The Earth’s climate is based on a sensitive balance of many factors. Will Steffen, University of Stockholm, Sweden, and colleagues have studied how stable the climate will be when mankind manages to limit global warming to 2 °C above pre-industrial levels. Using simulations, the researchers have found that tipping points and positive feedback destabilize our climate faster than previously thought and that there is the likelihood of setting off a domino effect.

The initial domino stones could form the tipping points, which already react to a relatively small increase in global temperatures, such as the melting of the West Antarctic and Greenland Ice Sheet and the Arctic sea ice. As this continues to fuel warming through positive feedback loops, it could then be “swept” by tipping points with slightly higher thresholds, such as the Gulf Stream ocean current or the Southern Ocean’s buffering of global CO₂. Once such a cascade is triggered, it might cause a runaway effect that could catapult Earth’s climate out of its stable phase even as human emissions are reduced. The earth could be 4–5 °C warmer than pre-industrial temperatures and have sea levels 10–60 m higher than today.

The researchers cannot rule out that such a cascade of feedback loops starts even when we are delivering the Paris climate agreement. This would mean to keep warming limited to 2 °C by the end of the century. Currently, global average temperatures already have reached 1 °C above pre-industrial levels. Global mean temperatures are rising 0.17 °C/decade. If nothing is done, we might achieve 2 °C of warming in less than 60 years.

To avoid this scenario, according to the researchers, it is necessary to take human action now and drastically to a new direction. In addition to reducing greenhouse gas emissions, the climate buffers of the earth system need to be better protected and regenerated. This could be achieved, for example, through improved forest, agricultural and soil management. Also, geoengineering (a technology that removes carbon dioxide from the atmosphere and stores it underground) can play an important role, according to the researchers.

• Trajectories of the Earth System in the Anthropocene,
  Will Steffen, Johan Rockström, Katherine Richardson, Timothy M. Lenton, Carl Folke, Diana Liverman, Colin P. Summerhayes, Anthony D. Barnosky, Sarah E. Cornell, Michel Crucifix, Jonathan F. Donges, Ingo Fetzer, Steven J. Lade, Marten Scheffer, Ricarda Winkelmann, Hans Joachim Schellnhuber,
  Proc. Nat. Acad. Sci. 2018.
  https://doi.org/10.1073/pnas.1810141115