Brian C. O'Neill

Global demographic trends and future carbon emissions

Substantial changes in population size, age structure, and urbanization are expected in many parts of the world this century. Although such changes can affect energy use and greenhouse gas emissions, emissions scenario analyses have either left them out or treated them in a fragmentary or overly simplified manner. We carry out a comprehensive assessment of the implications of demographic change for global emissions of carbon dioxide. Using an energy–economic growth model that accounts for a range of demographic dynamics, we show that slowing population growth could provide 16–29% of the emissions reductions suggested to be necessary by 2050 to avoid dangerous climate change. We also find that aging and urbanization can substantially influence emissions in particular world regions.

A unifying framework for metrics for aggregating the climate effect of different emissions

Multi-gas approaches to climate change policies require a metric establishing ?equivalences? among emissions of various species. Climate scientists and economists have proposed four classes of such metrics and debated their relative merits. We present a unifying framework that clarifies the relationships among them. We show that the Global Warming Potential, used in international law to compare greenhouse gases, is a special case of the Global Damage Potential, assuming (1) a finite time horizon, (2) a zero discount rate, (3) constant atmospheric concentrations, and (4) impacts that are proportional to radiative forcing. We show that the Global Temperature change Potential is a special case of the Global Cost Potential, assuming (1) no induced technological change, and (2) a short-lived capital stock. We also show that the Global Cost Potential is a special case of the Global Damage Potential, assuming (1) zero damages below a threshold and (2) infinite damage after a threshold. The UN Framework Convention on Climate Change uses the Global Warming Potential, a simplified cost-benefit concept, even though the UNFCCC frames climate policy as a cost-effectiveness problem and should therefore use the Global Cost Potential or its simplification, the Global Temperature Potential.

Mitigation implications of midcentury targets that preserve long-term climate policy options

Midcentury targets have been proposed as a guide to climate change policy that can link long-term goals to shorter-term actions. However no explicit mitigation analyses have been carried out of the relationship between midcentury conditions and longer-term outcomes. Here we use an integrated assessment modeling framework with a detailed representation of the energy sector to examine the dependence of climate change outcomes in 2100 on emissions levels, atmospheric concentrations, and technology characteristics in 2050. We find that midcentury conditions are crucial determinants of longer-term climate outcomes, and we identify feasibility thresholds describing conditions that must be met by midcentury to keep particular long-term options open. For example, to preserve the technical feasibility of a 50% likelihood of keeping global average temperature at < 2 °C above preindustrial in 2100, global emissions must be reduced by about 20% below 2000 levels by 2050. Results are sensitive to several assumptions, including the nature of future socio-economic development. In a scenario with high demand for energy and land, being below 2 °C with 50% likelihood requires a 50% reduction in emissions below 2000 levels by 2050, which is only barely feasible with known technologies in that scenario. Results suggest that a greater focus on midcentury targets could facilitate the development of policies that preserve potentially desirable long-term options.

Demographic change and carbon dioxide emissions

Relations between demographic change and emissions of the major greenhouse gas carbon dioxide (CO(2)) have been studied from different perspectives, but most projections of future emissions only partly take demographic influences into account. We review two types of evidence for how CO(2) emissions from the use of fossil fuels are affected by demographic factors such as population growth or decline, ageing, urbanisation, and changes in household size. First, empirical analyses of historical trends tend to show that CO(2) emissions from energy use respond almost proportionately to changes in population size and that ageing and urbanisation have less than proportional but statistically significant effects. Second, scenario analyses show that alternative population growth paths could have substantial effects on global emissions of CO(2) several decades from now, and that ageing and urbanisation can have important effects in particular world regions. These results imply that policies that slow population growth would probably also have climate-related benefits.

Climate impacts of geoengineering in a delayed mitigation scenario

Decarbonization in the immediate future is required to limit global mean temperature (GMT) increase to 2 degrees C relative to pre-industrial conditions, if geoengineering is not considered. Here we use the Community Earth System Model (CESM) to investigate climate outcomes if no mitigation is undertaken until GMT has reached 2 degree C. We find that late decarbonization (LD) in CESM without applying stratospheric sulfur injection (SSI) leads to a peak temperature increase of 3 degree C and GMT remains above 2 degrees for 160 years. An additional gradual increase and then decrease of SSI over this period reaching about 1.5 times the aerosol burden resulting from the Mt Pinatubo eruption in 1992 would limit the increase in GMT to 2.0 degrees for the specific pathway and model. SSI produces mean and extreme temperatures in CESM comparable to an early decarbonization pathway, but aridity is not mitigated to the same extent.

Interim targets and the climate treaty regime

Abstract We propose that international climate change policy would be strengthened by the development and adoption of targets for atmospheric concentrations of greenhouse gases 25–50 years in the future in addition to near- and long-term targets. ‘Interim concentration targets’, which could be accommodated under the current Convention/Protocol framework, would provide several advantages over the current focus on either the short term (e.g. Kyoto Protocol) or the long term (e.g. ultimate stabilization targets). Interim targets would help constrain rates of climate change (which are not sufficiently addressed by short- or long-term targets, even when paired together). They would also provide a means for keeping open the option of achieving a range of long-term goals while uncertainty (and political disagreement) over the appropriate goal is resolved. We substantiate a number of rationales for such an approach, discuss the use of interim targets in other contexts, and illustrate how such targets for climate change policy might be set.

Cairo and climate change: a win–win opportunity

Abstract The landmark Program of Action agreed to at the 1994 International Conference on Population and Development in Cairo calls for a wide range of population-related policies motivated primarily by the improvement of individual well being. Currently, a funding shortfall threatens continued progress toward the Cairo goals. This shortfall risks missing an opportunity not only to improve the lives of individuals around the world, but also to reduce the environmental consequences of population growth. Recent estimates of environmental externalities to childbearing associated with global climate change indicate that climate-related returns to investments in such policies could be of the same order of magnitude as the investments themselves. Thus, continued support of the Cairo program is clearly a “win–win” strategy.

A new toolkit for developing scenarios for climate change research and policy analysis

WWW.ENVIRONMENTMAGAZINE.ORG VOLUME 56 NUMBER 2 Y ogi Berra succinctly captured how economic growth, technology change, demographic change, and climate change are altering visions of what the future could bring: “the future ain’t what it used to be.”1 Understanding the range and character of possible futures is critical to furthering assessment of climate change, including the potential risks to physical, natural, and human systems in the context of different development pathways, and mitigation and adaptation options to avoid, prepare for, and manage those risks. Because concerns about climate change span the current to the far future, the field has a long history of using scenarios to explore and evaluate the extensive uncertainties associated with future climate change and development pathways. Projecting possible impacts under different futures and identifying the trade-offs and synergies of adaptation and mitigation policies require scenarios that include (1) the drivers of greenhouse gas emissions, (2) the resulting emissions, (3) assumptions about other drivers of socioeconomic development that will affect the magnitude and pattern of impacts, and/or the ability to avoid, prepare for, cope with, and recover from climate change, and (4) the adaptation and mitigation policy environment.

Comment on: 'The lifetime of excess atmospheric carbon dioxide' by Berrien Moore III and B. H. Braswell

Article original: Moore, B. III, and B. H. Braswell, Global Biogeochem. Cycles, 8, 23-38, 1994.

Developing Climate Model Comparisons

Since 1995, the worldwide climate modeling community has designed and participated in a series of intercomparison projects for the purposes of understanding and improving model performance, investigating scientific questions about the climate system, and projecting future climate conditions. These projects have been defined under the umbrella of the Coupled Model Intercomparison Project, and phase 6 of that project (CMIP6) is just getting under way. As in previous phases, many CMIP6 modeling activities interact and overlap with each other. For example, credible projections of future climate conditions require understanding and validating a variety of Earth system model responses, including those to changes in concentrations of greenhouse gases, aerosols and other air pollutants, and land use change.