I have previously written about special days on which certain interests ask that we please think about them (here and here, for instance). On Tuesday, it was "Earth Overshoot Day" or "Ecological Debt Day", another day of nonsense on which humanity's consumption of resources supposedly exceeds Mother Earth's capacity for regenerating them. This cannot mean that there isn't anything left to consume until New Year's Day of 2015, because when shopping, I have observed several well-stocked shelves since Tuesday. Rather, it must mean that "outtakes" of renewable resources are greater than the amount which the Earth can sustain in the long run, given some trends and estimates of future consumption, and that by-products of humanity's consumption, such as greenhouse gasses, are greater than the Earth can absorb.
Notice that this is about renewable resources, not non-renewable (or, more accurately, resources which regenerate very slowly) ones such as oil. So this particular day of nonsense is one which calls on people to be "aware" or the "problem" of things like deforestation or overfishing. Now there is the well-known tragedy of the commons argument to support some of these fears; if the individual's outtake has no discernible effect on total supply, very many people will take out too much, resulting in depletion. Overfishing, for instance, results from a lack of property rights of the oceans and seas (or of difficulties in introducing them, since fish are mobile creatures and pieces of the ocean are hard to close off). But fish farming, the successful raising of fish in tanks for human consumption, elegantly solves the problem. The fishermen in these cases can secure themselves of a future supply by leaving some of the current fish to reproduce. Not all kinds of fish thrive in such circumstances, but as far as I have been able to understand there is a great deal of progress in this field.
As for deforestation, I do not see any reason for rational fear. Landowners who leave nothing to grow for future cutting will suffer if future prices are expected to be sufficiently high. If landowners expect trees to be in short supply in a couple of decades, they will cut fewer trees and grow more trees now. There is a qualitative similarity here with the treatment of non-renewable resources. If I guess that some resource will near depletion in a couple of years, it would make sense for me to hoard said resource right now (or leave it unharvested), provided the eventual pay-off exceeds what I might expect from putting my money elsewhere. This means, as Harold Hotelling realized, that the price of non-renewable resources rises with the interest rate (actually, they have been fairly constant for many decades as far as I know, which is odd, but consistent with a vast array of explanations such as surprise finds). Renewable ones are not part of Hotelling's treatment. Hardly indicative of approaching doom. Those afraid of the message of the "Earth Overshoot" crowd should ask themselves what reasons there might be for renewable resources to be managed so much less well than are non-renewable ones.
In addition to the aforementioned commons problem, uncertain property rights may be such a reason. If I fear expropriation, I am unlikely to behave in the way described above to help stave off depletion. However, this reason seems a poor one in general; while property rights are insecure in many parts of the world, they should be sufficiently secure in other places so that more is conserved there in response to overzealous usage elsewhere; if supply will be very short in places with insecure property rights soon, it is an added pressure to conserve now. At any rate, this takes the discussion into one about non-renewable resources and so is a digression.
Many renewable resources are well-managed and will be in good supply in future thanks to property rights. Where property rights do not exist or may be impractical there are some reasons to worry, but as shown by the fish farming example there are solutions even there. There is no known solution to all of the commons problems, though. Global warming is the best-known such instance. The obvious solution is to tax emissions of greenhouse gasses, but if this is done, production is likely to shift to low-tax countries, putting a downward pressure on green taxes. Global agreements are therefore likely to fail. Perhaps a better approach is to tax the carbon content of final products, so that if so and so many tonnes of greenhouse gasses went into the production of my Weetabix, just tax the delicious breakfast cereal where it is sold. I can imagine this would impose harsh demands on producers and be tricky to calculate, but carbon taxation, though theoretically attractive, is certainly not going to work.
However, as economist David Friedman has repeatedly argued, in the case of global warming there is really no good reason to expect present temperatures to be optimal. Maybe greenhouse gasses should be subsidized instead? There is also a case to be made that any political solution will be polluted by the - completely rational - idiocy of the average citizen on the issue. For example, in this paper, Chicago Booth's Luigi Zingales Northwestern Kellogg's Paola Sapienza show that among the general public, almost two-thirds prefer car standards - such as CAFE standards - to a carbon tax, as compared to the economists' widespread (92.5 per cent) preference for carbon taxes (though I believe carbon taxes will fail they are certainly better than CAFE standards). At any rate, the worries of "Earth Overshoot Day" are vastly overblown.