Using a complex systems framework, we argue that a set of feedback processes intrinsic to the Earth System could form a planetary threshold which, if crossed, would not only speed up climate change, but also take the trajectory out of human control and propel the system irreversibly to a Hothouse Earth state.
The climate change challenge has fallen into a familiar pattern – more research papers on climate change, another UNFCCC COP (United Nations Framework Convention on Climate Change Conference of the Parties) meeting, more pledges by the world’s governments to do something, continuing rises in greenhouse gas emissions, worsening extreme weather events, reassurances that we can still solve the problem, yet another year of record high temperatures. Rinse and repeat.
Clearly, the nature of the climate change challenge requires a fundamental re-think.
I was one of a group of 16 Earth System scientists from around the world who took a fresh look at what climate change really is and what risks it holds for our future. Our recent paper “Trajectories of the Earth System in the Anthropocene”, published in the Proceedings of the National Academy of Sciences, USA, places climate change in a new framework based on long-term, complex system thinking.
Our paper challenges the almost universally accepted framework that human emissions of greenhouse gases will always be the dominant driver of climate change. That is, the more greenhouse gases we emit, the higher the Earth’s average surface temperature rises. This approach is most evident in the IPCC (Intergovernmental Panel on Climate Change) reports, in which the projected global average temperature out to 2100 scales approximately with the level of human emissions of greenhouse gases.
This rather linear view of the climate system – or more appropriately the Earth System – may be fundamentally flawed.
Our article offers an alternate framework. We argue that, far from being a generally linear, well-behaved system, the Earth System is more likely to behave as a ‘complex system’, with well-defined states and transitions between them driven largely by feedback processes internal to the system, not only by drivers external to the system. In essence, the external drivers act as triggers that initially drive the system away from a stable state but then intrinsic feedback processes become the dominant drivers of system change until a new stability domain is reached.
This framework argues that our current, very high human emissions of greenhouse gases can act as such a trigger of system change. Already there is evidence that the Earth System has been pushed out of the relatively stable Holocene state in which it has existed for the past 12,000 years. This is only state of the Earth System in which homo sapiens has developed agriculture, settlements and the complex societies we live in today.
Even more worrying is that several important feedback processes within the system are showing signs of instability as result of the human-driven temperature rise. Examples include melting of Arctic summer sea ice that accelerates warming in the north; increasing wildfires in the boreal forests and Amazon rainforest that release more carbon dioxide to the atmosphere; and the onset of melting in some permafrost regions in Siberia, which could release vast amounts of methane, a potent greenhouse gas, to the atmosphere. In fact, Arctic summer sea ice melt may have already crossed a tipping point leading to a complete loss of summer ice within a couple of decades.
Our analysis points to a risk that if enough of these feedback processes are activated, they could act like a row of dominoes that would form a global cascade. Ultimately, such a feedback cascade could take the trajectory of the Earth System out of human control and irreversibly far away from the Holocene state, and indeed towards states that were last seen millions of years ago. We call this possible future a ‘Hothouse Earth’ trajectory, with temperatures of 4 or 5°C above pre-industrial. Such conditions would have massive impacts on humans, particularly the most vulnerable, and could perhaps even threaten the viability of contemporary civilisation itself.
Critics may argue that our analysis lacks credibility. However, there is considerable evidence to suggest our complex systems framework is more realistic than the dominant linear one. Here are three such lines of evidence.
First, the Earth System has behaved as a complex system in the past 800,000 years at least, and certainly during the entire period that fully modern humans have been on the planet. The regular oscillations between ice ages and warm periods – like our current warm period, the Holocene – comprise a good example. The heavy lifting in the transitions between these states is done by feedback processes within the Earth System, not by the external forcing due to small changes in Earth’s orbit. These orbital changes acted as the triggers for much bigger, internally driven changes.
Second, not only do we understand the nature of several of these feedback processes that have operated in the past, we are beginning to see signs of instabilities in some of these processes today, as noted above. Most worrying are the increasing wildfires around the world that are releasing large amounts of carbon to the atmosphere and the loss of ice from the continental glaciers and polar ice sheets and surrounding seas. The decrease in ice is reducing the reflectivity of the Earth’s surface and thus accelerating warming, and the growing instability of land-based and grounded marine ice sheets threatens to lock in metres of sea-level rise over coming centuries.
Third, we know that the Earth System has existed in stable, much hotter conditions in the very recent geological past. For example, the current atmospheric carbon dioxide concentration of over 400 ppm (parts per million) was last seen in the mid-Pliocene, about 3-4 million years ago, when the temperature was 2-3°C hotter than pre-industrial and sea levels were 10-22 metres higher. If we push carbon dioxide concentrations to 450-500 ppm, entirely possible this century, we may commit the Earth System to mid-Miocene conditions, about 15-17 millions years ago, when temperatures were 4-5°C above pre-industrial and sea level was likely 20-30 metres above present levels.
We can’t yet put specific probabilities on the risk that a feedback cascade might propel us into a Hothouse Earth. That requires more research. But we know enough about the nature of the Earth System that this risk needs to be taken very seriously.
There is likely to be much more at stake in transgressing the Paris climate targets than just a slightly warmer planet. It may mean the difference between a planet we can live and one we can’t.
Will Steffen in an Earth System scientist. He is a Councillor with the Climate Council of Australia, an Emeritus Professor at the Australian National University and a Senior Fellow at the Stockholm Resilience Centre.
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),http://www.pnas.org/content/early/2018/07/31/1810141115
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