JP

The role of non-CO2 greenhouse gases in climate policy: Analysis using the MIT IGSM

First steps toward a broad climate agreement, such as the Kyoto Protocol, have focused on less than global geographic coverage. We consider instead a policy that is less comprehensive in term of greenhouse gases (GHGs), including only the non-CO2 GHGs, but is geographically comprehensive. Abating non-CO2 GHGs may be seen as less of a threat to economic development and therefore it may be possible to involve developing countries in such a policy even though they have resisted limits on CO2 emissions. The policy we consider involves a GHG price of about $15 per ton carbon-equivalent (tce) levied only on the non-CO2 GHGs and held at that level through the century. We estimate that such a policy would reduce the global mean surface temperature in 2100 by about 0.55 °C if only methane is covered that alone would achieve a reduction of 0.3 to 0.4°C. We estimate the Kyoto Protocol in its current form would achieve a 0.25°C reduction in 2100 if Parties to it maintained it as is through the century. Furthermore, we estimate the costs of the non-CO2 policies to be a small fraction of the Kyoto policy. Whether as a next step to expand the Kyoto Protocol, or as a separate initiative running parallel to it, the world could well make substantial progress on limiting climate change by pursuing an agreement to abate the low cost non-CO2 GHGs. The results suggest that it would be useful to proceed on global abatement of non-CO2 GHGs so that lack of progress on negotiations to limit CO2 does not allow these abatement opportunities to slip away. © 2006 IAEE

The role of nutricline depth in regulating the ocean carbon cycle

Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior (i.e., the "biological pump"), lowers the partial pressure of carbon dioxide (pCO2) in the upper ocean and facilitates the diffusive drawdown of atmospheric CO2. Conversely, precipitation of calcium carbonate by marine planktonic calcifiers such as coccolithophorids increases pCO2 and promotes its outgassing (i.e., the "alkalinity pump"). Over the past ≈100 million years, these two carbon fluxes have been modulated by the relative abundance of diatoms and coccolithophores, resulting in biological feedback on atmospheric CO2 and Earth's climate; yet, the processes determining the relative distribution of these two phytoplankton taxa remain poorly understood. We analyzed phytoplankton community composition in the Atlantic Ocean and show that the distribution of diatoms and coccolithophorids is correlated with the nutricline depth, a proxy of nutrient supply to the upper mixed layer of the ocean. Using this analysis in conjunction with a coupled atmosphere–ocean intermediate complexity model, we predict a dramatic reduction in the nutrient supply to the euphotic layer in the coming century as a result of increased thermal stratification. Our findings indicate that, by altering phytoplankton community composition, this causal relationship may lead to a decreased efficiency of the biological pump in sequestering atmospheric CO2, implying a positive feedback in the climate system. These results provide a mechanistic basis for understanding the connection between upper ocean dynamics, the calcium carbonate-to-organic C production ratio and atmospheric pCO2 variations on time scales ranging from seasonal cycles to geological transitions.

The role of nutricline depth in regulating the ocean carbon cycle

The role of science in policy: The climate change debate in the United States

Why is the U.S. finding it so difficult to agree on policies to address an ecological threat that, if it materializes, could have catastrophic consequences for itself and the rest of the world? Although much of the controversy surrounding global warming appears to revolve around scientific principles, political and economic forces actually dominate. This paper discusses the primary factors that determine policy outcomes (e.g., uncertainty, the structure of government, economic impacts, the media) and demonstrates how scientific knowledge interacts with the formulation of policy in the U.S.

The Role of Vegetation Dynamics in the Climate of West Africa

The Role of Vegetation in Sustaining Large-scale Atmospheric Circulations in the Tropics

The focus of this paper is the role of rain forests in large-scale atmospheric circulations. The significance of this role is investigated by studying the response of the tropical atmosphere to a perturbation in the state of vegetation (deforestation) over three regions: the Amazon, Congo, and Indonesia. A theory is developed to relate tropical deforestation and thc resulting changes in the large-scale atmospheric circulation. Field observations and numerical simulations support the argument that tropical deforestation reduces the total net surface radiation, including terrestrial and solar forms. However, the energy balance at the land-atmospheric boundary dictates that for equilibrium conditions, any reduction in net surface radiation has to be balanced by a similar reduction in the total flux of heat, including sensible and latent forms. Since these fluxes supply heat as well as entropy from the forest into the atmospheric boundary layer, a reduction in the total flux of heat reduces the boundary layer entropy. In a moist atmosphere, that satisfies a quasi equilibrium between moist convection and radiative forcing, the equilibrium temperature profile is uniquely related to the boundary layer entropy. Under such conditions, large-scale deforestation reduces boundary layer entropy relative to the surroundings, cools the upper troposphere, and results in subsidence, divergent flow in the boundary layer, and weakening of the large-scale circulation. These changes are simulated using a simple linear model of atmospheric flow. The comparison of the model predictions with observations of atmospheric circulations over the Amazon, Congo, and Indonesia suggests a significant role for vegetation in maintaining large-scale atmospheric circulations in the tropics.

The Role of Vegetation in the Dynamics of West African Monsoons

The focus of this paper is the role of meridional distribution of vegetation in the dynamics of monsoons and rainfall over West Africa. We develop a moist zonally symmetric atmospheric model coupled with a simple land surface scheme to investigate these processes. Four primary experiments have been carried out to examine the sensitivity of West African monsoons to perturbations in vegetation patterns. Each perturbation experiment is identical to the control experiment except that a change in vegetation cover is imposed for a latitudinal belt of 10° in width. The numerical experiments demonstrate that West African monsoons and therefore rainfall depend critically on the location of the vegetation perturbations. While the magnitude of local rainfall is sensitive to changes in local vegetation, the location of the Inter-Tropical Convergence Zone (ITCZ) is not sensitive to changes in the vegetation northward or southward from the location of ITCZ in the control experiment. However, the location of the ITCZ is sensitive to changes of the vegetation distribution in the immediate vicinity of the location of the ITCZ in the control experiment. The modeling results indicate that changes in vegetation cover along the border between the Sahara desert and West Africa (desertification) have a minor impact on the simulated monsoon circulation. On the other hand, coastal deforestation may cause the collapse of the monsoon circulation and have a dramatic impact on the regional rainfall. The observed deforestation in West Africa is then likely to be a significant contributor to the observed drought.

The Roles of Mixing, Geothermal and Surface Buoyancy Forcing in Ocean Meridional Overturning Dynamics

The response of the ocean's meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using a coupled model of intermediate complexity, including a dynamic 3D ocean subcomponent. Parameters are the increase in CO2 forcing (with stabilization after a specified time interval) and the model's climate sensitivity. In this model, the cessation of deep sinking in the north "Atlantic" (hereinafter, a "collapse"), as indicated by changes in the MOC, behaves like a simple bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO2-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas low CO2-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine how the rate of forcing, for a given CO2 stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models, we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise collapse.

The safety valve and climate policy

The "safety valve" is a possible addition to a cap-and-trade system of emissions regulation whereby the authority offers to sell permits in unlimited amount at a pre-set price. In this way the cost of meeting the cap can be limited. It was proposed in the U.S. as a way to control perceived high costs of the Kyoto Protocol, and possibly as a way to shift the focus of policy from the quantity targets of the Protocol to emissions price. In international discussions, the idea emerged as a proposal for a compliance penalty. The usefulness of the safety valve depends on the conditions under which it might be introduced. For a time it might tame an overly stringent emissions target. It also can help control the price volatility during the introduction of gradually tightening one, although permit banking can ultimately serve the same function. It is unlikely to serve as a long-term feature of a cap-and-trade system, however, because of the complexity of coordinating price and quantity instruments and because it will interfere with the development of systems of international emissions trade.

The Sensitivity of Tropical Convective Precipitation to the Direct Radiative Forcings of Black Carbon Aerosols Emitted from Major Regions

Previous works have suggested that the direct radiative forcing (DRF) of black carbon (BC) aerosols are able to force a significant change in tropical convective precipitation ranging from the Pacific and Indian Ocean to the Atlantic Ocean. In this in-depth analysis, the sensitivity of this modeled effect of BC on tropical convective precipitation to the emissions of BC from 5 major regions of the world has been examined. In a zonal mean base, the effect of BC on tropical convective precipitation is a result of a displacement of ITCZ toward the forcing (warming) hemisphere. However, a substantial difference exists in this effect associated with BC over different continents. The BC effect on convective precipitation over the tropical Pacific Ocean is found to be most sensitive to the emissions from Central and North America due to a persistent presence of BC aerosols from these two regions in the lowermost troposphere over the Eastern Pacific. The BC effect over the tropical Indian and Atlantic Ocean is most sensitive to the emissions from South as well as East Asia and Africa, respectively. Interestingly, the summation of these individual effects associated with emissions from various regions mostly exceeds their actual combined effect as shown in the model run driven by the global BC emissions, so that they must offset each other in certain locations and a nonlinearity of this type of effect is thus defined. It is known that anthropogenic aerosols contain many scattering-dominant constituents that might exert an effect opposite to that of absorbing BC. The combined aerosol forcing is thus likely differing from the BC-only one. Nevertheless, this study along with others of its kind that isolates the DRF of BC from other forcings provides an insight of the potentially important climate response to anthropogenic forcings particularly related to the unique particulate solar absorption.

MIT Global Change

JP

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