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Numerous terms are used to describe our home planet, such as the ‘earth’ or the ‘Earth’ or the ‘world’, but a more recent term that is becoming more common is the ‘Earth System’. This rather geeky term refers not just to the physical planet that we inhabit but, more importantly, to the intangible system – the massive ocean and atmospheric circulation systems and the flows of energy among the Earth’s physical components such as the atmosphere, ice, land and oceans that create a single planetary system with its own global-level characteristics.

Think of it this way – the Earth is the hardware and the Earth System is the software.

The Earth System has a long, evolutionary history stretching over 4.5 billion years, characterized by abrupt changes such as the bolide strike about 66 million years ago that led to sudden cooling and the extinction of the non-avian dinosaurs, but the planet has also experienced long periods of relatively little change, such as the ‘boring billion’ between about 1.7 and 0.7 billion years ago. But one critical feature of the long evolution of the Earth System stands out – the critical role of life in shaping the structure and functioning of the Earth System through time.

Life has not been just a passenger on the evolution of our planet but has actively shaped the Earth System just as much as the great geophysical forces like plate tectonics, ice sheet waxing and waning, and occasional bolide strikes.  

The role of life in the operation of the Earth System is not without controversy. In the 1970s, English scientist James Lovelock proposed that life and the non-living environment of Earth continually interact to form a single, self-regulating planetary system that he named Gaia, after the Greek goddess of the Earth. Lovelock argued that through its regulatory processes, Gaia maintains the planetary environment in a stable, resilient state via feedback processes that counteract shocks and disturbances that threaten to destabilize it.

The Gaia hypothesis was hotly debated in the decades after its introduction, with many scientists challenging the so-called ‘strong Gaia hypothesis’ that life would always keep the planet in a stable condition via its network of feedback processes. One of the challengers of the Gaia concept was the late Paul Crutzen, an atmospheric chemist and Nobel Laureate for his research on the chemistry of the stratospheric ozone hole. Crutzen pointed out that it was by sheer luck that chemical engineers used chlorine rather than bromine in the manufacture of CFCs, the chemicals that triggered the ozone hole. Had bromine been used, a massive ozone hole would have been created, covering the entire planet for all seasons, making it uninhabitable for many forms of life. According to Crutzen, we narrowly missed striking a potentially fatal blow to Gaia.

Crutzen didn’t stop with his critique of the Gaia hypothesis. Based on rapidly accumulating evidence of human disruption of many other features of the Earth System, not only the reduction in stratospheric ozone over the South Pole, Crutzen proposed in February 2000 that the Earth had left the 11,700-year-long Holocene, an epoch characterized by a relatively stable and accommodating environment for humans, and had entered a new epoch he called the Anthropocene. Humans had broken through Gaia and were sending the Earth System onto an accelerating trajectory into planetary terra incognita.    

The concept of Gaia and the human-driven destabilization of the planet are based on a growing field of science called complex system theory. ‘Complex’ doesn’t mean ‘complicated’, but rather the term ‘complex system’ refers to systems that can exist in well-defined, well-functioning states or conditions. The human body is a good example of a complex system.

When complex systems are perturbed, feedback mechanisms provide the means to counteract the perturbation and keep the system in its existing state. Our bodies have automatic feedback systems that keep us in a safe and well-functioning condition. For example, when our immediate environment becomes too hot, we sweat, and when the sweat evaporates, it cools our skin, thus counteracting the heat.

The Earth System, too, has feedback mechanisms that keep it in a stable state. For example, when the atmosphere begins to heat up, the Earth’s surface, particularly the oceans, evaporate more water, leading to a cooling of the surface, just as for a human body. Over its long history, the Earth System has evolved an elaborate set of feedback mechanisms to help stabilize it. In fact, the last 11,700 years of stability – the Holocene epoch – have proven to be essential for humans to expand and develop the complex societies we live in today.

However, like any complex system, the stability of the Earth System can be undermined if the destabilizing forces are so powerful and persistent that they overwhelm the stabilizing feedback processes inbuilt in the Earth System. The result is that the system begins to accelerate away from its previous stable state, creating a period of growing instability until it can, sometime in the future, reach another – but perhaps quite different – stable state.

This is precisely what is happening today, with the accelerating trajectory of the Earth System into the Anthropocene. The Anthropocene, however, is not yet a new, stable state of the Earth System. Rather, it is a very rapid trajectory into planetary no-man’s land, with an unknown endpoint sometime in the future. The rates at which the Earth System is being propelled away from stability by human actions are astounding.

The rate at which we are pouring carbon dioxide, the most important greenhouse gas, into the atmosphere is about 100 times greater than the maximum rate at which natural processes emitted carbon dioxide at the end of the last ice age. Global temperature is rising 200 times faster than the background rate of change. These rates of change in fundamental indicators for the state of the Earth System are almost unprecedented in the 4.5-billion-year history of our planet.

As noted above, the Earth System is on an accelerating human-driven trajectory. Where will it end? There is no answer to that yet, as it depends on how we humans react to the immense challenges that now clearly lie ahead. But science can provide some insights into the possibilities.

One potential future is a stabilized state of the Earth System resulting from rapid human actions to take the pressure off the planet – a rapid reduction in emissions of greenhouse gases until they are eliminated by mid-century, and a turn-around in our approach to the natural world around us – a rapid shift from one of exploitation to one of projection and stewardship. Such profound changes in contemporary human societies could indeed lead to a stabilised Earth System later this century, one in which humanity could continue to live, albeit in more challenging conditions.

But there is another, much more ominous, future that lies ahead if we continue on our current trajectory.  At present, the intrinsic feedbacks in the Earth System itself are still working to counteract our human pressures, slowing the trajectory of the system away from the Holocene and trying to stabilize it in a state which resembles the Holocene. An example of a stabilizing feedback is that the oceans and land together are currently absorbing more than half our emissions of carbon dioxide into the atmosphere.

But that ‘free service’ provided by the Earth System will not last forever. Escalating human pressures on the system could turn the feedback processes from friend to foe.

As the Earth heats up, forests are burning more often and permafrost is beginning to melt, emitting carbon dioxide back up into the atmosphere. Atlantic ocean circulation is slowing, changing rainfall patterns and drying the Amazon basin, leading to forest dieback. The melting of polar ice is increasing, pouring more water into the oceans and increasing the rate at which sea levels are rising.

There may lie a point ahead at which these destabilizing Earth System feedbacks become large enough that they overwhelm human efforts to stabilize the system and continue to drive it into ever more dangerous conditions. This could lead to ‘Hothouse Earth’, a much hotter but stable state of the Earth System that could exist for hundreds of thousands of years. But it would be a very hostile environment for humans, almost surely leading to significant drops in population and an end to the civilization that we enjoy today.

How can we turn around this trajectory towards a catastrophic future? Many solutions have been proposed. Some argue that new technologies, such as renewable energy, are the solution. Others point to new types of agriculture that are much less damaging to ecosystems. Others say that we must dig deeper and change the core values that drive our ever-expanding economic systems. The list could go on, but perhaps we need silver buckshot rather than a silver bullet.

One thing, though, is certain. The kind of thinking that has driven us into the Anthropocene is not going to lead us to the solutions.

Read the full Climate Change and Evolution series:

1. Introduction: The Nexus Between Climate Change and Evolution

2. The Anthropocene: A Shock in the Evolutionary History of the Earth System

3. Evolutionary Mismatch, Partisan Politics, and Climate Change: A Tragedy in Three Acts

4. A Climate of Change: To Combat Global Warming, We Need to Break the Law

Background reading:

IPBES (2019) Summary for policymakers of the global assessment report on biodiversity and ecosystem services of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. S. Diaz, J. Settele, E.S. Brondizio, H.T. Ngo, M. Gueze, J. Agard, A. Arneth, P. Balvanera, K.A. Brauman, S.H.M. Butchart, K.M.A., Chan, L.A. Garibaldi, K. Ichii, J. Liu, S.M. Subramanian, G.F. Midgley, P. Miloslavich, Z. Molnar, D. Obura, A. Pfaff, S. Polasky, A. Purvis, J. Razzaque, B. Reyers, R. Roy Chowdhury, Y.J. Shin, I.J. Visseren-Hamakers, K.J. Willis and C.N. Zayas (eds). IPBES Secretariat, Bonn, Germany, 56 pages.

Kaufman, D., McKay, N., Routson, C., Erb, M., Datwyler, C., Sommer, P.S., Heiri, O. and Davis, B. (2020) Holocene global mean surface temperature, a multi-method reconstruction approach. Scientific Data: https://doi.org/10.1038/s41597-020-0530-7

Lear, C.H., Anand, P., Blenkinsop, T., Foster, G.L., Mary Gagen, M., Hoogakker, B., Larter, R.D., Lunt, D.J., I. McCave, N., McClymont, E., Pancost, R.D., Rosalind E.M. Rickaby, R.E.M., Schultz, D.M., Summerhayes, C., Williams, C.J.R. and Zalasiewicz, J., 2020, Geological Society of London Scientific Statement: what the geological record tells us about our present and future climate. Journal of the Geological Society of London 178, https://doi.org/10.1144/jgs2020-239

Lenton, Tim (2016) Earth System Science: A very short introduction. Oxford University Press, 153pp,

Lenton, T.M., Rockström, J., Gaffney, O., Rahmstorf, S., Richardson, K., Steffen, W. and Schellnhuber, H.J. (2019) Climate tipping points – too risky to bet against. Nature 575: 593-596

Steffen, W., Richardson, K., Rockström, J., Schellnhuber, H.J., Dube, O.P., Dutreuil, S., Lenton, T.M. and Lubchenco, J. (2020) The emergence and evolution of Earth System Science. Nature Reviews: Earth and Environment 1:54-63

Steffen, W., Rockström, J., Richardson,, K., Lenton, T.M., Folke, C., Liverman, D., Summerhayes, C.P., Barnosky, A.D, Cornell, S.E., Crucifix, M., Donges, J.F., Fetzer, I., Lade, S.J., Scheffer, M., Winkelmann, R., and Schellnhuber, H.J. (2018) Trajectories of the Earth System in the Anthropocene. Proceedings of the National Academy of Sciences (USA), doi:10.1073/pnas.1810141115

Published On: December 2, 2021

Will Steffen

Will Steffen

Will Steffen is an Earth System scientist. He is an Emeritus Professor at the Australian National University (ANU); Canberra, a Senior Fellow at the Stockholm Resilience Centre, Sweden; and a member of the Anthropocene Working Group. From 1998 to mid-2004, Steffen was Executive Director of the International Geosphere-Biosphere Programme, based in Stockholm. His research interests span a broad range within Earth System science, with an emphasis on sustainability and climate change.

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