"...The amount of CO2 we produce depends on (1) how many people there are and (2) how much energy they use. The poorer people on Earth will seek a bet-ter standard of living, and that will require more energy. Almost all of our energy now comes from burning fossil fuels and therefore involves adding CO2 to the atmosphere. The hope expressed at the UN Conference on Environment and Development (Earth Summit; Rio de Janeiro, 1992) was that the production rate of CO2 could be held to its 1990 level, but the production rate has already risen well above that level.
Some politicians believe that the Earth Summit goal is achievable, but I don't. I suspect that we are going to generate 7 gigatons or more of carbon as CO2 every year. At this rate, the CO2 content of the atmosphere will rise at the rate of about 2 ppm per year (Fig. 6).
fig. 6
The CO2 content of the atmosphere will continue to increase; how much it increases depends on many variables. Maybe there will be a miracle, and we'll find some alternate energy source that is socially acceptable and economically fundable. I have little doubt, though, that late in the next century, the CO2 content of our atmosphere will reach 560 ppm, twice the preindustrial level. Before we're free from dependence on fossil fuels, we'll probably drive the CO2 up to 700 ppm or more. For this rise in CO2, models yield a range of global warmings, because they differ in the extents of water-vapor feedback.
As already stated, were there no such feedback, the warming would be only about 1.2°C and would not produce much difficulty. If the warming were 3, 4, or 5°C, as some models predict, then everybody would agree that there would be big trouble. What I've injected into this already complicated situation is the realization that in the past, climate changes haven't come gradually. Whatever pushed Earth's climate didn't lead to smooth changes, but rather to jumps from one state of operation to another. So the question naturally arises,
What is the probability that through adding CO2 we will cause the climate system to jump to one of its alternate modes of operation? I contend that since we can't yet reproduce any of these jumps in computer simulations, we don't really know how many modes of operations Earth has, and we certainly don't have any idea what it might take to push the system from one mode to another. We do know, however, that a substantial warming would surely reduce the density of polar surface water and thereby tend to cut off deep ventilation. So we're entering dangerous territory and provoking an ornery beast.
Our climate system has proven that it can do very strange things. Since we've only recently become aware of this capability, there's nothing concrete that we can say about the implications. This discovery certainly gives us even more reason to be prudent about what we do, though. We must prepare for the future by learning more about our change-able climate system, and we must create the wherewithal to respond if the CO2-induced climate changes are large, or, worse yet, if they come abruptly, changing agricultural conditions across the entire planet. We must think all this through. Even if there is only a 1% probability that such a change might occur during the next 100 years, its impact would be sufficiently catastrophic that the mere possibility warrants a lot of preparation.
My lifetime study of Earth's climate system has humbled me. I'm convinced that we have greatly underestimated the complexity of this system. The importance of obscure phenomena, ranging from those that control the size of raindrops to those that control the amount of water pouring into the deep sea from the shelves of the Antarctic continent, makes reliable modeling very difficult, if not impossible. If we're going to predict the future, we have to achieve a much greater understanding of these small-scale processes that together generate large-scale effects.
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