Introduction

Stabilization of the climate system requires stabilization of greenhouse-gas concentrations. Most work to date has considered only stabilization of CO2, where there are choices regarding both the concentration stabilization target and the pathway towards that target. Here we consider the effects of accounting for non-CO2 gases (CH4 and N2O), for different CO2 targets and different pathways. As primary cases for CO2 we use the standard “WRE” pathways to stabilization at 450 ppm or 550 ppm. We also consider a new “overshoot” concentration profile for CO2 in which concentrations initially exceed and then decline towards a final stabilization level of 450 ppm, as might occur if an initial target choice were later found to be too high.

Emissions reductions for CH4 and N2O are optimized for the different pathways using an energy-economics model (MERGE). The optimization procedure minimizes the total cost of emissions reductions. The CH4 and N2O emissions reductions lead to substantially reduced future warming and future sea-level rise relative to stabilization cases where likely emissions reductions for these gases are ignored. For central climate and sea level model parameter values the reductions are 0.3–0.4 °C and 2–3 cm in 2100 and 0.9–1.0 °C and about 14 cm in 2400. Reduced CH4 and N2O emissions also allow larger CO2 emissions by reducing the magnitude of climate feedbacks on the carbon cycle.

Summary

The following summary contains some material more fully discussed in Chapter 1 (which was extracted from the 1994 IPCC Report): Bullets containing significant new information are marked “***”; those containing updated information are marked “**”; and those which contain information which is essentially unchanged are marked “*”.

Climate change can be driven by changes in the atmospheric concentrations of a number of radiatively active gases and aerosols. We have clear evidence that human activities have affected concentrations, distributions, and life cycles of these gases. These matters, discussed in this chapter, were assessed at greater length in IPCC WGI report “Radiative Forcing of Climate Change” (IPCC 1994).

* Carbon dioxide concentrations have increased by almost 30% from about 280 ppmv in the late 18th century to 358 ppmv in 1994. This increase is primarily due to combustion of fossil fuel and cement production, and to land-use change. During the last millennium, a period of relatively stable climate, concentrations varied by about ±10 ppmv around the pre-industrial value of 280 ppmv. On the century time-scale these fluctuations were far less rapid than the change observed over the 20th century.

*** The growth rate of atmospheric CO2 concentrations over the last few years is comparable to, or slightly above, the average of the 1980s (∼1.5 ppmv/yr). On shorter (interannual) time-scales, after a period of slow growth (0.6 ppmv/yr) spanning 1991 to 1992, the growth rate in 1994 was higher (∼2 ppmv/yr).

Summary

Interest in the carbon cycle has increased because of the observed increase in levels of atmospheric CO2 (from ∼280 ppmv in 1800 to ∼315 ppmv in 1957 to ∼356 ppmv in 1993) and because the signing of the UN Framework Convention on Climate Change has forced nations to assess their contributions to sources and sinks of CO2n, and to evaluate the processes that control CO2 accumulation in the atmosphere. Over the last few years, our knowledge of the carbon cycle has increased, particularly in the quantification and identification of mechanisms for terrestrial exchanges, and in the preliminary quantification of feedbacks.

The Increase in Atmospheric CO2 Concentration Since Pre-Industrial Times

Atmospheric levels of CO2 have been measured directly since 1957. The concentration and isotope records prior to that time consist of evidence from ice cores, moss cores, packrat middens, tree rings, and the isotopic measurements of planktonic and benthic foraminifera. Ice cores serve as the primary data source because they provide a fairly direct and continuous record of past atmospheric composition. The ice cores indicate that an increase in CO2 level of about 80 ppmv paralleled the last interglacial warming. There is uncertainty over whether changes in CO2 levels as rapid as those of the 20th century have occurred in the past.

Since 1988, the Office for Interdisciplinary Earth Studies of the University Corporation for Atmospheric Research (UCAR) has run a series of annual Global Change Institutes (GCIs) on a range of topics under the broad theme of global environmental change. Participants in each GCI have come from a wide range of disciplines, including those peripheral to the main topic, in order to stimulate discussion and to ensure a multidisciplinary perspective. All GCIs have been highly successful and have led to important and useful proceedings volumes.

The sixth annual Global Change Institute was held over July 18–30, 1993, in Snowmass, Colorado. The topic of the institute was the carbon cycle. As a unique feature, the GCI focused on the practical problems of projecting future concentration changes for given emissions, estimating emissions for prescribed concentration profiles, and assessing the uncertainties in these calculations. Much of the discussion still involved processes, but the viewpoint fostered was as much that of the user of carbon cycle model output as of the “pure” scientist. Over the past decade, there has been a trend toward applied or socially relevant science. With the concern over future climatic change resulting from increasing greenhouse gas concentrations, and with the central role that CO2 plays in this problem, there is no area of science in which cognizance of the social and policy implications is more important than in carbon cycle research.