Earth, Life, and Time: What is Natural?

Life on Earth has always altered its environment.1 “Deep” time records major shifts in atmospheric composition, for example, as life evolved photosynthesis, leading to a massive transfer of CO2 from the atmosphere to the biosphere. Given this, is the large-scale injection into the atmosphere of anthropogenically released carbon a completely natural consequence of biotic activity?

Atmospheric levels of CO2 have always changed, attributable in part to life — and well before the influence of humans. On geologic time scales such fluctuation results from earth processes that either consume or emit CO2, such as sustained volcanism (emits), rock weathering (consumes), formation of coal and oil (consumes), and the evolution of photosynthesis (consumes). These well-recognized processes form the “adagio” movement of Earth’s carbon cycle, advancing on timescales of thousands to millions of years.

Other processes force CO2 emission or consumption at a more humanly pace. With each respiration, for example, we (and all animals) emit CO2 previously captured by a plant during photosynthesis days or months before our consumption. This is the “allegro” movement of Earth’s carbon cycle.

Fossil fuels represent carbon captured initially by plants (photosynthesis) as part of the short-term carbon cycle, but ultimately stored “permanently” into rocks by the long-term carbon cycle, over millions of years. When combusted, this carbon sequestered over millions of years is released geologically instantaneously. The adagio and allegro of Earth’s carbon cycle are juxtaposed— short-circuiting the cycle.

So the question is— is this natural? Because humans are of course natural. The issue is one of time—or more specifically, rates. We are injecting into the atmosphere carbon sequestered over millions of years. This produces a change in atmospheric composition, causing warming. Climate warming forced by CO2 has occurred in Earth history, but not at the rates we are currently experiencing. Earth can scrub anthropogenically produced carbon from the atmosphere, but these are the adagio movements, requiring several thousand to a few tens of thousands of years or so. The long-term carbon cycle largely regulates Earth’s thermostat, but the dial turns slowly.

Earth’s deep-time history records several revolutions of life; mass extinctions occurred, and life rebounded—changed, but resilient. The “Big Five”— the largest extinctions of life known—were all responses to unusual climate changes and shifts in atmospheric and oceanic chemistry. In some cases such changes were forced by intensive and sustained volcanic activity, and in at least one case by a catastrophic extraterrestrial impact.  Earth evolves. Life evolves. Earth forces life evolution, and life forces Earth evolution.

But evolution and extinction are also functions of time. All of the “Big Five” except perhaps the impact are thought to have occurred over tens of thousands to a couple million years. None were caused by life altering its environment geologically instantaneously. Are we in the midst of a 6th mass extinction? Barnosky (2011) suggested that, if our current extinction rates persist, then we will see a projected 75% species loss globally within a few centuries.2 This represents a rate of extinction that exceeds all but the impact-generated event, and caused by the activities of life itself—an event without precedent in Earth history.

The evolution of life has altered the environment for life before—oxygenation of Earth’s atmosphere in response to the rise of photosynthesis made conditions intolerable for anaerobic bacteria exposed to the atmosphere, for example. But the slow rate of change enabled further adaptation, evolution and migration. If it is true that life is altering its environment so rapidly that it threatens its own survival and the survival of many other life forms, if the allegro pace of environmental change prevents adaptation to that change, then it ceases to matter whether the forcing is “natural” or not.  It is a moral question. When faced with other moral questions, humanity has risen to the challenge. But the rise, at this point, must be an allegro movement.


  1. See Zev Trachtenberg’s post on niche construction theory
  2. Barnosky, A.D., Matzke, N., Tomiya, S., Guinevere, O., Wogan, U., Swartz, B., Quental, T.B., Marshall, C., McGuire, J.L., Lindsey, E.L., Maguire, K.C., Mersey, B., and Ferrer, E.A. 2011. “Has the Earth’s sixth mass extinction already arrived?” Nature, v. 471, p. 51-57. 
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2 thoughts on “Earth, Life, and Time: What is Natural?

  1. This is a great post, Lynn—helpful to me as an environmental anthropologist with limited firsthand knowledge of how these processes work and perfect for students (and others) who might be misled by cynical narratives about how climate change is “natural and inevitable.” I like your notion of an “allegro movement” response, and I hope that your future posts will give us a sense of what, in your mind, that should entail. For example, how do we allocate resources in terms of adaptation and mitigation? Does it even make sense to speak of adapting to changes that are happening at a rate unprecedented in the history of the planet? Or does the fact that this is a result of human technologies suggest that there might also technologies that can enable adaptation on a planetary scale? These are huge questions that go beyond any one person’s expertise, but I would still be keen to know your thoughts on them since you have such a comprehensive understanding of how these processes unfold at the macro scale.

  2. I just love the adagio/allegro comparison as well–and perhaps what’s called for now is even more of a presto response! But I also appreciate your observation that it does not matter whether anthropogenic climate forcing is natural or not. I take it that you mean that its being natural would not excuse it (or, more precisely, excuse our not responding to it). Natural or not it is a fact that must be dealt with. I think you express extremely clearly the idea that the “moral” and the “natural” are strictly separate conceptual domains. Whether or not they are is one of the basic philosophical questions that underlies much of what we are exploring in this blog–for example in the exchange on my creation myth post. I am working hard to think of a way of characterizing the way “nature” might be thought of as constraining “morality,” without fully determining it–I expect this issue will come up again here.

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