What is 'Hothouse Earth', and how bad would such a climate catastrophe be?

Oceans engulfing coastal cities, coral reefs eliminated and vast swathes of the Earth left completely uninhabitable.

This is what we have to look forward to in a future “Hothouse Earth” – a planet that has passed a “tipping point” beyond which its own natural processes trigger uncontrollable warming.

It is easy to assume you have heard it all before when it comes to climate change news stories, but the scenario outlined in a new paper by Professor Will Steffen and his colleagues is truly shocking.

Anyone feeling optimistic might hope it is shocking enough to make policymakers listen, and indeed the scientists laid out clearly what will be necessary to avoid this disaster.

Crucially, the new paper is not saying this will definitely happen, but scientists generally agree that it is feasible enough to be taken seriously.

Hothouse Earth is a term used to describe a scenario in which human activity causes a higher global temperature than at any time during the past 1.2 million years, due to a breakdown in the feedback loops that regulate the planet’s temperature.

Losing these safeguards would make warming largely beyond our control, no matter how much we subsequently reduced our greenhouse gas emissions.

Ultimately, the authors predicted things would start to stabilise at around 4-5C higher than pre-industrial times, with sea levels 60m higher than today.

By this point, warming would “likely exceed the limits of adaptation and result in a substantial overall decrease in agricultural production, increased prices, and even more disparity between wealthy and poor countries”.

The idea builds on previous work by Professor Hans Joachim Schellnhuber, one of the new paper’s authors, that suggested burning fossil fuels has postponed the next global ice age for 100,000 years.

The Earth goes through natural periods of heating and cooling, but by tampering with the planet’s natural feedback loops the idea is that we have knocked this cycle off its course.

This situation, as well as alternative “stabilised Earth” pathway, is explained in a new Proceedings of the National Academy of Sciences (PNAS) paper.

The scientists considered 10 natural feedback processes, some of which are considered “tipping elements” that will cause a cascade of further warming beyond a threshold limit.

These feedbacks include loss of permafrost, Arctic summer sea ice and Antarctic ice sheets, as well as dieback of Amazon and boreal forests and increased bacterial respiration in the oceans.

Forests, oceans and permafrost currently do us a great service by storing carbon. As rising temperatures cause these carbon “sinks” to weaken, some will actually start to emit more greenhouse gases into the atmosphere.

Some of these changes could be reversible, but others would be irreversible “on time frames that matter to contemporary societies”.

The PNAS paper is not based on a specific study that Professor Steffen and his team have carried out, and nor is it an outcome that has been considered in most existing climate models (which scientists use to make predictions about the Earth’s future under climate change).

“The paper is essentially an essay (or review of other’s work), rather than original research,” explained Professor Martin Siegert, codirector of the Grantham Institute at Imperial College London.

“But they’ve collated previously published ideas and theories to present a narrative on how the threshold change would work. It’s rather selective, but not outlandish.”

The scientists also looked at conditions on Earth in the distant past by way of comparison. The geological record can give a sense of ancient carbon dioxide levels and the resulting impact on life.

Though the scientists emphasised their theoretical tipping point is uncertain, they said “it could be only decades ahead at a temperature rise of 2.0C above preindustrial, and thus, it could be within the range of the Paris accord temperature targets”.

The Paris climate agreement calls on nations to limit warming to 1.5-2C by the end of the century, and while the authors still endorse these guidelines they suggest more ambitious targets may be necessary.

“What we do not know yet is whether the climate system can be safely ‘parked’ near 2C above preindustrial levels, as the Paris Agreement envisages. Or if it will, once pushed so far, slip down the slope towards a hothouse planet,” said Professor Schellnhuber.

“Research must assess this risk as soon as possible.”

In their paper, Professor Steffen and his colleagues do not attempt to put a number on the probability of a Hothouse Earth scenario arising. They only emphasise that given the uncertainty surrounding this projected future, drastic action is essential.

As ever, the refrain is that to avoid this outcome carbon emissions must be cut and the international community must meet the most ambitious targets set by international agreements.

But the authors also go further. They conclude that cutting emissions may not be enough, but ties into a growing consensus that we also need to start removing the gases we have already pumped into the atmosphere.

This could involve better management of forests and farms and developing technologies to suck carbon dioxide from the atmosphere. It will likely involve major societal changes, but seeing as Professor Schellnhuber has estimated the “carrying capacity” of a Hothouse Earth would drop to 1 billion people, it seems worth the effort.