IPCC’s New Emphasis on Risk May
Signal Broad Shift in Climate Thinking

In the northern hemisphere, there’s the outside, but potentially catastrophic, risk of disruption of the oceans’ conveyer belt. Technically a global pattern called “thermohaline circulation,” the watery conveyor belt that includes the Gulf Stream helps moderate the climate in northern zones, notably Europe, as warm water near the tropical oceans’ surface flows toward the poles. The northern water eventually sinks as it cools, then flows back toward the equator. In the climate change disaster movie, The Day After Tomorrow, northern cities in the United States and Europe paradoxically plunge into an Ice Age as the conveyer belt suddenly stops in a matter of a few days. That near-instantaneous catastrophe is not going to happen. But scientists have already seen some signs that seawater in the North Atlantic is freshening, likely thanks to an increasing volume of glacial melt and increased precipitation. Less saline surface seawater would be less dense, thus less inclined to sink, potentially triggering a disruption in circulation patterns over a period of years or decades. The 2007 IPCC report ranks this as “very unlikely” to occur before the end of this century and notes that Europe is much more likely to heat up than to cool in coming decades. But some scientists have predicted that the odds of the conveyer belt shutting down are greater than 50 percent if global warming eventually climbs into the highest ranges of some current projections, above 4 to 5 degrees Celsius (or about 7 to 9 degrees Fahrenheit). Some have also warned that by the time science can confirm that a conveyor belt shutdown has been triggered, it could be too late to stop it.

Yohe points out that some ugly surprises could be more local, but severe nonetheless. If hurricanes become more frequent and more intense, that would increase the odds of, say, a Category 3 hurricane hitting New York. That’s the intensity of the infamous 2005 Hurricane Katrina. 

“Risk boundaries went down,” Yohe says, “after Katrina taught us that our capacity to adapt might be lower than we thought.” For New York, he says, even the early, drenching phases of a Category 3 storm could flood and shut down major parts of the very subway system that city evacuation plans rely heavily on to move millions of at-risk New Yorkers out of harm’s way.

Early this year, climatologist Joel Smith and 14 co-authors, including Yohe, published an article called “Assessing Dangerous Climate Change Through An Update of the Intergovernmental Panel on Climate Change (IPCC) ‘Reasons for Concern’” in the Proceedings of the National Academy of Sciences. Smith was also lead author on the 2001 and 2007 IPCC assessments.

The IPCC’s 2001 report had published a figure often referred to as “burning embers.” That figure had highlighted climate disruptions that could lead to “dangerous” outcomes in five areas called “reasons for concern,” which ranged from species extinction to such “large-scale discontinuities” as major ice sheet collapse. For each of the five categories, the graph was color-coded from white to yellow, to orange, to increasing intensities of red, to indicate risks at various temperatures. The 2007 IPCC report did not include an update of the diagram, because a few governments felt that the vivid diagram seemed alarmist, and some of the participating scientists felt it oversimplified great complexity. Based on their analysis of rapidly growing scientific and economic knowledge, however, the authors of February’s National Academy of Sciences paper included an update of the burning-embers diagram to help illustrate their conclusion that “smaller increases in GMT [global mean temperature] are now estimated to lead to significant or substantial consequences. …” 

In other words, scientific evidence that has accumulated since the 2001 report points to rising levels of risk at lower levels of warming. Risks of widespread species extinction, coral reef damage and Inuit community collapse, for example, have moved well into the red zone, with as little as 1 or 2 degrees (Celsius) of additional warming. The new report also warns of “inevitable surprises” that could come as various gradual changes accumulate, just as “increasing pressure of a finger eventually flips a switch.”
  All the more reason that risk assessment matters. Yohe defines risk as “likelihood times consequences.” And from his perspective as an economist, he says he’s encouraged that the Valencia assessment might help policymakers understand why basing decisions on conventional approaches, such as cost-benefit analyses, can’t be enough. 

“For years, we’ve been asking ourselves questions like, would it be cheaper to just hunker down and adapt, build higher levees?,” he says.

Yet when it comes to some of the most risky outcomes, “cost-benefit is just out of its league,” he says. “Some of the sources of uncertainly are so profound, and some of the related risks are so profound, we’ll never find out enough in a timely enough fashion to inform our policies.”  

Yohe says a sort of mental light bulb appears to have gone on, with national representatives at the Valencia meeting starting to acknowledge the sort of paradigm the world’s finance ministers have long embraced: that just as serious risk assessment is key to sound fiscal policy, it’s critical to consider both likelihood and consequences when making climate policy.”

His own global warming bottom line: “Waiting to act makes no sense economically. Given the risks, uncertainty cannot be a reason for inaction.”

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