It is concentrations not emissions that matter
Climate change is related to the concentration of GHGs and not to any individual years emissions. Carbon dioxide (CO2) concentrations are closely related to the net accumulation of emissions over long periods of time. That is, it is the sum of emissions over time that determines the atmospheric concentration. Any individual years emissions are only marginally important1. Average residence times for GHGs can range up to thousands of years for some of the anthropogenic species. Strategies to control net emissions must account for long periods of time in a meaningful way. The ultimate objective of the UNFCCC is the stabilization of GHG concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system (UNFCCC 1993, Article 2).
Intergenerational transfers are inevitable
The consequences of climate change will be visited primarily on those who are alive in the future. The present generation has inherited its atmosphere and associated climate from its ancestors. While individuals and governments make many decisions that affect future generations, most of these decisions are undertaken inadvertently. It is impossible to avoid the intergenerational wealth-transfer issue when addressing the climate problem. That most of the affected parties are not present to participate in the decision-making process raises complicated ethical questions. The implications of their absence are not immediately obviously. Future generations have a stake both in the environmental resources, such as climate, that they inherit, and in other wealth that is passed down to them. Sacrifices that are made by the present generation for the good of its descendants will alter the composition of wealth (e.g., environmental versus material) that is transferred from the present to the future, as well as the magnitude of the transfer. As climate change is anticipated to be greater in the future than it is at present, those who live in the future will reap most of the benefits that accrue to near-term actions to limit emissions. Intergenerational asymmetry can lead to a form of public goods problem in which the willingness to undertake emissions mitigation in the near-term may be less than would have been the case if the decision makers lived infinitely. Also implied is a greater sensitivity to emission-limitation costs than would be the case if the present generation lived to benefit from its emissions-mitigation actions.
To limit the concentration of atmospheric carbon dioxide, global carbon
emissions must eventually peak and then decline
This result follows from the nature of the carbon cycle, as it is presently understood. While non-CO2 GHGs with relatively short life times, such as methane (CH4), have an atmospheric concentration that is stable with a stable rate of annual emission, CO2 does not. The cumulative net introduction of carbon emissions from terrestrial reservoirs, such as fossil fuels or biological carbon, through (for example) energy production and use or land-use change, determines the long-term, steady state, atmospheric CO2 concentration. Carbon cycle models require net emissions to asymptotically approach zero, though the process can take centuries. Most, but not all, emissions scenarios anticipate that, in the absence of a concern for climate change, future GHG emissions will continue to rise rather than fall (IPCC, 2000b). Where reference emissions scenarios exhibit increasing emissions over time, most of the emissions mitigation required to stabilize the concentration of carbon must occur in the future, with the deviation from the profile required for stabilization growing with time.
While emissions limitation is a policy response, it is not the only policy
response available to decision makers
In addition to emissions limitations, policymakers have a wide array of other tools at their disposal including knowledge gathering, research and development of technologies to reduce emissions and enhancing the resilience of societies experiencing climate change. The optimal and actual mix of policy responses will vary over time.
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