A slippery slope: How much global warming constitutes “dangerous anthropogenic interference”?

In a recent article (Hansen, 2004) I included a photograph taken by Roger Braithwaite with a rushing stream pouring into a hole in the Greenland ice sheet. The photo relates to my contention that disintegration of ice sheets is a wet, potentially rapid, process, and consequent sea level rise sets a low limit on the global warming that can be tolerated without risking dangerous anthropogenic interference with climate. I asked glaciologist Jay Zwally if I would be crucified for a caption such as: “On a slippery slope to Hell, a stream of snowmelt cascades down a moulin on the Greenland ice sheet. The moulin, a near-vertical shaft worn in the ice by surface water, carries water to the base of the ice sheet. There the water is a lubricating fluid that speeds motion and disintegration of the ice sheet. Ice sheet growth is a slow dry process, inherently limited by the snowfall rate, but disintegration is a wet process, spurred by positive feedbacks, and once well underway it can be explosively rapid.” Zwally replied “Well, you have been crucified before, and March is the right time of year for that, but I would delete ‘to Hell’ and ‘explosively”’. I thought immediately of the fellow who went over Niagara Falls without a barrel. Would not he consider that a joy ride, compared to slipping on the banks of the rushing melt-water stream, clawing desperately in the freezing water before being hurtled down the moulin more than a kilometer, and eventually being crushed by the giant grinding glacier? “A slippery slope to Hell” did not seem like an exaggeration. On the other hand, I was using “slippery slope” mainly as a metaphor for the danger posed by global warming. So I changed “Hell” to “disaster.” What about “explosively”? Consider the situation during past ice sheet disintegrations. In melt-water pulse 1A, about 14,000 years ago, sea level rose about 20 m in approximately 400 years (Kienast et al., 2003). That is an average of 1 m of sea level rise every 20 years. The nature of glacier disintegration required for delivery of that much water from the ice sheets to the ocean would be spectacular (5 cm of sea level, the mean annual change, is about 15,000 cubic kilometers of water). “Explosively” would be an apt description, if future ice sheet disintegration were to occur at a substantial fraction of the melt-water pulse 1A rate. Are we on a slippery slope now? Can human-made global warming cause ice sheet melting measured in meters of sea level rise, not centimeters, and can this occur in centuries, not millennia? Can the very inertia of the ice sheets, which protects us from rapid sea level change now, become our bête noire as portions of

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