Johan Rockstrom is one of the world’s most influential Earth scientists. As director of the Potsdam Institute for Climate Impact Research, he advises governments, corporations and activists, including his Swedish compatriot, Greta Thunberg, about the latest research on the climate and biodiversity and argues for better science communication. Last year, he co-edited Standing Up for a Sustainable World, a book that brought together essays from climatologists, economists, environmental defenders, financiers and school strike activists. In recent months, he has teamed up with David Attenborough to create a new Netflix series, Breaking Boundaries: The Science of Our Planet, participated in President Joe Biden’s climate summit and co-organised a declaration by more than 100 Nobel laureates.
The Nobel prize summitin April declared a planetary emergency. Why now? Scientists have known for at least three decades that human activity is destabilising the climate and accelerating the collapse of ecosystems. This statement, signed by 126 Nobel laureates, is a world record. Never before have we seen such a global uprising of Nobel prize winners. They issued an emergency call from science to humanity, calling for reason, truth and humanism in the transformation towards an equitable and prosperous future in a safe operating space.
An emergency is calculated by risk divided by time. Risk is probability multiplied by impact. Scientifically, we now have a very unfortunate set of data in front of us. We know that the likely impact on humans of climate disruption, mass extinction and air pollution is very, very high indeed. The probability is also uncomfortably high. This adds up to a very high risk. Now divide that by time. We have unequivocal evidence that we have entered a decisive decade. If we have any chance to prevent the loss of more than a million species, we must halt biodiversity loss now, not in 20 or 30 years. If we want to have any chance of keeping global warming to 1.5C, we need to cut emissions by half over the next nine years. That is what the Nobel laureates and other scientists are speaking out about. This isn’t just a matter of raising the volume on the same old data – it is a new juncture. We are running out of time.
In 2018, you co-authored the Hothouse Earthdiscussion paper, which warned that our planet could start drifting towards an uninhabitable state if warming exceeds the Paris Agreement target of 2 Celsius.. In the three years since, have you seen any evidence for this? My hypothesis still stands, though we have since seen new research that both strengthens and eases concerns. On the negative side, we have passed three important tipping points: Arctic summer ice, tropical coral reef systems and parts of the west Antarctic have, as far as the latest science shows, reached the point of no return.
We are also seeing more evidence of a weakening of the Atlantic meridional overturning circulation (AMOC) an effect of ice melt from Greenland and the Arctic that reduces the salinity and temperature gradient between warmer salty water flowing north from the equator and colder less salty water in the north atlantic, which in turn slows ocean and atmospheric currents, with impacts on regional climate systems such as the Amazon monsoon. Slower flow of warm waters from south to north, can also explain why the southern ocean is warming so fast, which in turn has led to accelerated melting of the west Antarctic ice sheets. These cascades are a core feature of a potential drift to a hothouse Earth.
We will see further support for our hypothesis in the forthcoming IPCC 6 (Intergovernmental Panel on Climate Change) assessment, which for the first time will show a change in the climate sensitivity range (the amount of warming projected by computer models if humanity doubles the amount of carbon dioxide in the atmosphere). In the five previous assessments, the IPCC estimated this range would be between 1.5C and 4.5C. In the sixth assessment that will be compacted. The higher point of 4.5C will remain, while the lower point will climb to 2.3C. This means we no longer have a range that goes from a manageable 1.5C to disastrous over 4C. Now the range is from very dangerous (above 2C, which can lead to a hothouse Earth) to catastrophic (over 4C). If we continue as we are, we are very unlikely to remain under 2C in 30 or 40 years.
The third piece of negative new research comes from the new climate model at the Potsdam Institute, which is the first to reproduce the temperature and ice on Earth over the past 3m years. Remarkably, this study shows that during this period, the world has not once passed 2C of warming – the hothouse Earth threshold that we postulated. It was not because of a gentle sun that the Earth remained in that temperature corridor, it was because of regulating feedbacks in the Earth system, such as ocean carbon uptake, terrestrial sinks, the albedo effect of the icecaps, and so on. We want this new model to examine whether 2C of human-forced warming will push the Earth into a runaway hothouse state or whether it can land back into an equilibrium state similar to that of the Holocene period.
How about evidence to the contrary? Are there studies that suggest we can avoid a hothouse state? On the more positive side, the biggest unknown is how the oceans will behave. What we see in several of the model inter-comparisons coordinated by the World Climate Research programme is that if we completely halt all greenhouse gas emissions, then the ocean will mop up the damage done to the climate system. That is under the assumption that ocean currents continue operating as now and that there are no further nasty surprises. It would come with very negative secondary effects: long-term ocean acidification, damage to marine life and perturbations of the heat conveyor belt in the ocean. Despite that, overall, I must admit I find it reassuring that the ocean might be able to continue buffering and ultimately clean up. But we urgently need more research on whether or not our planet can keep up its resilience and dampen warming or start self-amplifying warming.
The second positive is that there is evidence that it is never worth giving up. Regardless of whether we can phase out fossil fuels fast enough to stay within a 1.5C increase, there is a benefit to doing everything we can. Even after a period of overshoot, of challenging decades of air pollutants and climate extremes, if we can completely phase out greenhouse emissions and go to zero carbon in an absolute sense, then we still have a chance to return within a Holocene-like state.
You have argued for a new approach in science communication to convey more urgency and agency. In Standing Up for a Sustainable World, you urge a more positive message of prosperity and fairness that can attract an otherwise indifferent majority. How can this be achieved? After 50 years of trying to communicate risks, there is a rapidly rising recognition among environmental and Earth system scientists that we need to back off from an approach of just raising awareness and encouraging a willingness to pay. Indirectly, this creates a trade-off. People feel they have to choose between immediate wealth and protecting the planet. That is an obsolete and incorrect way of communicating. The indifferent majority will only come with us if the sustainable route is not just more attractive but easier. The moment we succeed is when people are sustainable without knowing it; they simply choose the right tomatoes because they taste better and they are cheaper.
The counterargument is that more strident activism and a wider sense of alarm are getting the scientific message out more effectively than science has managed by itself. Talk of prosperity and equality is nice, but isn’t this more effective when reinforced by a fear of the likely consequences if we don’t change? Isn’t it necessary to see the abyss in order to step back so that fear is not paralysing but motivating? Yes, I’m on your side here. We have a responsibility to communicate the danger, the fear. As scientists, we have long tended to play down the risks. We confused society with sophisticated terms about uncertainty ranges and probability levels. Fridays for Future activists make the threat much clearer.
All the cards need to be put on the table. The patient is sick and we must do a transparent diagnosis. We are clearly close to the point that can lead to disaster. A science based fear element is justified and important.