The current trajectory puts the world on track for a temperature rise between 2.1 °C and 3.9 °C by 2100. If all 2030 nationally determined contributions are fully implemented, warming of 2.4 °C (1.9 °C to 3.0 °C) is expected by 2100. IPCC’s best estimate for an equilibrium scenario is 3 °C  (66% likelihood), but there is 18% probability that it could be greater than 4.5°C.

The effects of climate change beyond 3°C will be catastrophic. There are feedbacks in the carbon cycle and potential tipping points that could generate high GHG concentrations even if human emissions are net zero or negative.

At or above 3°C warming, we could have breached several tipping points such as

• Arctic permafrost thawing that releases methane and CO2
• Carbon loss due to intense droughts and fires in the Amazon
• Slowing of dampening feedbacks such as natural carbon sinks
• Abrupt loss of stratocumulus cloud decks

We will be entering uncharted territory beyond this level of warming. There are many unknowns in the event of a “tipping cascade” – a scenario in which multiple tipping elements interact in such a way that tipping one threshold increases the likelihood of tipping another. Under these scenarios, emissions trajectories, future concentrations, future warming, and future impacts are all characterized by uncertainty.

There are four key reasons to be concerned over the potential of a global climate catastrophe.

First, there are warnings from history. Climate change has played a role in the collapse or transformation of numerous previous societies and in each of the five mass extinction events in Phanerozoic Earth history. The current carbon pulse is occurring at an unprecedented geological speed and, by the end of the century, may surpass thresholds that triggered previous mass extinctions, reversing 50 million years of cooler climates in the space of two centuries. Human societies are locally adapted to a narrow band of climatic envelope. The rise of large-scale, urbanized agrarian societies began with the shift to the stable climate of the Holocene ∼12,000 y ago. The cumulative impacts of warming may overwhelm societal adaptive capacity.

Second, climate change could directly trigger other catastrophic risks, such as international conflict, or exacerbate infectious disease spread, financial crises and spillover risk. These could be potent extreme threat multipliers.

Third, climate change could exacerbate vulnerabilities and cause multiple, indirect stresses (such as economic damage, loss of land, and water and food insecurity) that coalesce into system-wide synchronous failures. This is the path of systemic risk. Global crises tend to occur through such reinforcing “synchronous failures” that spread across countries and systems. A sudden shift in climate could trigger systems failures that unravel societies across the globe.

Finally, climate change could irrevocably undermine humanity’s ability to recover from another cataclysm. As climate disasters become more complex and difficult to manage, we will struggle to manage compounding climate hazards. Impacts that may be manageable during times of stability become dire when responding to and recovering from catastrophe.

Conceptual causal loop diagram of cascading global climate failure

The next 10 years will be our last chance to save our children from inheriting a brutal planet. Failing which, we will leave behind a planet that will be uninhabitable for our children.

“The planet has changed before, and life endured. In fact, over geologic time, we’ve seen ice ages come and go, asteroid impacts, continents adrift, and the rise of entirely new forms of life. Over millions and billions of years, change has been the only constant feature of our planet. And life persisted through it all.”

The future of our planet is the future of ‘US’ and it will be determined by our choices. “We didn’t ask to be born into this time — the time of our species’ inflection point, but here we are, and we have to decide what to do with it. “

Source: Climate Endgame: 1) Exploring catastrophic climate change scenarios, PNAS and 2) Dr. Jonathan Foley, Planetary Perspective