°C during coldhouse conditions. Earth occupied a hothouse state for nearly 60% of the last 485 million years, while icehouse conditions accounted for only about 11%.

An obvious question is how we can possibly know any of this. There were no thermometers for almost all of human history, and there weren’t even humans for almost all of the period shown in Figure 1.

We know far more about ancient climate than the absence of thermometers might suggest. Judd and colleagues assembled nearly 120,000 carefully screened temperature estimates from geological and fossil evidence spanning the Phanerozoic. They reconstructed where those samples were located on the ancient Earth and combined them with climate models using sophisticated data-assimilation methods similar to those used in modern meteorology.

The result is not a 485-million-year thermometer record, nor does it pretend to be one. It is a reconstruction built from an extraordinary body of evidence, and uncertainty increases as we move deeper into geological time. Different methods disagree about some absolute temperatures, particularly during very warm periods, but the broad succession of warmer and colder climate states is considerably more secure. We cannot know every detail of Earth’s past, but we know its broad outlines remarkably well.

Another obvious question raised by Figure 1 is what caused global temperature to rise and fall by more than 20°C across the Phanerozoic. To answer that, it helps to begin with a more fundamental question: what conditions made life on Earth possible in the first place?

That is a vast subject I will not attempt to cover comprehensively, but two essential ingredients are clear: liquid water and carbon. Carbon is abundant in Earth’s rocks and circulates among the solid Earth, oceans, atmosphere and living things. Water presented a different puzzle.

In the 1820s, Joseph Fourier recognized that Earth should be considerably colder than it actually is based on its distance from the Sun. In modern terms, Earth’s effective radiating temperature is about –18°C (0°F), compared with an average surface temperature near 15°C (59°F)—a difference of roughly 33°C.

A few decades later, John Tyndall supplied a crucial part of the explanation. His experiments showed that gases present in small concentrations—most importantly water vapor and carbon dioxide—strongly absorb infrared radiation. They help keep Earth’s surface warm enough for liquid water and therefore life.

This brings us back to carbon. Over geological time, that carbon cycle has become one of the principal regulators of Earth’s temperature. Understanding the enormous temperature changes in Figure 1 therefore begins not with CO₂ alone, but with the geological processes that control the movement of carbon through the Earth system.

The conventional way to think about the long-term carbon cycle is as a balance between geological sources and sinks. Much of the carbon mobilized at Earth’s surface comes from the weathering and erosion of carbon-bearing rocks, especially carbonate rocks. Carbon also moves through silicate weathering, where atmospheric CO₂ dissolved in rainwater forms weak carbonic acid that reacts with exposed rock.

Rivers carry the resulting dissolved material to the oceans, where carbon is incorporated into marine sediments that eventually become limestone and dolomite. Over geological time, these reactions transfer carbon among the continents, oceans, atmosphere and solid Earth. Silicate weathering is especially important because it consumes atmospheric CO₂ and generally accelerates under warmer, wetter conditions, providing a powerful negative feedback—a kind of long-term planetary thermostat.

But the other half of that thermostat is geological. Carbon buried in marine sediments and carbonate rocks does not simply disappear. Plate tectonics carries some of it back into Earth’s interior at subduction zones, where heating and metamorphism can release it again. Volcanoes, continental rifts, mid-ocean ridges, mantle plumes and magmatic activity at plate boundaries all return carbon to the atmosphere and oceans. Mountain building simultaneously exposes enormous volumes of fresh rock to weathering, strengthening the processes that remove CO₂. Tectonics therefore operates on both sides of the carbon cycle, continually altering its sources and sinks.

The balance can change substantially during periods of major tectonic reorganization. Continental breakup, large igneous provinces, changing rates of seafloor spreading, mountain building and the opening and closing of ocean basins can alter carbon degassing, weathering and burial for millions of years. Short-lived volcanic aerosols may temporarily cool the climate, but they operate on a vastly shorter timescale than the CO₂ released by sustained geological activity.

Life eventually became part of this geological apparatus. The spread of land plants during the Silurian and especially the Devonian transformed weathering by sending roots into rock and soil and altering water and chemical flows through the continents. Photosynthesis also transferred atmospheric CO₂ into organic matter, some of which escaped decomposition and was buried in sediments. Life was no longer simply adapting to Earth’s carbon cycle; it had become one of the processes changing it.

Figure 3 shows that temperature and CO₂ closely tracked one another through much of the Phanerozoic. Over the last 45 million years, the relationship is especially strong, with an r² of 0.97. From the Ordovician through the Triassic, the correlation was also robust at 0.74. But from about 166 to 52 million years ago, it fell to just 0.01—a discrepancy Emily Judd calls the “Mesozoic Conundrum.” This interval coincided with the breakup of the Pangea Supercontinent, a profound reorganization of the land and oceanic configuration. CO₂ remained an essential climate regulator, but it was operating within an Earth system whose boundary conditions and feedbacks were themselves changing.

The role of carbon in Earth’s climate history is hardly a new idea. Its importance became increasingly clear through the work of Fourier, Tyndall, Arrhenius and those who followed. Plate tectonics, by contrast, is a much more recent advance. Only in the last several decades have we begun to integrate the movement of continents, changing ocean basins, volcanism, weathering and the carbon cycle into a more complete picture of Earth’s climate history.

The broader point is that a deep-time perspective provides essential context for understanding and evaluating climate change today. We rightly worry about the consequences of 1.5 to 2°C of warming, yet Earth has experienced climate states many degrees warmer than the present. Life survived those worlds and adapted to extraordinary changes over geological time.

But that is not the relevant standard for us. Earth will continue to dissipate heat and reorganize around changing conditions as it always has. The question is not how much climate change Earth or life can survive, but how much change a complex human society and its infrastructure—built during an unusually cool and relatively narrow range of climate conditions—can accommodate.

Earth has adapted to enormous changes before. Whether civilization can adapt to rapid change on human timescales is another matter.