Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

Many empirical questions in economics and other social sciences depend on causal effects of programs or policies. In the last two decades, much research has been done on the econometric and statistical analysis of such causal effects. This recent theoreti- cal literature has built on, and combined features of, earlier work in both the statistics and econometrics literatures. It has by now reached a level of maturity that makes it an important tool in many areas of empirical research in economics, including labor economics, public finance, development economics, industrial organization, and other areas of empirical microeconomics. In this review, we discuss some of the recent developments. We focus primarily on practical issues for empirical research- ers, as well as provide a historical overview of the area and give references to more technical research.

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Recent developments in the econometrics of program evaluation

The Lancet Commission on pollution and health

A common approach to evaluating robustness to omitted variable bias is to observe coefficient movements after inclusion of controls. This is informative only if selection on observables is informative about selection on unobservables. Although this link is known in theory in existing literature, very few empirical articles approach this formally. I develop an extension of the theory that connects bias explicitly to coefficient stability. I show that it is necessary to take into account coefficient and R-squared movements. I develop a formal bounding argument. I show two validation exercises and discuss application to the economics literature. Supplementary materials for this article are available online.

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Unobservable Selection and Coefficient Stability: Theory and Evidence

A long-standing question in social science is to what extent differences in management cause differences in firm performance. To investigate this, the authors ran a management field experiment on large Indian textile firms, providing free consulting on modern management practices to a randomly chosen set of treatment plants and compared their performance to the control plants. They find that adopting these management practices had three main effects. First, it raised average productivity by 11 percent through improved quality and efficiency and reduced inventory. Second, it increased decentralization of decision making, as better information flow enabled owners to delegate more decisions to middle managers. Third, it increased the use of computers, necessitated by the data collection and analysis involved in modern management. Since these practices were profitable this raises the question of why firms had not adopted these before. Their results suggest that informational barriers were a primary factor in explaining this lack of adoption. Modern management is a technology that diffuses slowly between firms, with many Indian firms initially unaware of its existence or impact. Since competition was limited by constraints on firm entry and growth, badly managed firms were not rapidly driven from the market.

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Does Management Matter? Evidence from India

Designing public transport networks involves tradeoffs between extensive geographic coverage, frequent service on each route, and relying on interconnections as opposed to direct service. These choices, in turn, depend on individual preferences for waiting times, travel times, and transfers. We study these tradeoffs by examining the world's largest bus rapid transit system, in Jakarta, Indonesia, leveraging a large network expansion between 2016-2020. Using detailed ridership data and aggregate travel flows from smartphone data, we analyze how new direct connections, changes in bus travel time, and wait time reductions increase ridership and overall trips. We set up and estimate a transit network demand model with multi-dimensional travel costs, idiosyncratic heterogeneity induced by random wait times, and inattention, matching eventstudy moments from the route launches. Commuters in Jakarta are 2-4 times more sensitive to wait time compared to time on the bus, and inattentive to long routes. To study the implications for network design, we introduce a new framework to describe the set of optimal networks. Our results suggest that a less concentrated network would increase ridership and commuter welfare. Gabriel Kreindler Department of Economics Harvard University 1805 Cambridge Street Cambridge, MA 02138 and NBER gkreindler@fas.harvard.edu Arya Gaduh Department of Economics University of Arkansas 402 Business Building Fayatteville, AR 72701 and NBER AGaduh@walton.uark.edu Tilman Graff Harvard University Economics Department tgraff@g.harvard.edu Rema Hanna Kennedy School of Government Harvard University 79 JFK Street Cambridge, MA 02138 and NBER Rema_Hanna@hks.harvard.edu Benjamin A. Olken Department of Economics, E52-542 MIT 77 Massachusetts Avenue Cambridge, MA 02139 and NBER bolken@mit.edu

Optimal Public Transportation Networks:

Outsourcing government service provision to private firms can improve efficiency and reduce rents, but there are risks that non-contractible quality will decline and that reform could be blocked by vested interests exactly where potential gains are greatest. We examine these issues by conducting a randomized field experiment in 572 Indonesian localities in which a procurement process was introduced that allowed citizens to bid to take over the implementation of a subsidized rice distribution program. This led 17 percent of treated locations to switch distributors. Introducing the possibility of outsourcing led to a 4.6 percent reduction in the markup paid by households. Quality did not suffer and, if anything, households reported the quality of the rice improved. Bidding committees may have avoided quality problems by choosing bidders who had relevant experience as traders, even if they proposed slightly higher prices. Mandating higher levels of competition by encouraging additional bidders further reduced prices. We document offsetting effects of having high rents at baseline: when the initial price charged was high and when baseline satisfaction levels were low, entry was higher and committees were more likely to replace the status quo distributor; but, incumbents measured to be more dishonest on an experimental measure of cheating were also more likely to block the outsourcing process. We find no effect on price or quality of providing information about program functioning without the opportunity to privatize, implying that the observed effect was not solely due to increased transparency. On net, the results suggest that contracting out has the potential to improve performance, though the magnitude of the effects may be partially muted due to push back from powerful elites.

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Contracting out the Last-Mile of Service Delivery: Subsidized Food Distribution in Indonesia

The Marginal Disutility from Corruption in Social Programs: Evidence from Program Administrators and Beneficiaries

Social protection programs have become increasingly widespread in lowand middleincome countries, with their own distinct characteristics to match the environments in which they are operating. This paper reviews the growing literature on the design and impact of these programs. We review how to identify potential beneficiaries given the large informal sector, the design and implementation of redistribution and income support programs, and the challenges and potential of social insurance. We use our frameworks as a guide for consolidating and organizing the existing literature, and also to highlight areas and questions for future research. 1 We thank Amy Finkelstein, Nathan Hendren, the JEL editors, and four anonymous referees for helpful comments. We thank Emily Romano, Beatriz Velho and especially Kim Lan Mallon for outstanding research assistance. Financial support from the Wellspring Philanthropic Fund and the Australian Department of Foreign Affairs and Trade is gratefully acknowledged. The views expressed in this publication are the authors’ alone and are not necessarily the views of the Australian Government. Sverdlin-Lisker: This research was generously supported by the National Institute on Aging, Grant Number T32-AG000186 and is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1745302. 1

Rema Hanna

Papers

Optimal Public Transportation Networks:

Contracting out the Last-Mile of Service Delivery: Subsidized Food Distribution in Indonesia

The Marginal Disutility from Corruption in Social Programs: Evidence from Program Administrators and Beneficiaries

Social Protection in the Developing World1