Delay and pay: Climate tipping point costs quadruple after waiting

It turns out there's more to weigh than catastrophic environmental change as tipping points draw near, though. Another point to consider, a new study reveals, is the cost of undoing the damage.

The cost of reversing the effects of climate change -- restoring melted polar sea ice, for example -- quickly climbs nearly fourfold soon after a tipping point is crossed, according to new work published today in the journal npj Climate and Atmospheric Science. Much work has been done to explore the environmental costs tied to climate change. But this new study marks the first time researchers have quantified the costs of controlling tipping points before and after they unfold.

Common examples of Earth's tipping points include melting ice sheets and dwindling tropical coral reefs. As ice melts and reefs die off, drastic environmental effects like flooded coastal cities and lost biodiversity soon follow -- a shared trait among what the Intergovernmental Panel on Climate Change defines as "critical thresholds in a system that, when exceeded, can lead to a significant change in the state of the system."

Despite the well-established dangers of crossing a tipping point, little is known about the cost of controlling them. How much effort would it take to stop and reverse course just before crossing a tipping point?

In the case of polar sea ice, which is melting at a pace unrivaled by any period in the past 1,500 years, a reversal would entail halting melt and reestablishing ice cover. But how about after we tip -- how might the cost of intervention change if we wait?

Cross a tipping point threshold, said mathematician and lead author Parvathi Kooloth, and it costs nearly four times the effort to reverse the effects and reestablish the climate system to where it was just before tipping, as opposed to reversing course before the threshold. The message applies to most tipping points, said Kooloth, whether they involve tropical coral reefs or frigid sea ice.

"You either shoulder the cost now, just before the threshold is crossed," said Kooloth, "or you wait. And if you wait, the degree of intervention needed to bring the climate system back to where it was rises steeply. A key insight from this work is the confirmation that corrective action after the fact is much more costly and intrusive than preventive action."

Each tipping point is unique. The physical qualities that determine its behavior -- the extent of cloud cover or, say, the transport of heat in the nearest ocean waters -- determine how post-tipping changes take shape in the climate system. In turn, those qualities dictate the nuts and bolts of an actual intervention strategy.

At the heart of each tipping point, however, is a shared, core equation that describes its basic nature. This universality allows researchers like Kooloth to probe the fundamental, shared behavior of tipping points using simplified mathematical models.

From their findings, scientists can then glean broad stroke details that could inform future intervention plans and perhaps, as Kooloth hopes, even a way to identify early warning signals that a tipping point is drawing near.

"It's actually really hard to pin a tipping point down," said Kooloth. "We know a great deal about the climate system today. But even now we're never really sure how far or close we are to a tipping point. Could we one day use observable precursors to provide early warning? My hope is that we can."

The team behind the new study uncovered one other interesting phenomenon: that some tipping points have an "overshoot window." In this window of time just after a tipping point is crossed, the cost of intervention doesn't start its steep climb right away. Instead, cost only grows linearly with time. This can happen because nearby ocean waters take longer to heat up, for example, delaying the onset of rapid change.

It's a fortunate thing, Kooloth points out, a gift of additional time before dire changes start piling on. But this is "no free lunch," she adds. The extra leeway comes with an even steeper increase in intervention costs once the overshoot window is fully crossed. The bigger the overshoot window, the mightier the cost.

The authors point out that not all effects of climate change are reversible, like flora and fauna lost to rapid and prolonged environmental change. And some effects could demand a great deal of effort to reverse -- even more than the effort it took to push the climate system past a tipping point.

There is an asymmetry at play, said Kooloth. We could approach and pass a tipping point rather quickly, but the journey to revert the climate system to where it was could take much, much longer.

"The path forward and the path backward are often not the same," said Kooloth. "Imagine that we go down a high-emissions pathway, where the planet warms enough to melt all our sea ice by the end of the century. If we arrive in the year 2100 with no sea ice, it may not be sufficient to bring the ice back if we dialed our emissions down to the levels we're emitting now in 2024, when we still have some ice left. We may need to dial emissions down much further, to levels predating 2024 -- that asymmetry is important for us to consider as we choose our path forward."