The longer humans continue to pour carbon dioxide into the atmosphere, the closer we draw to leaving the next generation with an unmanageable climate problem, scientists say. A new study, just out Tuesday in the journal Earth System Dynamics, suggests that merely reducing greenhouse gas emissions may no longer be enough - and that special technology, aimed at removing carbon dioxide from the atmosphere, may also be necessary to keep the Earth's climate within safe limits for future generations.
The research was largely inspired by a landmark climate change lawsuit brought by 21 children against the federal government, which is scheduled to go to trial in February 2018, and will be used as scientific support in the case. In fact, its lead author, Columbia University climatologist and former NASA scientist James Hansen, is a plaintiff on the case, along with his now 18-year-old granddaughter.
The new paper argues that the Paris Agreement's target of keeping global temperatures within 1.5 to 2 degrees Celsius (2.7 to 3.6 degrees Fahrenheit) of their preindustrial levels isn't strong enough. During a previous warm period in the Earth's history, known as the Eemian, or the last interglacial period, the planet experienced similar levels of warming, the authors note - and the resulting consequences included the disintegration of ice sheets and six to nine meters of sea level rise.
Noting the dramatic changes that occurred during the last interglacial period, the paper calls for a more stringent target of bringing atmospheric carbon dioxide levels down from their current concentration of more than 400 parts per million to about 350 parts per million by the end of the century. This would bring global temperature closer to a 1-degree threshold, rather than 1.5 or 2 degrees, the authors say.
But the study has already come in from some criticism from other scientists, such as Kevin Trenberth of the National Center for Atmospheric Research, who told The Washington Post that some aspects of the study were "alarmist" and that if changes come slowly enough, society will be able to adapt to them. Trenberth said he disagreed that the 1 degree target is justified and thinks that even 1.5 degrees is "unrealistic."
Hansen is no stranger to controversy. In 2015, he and more than a dozen colleagues published a highly contested paper in the open-access journal Atmospheric Chemistry and Physics, suggesting that sea level rise may occur more rapidly in this century than previously predicted by the Intergovernmental Panel on Climate Change.
In the new study, the researchers suggest that allowing temperatures to creep into the Eemian range once again could eventually trigger the onset of certain slow-developing climate processes that may ultimately enhance global warming, once again inducing catastrophic ice melt, sea level rise and other harmful climate effects. For instance, continued loss of ice may reduce the Earth's reflectivity, they suggest, allowing more solar radiation to warm the planet's surface and melt more ice.
But to keep temperatures lower, the paper finds, would require not only significant emissions reductions efforts, but also the use of "negative emissions" technology, or special methods for pulling carbon dioxide back out of the atmosphere.
Using models, the researchers suggest that if immediate and significant emissions reduction efforts are undertaken - amounting to a decline in global carbon output by at least 3 percent annually starting in the next four years - then less carbon extraction will be needed. A majority of it could be accomplished through basic changes in agricultural and forestry practices to promote greater storage of carbon in vegetation and soil.
On the other hand, the longer global greenhouse gas emissions are allowed to remain at high levels, the more carbon extraction will be needed to reach this target, requiring additional, costlier forms of technology. These may include the burning of biomass for energy, accompanied with carbon capture and storage technology, or technology that directly sucks carbon dioxide out of the air.
If humans immediately began reducing global greenhouse gas emissions by a relatively high rate of 6 percent each year, the researchers estimate that the carbon extraction technology needed to get down to 350 parts per million could cost anywhere from $8 trillion to nearly $18.5 trillion. And if no emissions reductions occur, these costs could rise above $500 trillion through the end of the century.
"Some consequences [of climate change] are already becoming inevitable, but as yet it could be moderate if we begin to reduce emissions rapidly," Hansen said. "So that's the objective - to try to get the global community to understand the importance of beginning those emissions reductions soon, and keeping the task that we're leaving for young people one that they can manage."
But Trenberth said of the paper that while "it is a good point that some slow feedbacks do not kick in until temperatures have been sustained at a certain level," a great deal of the future human experience with climate change will depend not only on which thresholds we cross, but how quickly we cross them.
"If we can slow things down then a lot of adaptation can occur," he said.
Other researchers are a little more cautiously accepting of the paper's points.
Cristian Proistosescu, a postdoctoral researcher at the University of Washington who was not involved with the new research (but who recently led a major study, himself, on the potential future impact of slow-developing climate modes) expressed some skepticism about using the Earth's ancient history as an analogy for the future.
He noted that some of the conditions that were true during the Eemian - the existence of large ice sheets that have already disappeared, for instance - are not the same now. And because humans have not been around to witness some of the slow-developing climate processes that scientists fear will intensify in the future, there's uncertainty about how and even whether they will affect future climate change.
"But that would be the wrong way to think about it," he added in an email to The Post. "The more important point is that we cannot rule out the very real probability that there are slow feedbacks - and risk is probability times cost. ... Once you start thinking in terms of risks I would concur with Dr. Hansen that the current trajectory presents some unacceptable risks."