Monday, July 25, 2016

Data and Code for Our 1997 Paper in Nature

I got a request for the data in our 1997 paper in Nature on climate change. I didn't think I'd be able to send the actual data we used as I used to follow the practice of continually updating the datasets that I most used rather than keeping an archival copy of the data actually used in a paper. But I found a version from February 1997, which was the month we submitted the final version of the paper. I got the RATS code to read the file and with a few tweaks it was producing the results that are in the paper. These are the results for observational data in the paper, not those using data from the Hadley climate model. I have now put up the files on my website. In the process I found this website - - that can convert .wks to .xls files. Apparently, recent versions of Excel can't read the .wks Lotus 1-2-3 files that were a standard format 20 or more years years ago. For those that don't know, Lotus 1-2-3 was the most popular spreadsheet program before Microsoft introduced Excel. I used it in the late 80s and early 90s when I was in grad school.

The EKC in a Nutshell

The environmental Kuznets curve (EKC) is a hypothesized relationship between various indicators of environmental degradation and countries’ gross domestic product (GDP) per capita. In the early stages of economic growth environmental impacts and pollution increase, but beyond some level of GDP per capita (which will vary for different environmental impacts) economic growth leads to environmental improvement. This implies that environmental impacts or emissions per capita are an inverted U-shaped function of GDP per capita, whose parameters can be statistically estimated. Figure 1 shows a very early example of an EKC. A vast number of studies have estimated such curves for a wide variety of environmental impacts ranging from threatened species to nitrogen fertilizers, though atmospheric pollutants such as sulfur dioxide and carbon dioxide have been most commonly investigated. The name Kuznets refers to the similar relationship between income inequality and economic development proposed by Nobel Laureate Simon Kuznets and known as the Kuznets curve.

The EKC has been the dominant approach among economists to modeling ambient pollution concentrations and aggregate emissions since Grossman and Krueger (1991) introduced it in an analysis of the potential environmental effects of the North American Free Trade Agreement. The EKC also featured prominently in the 1992 World Development Report published by the World Bank and has since become very popular in policy and academic circles and is even found in introductory economics textbooks.

Despite this, the EKC was criticized almost from the start on empirical and policy grounds, and debate continues. It is undoubtedly true that some dimensions of environmental quality have improved in developed countries as they have become richer. City air and rivers in these countries have become cleaner since the mid-20th Century and in some countries forests have expanded. Emissions of some pollutants such as sulfur dioxide have clearly declined in most developed countries in recent decades. But there is less evidence that other pollutants such as carbon dioxide ultimately decline as a result of economic growth. There is also evidence that emerging countries take action to reduce severe pollution. For example, Japan cut sulfur dioxide emissions in the early 1970s following a rapid increase in pollution when its income was still below that of the developed countries and China has also acted to reduce sulfur emissions in recent years.

As further studies were conducted and better data accumulated, many of the econometric studies that supported the EKC were found to be statistically fragile. Figure 2 presents much higher quality data with a much more comprehensive coverage of countries than that used in Figure 1. In both 1971 and 2005 sulfur emissions tended to be higher in richer countries and the curve seems to have shifted down and to the right. A cluster of mostly European countries had succeeded in sharply cutting emissions by 2005 but other wealthy countries reduced their emissions by much less.

Initially, many understood the EKC to imply that environmental problems might be due to a lack of sufficient economic development rather than the reverse, as was conventionally thought, and some argued that the best way for developing countries to improve their environment was to get rich. This alarmed others, as while this might address some issues like deforestation or local air pollution, it would likely exacerbate other environmental problems such as climate change.

The existence of an EKC can be explained either in terms of deep determinants such as technology and preferences or in terms of scale, composition, and technique effects, also known as “proximate factors”. Scale refers to the effect of an increase in the size of the economy, holding the other effects constant, and would be expected to increase environmental impacts. The composition and technique effects must outweigh this scale effect for pollution to fall in a growing economy. The composition effect refers to the economy’s mix of different industries and products, which differ in pollution intensities. Finally the technique effect refers to the remaining change in pollution intensity. This will include contributions from changes in the input mix – e.g. substituting natural gas for coal; changes in productivity that result in less use, everything else constant, of polluting inputs per unit of output; and pollution control technologies that result in less pollutant being emitted per unit of input.

Over the course of economic development the mix of energy sources and economic outputs tends to evolve in predictable ways. Economies start out mostly agricultural and the share of industry in economic activity first rises and then falls as the share of agriculture declines and the share of services increases. We might expect the impacts associated with agriculture, such as deforestation, to decline, and naively expect the impacts associated with industry such as pollution would first rise and then fall. However, the absolute size of industry rarely does decline and it is improvement in productivity in industry, a shift to cleaner energy sources, such as natural gas and hydro-electricity, and pollution control that eventually reduce some industrial emissions.

Static theoretical economic models of deep determinants, that do not try to also model the economic growth process, can be summarized in terms of two parameters: The elasticity of substitution between dirty and clean inputs or between pollution control and pollution, which summarizes how difficult it is to cut pollution; and the elasticity of marginal utility, which summarizes how hard it is to increase consumer well-being with more consumption. It is usually assumed that these consumer preferences are translated into policy action. Pollution is then more likely to increase as the economy expands, the harder it is to substitute other inputs for polluting ones and the easier it is to increase consumer well-being with more consumption. If these parameters are constant then either pollution rises or falls with economic growth. Only if they change over time will pollution first rise and then fall. The various theoretical models can be classified as ones where the EKC is driven by changes in the elasticity of substitution as the economy grows or models where the EKC is primarily driven by changes in the elasticity of marginal utility.

Dynamic models that model the economic growth process alongside changes in pollution, are harder to classify. The best known is the Green Solow Model developed by Brock and Taylor (2010) that explains changes in pollution as a result of the competing effects of economic growth and a constant rate of improvement in pollution control. Fast growing middle-income countries, such as China, then having rising pollution, and slower growing developed economies, falling pollution. An alternative model developed by Ordás Criado et al. (2011) also suggests that pollution rises faster in faster growing economies but that there is also convergence so that countries with higher levels of pollution are more likely to reduce pollution faster than countries with low levels of pollution.

Recent Empirical Research and Conclusion 
Recent empirical research builds on these dynamic models painting a subtler picture than did early EKC studies. We can distinguish between the impact of economic growth on the environment and the effect of the level of GDP per capita, irrespective of whether an economy is growing or not, on reducing environmental impacts. Economic growth usually increases environmental impacts but the size of this effect varies across impacts and the impact of growth often declines as countries get richer. However, richer countries are often likely to make more rapid progress in reducing environmental impacts. Finally, there is often convergence among countries, so that countries that have relatively high levels of impacts reduce them faster or increase them slower. These combined effects explain more of the variation in pollution emissions or concentrations than either the classic EKC model or models that assume that either only convergence or growth effects alone are important. Therefore, while being rich means a country might do more to clean up its environment, getting rich is likely to be environmentally damaging and the simplistic policy prescriptions that some early proponents of the EKC put forward should be disregarded.

Brock, W. A. and Taylor, M. S. (2010). The green Solow model. Journal of Economic Growth 15, 127–153.

Grossman, G. M. and Krueger, A. B. (1991). Environmental impacts of a North American Free Trade Agreement. NBER Working Papers 3914.

Ordás Criado, C., Valente, S., and Stengos, T. (2011). Growth and pollution convergence: Theory and evidence. Journal of Environmental Economics and Management 62, 199-214.

Panayotou, T. (1993). Empirical tests and policy analysis of environmental degradation at different stages of economic development. Working Paper, Technology and Employment Programme, International Labour Office, Geneva, WP238.

Smith, S. J., van Ardenne, J., Klimont, Z., Andres, R. J., Volke, A., and Delgado Arias S. (2011). Anthropogenic sulfur dioxide emissions: 1850-2005. Atmospheric Chemistry and Physics 11, 1101-1116.

Stern, D. I. (2015). The environmental Kuznets curve after 25 years. CCEP Working Papers 1514.

Stern, D. I., Common, M. S., and Barbier, E. B. (1996). Economic growth and environmental degradation: the environmental Kuznets curve and sustainable development. World Development 24, 1151–1160.

Thursday, July 21, 2016

Dynamics of the Environmental Kuznets Curve

Just finished writing a survey of the environmental Kuznets curve (EKC) for the Oxford Research Encyclopedia of Environmental Economics. Though I updated all sections, of course, there is quite a bit of overlap with my previous reviews. But there is a mostly new review of empirical evidence reviewing the literature and presenting original graphs in the spirit of IPCC reports :) I came up with this new graph of the EKC for sulfur emissions:

The graph plots the growth rate from 1971 to 2005 of per capita sulfur emissions in the sample used in the Anjum et al. (2014) paper against GDP per capita in 1971. There is a correlation of -0.32 between the growth rates and initial log GDP per capita. This shows that emissions did tend to decline or grow more slowly in richer countries but the relationship is very weak -  only 10% of the variation in growth rates is explained by initial GDP per capita. Emissions grew in many wealthier countries and fell in many poorer ones, though GDP per capita also fell in a few of the poorest of those. So, this does not provide strong support for the EKC being the best or only explanation of either the distribution of emissions across countries or the evolution of emissions within countries over time. On the other hand, we shouldn't be restricted to a single explanation of the data and the EKC can be treated as one possible explanation as in Anjum et al. (2014). In that paper, we find that when we consider other explanations such as convergence the EKC effect is statistically significant but the turning point is out of sample - growth has less effect on emissions in richer countries but it still has a positive effect.

The graph below compares the growth rates of sulfur emissions with the initial level of emissions intensity. The negative correlation is much stronger here: -0.67 for the log of emissions intensity. This relationship is one of the key motivations for pursuing a convergence approach to modelling emissions. Note that the tight cluster of mostly European countries that cut emissions the most appears to have had both high income and high emissions intensity at the beginning of the period.

Tuesday, July 12, 2016

Legitimate Uses for Impact Factors

I wrote a long comment on this blogpost by Ludo Waltman but it got eaten by their system, so I'm rewriting it in a more expanded form as a blogpost of my own. Waltman argues, I think, that for those that reject the use of journal impact factors to evaluate individual papers, such as Lariviere et al., there should be then no legitimate uses for impact factors. I don't think this is true.

The impact factor was first used by Eugene Garfield to decide which additional journals to add to the Science Citation Index he created. Similarly, librarians can use impact factors to decide on which journals to subscribe or unsubscribe from and publishers and editors can use such metrics to track the impact of their journals. These are all sensible uses of the impact factor that I think no-one would disagree with. Of course, we can argue about whether the mean number of citations that articles receive in a journal is the best metric and I think that standard errors - as I suggested in my Journal of Economic Literature article - or the complete distribution as suggested by Lariviere et al., should be provided alongside them.

I actually think that impact factors or similar metrics are useful to assess very recently published articles, as I show in my PLoS One paper, before they manage to accrue many citations. Also, impact factors seem to be a proxy for journal acceptance rates or selectivity, which we only have limited data on. But ruling these out as legitimate uses doesn't mean rejecting the use of such metrics entirely.

I disagree with the comment by David Colquhoun that no working scientists look at journal impact factors when assessing individual papers or scientists. Maybe this is the case in his corner of the research universe but it definitely is not the case in my corner. Most economists pay much, much more attention to where a paper was published than how many citations it has received. And researchers in the other fields I interact with also pay a lot of attention to journal reputations, though they usually also pay more attention to citations as well. Of course, I think that economists should pay much more attention to citations too.