Can we defuse the global warming time bomb?
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My opinion: Scientific uncertainties
The above assessment involves personal judgments, even though it is based on data and published papers. I included estimates of prime uncertainties, e.g., for climate sensitivity and climate forcings. However, there will surely be surprises as we obtain more information about climate forcings, observe actual climate change, and improve global climate models. In this section I discuss two areas of uncertainty that I believe deserve special attention.
Dangerous anthropogenic interference (DAI)
Establishing the level of global warming that constitutes DAI deserves greater attention than it has received. I argue that DAI will be determined by the level of warming that threatens eventual large-scale disintegration of the ice sheets. That is probably a good assumption if, indeed, a global warming of the order of only 12 °C is enough to initiate eventual removal of large portions of the Greenland or Antarctic ice sheets.
Why choose 1 °C (relative to present global mean temperature) as a first estimate of the level of DAI? This is based in part on the assertion that global mean temperature at the peaks of the current (Holocene) and previous (Eemian) interglacial periods were only 0.5 and 1.5 °C warmer, respectively, than the mid-twentieth century temperature, and the fact that the Earth has already warmed 0.5 °C in the past 50 years. In presenting that argument, I used records of polar temperature and the assumption that polar temperature changes are amplified by at least a factor of two over global mean changes. However, in addition, global climate models driven by early Holocene and Eemian boundary conditions provide strong supporting evidence that global mean temperatures were not warmer than these estimated levels.
Michael Oppenheimer (Reference 2b) also has used ice sheet stability as a basis to infer the level of DAI, concluding that 2 °C was his best estimate. His larger value is primarily a result of differing estimates for the global temperature in previous warm periods. I agree that the total uncertainties in the level of DAI, including those discussed below, encompass both the 1 °C and 2 °C estimates. Furthermore, other scientists will argue that the level of DAI could be even larger than 2 °C. Indeed, Wild et al. (Reference 2c), using one of the most sophisticated global climate models with 1° resolution, calculated that the Greenland and Antarctic ice sheets will grow with doubled CO2, resulting in only a modest sea level rise due mainly to thermal expansion of ocean water. In my opinion, the IPCC calculations, epitomized by the Wild et al. result, omit the most important physics, especially the nonlinear effects of meltwater and secondarily the effects of black carbon. Clearly it is crucial to define DAI more accurately. For example, if there is now 0.5 °C global warming "in the pipeline," then DAI = 2 °C would permit three times as much additional anthropogenic climate forcing as would DAI = 1 °C. The Wild et al. results predict an even higher DAI level.
The time required for ice sheets to respond to global warming, commonly assumed to be thousands of years, is another, related, aspect of the uncertainty in estimating DAI. The IPCC presumes a negligible change in ice sheet dynamics in the 21st century. I doubt that assumption, because increased ice sheet movement surely must be driven by surface melt and percolation to the ice sheet base, rather than by penetration of a thermal wave through the solid ice. Surface melt and summer precipitation associated with human-induced warming and planetary energy imbalance are likely to be unusual by paleoclimate standards, and even the paleoclimate record reveals instances of rapid ice sheet disintegration. The Bølling warming about 14 000 years ago, for example, was accompanied almost simultaneously by sea level rise at a rate of 45 meters per century (Reference 2d).
Still another uncertainty is the magnitude of actual sea level rise during the Eemian period. This is uncertain because uneven motions of the Earth's crust make it difficult to determine mean sea level change from the data available for a small number of sites. If Eemian sea level was not much higher than that in the Holocene, our estimate for DAI would be called into question. However, it would not eliminate concern about the possibility of large sea level rise due to the unique climate forcings in the budding "Anthropocene" era.
There are additional interesting issues that could alter the ice sheet response to human forcings. As discussed below, surface melt may be abetted by a slight aerosol darkening of the ice sheet surface, which becomes especially effective in the warm season. Another curiosity is that Antarctica (except the Antarctic Peninsula) and Greenland may have been "protected" in recent decades by amplification of the polar vortices, i.e., a strengthening of the zonal winds that has limited the warming in Greenland and Antarctica. To the extent that these enhanced zonal winds are driven by ozone depletion, this "protection" may decrease in coming decades as the Earth's ozone layer recovers.
It is apparent that there is considerable uncertainly about the level of global warming that will constitute DAI. This should be an area of focused research in coming years, especially since precise monitoring of ice sheet behavior is now possible. The NASA IceSat mission, monitoring ice sheet topography with centimeter scale precision, should be used to revitalize glaciological studies and test ice sheet modeling capabilities.
Climate modelers should be puzzled by the large negative forcings that aerosol scientists estimate as the direct and indirect effects of human-made fine particles in the air. If these forcings were included in full in global climate models, the models would tend to have cooling at middle latitudes in the Northern Hemisphere where the aerosols are most abundant, as has been stressed by Peter Stone and associates (Reference 7). In reality, moderate warming has been observed there.
It is possible that the negative aerosol forcings have been overestimated. Certainly better measurements are needed. However, we suggested (References 1a, 1b, 6) an alternative interpretation: positive human-made climate forcings (in the same regions), especially black carbon aerosols, have been underestimated. Recent analyses of measurements by a global network of sun photometers (Reference 8) provide partial confirmation of this interpretation, revealing that black carbon aerosols absorb about twice as much sunlight as previously estimated.
There is another, indirect, forcing of black carbon aerosols that seems to have been overlooked by the IPCC: the effect of black carbon aerosols on the albedo (reflectivity) of snow and ice. This effect is no surprise to a number of researchers (Reference 9) who have pointed out that the amount of absorbing aerosols in snow determines its maximum albedo. Snow albedos in the Arctic are seldom found to be much more than 90% at visible wavelengths, even though pure snow should have a visible albedo of at least 98%. Soil dust provides some of the aerosol absorption, but black carbon is believed to be the primary source of absorption.
Using the radiative transfer theory of Steve Warren, Warren Wiscombe, Petr Chylek and associatess (Reference 9a,b), we estimate that this indirect black carbon climate forcing is about 0.5 W/m2 in the Northern Hemisphere and about 0.3 W/m2 globally. Probably two thirds of this, 0.2 W/m2, is anthropogenic. This positive forcing not only adds to global warming, it also contributes to (1) thinning of Northern Hemisphere sea ice and reduction of sea ice cover, (2) softening and loss of permafrost, (3) melting of alpine glaciers, and (4) enhancement and expansion of the summer melt season on the Greenland ice sheet.
The black carbon forcing of snow and ice is seasonally dependent. Black carbon has little effect on fresh snow, but as the snow ages and partially melts, black carbon remains as crud on the surface, noticeably decreasing the albedo of snow and ice. As a result, spring snowmelt is completed earlier, summer melt of glaciers is increased, and sea ice is thinned and reduced in area. I believe that these effects partially account for several otherwise puzzling phenomena: (1) alpine glaciers have retreated faster than expected for the magnitude of global warming; (2) arctic sea ice has thinned in the past 50 years and has decreased in area, while the Southern Hemisphere sea ice has changed little; and (3) spring in the Northern Hemisphere is coming noticeably earlier in recent decades, while fall has not been extended by an equal amount.
Unlike well-mixed greenhouse gases, the efficacy of black carbon as a climate forcing probably depends a good deal on the mechanism producing the black carbon. Tropical outdoor biomass burning, for example, produces a lot of black carbon but much more organic carbon. The biomass burning lofts these aerosols into the middle troposphere where their effect on surface temperature is small, or even results in a slight cooling. In contrast, diesel fuels and biofuels produce a greater proportion of black carbon that remains mainly in the planetary boundary layer (the lowest mile or two), where it has a direct warming effect and an indirect warming effect after deposition on snow and ice surfaces.
"Die ganze welt erstickt im russ" (the whole world is suffocating in soot) was a headline of a local newspaper during an international conference on black carbon held in Austria in 1983. However, climate science has never fully investigated the role of black carbon in climate change. Global measurements of aerosols, including their effects on snow and ice albedos and their effects on clouds, and realistic modeling of all these phenomena are needed. It will not be possible to optimize strategies for dealing with global warming until all important climate forcings, including carbonaceous aerosols, have been well quantified.
My opinion: Practical uncertainties
Science and politics don't mix. I believe that active researchers should offer objective assessment of the science problem and leave it to others to extract policy implications. The complication is that the scenarios for climate forcings and climate change are a function of people's actions. Unless we make clear the relation between those actions and climate change, policy makers will not have the information they need.
Perhaps the best way to handle this situation is to point out the positive aspects in the positions of all three of the relevant parties in the climate change discussion: the Kyoto parties, the United States, and the developing countries. It turns out that each of the three parties is in a position to make unique contributions to reducing climate forcings, and, furthermore, the sum of these is what is needed to achieve a stable atmospheric composition and a stable climate, as universally agreed upon with the Framework Convention on Climate Change.
Let's start with the Kyoto parties. These countries have agreed to cut their greenhouse gas (GHG) emissions to a level several percent below their 1990 level. This will not be easy but it is achievable, based in part on fortuitous happenings: discovery of North Sea gas that allowed Britain to close coal mines; German reunification, which resulted in closing of inefficient East German industry; the collapse of the Soviet Union, which reduced Russian CO2 emissions by 30%; and a stagnant Japanese economy, which has slowed their CO2 emission growth. Adherence to the Kyoto Protocol by its signatories will engender improvement of energy efficiencies and development of renewable energies. The implied technological developments will have world-wide applications, reducing GHGs by more than the emission reductions within Kyoto party countries themselves.
The United States cannot practically meet proposed Kyoto Protocol GHG emission targets (which are based on 1990 emission levels) given the rapid growth of its economy and CO2 emissions in the 1990s. Because of that growth, it is estimated that two thirds of the cost of the Kyoto targets, if they were extended to the U.S., would be borne by the U.S., so there is no expectation that the U.S. will join that accord. However, President Bush indicated in a June 2001 "Rose Garden" speech that the U.S. would take a leadership role in addressing global climate change. He said that the United States would work aggressively on energy efficiencies, renewable energies, and longer-term technologies including fuel cells and hydrogen, next generation nuclear power, and CO2 sequestration. In an advance beyond Kyoto, he recognized the importance of reducing air pollution climate forcings, specifically mentioning black soot, ozone, and its precursors. He said: "Our approach must be consistent with the long-term goal of stabilizing greenhouse gas concentrations in the atmosphere." And: "We will act, learn, and act again, adjusting our approach as science advances and technology evolves." This approach, together with the planned actions of the Kyoto parties, comprises essential ingredients needed for the "alternative scenario" to be achieved.
Developing countries, located primarily at low latitudes, stand to suffer the most if climate is not stabilized, and they already have punishing air pollution. The common presumption that their CO2 emissions will soon explode ignores the fact that developing countries will wish to pursue high-efficiency clean technologies for their own good. The experience with CFCs, which India and China agreed to limit production of in exchange for assistance with replacement technology, illustrates that such cooperation is possible. Climate mitigation and pollution reduction will benefit developing countries most of all, so attainment of their cooperation must be achievable. How to carry out these discussions and cooperation is a matter for policy makers and beyond the scope of this paper.
We note, however, that cooperation of developed and developing countries will be needed on CO2 emissions as well as air pollution. Figure 13 shows that the Far East (defined as Japan, Korea, China, Taiwan, Mongolia) and the Rest of Asia (includes the Middle East) have had the fastest growing CO2 emissions in recent decades and are now near the same level of emissions as the United States. Future global CO2 emissions depend upon the path of Asian emissions, yet the U.S. emissions (blue curve in Figure 13) remain critical for defining the global emissions curve.
Figure 13. Fossil fuel CO2 emissions by global region (data from Reference 10a).
Figure 13 also reveals a period with no growth in emissions in the U.S. during the 1970s and 1980s, which was mainly a consequence of energy efficiencies engendered by an oil supply disruption that caused a large increase in energy prices. Economists agree that the most efficient way to slow emissions growth would be an increase in energy costs, but the growth in cost should be slow and steady to avoid economic disruption and social hardships.
Limitations on global supplies of oil and gas, especially if environmental pressures restrict regions of exploration, might themselves tend to increase energy costs. However, improving technologies are likely to increase accessible hydrocarbon resources, and shortages that occur are usually in irregular disruptive bursts that create hardships and are less effective for improving energy efficiency. Governments could alleviate these problems via flexible assessments that yield a smooth growth in energy costs.
Coal is both the principal root of the CO2 climate problem and the potential solution. Even if all accessible oil and gas is utilized, atmospheric CO2 growth can be kept within the 2 °C or even the "alternative" (1 °C) scenarios, provided that CO2 emissions from coal are limited. Coal produces more CO2 per unit energy than oil or gas, and the CO2 in coal resources is ten times greater than the CO2 in oil resources (Reference 12), enough to cause global warming of several degrees Celsius and certain devastation of the ice sheets. Note that updating Table 2 in Reference 12 to 2002 indicates that the 86 ppm increase of atmospheric CO2 since 1850 is composed of 40 ppm from coal, 34 ppm from oil and 12 ppm from gas.
A flattening of CO2 emissions and a decline as the 21st century progresses thus could be obtained by requiring that new uses of coal be permitted only in cases where the resulting CO2 is sequestered. This approach would make good economic sense, as the costs of sequestration would be attached to coal use, with coal then permitted to compete with other energy sources.
The largest reservoirs of coal are in the United States, China and Russia, although significant amounts exist in other countries. The international community may need to supply technological assistance to developing countries for sequestration capabilities, as it provided assistance for CFC replacements. International cooperation on coal use and sequestration is probably the most important action needed to stabilize atmospheric composition and climate.
Although coal is the key to solving the CO2 problem, this does not mean that other actions are unneeded. Halting the growth of the non-CO2 forcings is essential for staying beneath the most plausible levels of dangerous anthropogenic interference with the climate system, as are concerted efforts to improve energy efficiencies, increase use of renewable energies, and develop energy technologies that produce little or no CO2.
The bottom line
How can I be optimistic if, as I have argued, climate is now in the hands of humans and it is closer to the level of "dangerous anthropogenic interference" than has been realized? If we compare the situation today to that 1015 years ago, we realize that the main elements required to halt climate change, as summarized above, have come into being with remarkable rapidity. I realize that it will not be easy to stabilize greenhouse gas concentrations, but I am optimistic because I expect empirical evidence for climate change and its impacts to continue to accumulate, and that this will influence the public, public interest groups, industry, and governments at various levels. The question is: Will we act soon enough. It is a matter of time.