Are we racing towards Earth’s ‘Hothouse’ tipping point?

Recently, myself and academic colleagues published an analysis exploring the risk of biophysical systems that regulate the state of the planet, crossing critical tipping points. This is important because it could push the planet away from the stable state it has been in since the last Ice Age some 12,000 years ago. 

This research effort was a daunting and complex task. Since the advent of the Anthropocene and the great acceleration of human environmental pressures on Earth, the scientific community is now forced, for the first time, to seek answers to the following existential question - are we as humanity now in a position to destabilize the entire Earth system? The research we just published is an attempt to address this question, by summarizing our current knowledge, setting the stage for further science, and presenting additional scientific support for urgent climate action. 

And, despite complexity and uncertainty, we already know quite a lot. We know that Earth, for the last 1.2 million years, has naturally fluctuated 6-8 times, between Ice Ages lasting 100,000 years and warmer inter-glacial periods, similar to the one we have today. These are the only two natural stable states of our planet in this time.

We also know that we humans have increased the temperature on Earth by a global average of 1 degree Celsius (1.8°F), due to burning of fossil fuels, polluting air, and degrading forests and ecosystems. This is the warmest period on Earth since the last Ice Age, and we are reaching the edge of the warmest temperatures on Earth during inter-glacials. We thus have increasingly strong scientific evidence that we are hitting the ceiling of the warmest conditions on Earth over the past 1 million years. 

Furthermore, we also know the Holocene state we have been in since the last ice age is stable – it has lasted 11,000 years and could have lasted another 50,000. We also have overwhelming evidence that the Holocene is the only state of Earth we know for certain can support our modern world - thanks to its extraordinary stability relative previous states.  

And we also know that before Earth entered the limit cycle between inter-glacial and glacial state during the last 1 million years, Earth has at times been much warmer than now – at least 4-5°C warmer – and that this state lasted millions of years. We call it “Hothouse Earth”. 

Finally, and most importantly, what we also know, is that the only reason global temperatures have not risen even higher than 1°C already, with the increasingly unsustainable exploitation of the biosphere, is Earth’s natural resilience. What explains Earth's trajectory between Ice Age and inter-glacial is not only the physics related to Earth's distance and angle to the sun. It is also biosphere carbon feedbacks, i.e., the capacity and dynamics of the biosphere on Earth to maintain so-called feedback mechanisms.

Negative feedbacks dampen and thereby cool Earth, while positive feedbacks self-reinforce and thereby warm Earth. It is when large systems that regulate the state of the Earth cross tipping points that feedbacks shift -- from self-cooling to self-warming. We know, as a fact, that during the last 200 years, since we embarked on the fossil-fuel driven Industrial revolution, Earth has applied its biophysical processes to buffer and reduce the warming (climate forcing) caused by us humans. Just over the past 50 years, the Earth system has doubled its carbon sinks in oceans and on land, and has sucked up 95 % of heat caused by us in the oceans. A remarkable proof of Earth resilience, and the recognition that Earth's biophysical processes are operating to maintain the Earth system in its Holocene equilibrium state. 

Now, we start seeing signs of cracks in this stable fabric. We are starting to see the first signs of weakening carbon sinks in terrestrial ecosystems, albedo shifts in ice sheets as they darken when melting, worrying trends of methane release from thawing permafrost, forest dieback, and weaker soil carbon sinks. 

We thus have increasingly strong scientific evidence that we can no longer rely on a stable and resilient Earth system dominated by cooling feedbacks. 

It is only in the last 1 million years that Earth has started to oscillate in its natural limit cycle between glacial and inter-glacial states. Among the warm inter-glacials, the Holocene is the most stable with benign conditions for human life. The great acceleration of the human entreprise in the 1950s, with rapid rise of human pressures on Earth, has placed the Earth system in a new geological Epoch, the Anthropocene. Today, we have pushed the planet close to the edge of inter-glacial states on Earth. Rising evidence indicates the risk of crossing a planetary threshold where biosphere feedbacks on Earth shift from dampening and cooling Earth to self-reinforcing and warming Earth, which would put the planet irreversibly on a Hothouse Earth trajectory. There is evidence suggesting that the planetary threshold may be reached already at 2 degrees C global warming caused by human forcing. 

Is Earth at risk of flipping over from being our best friend -- a cooling agent, to becoming a foe -- a self-reinforcing warming agent? And, if this is a true fear, the grand question, of course, is whether there is a third state of the planet we should be concerned about, i.e., that we push Earth across a planetary tipping point taking us irreversibly towards a Hothouse Earth, which will be catastrophic for humanity, and if this is a true risk, where is the tipping point? 

And of course, for our future on Earth. Is there an intermediate stable state of Earth between the mild warmth today, and Hothouse Earth? if there is a planetary tipping point, and if we are approaching it fast, what will be required from us to successfully navigate our world development within a reasonably stable Earth?

The media response to our paper was overwhelming with about 500 articles published. The interest surprised us – several of the authors were on vacation when it was published disrupting their holidays. Perhaps it should not have: by coincidence the paper came out as the northern hemisphere baked in an extraordinary heatwave. Is this a sign of things to come? Yes. 

While there remains much uncertainty, we concluded that we cannot exclude that a hard-wired biophysical tipping point may be crossed as early as at 2°C global warming, caused by us humans. This means that even if the world makes grand efforts to rein in greenhouse gas emissions, compared to the current depressing trajectory of emissions taking us to 3 degrees and beyond just through human action, we cannot rule out the risk of pushing the planet into an irreversible Hothouse Earth pathway. 

This would not mean that the planet would abruptly collapse. What it means is that we have increasing scientific support that at 2°C global warming, we face the risk of pushing the "on-button" kick-starting irreversible tipping points and domino effects that would take us towards a Hothouse Earth. 

We do not know for certain that a planetary threshold resides at 2°C. The rising scientific suspicion that it may be there or in the vicinity is, in itself a warming of the urgent need for planetary precaution. Alarmingly, not only do we not have better precision of where the planetary threshold is, we are not at this point able to quantify the risk of crossing it. But we do know the following…

Under current trajectory, we will reach 2°C global warming within 30-50 years with a high probability of hitting 2°C within two generations. That is very soon. This is the key new insight. This paper provides strong scientific support to the goal of the Paris Climate Agreement -- of holding global warming well below 2°C and aiming for 1.5°C. But it does so from a new scientific standpoint -- that staying well away from 2°C is supported as a way for us to be reasonably sure that Earth remains in its current inter-glacial regime, capable of providing manageable conditions for world development. 

If we cross a planetary tipping point, this would be a point of no return. Yet, the full impact of such a trajectory may not be felt for hundreds, if not thousands, of years.

We identify ten key biophysical systems that contribute in regulating the state of the planet:

1. Thawing permafrost in tundra and seabed methane release
2. Weakening of land and ocean carbon sinks
3. Rainforest dieback (Amazon in focus)
4. Temperate and boreal forest dieback
5. Increased bacterial respiration in oceans
6. Reduced snow- and ice coverage in the Arctic and Antarctica
7. Shifts in the thermohaline circulation in oceans
8. Shifts in the jetstream, high in the atmosphere over the northern hemisphere
9. Shifts in the El Nino Southern Oscillation
10. Shifts in the monsoon systems

We estimate, based on published scientific literature, that the first 5 of these, would, from the knowledge we have today, probably bump up global mean temperature with an additional 0,4-0,5°C, if we humans cause temperature rise to 2°C through burning of fossil-fuels. This biosphere temperature bump, though seemingly small, may be enough to trigger a knock-on effect on other more temperature resistant shifts.  

Hothouse Earth would of course be catastrophic for humanity. We are in an emergency situation. So, what do we need to do? 

1) Our latest analysis provides further support for the Paris Agreement and even more rapid action. Greenhouse gases should peak no later than 2020 and be cut in half or more every decade out to 2050 and beyond – a trajectory we call the Global Carbon Law. The world should wake up to the fact that 2°C may not be only a challenging state in terms of adaptation, but also a threshold triggering an irreversible journey towards a Hothouse Earth.

2) There is a high degree of uncertainty, so we need more research. We cannot give precision on if it is exactly at 2°C and we cannot say the probability of crossing a planetary threshold. Therefore, as researchers we advise precaution.

3) And finally, enhancing Earth resilience is a real priority for humanity and will require integration of ecosystem and climate research and political agendas.

While renewable energy solutions are scaling rapidly, elsewhere progress is nowhere near the scale we require. That is why in September, myself and Christiana Figueres, the former head of the UN’s climate body, will join some of the world’s climate leaders from business, investment and cities in San Francisco for the Global Climate Action Summit. With colleagues we will publish a Global Climate Action Roadmap in advance of the summit to shift gear and move from incremental to exponential action. 

Despite the findings of our research, I remain confident the world can make the just and equitable transition to a sustainable future and that this coming decade will be decisive for our future. 

Last updated August 17, 2018

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