Throughout Earth’s history, our planet experienced vastly different global climates, influenced by forces as diverse as meteor impacts, volcanic eruptions, and shifts in the earth’s tilt and spin. For the most part, these events didn’t change Earth’s average temperature instantaneously: They altered Earth’s systems in such a way that global temperatures gradually rose or fell over thousands to millions of years. These long, steady temperature shifts pushed the planet to super hot highs and frigid lows. 

Earth’s climate has been in a glacial-interglacial cycle for over a million years, fluctuating over tens of thousands of years between severe ice ages and milder, warmer conditions. The unprecedented stable Holocene climate of the last 12,000 years is a milder interglacial period during which moderate warming forces offset the natural tendency for the earth to gradually head back towards another severe ice age.

Humans are now causing so much warming that we have overwhelmed the glacial-interglacial cycle. As greenhouse gas emissions trap heat, atmospheric temperatures rise, causing a new risk to emerge. We are not only reaching higher temperatures than any point in the last 1.2 million years, but we might push Earth’s climate into something called the Hothouse Earth trajectory.

What is the Hothouse Earth trajectory?

“Hothouse Earth” was coined in a 2018 paper by Johan Rockström, Katherine Richardson, and Hans Joachim Schellnhuber and refers to a possible trajectory in which human-caused climate change sets our planet on a warming path that would be difficult or impossible to control.

In a Hothouse Earth scenario, human greenhouse gas emissions activate mechanisms in Earth’s climate that eventually push the global climate over a threshold into continued, self-perpetuating warming, independent of human emissions.

The mechanics of Hothouse Earth

A series of mechanisms is working together to potentially put us on the Hothouse Earth trajectory. Those mechanisms are greenhouse gas emissions, climate feedbacks, and tipping points.

Greenhouse gas emissions

Humans have now contributed enough greenhouse gases to the atmosphere through the widespread use of fossil fuels to raise global temperatures. Global average temperatures stabilized after the planet emerged from the Pleistocene ice age roughly 12,000 years ago and entered the Holocene, which was distinctive for its stability. Temperatures are now rising more quickly than at any time in known history. These rising temperatures could trigger the series of events that would put Earth on the Hothouse Earth trajectory.

Climate feedbacks

Warmer atmospheric temperatures could activate looped responses in Earth’s systems, called climate feedbacks, that generate further warming.

For an example of how these feedbacks work, consider the carbon cycle in a forest. Forests absorb carbon dioxide from the atmosphere, locking it into the leaves, trunks, and roots of trees. As global temperatures rise, hotter, drier conditions lead to more wildfires. These wildfires burn trees, releasing all of that stored carbon dioxide back into the atmosphere, where it acts as a greenhouse gas warming the planet further. This is the feedback loop: More carbon in the atmosphere leads to higher temperatures which leads to more fires which leads back to more carbon in the atmosphere.

Here are examples of other powerful feedbacks that can raise global temperatures:

• Melting ice sheets and glaciers reduce the amount of reflective ice cover, increasing the amount of sunlight the oceans absorb, which leads to higher temperatures and more melting.
• Thawing permafrost releases greenhouse gasses as previously frozen organic matter decays. More heat releases more greenhouse gasses, which lead to more heat, which leads to more thawing.
• Disruption to ocean circulation patterns from melting ice can intensify the El Nino southern oscillation, a powerful weather pattern that influences global temperature. Warmer water comes to the surface, warming the atmosphere, and causing more changes in the circulatory pattern.

There are climate feedback loops around the world that can add, either in small or large amounts, to the warming caused directly by human activity, engaging the next important mechanism in activating Hothouse Earth: tipping points.

Tipping points

The combined warming effect of human emissions and climate feedback loops could trigger transformative events in the global climate called tipping points. “Tipping point” describes the mechanism by which a feedback loop starts having such a profound warming effect that it permanently and drastically changes the global climate. 

For example, if human greenhouse gas emissions cause temperatures to rise so much that wildfire, drought, and forest dieback cause the Amazon rainforest to no longer absorb greenhouse gasses but instead release them, this release would raise greenhouse gas concentrations in the atmosphere even if human emissions stopped. Moreover, a drying, changing Amazon could disrupt weather patterns around the world. Once underway, a change like this would likely be irreversible.

Another tipping point is the loss of the Greenland ice sheet, which currently helps to cool the planet by reflecting sunlight and heat. If sustained warmer temperatures were to cause the Greenland ice sheet to lose critical mass, not only would sea levels rise, but the warming effect would drastically change ocean temperatures, disrupting crucial ocean currents that help to regulate global weather conditions. 

Passing tipping points like these could generate enough additional warming that they could trigger other tipping points, generating even more warming. This cascading chain of impacts would amount to runaway planetary warming that propels itself, rather than stabilizing: This is Hothouse Earth.

The speed with which we pass tipping points matters, as does the amount by which we pass them. For example, if Greenland is destined to lose its ice sheet, there are impacts that will further climate instability. It’s much worse if Greenland loses it in 50 years than in 1,000 years, and the difference between those two is a function of how much warmer we get and how fast we get there. In other words, barely crossing a tipping point is very different from zooming past it.

What happens if we activate the Hothouse Earth trajectory?

While there are aspects of the Hothouse Earth trajectory we can’t predict, here’s what we do know.

Losing control

At the moment, humans account for 100% of climate change. If we trigger a Hothouse Earth trajectory, the planet would become self-warming. If humans are driving the car of climate change, initiating Hothouse Earth would be like losing control of the steering wheel, the accelerator, and the brakes. The more warming there is when we cross tipping points, the more dramatic our actions would have to be to retain control.

An unrecognizable planet

According to the authors of the 2018 paper, planetary conditions on the Hothouse Earth trajectory would eventually “resemble planetary states… last seen several million years ago.” We don’t know precisely what this would look like, but it is likely that many of the things that make our planet recognizable to us—plants, animals, and other organisms—would be unable to adapt to a trajectory of continuous warming.

Irreversible changes

The changes described above, like the loss of the Amazon rainforest, or the Greenland ice sheet, would be irreversible on time scales that matter to humans, and it would be millennia before the planet returned to a climate similar to the stable one we enjoy now, if ever.

How close are we to activating Hothouse Earth?

Scientists do not know the precise temperature at which the planet and its atmosphere would transition into a Hothouse Earth trajectory. While both feedback loops and tipping points can make the future riskier and more unstable, feedback loops can be strong or weak, and tipping points can be passed gradually or rapidly.

But there is evidence that activation of some of the key climate feedbacks has already begun. This does not mean that we are on the Hothouse Earth Trajectory, but the risks are rising. Because some relevant tipping points are estimated to occur between 1.5°C and 2°C of warming, each incremental bit of warming grows in significance.

Our best chance at avoiding these crucial tipping points is to reduce and eventually stop emitting greenhouse gasses. Tipping points amplify whatever warming level the planet is at, so if we still can’t avoid activating them, we will be better off the cooler the planet is when we do. Moreover, gradual, steady warming is less dangerous than the rapid warming we are now causing: Barely crossing a tipping point may prove to be reversible, while zooming past a tipping point increases the risk of losing control.

Want to see how temperatures are changing in places you care about? Read Heat in our climate handbook to learn more about Hothouse Earth.