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Appendix: Climate forcing scenarios

The IPCC (2001) uses a "storyline" approach to produce a useful plethora of scenarios with a broad range of forcings. However, this approach does not show how much current emission trends must be modified in order to stay below an estimated level for dangerous anthropogenic interference. Also, the IPCC predilection for exaggerated growth rates of population, energy intensity, and pollution calls into question the realism of their results. Let's try an alternative approach that begins with observed rates of change in the forcings.

CO2 growth rates
CO2 is the most important forcing. Its growth depends upon the rate at which we add CO2 to the air and upon how fast this human increment is removed via uptake by the ocean and the land. In the past three decades, since the oil embargo of 1973, the growth rate of fossil fuel CO2 emissions has been 1.4% per year (Figure 10), yielding an increase of about 49% in annual CO2 emissions between 1973 and 2002. The annual growth of CO2 in the air increased by a comparable proportion (Figure 9). If we want the growth rate of CO2 in the air to stabilize at the current rate, we probably need to decrease the CO2 emissions growth rate to about 0% per year, i.e., CO2 emissions (and thus fossil fuel burning) would need to remain approximately the same as today (unless CO2 is captured and sequestered, in which case fossil fuel burning could increase).

Actual growth rate of CO2 emissions in the 1990s, based on the recent update of DOE (Reference 10a), was 0.7% per year. In the IPCC CO2 scenarios (constructed before data for the full decade were available), the growth rate of CO2 emissions in the 1990s is 1.5% per year, about twice the actual growth rate. Is it practical to achieve flat CO2 emissions during the next few decades, setting the stage for still lower emission rates later in the century? Such a scenario surely requires all of the following: (1) near-term and long-term emphasis on energy efficiency, (2) increased use of renewable energies that produce little or no net CO2, and (3) long-term development of large energy sources that produce no CO2 (e.g., next-generation nuclear power) and/or technologies to capture and sequester CO2. By the second half of the century it is possible that there will be new technologies that help reduce climate forcings, e.g., by removing CO2 from the air. In the near-term, experience of recent decades suggests that it would be feasible to achieve flat CO2 emissions via the multi-pronged effort mentioned above (efficiencies, renewables, other new technologies).

It is sometimes suggested that the recent ~1% per year growth rate of CO2 emissions is an aberration resulting from the collapse of the Soviet Union's economy and is affected by possible under-reporting of China's emissions. On the contrary, the demise of inefficient systems is natural and there is much room for further gains in efficiency. Reported reductions of coal use in China in the late 1990s were probably exaggerated, as indicated by a 28% increase in reported coal use between 2001 and 2002 (Reference 10b). But such uncertainties do not modify the conclusion that a realistic description of business-as-usual is 1–1.5% per year growth of global CO2 emissions, not 4% per year (see note 10c in References). The presumption inherent in the fast-growth IPCC scenarios—that the entire world will follow the energy path of the U.S. between 1945 and the early 1970s, developing a comparable dependence on fossil fuel supplies, with all the disadvantages that entails—is highly dubious.

These arguments do not imply that the transition from a CO2 growth rate of 1–1.5% per year, which is a realistic description of "business-as-usual," to the 0% per year growth rate of the "alternative" scenario would be easy. On the contrary, it requires a concerted global effort of developed and developing countries. However, the change is small enough that it can be attained via appropriate emphasis on improved energy efficiencies, renewable energies, and other advanced technologies such as carbon sequestration and next-generation nuclear power. This further reduction in the CO2 growth rate is needed not so much because of its effect on climate change during the next few decades, which will be small, but rather because of its impact on our ability to stabilize atmospheric composition later in the century.

The change seems moderate, but it is crucial. Growth of 1% per year for 50 years yields an increase of 70% in the emission rate. Growth of 1.5% per year yields a factor of 2.1. The first steps that are taken in the 21st century are important, as they will determine the direction that we are headed.

Non-CO2 forcings
Methane (CH4) causes the second largest GHG climate forcing. Hansen and Sato (Reference 1a) show that actual growth rate of CH4 is falling below all IPCC scenarios. In the past two years, the gap between the IPCC CH4 scenarios and reality has widened. Other large anthropogenic forcings are those of black carbon and O3. Unfortunately, neither of these is being measured well enough globally to determine its rate of change. I leave it to the reader to mull: do you believe that the amount of these air pollutants will be larger in 2050 than it is today, as it is in the IPCC scenarios? If it is not, their added forcing will be zero or negative. Finally, note that IPCC assumes that the net climate forcing by CFCs and their replacements will increase this decade. Observations show that the CFC forcing is below the IPCC scenarios and may shift to a small negative annual change by 2005.

It is reasonable to project that further change in non-CO2 forcings could be minimal in the 21st century. Small decreases in CFCs and some air pollutants could tend to balance modest increases in other pollutants and N2O. However, such a near balance will not happen automatically. It will require concerted actions and international cooperation.

"Alternative" and "2 °C" scenarios
Let's consider two target scenarios: the "alternative" scenario, which yields a maximum additional global warming of about 1 °C, and a "2 °C" scenario. Warmings are defined relative to 2000. It is assumed that climate sensitivity is about 3 °C for doubled CO2 and that net additional non-CO2 forcings in the 21st century are small. Maximum global warmings of ~1 °C and ~2 °C for these two scenarios occur in 2125–2150, based on simulations with the GISS climate model.

The "alternative" scenario is an extension of the scenario we defined for 2000–2050 (Reference 6), with the annual CO2 growth decreasing linearly to zero between 2050 and 2100 such that atmospheric CO2 stops growing by 2100. Such an assumption, which is required for any scenario that achieves stabilization, implies at least a 50% reduction in fossil fuel use or CO2 capture and sequestration.

The "2 °C" scenario permits larger annual CO2 growth, but after 2050 its annual CO2 growth also decreases linearly to achieve zero CO2 growth in 2100. The annual CO2 increment in the "2 °C" scenario almost doubles by mid-century, reaching 3 ppm per year in 2050. Thus the "2 °C" scenario permits a realistic "business-as-usual" CO2 growth rate (more than 1% per year) to persist for 50 years, but it would require a steep reduction in emissions after 2050.

Amounts of CO2 in these scenarios are shown in Figure 14. CO2 peaks at ~475 ppm in 2100 in the "alternative" scenario and at ~560 ppm in 2100 in the "2 °C" scenario. It is perhaps unlikely that actual CO2 growth (in the next 50 years) will exceed that of the "2 °C" scenario, given the existence of concerns about global climate change.

Figure 14. CO2 in the range of IPCC (2001) "marker" scenarios, and in our "alternative" and "2 °C" scenarios. In the alternative scenario, ΔCO2 decreases linearly from 1.7 ppm per year in 2000 to 1.3 ppm per year in 2050 and then linearly to zero in 2100; CO2 peaks at ~475 ppm in 2100. In the "2 °C" scenario, ΔCO2 increases linearly from 1.7 ppm per year in 2000 to 3 ppm per year in 2050 and then decreases linearly to zero in 2100; CO2 peaks at ~560 ppm in 2100. Upper and lower limits of the IPCC range are their scenarios A1FI and B1 (IPCC, 2001, Appendix II, pp 807 and Figure 18, pp 65). IS92a is the updated version of that scenario in IPCC (2001), which incorporates recent carbon cycle modeling. A still broader range of IPCC scenarios is included in their Special Report on Emission Scenarios (SRES) document (Reference 11b). CO2 scenarios for the alternative and "2 °C" scenarios are given at

It is informative to compare these two scenarios with the IPCC scenarios. The manifold "story lines" in IPCC (2001) produce a plethora of scenarios, but when new scenarios are devised with each report it is hard to judge how well prior scenarios have fared against reality. Fortunately the standard emission scenario of previous reports, IS92a, has been retained in IPCC (2001) with atmospheric CO2 amounts obtained from updated carbon cycle calculations. The "alternative" and "2 °C" scenarios both fall far below IS92a. The "alternative" scenario falls far below the range of IPCC (2001) marker scenarios, while the "2 °C" scenario is near the bottom of that IPCC range. Of late the real world has been close to the "alternative" scenario (Figure 12).

The conclusion that the real world is likely to fall somewhere in the range between the "alternative" and "2 °C" scenarios (at least for the next several decades) has the practical implication of heightening the importance of the non-CO2 forcings. The large CO2 forcing in most IPCC scenarios had left the impression that nothing except CO2 was important. The figure in Box 3 is a better measure of the relative importance of different forcings. The non-CO2 forcings deserve emphasis comparable to that placed on CO2.

Summary of opinion regarding scenarios
Emphasis on extreme scenarios may have been appropriate at one time, when the public and decision-makers were relatively unaware of the global warming issue, and energy sources such as "synfuels," shale oil and tar sands were receiving strong consideration. Now, however, the need is for demonstrably objective climate forcing scenarios consistent with what is realistic under current conditions. Scenarios that accurately fit recent and near-future observations have the best chance of bringing all of the important players into the discussion, and they also are what is needed for the purpose of providing policy-makers the most effective and efficient options to stop global warming.

The IPCC scenarios encompass a great range, especially in the IPCC SRES document (Reference 11b), which includes CO2 growth rates faster and slower than the range of "marker" scenarios that are included in IPCC (2001) and illustrated in our Figure 14. The IPCC, however, does not specify the likelihood of the scenarios or examine the direction of current real-world growth rates. A realistic "business-as-usual" scenario would have CO2 growth rates in the range of 1–1.5% per year, thus on a course comparable to our "2 °C" scenario for the next few decades.

I have argued that achievement of a 1 °C scenario would be feasible based on increased emphasis on energy efficiencies, renewable energies, and advanced technologies. However, I am not implying that this "alternative scenario" would be easy to achieve. Indeed, it surely requires concerted world-wide actions. Furthermore, stabilization of atmospheric composition by the end of the century eventually will require substantial reductions in CO2 emissions. If fossil fuels remain the primary source of energy, this implies the need for large-scale sequestration of CO2. I have not discussed propositions to counterbalance global warming with geo-engineered cooling, because the suggestions that have been made, such as a large shade in space or human-injected aerosols in the stratosphere, appear to be uneconomic and fool-hardy in comparison with the actions that would slow global warming.

The great uncertainty about scenarios concerns the level of global warming that would constitute dangerous anthropogenic interference. I have argued that ice sheet stability may require that global warming be kept less than about 1 °C. Hopefully I am wrong, because that may be a difficult scenario to achieve. Others have suggested 2 °C, and IPCC implies that even larger warming would have little effect on sea level. Research on the stability of the ice sheets deserves high priority. A curious point that we have raised concerns the contribution of black carbon to the disintegration of ice sheets. The implication is that by reducing black carbon emissions we could raise somewhat the level of warming that would constitute dangerous anthropogenic interference. However, I am not suggesting that black carbon is the primary factor affecting ice sheet stability.

Lay person's CO2 emissions graph

The presentation of fossil-fuel CO2 emissions in Figure 11 reveals the fundamental changes in growth rate that have occurred over long periods and the time scales over which different energy sources have penetrated global energy use (an estimate for wood is added to that figure in Reference 6a). However, the logarithmic scale for emissions might mislead a lay person. An alternative (linear) presentation (Figure 15) reveals additional information for a limited period.

Figure 15. Fossil fuel CO2 emissions as in Figure 11, but with a linear scale. IPCC/SRES emission scenarios were defined in the mid 1990s, which accounts for their offset in 2000.

The sea change in energy growth rates that occurred in 1973, with the oil embargo and energy price increase, is less apparent in Figure 15 than in Figure 11, although the discerning eye might note the change from exponential growth prior to 1973 to essentially linear growth (constant growth) since 1973. A realistic projection of current trends is a continuation of that constant growth rate, the dash-dot line in Figure 15.

"Constant growth" at the rate of the past three decades falls below the IPCC scenarios, and "constant emissions" falls far below the IPCC scenarios. The dark blue area is the range of "marker" scenarios in the primary IPCC publication (Reference 6a), while the lighter blue area adds the full range of scenarios in the IPCC SRES publication (Reference 6b). The IPCC scenarios that extend far off-scale (high) are impractical to show in entirety with a linear scale, but they do not need to be shown as they are unrealistic.

The "constant growth" and "constant emissions" tracks are approximately what is needed to achieve the "2 °C" and "alternative" climate scenarios, which are designed to keep additional global warming below 2 °C and 1 °C, respectively. Keeping CO2 emissions from exceeding the "constant growth" track for the next few decades may be, comparatively, "easy." Achieving the "constant emissions" path, on the other hand, requires a second sea change in fossil fuel use trends. We will present quantitative evidence elsewhere that this "alternative" scenario could be achieved via feasible emphasis on energy efficiencies, renewable energies and other advanced technologies.

This discussion refers to CO2 emissions during the next few decades. The (uncaptured) CO2 emissions in both the 2 °C and 1 °C scenarios must begin to decrease prior to mid-century to achieve stabilization of atmospheric CO2 amount, as agreed in the Framework Convention on Climate Change. To keep additional global warming from exceeding 1 °C, which I have argued is the most plausible value for the level of dangerous anthropogenic interference, implies the need for a change in CO2 emission rates at least as dramatic as that initiated in 1973. This will require an unprecedented level of international cooperation.

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    3. Some analysts argue that the reported decline of coal use in China beginning in the late 1990s was overstated, while others argue that the reported coal use for 2002 (28% higher than 2001) is exaggerated. Despite these uncertainties, the global fossil fuel CO2 emissions growth rate since 1973 is in the range 1.3–1.4% per year. The growth rate required to go from the 1973 emission rate to the 2001 emission rate (with its low China emissions) is 1.312% per year, while the rate required to reach the reported 2002 emissions (with its 28% increase in China coal use) is 1.375% per year.
    4. Some analysts argue that the reported decline of coal use in China beginning in the late 1990s was overstated, while others argue that the reported coal use for 2002 (28% higher than 2001) is exaggerated. Despite these uncertainties, the global fossil fuel CO2 emissions growth rate since 1973 is in the range 1.3–1.4% per year. The growth rate required to go from the 1973 emission rate to the 2001 emission rate (with its low China emissions) is 1.312% per year, while the rate required to reach the reported 2002 emissions (with its 28% increase in China coal use) is 1.375% per year.
    5. Black carbon climate effects: J. Hansen and L. Nazarenko, to be submitted to PNAS, 2003.
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    Hansen, J., D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind and G. Russell. 1981. Climate impact of increasing atmospheric carbon dioxide. Science 213:957–966. (The contributions of coal, oil and gas to airborne CO2 in Table 2 and the update here account for the history of emissions and the decay time of CO2 incremental additions.)
  13. Terzian, Y. and E. Bilson (Eds.). 1997. Carl Sagan's Universe. Cambridge University Press, 282 p.

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