The evidence of our rapidly changing climate – and the Earth System as a whole – is literally raining down around us. Over the past few years, it has been almost impossible to ignore the growing list of tropical storms and severe precipitation events that have devastated communities across the globe. Mother Nature has spared neither modern metropolises (e.g. Tropical Storm Harvey, around Houston USA, August 2017), nor more vulnerable parts of the developing world (e.g. cyclone Idai in south-east Africa, March 2019). As scientists, we greet these human and natural catastrophes with sadness, but not surprise. This increase in extreme storm activity is a prediction of our modern understanding of the Earth System’s response to rising global temperatures, which are in turn the result of the burning of fossil fuels, and the degradation of the Earth’s natural compensatory systems (e.g. carbon sinks in the biosphere). In other words, we are collecting abundant evidence that we have entered the Anthropocene, an epoch in which human activity is disrupting essential planetary processes, and where human interference is driving the Earth out of the Holocene epoch in which agriculture, sedentary communities, and socially- and technologically-complex human societies emerged.
Scientists are now asking themselves, where is this all headed? And what can we do to influence the path we, as a global community, are taking? To answer the first of these questions, we must quantify the driving forces (how much carbon dioxide will be emitted into the atmosphere over the coming years and decades), and require a deep understanding of how the Earth System will respond to these driving forces.
The publication in 2018 of the Intergovernmental Panel on Climate Change’s (IPCC) Special Report on the impacts of 1.5°C anthropogenic global warming and related greenhouse gas emissions pathways made clear that “every half degree of warming matters” (Hoesung Lee, IPCC Chair), and that even 1.5°C warming will have dire consequences for many vulnerable communities. And yet a stocktake of current political ambition in terms of greenhouse gas emission reductions on a country-by-country basis puts us on a path to anywhere between 2.5° and 4.7°C global mean temperature rise by 2100 (Climate Action Tracker, September 2018).
It will not be a smooth ride towards a new state of the Earth System in a warmer world. The road is dotted with thresholds, beyond which key systems may be tipped into an irreversible and self-perpetuating new state (Lenton et al. 2008; Schellnhuber et al. 2016), such as the ice sheets, the tropical rainforests and global circulation patterns in the ocean and the atmosphere (see Figure 1). A set of these ‘tipping elements’ may already be activated by global temperature rises within the range set by the Paris Climate Agreement (1.5°-2°C). These include the West-Antarctic Ice Sheet, which may enter a state of self-amplifying melting, resulting on average in around 3m of sea-level rise (Bamber et al. 2009); or the permanent demise of coral reefs, which will not have time to recover between the increasingly frequent bleaching events (Zemp et al. 2017; Lucht et al. 2006). Between 3°C and 5°C global temperature rise, irreversible systems changes such as a weakening of the Indian summer monsoon (Schewe and Levermann 2012), the dieback or functional change of the Amazon or boreal forests (Zemp et al. 2017; Lucht et al. 2006), or a transformation of the jet stream (Petoukhov et al. 2013) are on the cards. And beyond 5°C warming, permafrost thawing, the disappearance of the Arctic winter sea-ice, and runaway melting of the East Antarctic Ice Sheet may be triggered.
Nota bene that this global-warming-dependent landscape of risks should not be seen as a decision-making tool. There is no compelling evidence to support the belief that the Earth System can be ‘parked’ safely at a chosen level of global warming, even at the relatively low warming levels addressed in the Paris Agreement. In fact, some of the relevant tipping elements, especially those linked to the global carbon cycles, have the potential to accelerate the climate change that pulled the trigger in the first place (Steffen et al. 2018). Such runaway processes would have a devastating effect on human settlements and the natural world.
Moreover, these tipping elements do not present isolated threats, but are rather part of a network of interdependent components of the Earth System. Their interconnectedness, whilst fascinating in its intricacy, delicate balance and resilience thus far, will become a point of vulnerability as global temperatures rise. Triggering a ‘low-temperature’ tipping element, could lead to further temperature rise, which in turn triggers a higher-temperature tipping element. These domino-like cascades could see global temperatures skyrocket, with no time or opportunity for the global community to react. We might be standing at a fork in the road, with paths defined by a rapid assumption of human stewardship towards a ‘Stabilized Earth’; or an unchecked trajectory across a planetary threshold towards a ‘Hothouse Earth’ (Steffen et al. 2018 and Figure 2 below).
On a stabilized Earth, global temperature rise does not exceed 2°C, keeping the likelihood of triggering a given tipping element, and subsequent cascade, relatively low. However, this risk reduction can only be achieved with the rapid adoption of policies and practices to protect and enhance natural marine and terrestrial carbon sinks, deep cuts in greenhouse gas emissions, and promotion of sensible efforts to remove carbon dioxide from the atmosphere. In other words, this future is only achievable if humankind becomes a steward of its own future.
Without that stewardship, we will hold the path towards a potential planetary threshold, beyond which a Hothouse Earth may become unavoidable. We only need to look into the past to understand the acuteness of the Hothouse vs. Stabilized Earth dichotomy. A Stabilized Earth would experience conditions similar to those in a period known as the Pliocene (some three to four million years ago, see Figure 3 below), with global temperatures between 2°C and 3°C above pre-industrial levels (another 1°-2°C higher than today), and sea levels between 10m and 22m higher than today. Even this sobering ‘best-case’ scenario is only accessible if humankind immediately acts to ensure the promises of the Paris Agreement, signed by 195 nations, but as yet implemented by very few, are kept.
However, we are still on a trajectory towards a much higher global temperature rise. If a planetary threshold does indeed exist, the current trajectory might lead us directly towards a Hothouse Earth, without the possibility to park the Earth System at intermediate global-warming levels (see Figure 2). This would put the Earth System in a state similar to that of the Middle Miocene (some 15-17 million years ago) (Burke et al. 2018, see Figure 3 below). At this time, global temperatures were 4°C-5°C higher than in pre-industrial times, and sea levels 10m-60m higher. In such a world, severe risks would be posed to political and economic stability, human health (especially for the most vulnerable), efforts to establish global equality, and ultimately to the habitability of planet Earth for humans.
Depending on the path taken, we may be on the cusp of being catapulted, in just 200 years of industrialization, into conditions last seen on Earth between 3 and 17 million years ago! While looking into the past offers an insight into a near-equilibrium state of the Earth System under particular climatic conditions, we are facing a situation in which these conditions will be imposed in the blink of an eye, subjecting ecosystems to an unwinnable game of catch up.
Since entering the Anthropocene, human activity has taken the wheel in determining the state of the Earth System. The efficacy of this takeover is demonstrated in the analysis of the interplay between the variations of solar forcing due to tiny changes in Earth’s orbit around the sun, and global CO2 concentration, which shows that human activity has already delayed the onset of the next glaciation by some 50 000 years (Ganopolski, Winkelmann, and Schellnhuber 2016). So the potency of human interference is being understood in ever finer detail, and our understanding of the paths that lay ahead is becoming increasingly sophisticated. And yet, the stewards at the helm of our shared future have fallen asleep at the wheel. There is no better evidence of this than the insufficient pledges being offered by industrialized countries to reduce emissions of greenhouse gases, whose cumulative effect would launch us on a terrifyingly fast journey 15 million years into the past.
Despite vociferous warnings from scientific and lay communities, international bodies tasked with curbing greenhouse gas emissions are currently too slow to divert us away from a potentially Hothouse Earth. It seems that we are living in an age of global cognitive dissonance, in which imminent threats, laid out in graphic and meticulous detail in internationally agreed-upon reports, have become a driver of inaction.
Figure 1: This graph (taken from Schellnhuber et al. 2016) shows the global-temperature rise (above pre-industrial levels) at which certain tipping elements could be triggered. The colour scale of the vertical columns depicts the temperature range in which the tipping element could be triggered with increasing probability (from yellow to red). The blue curve shows the history of the development of mean global temperature from 200 000 in the past, and projected 500 years into the future based on four global greenhouse gas emissions scenarios (RCP2.6, RCP4.5, RCP6.0 and RCP8.5).
Figure 2: The trajectory of Earth from the Holocene into the Anthropocene, and thus away from the glacial-interglacial dynamics of the past 10 000 years. The Earth system stands at a cusp, which, depending on action (not) taken by the global community, could lead us towards a planetary boundary, beyond which we may face a ‘Hothouse Earth’; or towards a ‘Stabilized Earth’, reached via human stewardship of the Earth system. (Graphic taken from (Steffen et al. 2018))
Figure 3: Wall paintings from the Smithsonian Museum depicting the flora and fauna of the Pliocene (left) and Miocene (right). In the Pliocene, global mean temperature was 2°-3°C warmer than pre-industrial conditions, compared to 4°-5°C warmer in the Miocene.
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