We present a pedagogical paper on the detection of climate change and its attribution to anthropogenic influences. We attempt to separate the key thought processes and tools that are used when making qualitative statements about the level of human influence on climate. / Includes bibliographical references (p. 9). / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)

Given the large uncertainties regarding potential damages from climate change and the significant but also uncertain costs of reducing greenhouse emissions, the debate over a policy response is often framed as a choice of either acting now or waiting until the uncertainty is reduced. Implicit behind the "wait to learn" argument is the notion that the ability to learn in the future necessarily implies that less restrictive policies should be chosen in the near-term. I demonstrate in the general case that the ability to learn in the future can lead to either less restrictive or more restrictive policies today. I also show that the initial decision made under uncertainty will be affected by future learning only if the actions taken today change the marginal costs or marginal damages in the future. Without this interaction, learning has no effect on what we do today, regardless of what we learn in the future. Results from an intermediate-scale integrated model of climate and economics indicate that the choice of current emissions restrictions is independent of whether or not uncertainty is resolved before future decisions, because the cross-period interactions in the model are minimal. Indeed, most climate and economic models fail to capture potentially important cross-period interaction effects. I construct a simple example to show that with stronger interactions, the effect of learning on initial period decisions can be more important. / Includes bibliographical references (p. 21). / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Sponsored in part by the U.S. Dept. of Energy (901214-HAR DE-FG02-94ER61937, DE-FG02-93ER61713)

In March 2000, the European Commission presented a Green Paper on greenhouse gas emissions trading within Europe, supporting implementation of a Community-wide scheme in which the design and regulation of all essential elements would be harmonized at the Community level. The present paper analyzes economic arguments used to justify such a coordinated scenario, showing these arguments to be based on misleading rhetoric about fair trade and harmonization. Diverse allocations of emissions allowances across Member States are justified in theory. In practice, too, no empirical evidence or model-based results demonstrate that an uncoordinated European trading scheme would adversely affect competitiveness to any significant extent or substantially increase industrial relocations. / Includes bibliographical references (p. 9-11). / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Supported in part by the Institut Français de l'Energie.

Includes bibliographical references (p. 16). / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)

Includes bibliographical references (p. 14). / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)

We have incorporated a reduced-form urban air chemistry model in MIT's 2D-LO coupled chemistry-climate model. The computationally efficient reduced-form urban model is derived from the California Institute of Technology-Carnegie Institute of Technology (at Carnegie Mellon University) Urban Airshed Model by employing the probabilistic collocation method. To study the impact of urban air pollution on global chemistry and climate we carried out three simulations each including or excluding the reduced-form urban model for the time period from 1977 to 2100. In all three runs we use identical emissions, however in the two runs involving the reduced-form urban model the emissions assigned to urban areas are allocated in different ways depending on the scenario we assume for the future development of polluted urban areas. These two simulations are compared to the reference, which does not utilize the reduced-form urban model. We find that the incorporation of the urban air chemistry processes leads to lower global tropospheric NOx, ozone, and OH concentrations, but to a higher methane mole fraction than in the reference. The tropospheric mole fraction of CO is altered either up or down depending on the projections of urban emissions. The global mean surface temperature is effected very little by the implementation of the reduced-form urban model because predicted increases in CH4 are offset in part by decreases in O3 leading to only small changes in overall radiative forcing. / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Includes bibliographical references (p. 26-29).

We present a method for constraining key properties of the climate system that are important for climate prediction (climate sensitivity and rate of heat penetration into the deep ocean) by comparing a model's response to known forcings over the 20th century against climate observations for that period. We use the MIT 2D climate model in conjunction with results from the Hadley Centre's coupled atmosphere-ocean general circulation model (AOGCM) to determine these constraints. The MIT 2D model is a zonally-averaged version of a 3D GCM which can accurately reproduce the global-mean transient response of coupled AOGCMs through appropriate choices of the climate sensitivity and the effective rate of diffusion of heat into the deep ocean. Vertical patterns of zonal mean temperature change through the troposphere and lower stratosphere also compare favorably with those generated by 3-D GCMs. We compare the height-latitude pattern of temperature changes as simulated by the MIT 2D model with observed changes, using optimal fingerprint detection statistics. Interpreted in terms of a linear regression model as in Allen and Tett (1998), this approach yields an objective measure of model-observation goodness-of-fit (via the normalized residual sum of squares). The MIT model permits one to systematically vary the model's climate sensitivity (by varying the strength of the cloud feedback) and rate of mixing of heat into the deep ocean and determine how the goodness-of-fit with observations depends on these factors. This approach provides an efficient framework for interpreting detection and attribution results in physical terms. For the aerosol forcing set in the middle of the IPCC range, two sets of model parameters are rejected as being implausible when the model response is compared with observations. The first set corresponds to high climate sensitivity and low heat uptake by the deep ocean. The second set corresponds to low sensitivities for all values of heat uptake. These results demonstrate that fingerprint patterns must be carefully chosen, if their detection is to reduce the uncertainty of physically important model parameters which affect projections of climate change. / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Includes bibliographical references (p. 28-31).

The experience of other environmental problems suggests that policies yielding uniform marginal costs across sectors, as most analyses assume, are not likely to be realized in practice. Some sectors will be favored over others, yielding different levels of control. Using the MIT Emissions Prediction and Policy Analysis Model, the national cost of such differentiation across sectors is shown to be very high. Moreover, because of interactions and feedbacks in the economy, measures that differentiate in this way may not even aid the sectors they are intended to protect. / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Includes bibliographical references (p. 15).

Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Includes bibliographical references (p. 26-28).

Article 17 of the Kyoto Protocol allows Annex B parties to meet their commitments by trading greenhouse gas emissions reductions "supplemental" to domestic emissions control. We demonstrate that implementing supplementarity by imposing concrete ceilings on imports of allowances in a market for tradable emissions rights gives rise to monopsonistic effects, even with price-taking behavior by both buyers and sellers. We assess the importance of this finding for Annex B emissions trading, in the context of the import and export provisions of the recent EU Proposal on supplementarity. Our results show that the proposal would reduce efficiency, and could significantly alter the distribution of the gains from trade in an Annex B tradable permits market. / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/) / Includes bibliographical references (p. 16).