Storms of My Grandchildren (38 page)

I am optimistic that reason might prevail soon enough to avoid planetary calamity. Yet it is not too difficult to imagine a tragic course, one in which political calculations result in continued kowtowing to the coal industry and the figment of clean coal, and even the development of unconventional fossil fuels. That could bring on the storms of my grandchildren.

Storms. That is the one word that will best characterize twenty-first-century climate, as policy makers continue along their well-trodden path of much talk without a fundamental change of direction. Our grandchildren are in for a rough ride.

The picture, of a dynamic, chaotic climate transition as ice sheets begin to disintegrate, must be painted with little assistance from climate models. Indeed, early climate models suggested a picture that was surely quite wrong.

Primitive global climate models treated the ocean in simple ways and omitted ice sheet dynamics altogether. The consequence in these early models was pervasive large warming at polar latitudes, lesser warming at low latitudes, and a resulting pronounced reduction of the temperature differences (temperature gradients) from equator to pole. The conclusion, therefore, from these climate models was that storms driven by large-scale north-south temperature gradients would be diminished.

Tragically, real-world temperature gradients this century will not be so simple. In the first decade of this century, while the large ice sheets are just beginning to be softened up, we have seen significant increased warming in the high latitudes of the northern hemisphere, especially in central Asia and the Arctic. But once ice sheet disintegration begins in earnest, our grandchildren will live the rest of their lives in a chaotic transition period. The transition period necessarily will last at least several decades, even if methane hydrates kick in and hasten explosive change, because of the large amount of ice involved.

Business-as-usual greenhouse gas emissions, without any doubt, will commit the planet to global warming of a magnitude that will lead eventually to an ice-free planet. An ice-free planet means a sea level rise of about 75 meters (almost 250 feet).

Ice sheet disintegration will not occur overnight. But concepts about the response time of ice sheets that paleoclimate scientists have developed based on Earth’s history are misleading. Those ice sheet changes were in response to forcings that changed slowly, over millennia. Ice sheet responses in the past often occurred in fairly rapid pulses, but disintegration of an entire continental-scale ice sheet required more than a thousand years.

Humans are beginning to hammer the climate system with a forcing more than an order of magnitude more powerful than the forcings that nature employed. It will not require millennia for the ice sheets to respond to the human forcing, but the same inertial forces that slowed the natural response will be in play. It requires a lot of energy to melt ice.

Consider ice initially at a temperature of −10 degrees Celsius. To melt one gram of that ice and bring the water up to the average temperature of Earth’s surface (about 15 degrees Celsius) requires about 100 calories of energy. Let’s put that into units relevant to the planet: Melting enough ice to raise the sea level 1 meter requires an average 9 watt-years of energy over the entire planet. In other words, if the planet is out of energy balance by 1 watt per square meter, it will take the planet nine years to gain enough energy to melt enough ice to raise sea level 1 meter, if all that energy gain goes into melting ice.

Earth at present, averaged over a decade, is out of energy balance, gaining slightly more energy from absorbed sunlight than it radiates to space as heat radiation. The positive energy balance is due to increasing greenhouse gases, mainly carbon dioxide, which is the dominant climate forcing. However, the imbalance is reduced by human-made aerosols, which reflect sunlight to space. And during the past six years, since 2003, the planet’s energy imbalance has been small, at least in part because of the diminishing solar irradiance, as the sun has gone into the deepest and longest solar minimum during the period of accurate solar measurements.

Averaged over a decade, Earth’s recent energy imbalance is probably about one-half watt per square meter—but we are not measuring ocean temperature well enough to define the imbalance precisely. However, so far only a small fraction of this energy imbalance is being used to melt ice—most of the energy imbalance is warming the ocean. This division of the excess energy between melting the ice and warming the ocean will shift more to ice melt as the ice sheets are softened up by global warming and begin to discharge ice to the ocean more rapidly.

One effect of increased ice discharge will be to cool the neighboring ocean. So far, the cooling effect of ice discharge is relatively small, although extensive ice shelf melt around Antarctica already has a detectable influence on ocean surface temperature. As greenhouse gases increase under business-as-usual emissions, it is inevitable that the ice sheets will begin to discharge ice more rapidly and have a larger cooling effect on nearby ocean regions.

West Antarctica, the most vulnerable ice sheet, will begin to shed ice at a substantial rate as climate change continues. Ice from West Antarctica will probably be the largest contributor to rising sea level in the twenty-first century and keep the ocean surface around Antarctica near the freezing point, similar to present temperatures.

The Greenland ice sheet rests mostly on land above sea level, so it is not as vulnerable to rapid collapse as West Antarctica, but it can lose mass fast enough to influence North Atlantic Ocean surface temperature. Greenland cannot contribute as much to sea level rise as Antarctica, but freshwater from melting Greenland ice can have a huge impact on the North Atlantic region via its effect on the ocean “conveyor” circulation.

Ocean water in the North Atlantic is rather salty, compared with, say, the North Pacific, in part because of the contribution of very salty Mediterranean water that passes through Gibraltar and moves into the North Atlantic. The combination of high salt content and winter cooling causes North Atlantic surface water to become dense enough to sink to the ocean bottom. As that deep water moves south, warmer water at intermediate levels moves north to replace it.

In any case, once Greenland starts shedding ice at a substantial rate, the ice will keep the temperature of parts of the North Atlantic relatively cool. If deepwater formation slows down, regional North Atlantic cooling will be enhanced.

Meanwhile, throughout low latitudes, the atmosphere and the ocean surface will be getting warmer and warmer during the twenty-first century. The effects of increased global warming will exacerbate trends that are already apparent, including melting of mountain glaciers, expansion of dry subtropical regions, more intense forest fires, and competition for diminishing freshwater supplies. A warmer atmosphere causes greater desiccation, but at other times and places it can deliver heavier rain and cause larger floods.

Increased warming’s greatest impact on storms will occur through its influence on atmospheric water vapor. The amount of water vapor that the air can hold is a strong function of temperature. The fact that atmospheric water vapor increases rapidly with only a small temperature rise is the basis for the runaway greenhouse effect. But the storms of our grandchildren will begin long before the planet approaches the runaway greenhouse effect.

Even without the chaos that disintegrating ice sheets will bring, the strongest storms will become more powerful this century. That statement is true for storm types that are driven by latent heat. That’s a big deal, because storms fueled by latent heat include thunderstorms, tornadoes, and tropical storms such as hurricanes and typhoons.

Latent heat is the energy that water vapor acquires when it evaporates from the liquid state or sublimates from ice. To evaporate water requires a lot of energy—more than 500 calories per gram of water at normal atmospheric pressure—which is needed to break the strong forces of attraction between water molecules. When the water vapor condenses, that latent energy is released as heat that is potentially available to fuel a storm.

Not each individual storm fueled by latent heat will be stronger as the world becomes warmer. Just because there is more fuel around does not assure it will be used; instead, each storm’s strength depends on specific meteorological circumstances. However, the strongest storms of the future will have greater wind velocities. That’s important, because damage caused by winds is a sharp function of wind speed. Just a 10 percent rise in wind speed increases the destructive potential of the wind by about one third.

Because a warmer atmosphere holds more water vapor and thus has greater latent heat, the strength of the strongest storms will increase as global warming increases. The greater moisture content of the air also increases the amount of rainfall and the magnitude of floods. Already, as we’ve seen, many places around the world have experienced an unnatural increase of “hundred-year” floods, which are occurring more often than their name would imply. In some places the effect of increased rainfall amounts is exacerbated by deforestation or other human activities that reduce the ability of the surface to retain water.

The strongest hurricanes and other tropical storms will become stronger, because of the increased “fuel” for the storms. The impact of warming on the frequency of tropical storms is more difficult to predict, because hurricane formation depends on various meteorological factors that can change as climate changes. However, one of the requirements for hurricanes is a sufficiently warm sea surface. Thus the region in which tropical storms can form almost surely will expand as sea surface temperatures rise. Some confirmation of that expectation was provided by Cyclone Catarina, which developed wind speeds of 100 miles per hour in the South Atlantic Ocean in March 2004 before it made landfall in southeastern Brazil. It was the first recorded tropical storm in the South Atlantic.

Even thunderstorms can produce great damage. Thunderstorms usually develop where a warm, moist air front collides with a cool air front. As the warm, moist air rises within the cooler surrounding air, water vapor condenses, releasing its latent heat, which fuels and speeds the updraft. The surrounding compensating downdraft is what causes wind damage on the ground. Unstable air masses along a cold front can produce severe thunderstorms, including large supercell storms with wind speeds of 80 miles per hour or more. Such supercells are the principal spawning ground of tornadoes. In addition to direct wind damage, these supercell storms can produce heavy hail and flash floods.

But an increase in maximum storm strength and an expansion of the regions with severe storms—thunderstorms, tornadoes, and tropical storms—are just the beginning of the storm story. As global warming continues, storm effects will ratchet upward in three major ways.

One of these ratchetings will be the development of more powerful and destructive midlatitude or frontal cyclones. Frontal storms will be more powerful, because they depend upon the temperature difference between the cold and warm air masses as well as upon the amount of moisture in the atmosphere behind the warm front. This intensification of frontal cyclones will be an effect of melting ice sheets, once ice sheets begin to disintegrate rapidly enough to keep regional ocean surface temperature from rising as fast as continental temperatures and temperatures at lower latitudes. The most important point is that there will be places and occasions in which the warm air masses will be loaded with far more water vapor than would be the case in a cooler world.

A taste of this ratcheting’s future consequences was provided by the cyclonic blizzard, the Superstorm, that hit North America in mid-March 1993. That storm, referred to in some regions as the “Storm of the Century,” was caused by a collision of a cold Arctic air mass and a moisture-laden low-pressure air mass from the Gulf of Mexico. A squall line, a line of severe thunderstorms, formed along the frontal boundary, which moved from the Gulf of Mexico over Cuba and Florida and then up the East Coast of the United States. The storm stretched from Central America to Nova Scotia, Canada. Straight-line winds reached hurricane force in the gulf region, and well over 100 miles per hour in Cuba. The squall line produced “thundersnow” (a snowstorm with thunder and lightning) and a blizzard from Texas to Pennsylvania. Birmingham, Alabama, had seventeen inches of snow and parts of Pennsylvania received two to three feet. Ten million people lost electric power, and three hundred people died from the storm.

Yet the ’93 Superstorm will readily be eclipsed by storms in the twenty-first century, as the moisture content of low- and midlatitude air increases and coexists with ice-cooled polar air masses. The intensity of frontal cyclones will increase through the twenty-first century as the rate of ice sheet mass loss increases and warming continues to grow at low- and midlatitudes. This first ratcheting, though, will pale in comparison to the effects of the second ratcheting: when ice sheets’ rapid disintegration causes a sea level rise measured in meters.