Some personal remarks

In this last section, I would like to emphasize two aspects about global warming that do not, I think, get sufficient attention: We need to think in terms of probabilities or

GLOBAL WARMING IN A NUTSHELL

Causes and evidence

• Carbon dioxide (CO2) and a few other gases are greenhouse gases, meaning that they absorb and re-emit longwave radiation that is emitted from Earth's surface, maintaining the surface at a higher temperature than it would be in their absence (about 15°C as opposed to — 18°C).

• Greenhouse gases, and in particular CO2, are added to the atmosphere by the burning of fossil fuels. CO2 concentration has steadily increased since the beginning of industrialization, from about 270 ppm in 1750 to about 295 ppm in 1900 and 390 ppm in 2010. It will probably exceed 400 ppm some time in 2014.

• The average surface temperature has also increased since preindustrial times and by about 0.8°C over the past century.

• The increase in radiative forcing at the surface is approximately linear—a little less than 4 Wm—2 for each doubling of cO2. This increased forcing (along with other greenhouse gases, and ameliorated by aerosols) can readily account for the observed increase in temperature over the past century.

Projections

• If carbon dioxide levels keep steadily rising and double in the coming century, average temperature increase is expected to be within a range of 1.3°C to 2.5°C, and most likely about 1.8°C, from preindustrial levels by the time of doubling.

• Even if carbon dioxide levels were to stabilize at twice the preindustrial value some time in this century, the temperature would keep on slowly rising to finally reach between about 2°C and 4.5°C higher than the preindustrial value when the ocean equilibrates after a number of centuries.

If anthropogenic carbon dioxide emissions were completely halted, it would take several centuries for the level of CO2 in the atmosphere to revert to its preindustrial value. Hence, temperatures would remain high for several centuries.

other effects on and of the ocean

• Sea level is projected to rise on average by somewhere in the region of 0.4 m over the next century, mostly because of the expansion of seawater as it warms, plus some ice melt, but the uncertainty is large (a factor of 2 either way) and the rise may not be uniform.

• A major melting of the major land-ice sheets is unlikely over the coming century, but if one were to begin, the consequences could eventually be catastrophic, with a sea-level rise of about 6 m if either the Greenland or West Antarctica ice sheets were to completely melt.

• Global warming, once it has occurred, will persist for centuries. Thus, a significant reduction of sea ice and a melting of the Greenland and West Antarctica ice sheets, with concomitant changes in ocean overturning circulation, cannot be ruled out on these timescales. But our ignorance is profound on such matters.

likelihoods, and we need to think clearly about the tim-escales involved.

Thinking about probabilities is necessary because we don't understand the climate system fully, so we don't know for sure what will happen in the future. The probability we assign to something is then really a measure of the confidence we have in that outcome. However, unlike a weather forecast, it is not as if the climate will warm by either 1°c or 5°c depending on the outcome of some chance weather event, some metaphorical coin toss, that might occur somewhere in the system. Rather, if we build up the level of greenhouse gases to some particular amount, then some definite amount of warming will occur, with 100% certainty. We just don't know what that amount is! We might never know, until the warming actually occurs. Thus, the probability represents our degree of belief in something. It is not just a wholly subjective degree of belief, because it is based on calculations that in turn are based on sound physical principles and the laws of nature, but the probability does represent our lack of knowledge of the system. It is useful to try to be quantitative about our uncertainty, by saying something like "We are 90% sure of something," both because it is useful to be as definite about something as we can possibly be and because there are then ways to calculate the probabilities of other uncertain events occurring, using various statistical procedures. Still, it is good to be somewhat skeptical about such quantitative measures when they primarily reflect our own ignorance.

A particularly disturbing aspect of global warming is that there is a small likelihood that something catastrophic will happen—that the Greenland ice sheets will melt and raise the sea level by several meters, for example. We currently believe that the chances of this are very small, at least in this century, but the consequences would be catastrophic to many countries of the world. I personally would not cross a street if the chances of being run over were one in a thousand. The chances might be small, but the consequences, at least to me, would be large. Whether we should live with the risk of potentially catastrophic global warming or "take insurance" by trying to curb emissions today is a question for society as a whole. (Some degree of global warming is of course inevitable.) When dealing with risk, we have to take into account both the likelihood of something happening and the consequences if that something does happen, and we need to weigh the overall risk against the cost of taking insurance. We "buy" insurance by investing in alternative sources of energy, renewable and nuclear, and by living, where possible, less wastefully. There is little downside to this for the developed nations: the cost is not prohibitive compared to the consequences, and in the worst case we prepare for global warming by being efficient and environmentally sound and then the warming turns out to be less than anticipated. For the undeveloped nations and emerging economies such as China and India, the transition to an economy less dependent on fossil fuels may be far more problematic.

The second aspect is the one of timescales, and this is where the ocean in particular comes in. Over the next several decades it is quite plausible (although take heed of the previous paragraphs!) that global warming will continue at about the pace we have seen in the last century. If, let us say, carbon dioxide levels increase in the atmosphere at about 1% per year, then in somewhat less than half a century, they will reach double their preindustrial level and we might expect temperatures to further increase by somewhere around a degree, for a total increase in a range of 1.3°C-2.5°C over the pre-industrial value, perhaps most likely about 1.8°C (this figure is known as the transient climate response). Let us imagine that by this time technology has improved sufficiently that emissions can be considerably reduced at little cost to our standard of living and that levels of greenhouse gases in the atmosphere then stabilize. However, as we discussed earlier in this chapter, temperatures will almost certainly keep on rising. And they will do so until the deep ocean finally equilibrates in hundreds of years, or until emissions virtually cease. There is much more uncertainty as to what the value of the final equilibrated temperature increase (the equilibrium climate sensitivity) will be than for the transient value, because with larger responses and on long timescales, other effects and feedbacks potentially start to come into play (possible changes in cloudiness, the wholesale melting of the ice sheets, and the positive ice-albedo feedback, for example). We noted earlier that the likely equilibrium response is between 2°C and 4.5°C, but there is some possibility of still higher values.

One might of course hope that once anthropogenic emissions have stabilized there will be a drawdown of CO2 by the ocean and by vegetation on land. However, calculations of the carbon cycle suggest that levels of carbon dioxide in the atmosphere will not, in fact, significantly diminish for many centuries after human emissions cease. Let us also note that if we do burn all Earth's resources of coal and petroleum (including heavy oil and shale), carbon dioxide levels may well go up by a factor of six or more with a corresponding warming of almost certainly more than 5°C and possibly more than 10°C, staying at that level for centuries and giving the great ice sheets on Greenland and West Antarctica plenty of time to melt.

Given all the above, one scenario of the future is the following. Let us suppose that we continue to burn fossil fuels for the next few decades, but (rather optimistically) let us also suppose we make good efforts to curb emissions and finally succeed in doing so a few decades hence, and that we are able to stabilize the level of greenhouse gases in the atmosphere at about double the preindustrial level. on this timescale (the short timescale, given the nature of the problem), the additional global warming will likely be a degree or so celsius, and although there will be some significant regional changes (perhaps especially in precipitation and in extremes of climate), climate change overall may well be less dramatic than the dire scenarios that are sometimes portrayed in the media. However, in the longer term (several decades to centuries and beyond) the problem may be worse that is often expected because global warming will continue relentlessly with consequences to match. An eventual 3°c rise in temperature, which is quite likely if carbon dioxide levels modestly double and stay doubled, will have very large consequences if and when it persists for centuries. Eventually, of course, emissions will diminish or cease if only because fossil fuels run out or become uneconomical, and a scenario for that is illustrated in figure 7.7. If co2 levels were to increase to double (or triple) the present value and emissions were, optimistically, to completely cease a century or two after that, the temperature would remain at more than 2°C (or 3°C for tripling) above the preindustrial value for several centuries.

Set against this bleak scenario is the likelihood that society itself will evolve in unforeseen ways, potentially making our current fears moot. Perhaps, then, we simply should not plan for the long term? Perhaps, as Estragon said so memorably in Waiting for Godot, there is "Nothing to be done," as the short term will not be so bad and in the long term human development itself is unpredictable. But if we do nothing, then almost certainly carbon dioxide levels will more than double this century and may well double again the following century. Unless we are somehow able to engineer our way out of trouble (for example, by trapping and sequestering the CO2 emitted when fossil fuels are burned, or extracting CO2 from the atmosphere), the climate change that will inevitably follow will almost certainly significantly affect the planet Earth itself and all the life on it.

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