Storms of My Grandchildren (13 page)

One mechanism is the melting of ice shelves by the warming ocean. Ice shelves—tongues of the ice sheets that extend into the ocean, usually at least partially grounded on the ocean floor—buttress the ice sheet, limiting the rate at which ice is discharged to the ocean. This buttressing opposes the natural “plastic” flow of ice toward the ocean, which is driven by the weight of the ice sheet as snowfall piles up on its interior. If a warming ocean melts ice shelves, the ice “streams” coming from the ice sheet, which discharge giant icebergs into the ocean, begin to move more rapidly, discharging more ice. It is somewhat analogous to pulling the cork from a wine bottle—removing the impediment allows rapid flow.

The West Antarctic ice sheet is especially vulnerable to removal of its ice shelves, because much of that ice sheet rests on bedrock several hundred meters below sea level. Loss of the entire West Antarctic ice sheet would raise sea level 6 to 7 meters (20 to 25 feet) and eventually open a path to the ocean for part of the much larger East Antarctic ice sheet. Once the ice sheets’ collapse begins, global coastal devastations and their economic reverberations may make it impractical for humanity to take actions to rapidly reverse climate forcings. Thus if we trigger the collapse of the West Antarctic ice sheet, sea level rise may continue to even much higher levels via contributions from the Greenland and East Antarctica ice sheets.

Most of the Greenland ice sheet sits on bedrock above sea level, but some of Greenland’s major ice streams are in fjords with the bedrock well below sea level. The termini of these ice streams are retreating into the ice sheet as warming ocean melts the ice front. If the warming continues and termini are pushed farther back into the ice sheet, walls of ice sheet on both sides of fjords may begin to collapse, increasing the rush of giant icebergs to the ocean.

Disappearing ice shelves, ice stream dynamics, and iceberg melting were not included in global climate models used for IPCC studies. This failure to take into account the increased discharge of icebergs to the ocean, where they melt much more rapidly than they would if they had remained as an ice block on land, probably explains the models’ inability to predict realistic sea level change. It is not necessary to move excess heat from the ocean to the ice sheets in order for ice sheets to shrink. Rather the mountain can come to Muhammad: Chunks of the ice sheet (icebergs) are dispersed over a broad area, where they melt by drawing heat from ocean water.

Melting ice shelves is the critical mechanism in initiating ice sheet collapse. However, other contributing factors and feedbacks speed ice sheet disintegration. As the atmosphere becomes warmer, “aging” of snow accelerates—that is the process in which snow crystals vaporize on a microscopic scale and re-form into larger, darker crystals, which absorb more sunlight. Also, snowmelt begins earlier in the spring, causing the ice sheet to also become darker and absorb more sunlight. Human-made black soot aerosols, which are now deposited in measurable quantities on the Greenland ice sheet, contribute to this process as well. And as ice sheet mass loss becomes substantial, the ice sheet surface sinks to a lower level, where the temperature is warmer, which is another amplifying feedback.

Given these amplifying feedbacks, it is no wonder that the glacial-interglacial climate cycles depicted in figure 3 (page 37) are asymmetric, with the wet process of ice sheet disintegration proceeding much more rapidly than ice sheet growth. Sea level rise at a rate of a few meters per century is not uncommon in the paleoclimate record. Instead, it is the stability of sea level for the past 7,000 years that is unusual. Earth in recent millennia was warm enough to prevent an ice sheet from forming in Canada but cool enough to keep the Greenland and Antarctic ice sheets stable. Also, any tendency for continued ice sheet mass loss after the demise of the large Laurentide (North American) ice sheet was opposed by the slight global cooling trend since peak early Holocene temperatures (6,000 to 10,000 years ago).

As mentioned earlier, the sea level stability of the past 7,000 years probably contributed to the development of civilization, because stable sea level led to high biologic productivity and thus ample amounts of fish in coastal areas. With the exception of Jericho, the first cities that developed on several continents 5,000 to 7,000 years ago were all coastal cities. Even today a large portion of the world’s cities are located along the coasts; more than a billion people live within a 25-meter elevation of sea level.

If ice sheets begin to disintegrate, there will not be a new stable sea level on any foreseeable time scale. Instead, we will have created a situation with continual change, with intermittent calamities at thousands of cities around the world. Because the ocean and ice sheets each have response times of at least centuries, change will continue for as many generations as we care to think about. Change will not be smooth and uniform. Instead, local catastrophes will occur in association with regional storms. Given the enormous infrastructure and historical treasures in our coastal cities, it borders on insanity to suggest that humans should work to “adapt” to climate change, as opposed to taking actions needed to stabilize climate.

Would coastal cities be rebuilt, given the knowledge that sea level will continue to rise? It is hard to imagine that humanity would decide to abandon coastlines—although look at New Orleans. But where would people in low-lying regions such as Bangladesh migrate to? Global chaos will be difficult to avoid if we allow the ice sheets to become unstable.

Was sea level stable during prior interglacial periods, some of which were warmer than the Holocene? Bill Thompson of the Woods Hole Oceanographic Institute deduced, from heights of ancient coral reefs on eroded shorelines, that sea level fluctuated several meters during the last interglacial period, about 120,000 years ago. Geologist Paul Hearty used another indicator of past sea level, wave-formed shoreline terraces, to draw similar conclusions. Recently Paul Blanchon and colleagues at the University of Mexico presented evidence that a 2- to 3-meter sea level rise probably occurred in a period of 50 years or less during that interglacial period. Such a rapid change would imply ice sheet collapse, most likely on West Antarctica.

Sea level changes to heights at least several meters greater than today’s level occurred in interglacial periods that were at most 1 to 2 degrees Celsius warmer than today. As this knowledge was developing and becoming more convincing to me, I argued that we must keep additional global warming to less than 1 degree Celsius, much below the 2- to 3-degree “dangerous” level that IPCC suggested with their well-known “burning embers” diagram—used to indicate the probablility of danger as a function of global warming, it begins to glow red, for danger, only when global warming exceeds 2 to 3 degrees Celsius. It was this rationale that led me to argue for a maximum CO2 level of about 450 ppm, as discussed in the “Time Bomb” and other papers.

The paleoclimate sea level data were complemented by disturbing data on ongoing changes in polar regions. Eric Rignot of the Jet Propulsion Laboratory reported that most of the ice shelves around Antarctica were melting from below at a rate of several meters per year. This melting clearly was due to warming ocean waters, although there was no proof that the warming was human-caused.

Melting on the Greenland ice sheet also increased. Summer melting fluctuates year to year, depending on the weather, but there was a clear long-term increase of melt area. In fact the area with melting has almost doubled since the beginning of satellite measurements in the late 1970s. Estimates of ice sheet mass balance, gains from snowfall and loss from melting, show that both the Greenland and West Antarctica ice sheets are beginning to lose mass. Sea level is observed to be rising at a rate of more than 3 centimeters per decade. That is a rate of about a foot per century, twice as large as the rate of sea level rise in the twentieth century.

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 a fellow who had gone over Niagara Falls a year earlier without a barrel (and lived to tell about it). That would seem like a joy ride compared with slipping on the banks of the rushing meltwater, clawing desperately in the freezing water before being hurtled down the moulin more than a kilometer, eventually being crushed by the giant, grinding glacier. But 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”? Paleoclimate sea level increase as great as 1 meter in 20 years is 15,000 cubic kilometers of water per year. Ice sheet disintegration at even a fraction of that rate would seem pretty explosive to most people.

That photograph caption first caused me to think about “scientific reticence.” Reticence leaped to mind again a year later as I was being grilled by a nasty lawyer for the plaintiff in an automobile manufacturers versus the California Air Resources Board lawsuit. He demanded that I name a glaciologist who agreed, on the record, with my assertion that business-as-usual greenhouse gas emissions would likely cause a sea level rise of at least a meter in a century: “Name one!”

I could not, instantly. I was dismayed, because I sensed a deep concern among relevant scientists about likely consequences of continued emissions growth. I remembered a field glaciologist saying, in reference to a moulin, “The whole damned ice sheet is going to go down that hole!”

Scientific reticence, in some cases, may hinder communication with the public. Reticence may be a consequence of the scientific method—success in science depends on continual objective skepticism. Caution has its merits, but we may live to rue our reticence if it serves to help lock in future disasters.