Epochs, Urgency, and Understanding: Why Today's Climate Change Demands a Different Perspective

Anthropocene Acceleration and Climate Risk

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A Response to Mr. Guy DeWhitney's article;

Introduction: Addressing Mr. DeWhitney's Timescale Argument

Observations, such as those likely presented by Mr. Guy DeWhitney, highlighting Earth's long history of profound climatic shifts and fluctuating atmospheric carbon dioxide (CO2) levels, possess an intuitive appeal. It is indeed a fact that our planet has witnessed CO2 concentrations far exceeding those of today and has transitioned between hothouse states and widespread glaciations. However, to draw from this deep history the conclusion that current climatic changes are merely another chapter in an old story, and therefore not of unique concern, is to overlook critical distinctions – most notably, the rate of contemporary change and the adaptive capacity of present-day ecosystems and human civilization.

The "climate has always changed" narrative, while factually correct in its basic premise, can inadvertently become a pathway to disengagement from the realities of the current climate situation. This response aims not to dispute the rich tapestry of Earth's geological climate history, but rather to illuminate why superficial comparisons between past and present can obscure the true nature and pressing urgency of the climate crisis unfolding today. Paleoclimatology, the study of past climates, does indeed teach us about the Earth's climate system's vast dynamic range. Yet, it also provides stark warnings when current events are placed in their proper, rapid context. The fundamental difference in how geologists might view Earth's history, characterized by long, slow processes, versus how climate scientists assessing current risk must view it—focused on rapid, impactful changes on human-relevant timescales—needs careful consideration. This article endeavors to bridge this conceptual gap, fostering an appreciation for geological history while underscoring the unprecedented nature of the challenges we now face.

Earth's Deep Climate Past – A World of Gradual Transformations

The Earth's climatic history, stretching back over eons, is a chronicle of immense transformations driven by natural forces operating over millions of years. These gradual shifts allowed life to adapt, evolve, or, in some cases, be replaced.

Atmospheric CO2 concentrations have varied dramatically. During the Cambrian period, about 500 million years ago (Mya), CO2 levels were as high as 4,000 parts per million (ppm).¹ The Devonian period (around 400 Mya) saw CO2 peak at approximately 2,000 ppm, with another significant peak occurring in the Triassic period (220–200 Mya).¹ These ancient "greenhouse" periods contrast sharply with "icehouse" times, such as the Quaternary glaciations of the last two million years, when CO2 concentrations dipped as low as 180 ppm.¹ For a significant portion of the Phanerozoic Eon (the last ~540 million years), CO2 levels were generally higher than today, often four to six times current concentrations during the Mesozoic era, and ten to fifteen times during the early Paleozoic era.¹ However, more recent research, leveraging an expanded proxy database, suggests that for most of the Phanerozoic, from the Devonian onwards, CO2 concentrations may have been less than 1,000 ppm, coinciding with the global proliferation of forests.⁴ This evolving understanding underscores the importance of continuous scientific inquiry.

These ancient climatic shifts were driven by a variety of natural factors. The geological carbon cycle involves the slow release of CO2 into the atmosphere through volcanic outgassing and its gradual removal through processes like the formation of carbonate rocks and the burial of organic matter in peat bogs and swamps.¹ These sequestration processes can lock away carbon for immense periods, sometimes exceeding 70,000 years before its eventual release.⁵ The evolution of life itself played a role; the spread of land plants during the late Devonian period, for example, is thought to have significantly reduced atmospheric CO2 concentrations.¹ This was a profound biological feedback, but one that unfolded over geological timescales.

Crucially, any comparison of past CO2 levels with the present must account for changes in solar luminosity. The Sun's output was lower in the distant past. During the Late Ordovician period, for instance, solar luminosity was approximately 4% less than it is today.⁷ This means that a higher CO2 concentration would have been required to maintain global temperatures comparable to those achieved with lower CO2 levels under a more luminous Sun. For example, if a CO2 threshold for initiating widespread glaciation today is around 500 ppm, the equivalent threshold during the Late Ordovician, with its fainter sun, might have been closer to 3,000 ppm.⁷ Thus, simply pointing to high absolute CO2 values in the deep past without this solar context is an oversimplification; 4,000 ppm CO2 during the Cambrian did not exert the same warming influence as 4,000 ppm CO2 would today.

The Earth's climate system possesses powerful natural feedback mechanisms that regulate CO2 over geological time, but these operate on scales far too slow to counteract the current rapid anthropogenic pulse. The formation of major ice sheets, such as the Antarctic ice sheet which began to form around 34 Mya, provides a clear example of the climate's sensitivity to CO2. This glaciation event was associated with a drop in atmospheric CO2 to about 760 ppm, with a critical tipping point for significant ice sheet expansion identified around 600 ppm.¹ This historical event underscores the potent role of CO2 as a greenhouse gas, demonstrating that decreases lead to cooling and glaciation, and by implication, increases lead to warming and deglaciation. These grand transformations, while immense, typically occurred over millions of years, allowing for the gradual evolutionary adaptation of species and the slow restructuring of ecosystems.

The Anthropocene Acceleration – An Unprecedented Rate of Change

The defining characteristic of the current era of climate change is not merely the absolute level of atmospheric CO2, but its extraordinarily rapid rate of increase, overwhelmingly driven by human activities. This acceleration has no parallel in at least the last 800,000 years, and likely much longer.

For approximately 800,000 years leading up to the mid-18th century, atmospheric CO2 concentrations oscillated naturally between about 180 ppm during glacial periods and 280 ppm during interglacial periods.¹ Since the dawn of the Industrial Revolution, this relative stability has been shattered. Atmospheric CO2 has surged by over 50%, from approximately 280 ppm to 427 ppm as of 2024.¹ This concentration is the highest in at least 800,000 years ⁸, very likely the highest in the last 3 to 5 million years ⁸, and potentially the highest in the last 14 million years.¹

The rate of this increase is what truly sets the current period apart. During natural transitions from a glacial to an interglacial period over the past 800,000 years, CO2 levels typically rose by about 100 ppm over a span of roughly 10,000 to 20,000 years.³ This translates to an average rate of increase of approximately 0.005 to 0.01 ppm per year. In stark contrast, atmospheric CO2 has risen by a comparable 100 ppm in merely the last 60 to 65 years (since roughly 1960) ³, an average rate of over 1.6 ppm per year. Current annual increases can be even higher, often exceeding 2 ppm per year. Analysis of ice core data and direct atmospheric measurements indicates that the current rate of CO2 increase due to human activities is approximately 250 times faster than the rate at which it rose from natural sources after the last Ice Age.⁹ The associated global warming is occurring roughly 10 times faster than the average rate of warming observed during the recovery from past ice ages.⁹

The source of this rapid increase is unequivocal: human activity. The overwhelming majority of this excess CO2 comes from the burning of fossil fuels for energy, cement production, and changes in land use such as deforestation.¹ Human-caused CO2 emissions are staggering; in 2015, for example, human activities released roughly 40 billion metric tons of CO2 into the atmosphere.¹¹ These emissions vastly dwarf those from all natural geological sources combined. For instance, global volcanic CO2 emissions are estimated to be around 0.6 billion metric tons per year, or less than 1 billion metric tons annually.¹⁰ This means human emissions are at least 60 times greater than those from all volcanoes on Earth each year.¹¹ The argument that current CO2 rise is primarily due to natural geological activity is therefore untenable; human activity has become a dominant force in the global carbon cycle, a hallmark of the proposed geological epoch known as the Anthropocene.

The following table provides a direct comparison of these rates, underscoring the unique character of the present-day increase:

Period	Approximate CO2 Change (ppm)	Timescale (years)	Average Rate (ppm/year)	Source Examples
Glacial-Interglacial (avg. natural)	~100	~10,000-20,000	~0.005 – 0.01	3
Post-Last Ice Age (natural warming)	(Rate implied relative to current)	(Thousands of years)	(Implied ~0.008-0.012 if current is 250x faster at ~2-3 ppm/yr)	9
Post-Industrial (approx. 1960-Present)	~100	~60-65	~1.6+	1
Current Annual Rate (approx.)	~2-3	1	~2-3	(Consistent with 1)

Table 1: Comparative Rates of Atmospheric CO2 Increase

This dramatic acceleration represents more than just an incremental step; it is a fundamental shock to the Earth's carbon cycle. Natural systems possess adaptive and buffering capacities, but these operate on much slower timescales. A rate of change that is orders of magnitude faster, as illustrated, overwhelms these natural capacities. The iconic Keeling Curve, which has tracked atmospheric CO2 concentrations at Mauna Loa Observatory since 1958, provides a continuous, real-time measurement of this ongoing anthropogenic experiment.¹ Its relentless upward trajectory, superimposed on the seasonal oscillations of Earth's biosphere, is a powerful visual testament to the accumulating burden of human-emitted CO2 in the atmosphere.

Why This Rapid Change Matters: Impacts on a Human Timescale

The rapid, human-caused increase in atmospheric CO2 and the associated global warming are of critical concern because they are already inflicting severe and widespread impacts on ecosystems and human societies. These systems are adapted to the relatively stable climate of the Holocene epoch (the last ~11,700 years), and their capacity to adjust to such swift alterations is profoundly limited.

There is an overwhelming scientific consensus that the Earth is warming and that human activities are the primary driver of this warming.¹³ The Intergovernmental Panel on Climate Change (IPCC) has stated that it is "extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by human activity".¹⁵ In fact, the best estimate suggests that the human contribution to warming over this period was approximately 110% of the observed warming, as natural factors alone would likely have caused a slight cooling.¹⁵ The likely range of human contribution to the observed 1951–2010 temperature change is 93%–123%.¹⁶

The consequences of this rapid change are already observable and are projected to intensify:

Ocean Acidification: The world's oceans have absorbed between 20% and 30% of the total anthropogenic CO2 emissions in recent decades.⁹ This absorption has led to a decrease in ocean surface pH by approximately 0.1 units since around 1850, which corresponds to a 26% increase in acidity.¹⁷ This rate of acidification is roughly 10 times faster than any experienced in the last 55 million years.¹⁷ This chemical change directly impacts marine calcifying organisms, such as corals, shellfish, and some plankton, by making it more difficult for them to build their shells and skeletons from calcium carbonate. Existing shells can also become vulnerable to dissolution.³ This is a direct chemical consequence of increased atmospheric CO2, distinct from its warming effect, creating a dual stressor for marine life already contending with rising water temperatures, which also reduce oxygen solubility.¹⁹
Sea Level Rise: Global average sea level has risen by 20–25 cm since 1900, with about half of that increase occurring since 1980.¹³ This rise is occurring at the fastest rate in "at least the last 3000 years" ¹³ and is driven by the thermal expansion of warming ocean water and the melting of glaciers and continental ice sheets.⁹
Extreme Weather Events: There is clear evidence of an increase in the frequency and intensity of many extreme weather events. This includes more frequent and intense heatwaves, heavy precipitation events leading to flooding, more severe droughts in some regions, and more powerful storms.³ As temperatures rise, more moisture evaporates from land and water surfaces, providing additional energy and moisture for storms.²⁰
Biodiversity Loss and Ecosystem Disruption: Climate change poses a significant threat to the survival of countless species on land and in the oceans, with risks escalating as temperatures climb.²⁰ The world is estimated to be losing species at a rate 1,000 times greater than natural background rates, with as many as one million species currently at risk of extinction within the next few decades.²⁰ Climate change currently affects at least 10,967 species on the IUCN Red List of Threatened Species™.²² These impacts create cascading failures within ecosystems; for example, the decline of krill in the Arctic Ocean due to sea-ice melt and ocean acidification threatens whales, penguins, and seals that depend on them as a primary food source.²² The concept of "climate velocity"—the speed at which climate zones are shifting across landscapes and in oceans—is critical. If this velocity exceeds the rate at which species can migrate or adapt, local extinctions and ecosystem reorganizations become inevitable.²⁰
Threats to Food Security: Changes in climate and the increased frequency of extreme weather events are contributing to a global rise in hunger and malnutrition.²⁰ Fisheries are threatened by warming waters and ocean acidification, while crops face challenges from droughts, heat stress, and shifting patterns of pests and diseases. Livestock productivity is also affected.¹⁹
Human Health Risks: Climate change has been identified as the single biggest health threat facing humanity.²⁰ Impacts include heat-related illnesses and deaths, the expansion of ranges for vector-borne diseases, respiratory illnesses exacerbated by air pollution and wildfires, malnutrition from disrupted food supplies, and mental health impacts associated with displacement and disasters.²⁰

The rapid pace of these changes is crucial because it severely limits the adaptive capacity of both natural ecosystems and human societies.³ Evolutionary adaptation in species typically occurs over much longer timescales. Human infrastructure, agricultural practices, and water management systems are all finely tuned to the climatic conditions of the recent past. Rapid alterations impose immense adaptation costs and challenges. Arguments that higher CO2 is simply "plant food" overlook the complexities. While plants require CO2 for photosynthesis (with a minimum threshold around 150 ppm for some C3 plants to successfully complete their life cycles ²³), the current issue is not a deficit but a rapidly accumulating, destabilizing surplus. Indeed, some plant types, such as C4 plants which evolved in lower CO2 environments, can be disadvantaged at very high CO2 levels relative to C3 plants, potentially shifting competitive balances in ecosystems and agriculture.²⁴

The Climate Scientist's Lens: Focusing on Relevant Risks

Climate scientists emphasize current, rapid changes because these pose immediate and severe threats to human civilization and contemporary ecosystems. These systems are not adapted to the conditions of deep geological time, nor are they equipped to handle the unprecedented speed of current alterations.

While Earth's climate system is inherently dynamic, as evidenced by its geological past, the drivers and rates of those ancient changes were fundamentally different from what is observed today. Paleoclimatology provides invaluable insights into climate sensitivity—for instance, how much global temperature tends to change in response to a given increase in CO2. The formation of the Antarctic ice sheet when CO2 levels fell below a threshold of roughly 600-760 ppm serves as a powerful illustration of this sensitivity.¹ The Phanerozoic Eon witnessed dramatic climatic shifts, but these occurred when life forms, the configuration of continents, and even the Sun's energy output were vastly different from today.⁷ Ecosystems of those ancient times co-evolved with these gradual changes.

Consequently, using periods of exceptionally high CO2 in the deep past, such as the Cambrian with its 4,000 ppm CO2 levels ¹, to imply that current or projected CO2 concentrations are inherently safe is a misleading comparison. Human civilization, modern agriculture, and current global biodiversity evolved and are intricately adapted to the relatively stable, lower-CO2 conditions of the Holocene. During the Cambrian, the sun was fainter, complex land plants had not yet evolved, and the Earth's biosphere was populated by organisms vastly different from those present today. Even looking at a more recent past, about 3 to 5 million years ago (the Pliocene epoch), when atmospheric CO2 concentrations last approached 400 ppm, global average surface temperatures are estimated to have been about 2 to 3.5°C higher than in the pre-industrial period, and sea levels were substantially elevated.⁸ Such conditions, were they to manifest today, would be catastrophic for modern human society and its coastal infrastructure.

The stability of the Holocene climate was a critical enabling factor for the development of agriculture and complex human societies. Our cities, agricultural zones, water resource management systems, and coastal settlements are all predicated on this period of relative climatic calm. The current rapid changes are pushing the climate system out of this Holocene envelope at a speed that makes adaptation extremely difficult and costly. Therefore, the "relevant risk" is not merely to an abstract concept of "nature," but to the specific environmental conditions that have underpinned human flourishing.

Climate science, particularly when it informs policy and risk assessment, must prioritize threats to current human well-being, infrastructure, food and water security, and the contemporary ecosystems upon which humanity depends. These are fundamentally threatened by changes occurring over decades and centuries, not millennia or eons. The overriding factor for concern is the unprecedented rate of anthropogenic CO2 increase and the resultant warming ³, as this rate overwhelms both natural and societal adaptive capacities.¹⁹ The evolution of different photosynthetic pathways in plants, such as C3 and C4 mechanisms, in response to changing CO2 levels over millions of years ¹, further highlights this timescale mismatch. Natural biological adaptation processes that unfolded over eons are now being confronted with dramatic shifts in a fundamental atmospheric resource within mere centuries, creating an evolutionary whiplash for plant communities and agricultural systems. Paleoclimate data, rather than offering reassurance, actually underscores the potential severity of current changes by revealing how sensitive the Earth system is to greenhouse gas forcings and by documenting the magnitude of past environmental shifts (e.g., sea levels at least 60 metres higher when CO2 may have reached 1,000 ppm around 50 Mya ⁸) associated with different CO2 regimes.

Conclusion: Reconciling Geological Time with Present-Day Urgency

The Earth's climate has indeed always been a dynamic system, subject to profound changes over geological timescales. However, the human-driven climatic changes occurring today are distinguished by their unprecedented rate and attribution, setting them apart from the gradual shifts of the deep past. This rapidity is profoundly dangerous for contemporary life, which is adapted to the far more stable conditions of the recent Holocene epoch.

Understanding Earth's deep climate history enriches our perspective and provides vital lessons about the climate system's sensitivity to forcings like greenhouse gases. It does not, however, offer a reprieve from the responsibilities of addressing the present crisis. The Earth system will, undoubtedly, eventually find a new equilibrium in response to current perturbations. But the transition path, driven by such rapid anthropogenic forcing, is fraught with severe risks and disruptions for current ecosystems and human societies. The pertinent argument is not that "Earth has never seen this much CO2" – it has, in very distant, vastly different geological eras.¹ Rather, the crucial point is that "Earth has not experienced such a rapid increase to these CO2 levels in millions of years, a period during which current life forms, including human civilization, evolved and adapted to very different atmospheric and climatic conditions".⁸

An informed discourse on climate change must acknowledge the overwhelming scientific consensus on its anthropogenic nature and focus on mitigating the acute risks posed by these rapid alterations to the climate system upon which we all depend. The ultimate "epochal view," informed by paleoclimatology, should lead not to complacency but to a profound respect for the power of the Earth's systems and a cautious awareness of humanity's newfound capacity to trigger state shifts with long-lasting, and potentially irreversible, consequences on human timescales. Effective climate communication must continue to bridge the gap between the abstract concept of geological time and the concrete, lived realities of climate impacts, ensuring that scientific understanding translates into meaningful action for the well-being of current and future generations.