A rise in sea level, necessarily, begins slowly. Massive ice sheets must be softened and weakened before rapid disintegration and melting occurs and the sea level rises. It may require as much as a few centuries to produce most of the long-term response. But the inertia of ice sheets is not our ally against the effects of global warming. The Earth’s history reveals cases in which sea level, once ice sheets began to collapse, rose one meter (1.1 yards) every twenty years for centuries. That would be a calamity for hundreds of cities around the world, most of them far larger than New Orleans. Devastation from a rising sea occurs as the result of local storms which can be expected to cause repeated retreats from transitory shorelines and rebuilding away from them.
Satellite images and other data have revealed the initial response of ice sheets to global warming. The area on Greenland in which summer melting of ice took place increased more than 50 percent during the last twenty-five years. Meltwater descends through crevasses to the ice sheet base, where it provides lubrication that increases the movement of the ice sheet and the discharge of giant icebergs into the ocean. The volume of icebergs from Greenland has doubled in the last ten years. Seismic stations reveal a shocking increase in “icequakes” on Greenland, caused by a portion of an ice sheet lurching forward and grinding to a halt. The annual number of these icequakes registering 4.6 or greater on the Richter scale doubled from 7 in 1993 to 14 in the late 1990s; it doubled again by 2005. A satellite that measures minute changes in Earth’s gravitational field found the mass of Greenland to have decreased by 50 cubic miles of ice in 2005. West Antarctica’s mass decreased by a similar amount.
The effect of this loss of ice on the global sea level is small, so far, but it is accelerating. The likelihood of the sudden collapse of ice sheets increases as global warming continues. For example, wet ice is darker, absorbing more sunlight, which increases the melting rate of the ice. Also, the warming ocean melts the offshore accumulations of ice—“ice shelves”—that form a barrier between the ice sheets and the ocean. As the ice shelves melt, more icebergs are discharged from the ice sheets into the ocean. And as the ice sheet discharges more icebergs into the ocean and loses mass, its surface sinks to a lower level where the temperature is warmer, causing it to melt faster.
The business-as-usual scenario, with five degrees Fahrenheit global warming and ten degrees Fahrenheit at the ice sheets, certainly would cause the disintegration of ice sheets. The only question is when the collapse of these sheets would begin. The business-as-usual scenario, which could lead to an eventual sea level rise of eighty feet, with twenty feet or more per century, could produce global chaos, leaving fewer resources with which to mitigate the change in climate. The alternative scenario, with global warming under two degrees Fahrenheit, still produces a significant rise in the sea level, but its slower rate, probably less than a few feet per century, would allow time to develop strategies that would adapt to, and mitigate, the rise in the sea level.
Both the Department of Energy and some fossil fuel companies insist that continued growth of fossil fuel use and of CO2 emissions are facts that cannot be altered to any great extent. Their prophecies become self-fulfilling, with the help of government subsidies and intensive efforts by special interest groups to prevent the public from becoming well-informed.
In reality, an alternative scenario is possible and makes sense for other reasons, especially in the US, which has become an importer of energy, hemorrhaging wealth to foreign nations in order to pay for it. In response to oil shortages and price rises in the 1970s, the US slowed its growth in energy use mainly by requiring an increase from thirteen to twenty-four miles per gallon in the standard of auto efficiency. Economic growth was decoupled from growth in the use of fossil fuels and the gains in efficiency were felt worldwide. Global growth of CO2 emissions slowed from more than 4 percent each year to between 1 and 2 percent growth each year.
This slower growth rate in fossil fuel use was maintained despite lower energy prices. The US is still only half as efficient in its use of energy as Western Europe, i.e., the US emits twice as much CO2 to produce a unit of GNP, partly because Europe encourages efficiency by fossil fuel taxes. China and India, using older technologies, are less energy-efficient than the US and have a higher rate of CO2 emissions.
Available technologies would allow great improvement of energy efficiency, even in Europe. Economists agree that the potential could be achieved most effectively by a tax on carbon emissions, although strong political leadership would be needed to persuasively explain the case for such a tax to the public. The tax could be revenue-neutral, i.e., it could also provide for tax credits or tax decreases for the public generally, leaving government revenue unchanged; and it should be introduced gradually. The consumer who makes a special effort to save energy could gain, benefiting from the tax credit or decrease while buying less fuel; the well-to-do consumer who insisted on having three Hummers would pay for his own excesses.
Achieving a decline in CO2 emissions faces two major obstacles: the huge number of vehicles that are inefficient in their use of fuel and the continuing CO2 emissions from power plants. Auto makers oppose efficiency standards and prominently advertise their heaviest and most powerful vehicles, which yield the greatest short-term profits. Coal companies want new coal-fired power plants to be built soon, thus assuring long-term profits.
The California legislature has passed a regulation requiring a 30 percent reduction in automobile greenhouse gas emissions by 2016. If adopted nationwide, this regulation would save more than $150 billion annually in oil imports. In thirty-five years it would save seven times the amount of oil estimated by the US Geological Services to exist in the Arctic National Wildlife Refuge. By fighting it in court, automakers and the Bush administration have stymied the California law, which many other states stand ready to adopt. Further reductions of emissions would be possible by means of technologies now being developed. For example, new hybrid cars with larger batteries and the ability to plug into wall outlets will soon be available; and cars whose bodies are made of a lightweight carbon composite would get better mileage.
If power plants are to achieve the goals of the alternative scenario, construction of new coal-fired power plants should be delayed until the technology needed to capture and sequester their CO2 emissions is available. In the interim, new electricity requirements should be met by the use of renewable energies such as wind power as well as by nuclear power and other sources that do not produce CO2. Much could be done to limit emissions by improving the standards of fuel efficiency in buildings, lighting, and appliances. Such improvements are entirely possible, but strong leadership would be required to bring them about. The most effective action, as I have indicated, would be a slowly increasing carbon tax, which could be revenue-neutral or would cover a portion of the costs of mitigating climate change.
The alternative scenario I have been referring to has been designed to be consistent with the Kyoto Protocol, i.e., with a world in which emissions from developed countries would decrease slowly early in this century and the developing countries would get help to adopt “clean” energy technologies that would limit the growth of their emissions. Delays in that approach—especially US refusal both to participate in Kyoto and to improve vehicle and power plant efficiencies—and the rapid growth in the use of dirty technologies have resulted in an increase of 2 percent per year in global CO2 emissions during the past ten years. If such growth continues for another decade, emissions in 2015 will be 35 percent greater than they were in 2000, making it impractical to achieve results close to the alternative scenario.
The situation is critical, because of the clear difference between the two scenarios I have projected. Further global warming can be kept within limits (under two degrees Fahrenheit) only by means of simultaneous slowdown of CO2 emissions and absolute reduction of the principal non-CO2agents of global warming, particularly emissions of methane gas. Such methane emissions are not only the second-largest human contribution to climate change but also the main cause of an increase in ozone—the third-largest human-produced greenhouse gas—in the troposphere, the lowest part of the Earth’s atmosphere. Practical methods can be used to reduce human sources of methane emission, for example, at coal mines, landfills, and waste management facilities. However, the question is whether these reductions will be overwhelmed by the release of frozen methane hydrates—the ice-like crystals in which large deposits of methane are trapped—if permafrost melts.
If both the slowdown in CO2 emissions and reductions in non-CO2 emissions called for by the alternative scenario are achieved, release of “frozen methane” should be moderate, judging from prior interglacial periods that were warmer than today by one or two degrees Fahrenheit. But if CO2 emissions are not limited and further warming reaches three or four degrees Fahrenheit, all bets are off. Indeed, there is evidence that greater warming could release substantial amounts of methane in the Arctic. Much of the ten-degree Fahrenheit global warming that caused mass extinctions, such as the one at the Paleocene-Eocene boundary, appears to have been caused by release of “frozen methane.” Those releases of methane may have taken place over centuries or millennia, but release of even a significant fraction of the methane during this century could accelerate global warming, preventing achievement of the alternative scenario and possibly causing ice sheet disintegration and further long-term methane release that are out of our control.
Any responsible assessment of environmental impact must conclude that further global warming exceeding two degrees Fahrenheit will be dangerous. Yet because of the global warming already bound to take place as a result of the continuing long-term effects of greenhouse gases and the energy systems now in use, the two-degree Fahrenheit limit will be exceeded unless a change in direction can begin during the current decade. Unless this fact is widely communicated, and decision-makers are responsive, it will soon be impossible to avoid climate change with far-ranging undesirable consequences. We have reached a critical tipping point.
The public can act as our planet’s keeper, as has been shown in the past. The first human-made atmospheric crisis emerged in 1974, when the chemists Sherry Rowland and Mario Molina reported that chlorofluorocarbons (CFCs) might destroy the stratospheric ozone layer that protects animal and plant life from the sun’s harmful ultraviolet rays. How narrowly we escaped disaster was not realized until years later.