领域：学术论文 · 社会科学

Natural resources, pollution, and other environmental considerations are absent from the Solow model. But at least since Malthus (1798) made his classic argument, many people have believed that these considerations are critical to the possibilities for long-run economic growth. For example, the amounts of oil and other natural resources on earth are fixed. This could mean that any attempt to embark on a path of perpetually rising output will eventually deplete those resources, and must therefore fail. Similarly, the fixed supply of land may become a binding constraint on our ability to produce. Or ever-increasing output may generate an ever-increasing stock of pollution that will bring growth to a halt.   
This section addresses the issue of how environmental limitations affect long-run growth. In thinking about this issue, it is important to distinguish between environmental factors for which there are well-defined property rights—notably natural resources and land—and those for which there are not—notably pollution-free air and water.   
The existence of property rights for an environmental good has two important implications. The first is that markets provide valuable signals concerning how the good should be used. Suppose, for example, that the best available evidence indicates that the limited supply of oil will be an important limitation on our ability to produce in the future. This means that oil will command a high price in the future. But this in turn implies that the owners of oil do not want to sell their oil cheaply today. Thus oil commands a high price today, and so current users have an incentive to conserve. In short, evidence that the fixed amount of oil is likely to limit our ability to produce in the future would not be grounds for government intervention. Such a situation, though unfortunate, would be addressed by the market.   
The second implication of the existence of property rights for an environmental good is that we can use the good’s price to obtain evidence about its importance in production. For example, since evidence that oil will be an important constraint on future production would cause it to have a high price today, economists can use the current price to infer what the best available evidence suggests about oil’s importance; they do not need to assess that evidence independently.   
With environmental goods for which there are no property rights, the use of a good has externalities. For example, firms can pollute without compensating the people they harm. Thus the case for government intervention is much stronger. And there is no market price to provide a handy summary of the evidence concerning the good’s importance. As a result, economists interested in environmental issues must attempt to assess that evidence themselves.  
The stock of land is fixed, and resource use must eventually fall. Thus even though technology has been able to keep ahead of resource and land limitations over the past few centuries, it may still appear that those limitations must eventually become a binding constraint on our ability to produce.   
The reason that this does not occur in our model is that production is Cobb–Douglas. With Cobb–Douglas production, a given percentage change in A always produces the same percentage change in output, regardless of how large A is relative to R and T. As a result, technological progress can always counterbalance declines in R/L and T/L.   
This is not a general property of production functions, however. With Cobb–Douglas production, the elasticity of substitution between inputs is 1. If this elasticity is less than 1, the share of income going to the inputs that are becoming scarcer rises over time. Intuitively, as the production function becomes more like the Leontief case, the inputs that are becoming scarcer become increasingly important. Conversely, if the elasticity of substitution is greater than 1, the share of income going to the inputs that are becoming scarcer is falling. This, too, is intuitive: as the production function becomes closer to linear, the abundant factors benefit.   In terms of our earlier analysis, what this means is that if we do not restrict our attention to Cobb–Douglas production, the shares in expression (1.51) for the growth drag are no longer constant, but are functions of factor proportions. And if the elasticity of substitution is less than 1, the share of income going to resources and land is rising over time—and thus the growth drag is as well. Indeed, in this case the share of income going to the slowestgrowing input—resources—approaches 1. Thus the growth drag approaches b + n. That is, asymptotically income per worker declines at rate b + n, the rate at which resource use per worker is falling. This case supports our apocalyptic intuition: in the long run, the fixed supply of resources leads to steadily declining incomes.   
In fact, however, recognizing that production may not be Cobb–Douglas should not raise our estimate of the importance of resource and land limitations, but reduce it. The reason is that the shares of income going to resources and land are falling rather than rising. We can write land’s shareas the real rental price of land multiplied by the ratio of land to output. The real rental price shows little trend, while the land-to-GDP ratio has been falling steadily. Thus land’s share has been declining. Similarly, real resource prices have had a moderate downward trend, and the ratio of resource use to GDP has also been falling. Thus resources’ share has also been declining. And declining resource and land shares imply a falling growth drag.   
The fact that land’s and resources’ shares have been declining despite the fact that these factors have been becoming relatively scarcer means that the elasticity of substitution between these inputs and the others must be greater than 1. At first glance, this may seem surprising. If we think in terms of narrowly defined goods—books, for example—possibilities for substitution among inputs may not seem particularly large. But if we recognize that what people value is not particular goods but the ultimate services they provide—information storage, for example—the idea that there are often large possibilities for substitution becomes more plausible. Information can be stored not only through books, but through oral tradition, stone tablets, microfilm, videotape, DVDs, hard drives, and more. These different means of storage use capital, resources, land, and labor in very different proportions. As a result, the economy can respond to the increasing scarcity of resources and land by moving to means of information storage that use those inputs less intensively.   
Declining quantities of resources and land per worker are not the only ways that environmental problems can limit growth. Production creates pollution. This pollution reduces properly measured output. That is, if our data on real output accounted for all the outputs of production at prices that reflect their impacts on utility, pollution would enter with a negative price. In addition, pollution could rise to the point where it reduces conventionally measured output. For example, global warming could reduce output through its impact on sea levels and weather patterns.   
Economic theory does not give us reason to be sanguine about pollution. Because those who pollute do not bear the costs of their pollution, an unregulated market leads to excessive pollution. Similarly, there is nothing to prevent an environmental catastrophe in an unregulated market. For example, suppose there is some critical level of pollution that would result in a sudden and drastic change in climate. Because pollution’s effects are external, there is no market mechanism to prevent pollution from rising to such a level, or even a market price of a pollution-free environment to warn us that well-informed individuals believe a catastrophe is imminent.   
Conceptually, the correct policy to deal with pollution is straightforward. We should estimate the dollar value of the negative externality and taxpollution by this amount. This would bring private and social costs in line, and thus would result in the socially optimal level of pollution.   
Although describing the optimal policy is easy, it is still useful to know how severe the problems posed by pollution are. In terms of understanding economic growth, we would like to know by how much pollution is likely to retard growth if no corrective measures are taken. In terms of policy, we would like to know how large a pollution tax is appropriate. We would also like to know whether, if pollution taxes are politically infeasible, the benefits of cruder regulatory approaches are likely to outweigh their costs. Finally, in terms of our own behavior, we would like to know how much effort individuals who care about others’ well-being should make to curtail their activities that cause pollution.   
Since there are no market prices to use as guides, economists interested in pollution must begin by looking at the scientific evidence. In the case of global warming, for example, a reasonable point estimate is that in the absence of major intervention, the average temperature will rise by 3 degrees centigrade over the next century, with various effects on climate (Nordhaus, 2008). Economists can help estimate the welfare consequences of these changes. To give just one example, experts on farming had estimated the likely impact of global warming on U.S. farmers’ ability to continue growing their current crops. These studies concluded that global warming would have a significant negative impact. Mendelsohn, Nordhaus, and Shaw (1994), however, note that farmers can respond to changing weather patterns by moving into different crops, or even switching their land use out of crops altogether. They find that once these possibilities for substitution are taken into account, the overall effect of global warming on U.S. farmers is small and may be positive (see also Deschenes and Greenstone, 2007).   
After considering the various channels through which global warming is likely to affect welfare, Nordhaus (2008) concludes that a reasonable estimate is that the overall welfare effect as of 2100 is likely to be slightly negative—the equivalent of a reduction in GDP of 2 to 3 percent. This corresponds to a reduction in average annual growth of only about 0.03 percentage points. Not surprisingly, Nordhaus finds that drastic measures to combat climate change, such as policies that would largely halt further warming by cutting emissions of greenhouse gases to less than half their 1990 levels, would be much more harmful than simply doing nothing. Similarly, mainstream estimates of the social cost of carbon (that is, the size of the appropriate Pigovian tax to address the negative externalities from carbon emissions through their impact on climate), while not trivial, are only moderate. Both Greenstone, Kopits, and Wolverton (2013) and Nordhaus(2014) estimate the cost as about $20 per ton. One way of describing the size of such a tax is that it would add about $0.20 to the cost of a gallon of
gasoline.
Of course, it is possible that these attempts to interpret the scientific evidence and estimate the likely welfare effects are far from the mark. There appear to be two main considerations that could lend support to much stronger views of the costs of climate change and the value of measures to address it. The first is tail risks (or tipping points) that is, the perhaps small chance that outcomes will be vastly worse than the point estimates. Nordhaus (2013) tries to account for uncertainty and concludes that it does not greatly change his conclusions; one reason is simply that just as outcomes could be worse than his point estimates, they could also be better. Likewise, Greenstone, Kopits, and Wolverton (2013) find that there are alternative assumptions that lead to, say, a doubling of the estimated social cost of carbon, but that it is hard to make a case for estimates that are qualitatively different from their baseline. In contrast, Weitzman (2009) argues that tail risks fundamentally change the analysis of climate change and support much more dramatic policy changes.
The second important issue is the appropriate discount rate: even small changes in the discount rate have very large effects on analyses of policies that involve costs today in exchange for benefits extending decades into the future. And with a sufficiently low discount rate, impacts at horizons beyond the 50 to 100 years usually examined in analyses of climate change could have large effects on the conclusions. A good introduction to the question of how to discount the costs and benefits of actions to mitigate climate change is the debate between Nordhaus (2007) and Stern (2008). Despite these complications, the fact remains that most (though certainly not all) economists who have studied climate change seriously, even ones whose initial positions were very sympathetic with environmental concerns, have concluded that the impact of climate change on growth is likely to be no more than moderate.25
Finally, it is important to remember that climate change is not the only type of pollution. Indeed, using an approach similar to his analysis of climate change, Nordhaus (1992) estimates that the welfare costs of the externalities from other types of pollution are probably slightly larger than those from climate change; his point estimate is that they are lowering appropriately measured annual growth by roughly 0.04 percentage points. Thus, policymakers and concerned citizens should not lose sight of more conventional types of pollution.


翻译考点：
考点1："narrowly defined goods“推荐译为“狭义的产品“。
考点2：“pollution’s effects are external”应译为“污染的影响是外生的”，”外生性“是经济学模型中常见概念，此处应准确翻译。
考点3：“ tipping points”推荐译为“临界点理论“。
考点4:  "apocalyptic intuition"推荐译为“末世直觉”
考点5:  "sympathetic with environmental concerns"中的“sympathetic”不能译为“同情”