by Jean Jouzel
These exceptional conditions could be a true piece of luck in forecasting our future. None of the warm periods of the interglacial stages 5.5, 7.3, and 9.3 lasted more than 10,000 years. And since our Holocene has already lasted for 10,000 years, we should ask questions about the rapid arrival of the next glaciation, which would mark the end of our interglacial period. Ruddiman pushes his reasoning in that direction: not only does he propose—and we have seen the weakness of this—the hypothesis that the increase in the greenhouse effect observed since the first half of the Holocene is mainly linked to human activity, but he argues that without that increase we would be, let’s say naturally, on the verge of entering into a glacial period or even already in the process of undergoing the first consequences of one.
The full story is not yet clear, but the reason the Holocene would be continuing in the absence of the human impact could well be related to the astronomical configuration of weak eccentricity from which we are currently benefiting and which would continue for several more tens of thousands of years. It isn’t possible over such long periods to use models that represent the behavior of the atmosphere, the ocean, and the polar ice caps in a detailed way. Such an exercise is in any case beyond our reach regarding the available means of calculation. Modelers can nevertheless project into that very long-term future by relying on either very simple conceptual models or intermediate complex models. None of these projections foretells an entrance into the next glacial period before several tens of thousands of years.
These projections, based on the perfectly known evolution of astronomical parameters, are remarkably corroborated by the available data on the closest stage 11.3, as we have said, of the current period and of our future in terms of variations of insolation. Marine sediments attest that this warm period has been exceptionally long compared to the three last interglacial periods, and the data from the ice cores at Dôme C provide a remarkable confirmation of this. The amount of time that the temperature has been the same or above that which on average the Holocene has known is close to 30,000 years.
Everything thus indicates that the next glacial period will not be anytime soon. This is not the problem that future generations must face; rather it is that of the warming put into motion by an increase in the greenhouse effect linked to human activity since the beginning of the industrial era around two hundred years ago and its increase during the twenty-first century and beyond.
PART THREE
THE WHITE PLANET TOMORROW
CHAPTER 11
The Climate and Greenhouse Gases
Through the eyes of glaciologists we have led you to the discovery of our white planet, of the world of ice with such variable shapes and with such a rich memory. What will become of this ice on a planet whose climatic history is no longer written by Mother Nature but quite probably is already influenced by the activities of humans and which will probably be even more so in the decades and centuries to come? This question, which bears on the role of human activities in the warming we have been experiencing for a few decades and on the evolution of our climate from now until the end of the century and beyond, is the focus of the next several chapters. But let’s first go back to the greenhouse effect, to its role in the climatic machine, and to its evolution during the last centuries in response to human activity.
The Greenhouse Effect: A Truly Beneficial Natural Phenomenon
To explain the greenhouse effect we must look at it as climatologists, which will enable us to say a bit more about the functioning of our climate, of which the Sun is the initial driving force. For even if the Earth absorbs only a small part of the energy the Sun emits, that energy is much greater (around 7,000 times) than the geothermal flux that comes from inside the Earth. That flux thus has no notable influence on the average climate of our planet.
In a given place, the solar energy received at the top of the atmosphere—insolation—is extremely variable. It follows the rhythm of the succession of days and nights, which is connected to the rotation of the Earth on its axis, and of that of the seasons, which result from the obliquity, the tilt of that axis in relation to the plane of the Earth’s orbit. It also depends on the average incline of the Sun’s rays and thus on latitude. The result of these diurnal, seasonal, and geographic variations: over an entire year, the average insolation received at the top of the atmosphere is indeed much less than the energy that would go through that same surface if it were placed permanently perpendicular to the Earth-Sun direction. This latter, which we designate wrongly by the name of “solar constant,” is close to 1,365 Wm–2, whereas the average energy received, approximately 342 Wm–2, is only a fourth of that. This ratio owes nothing to chance but to the fact that the surface of a sphere of radius R (4πR2) is four times that of a circle passing in its center (πR2). This is, quite simply, the way in which our Earth is viewed from the Sun.
On a longer scale of time other factors come into play. There are astronomical parameters (eccentricity, obliquity, and precession), which we have already mentioned. They enable us to calculate the insolation at a given time in the past, present, or future, as a function of latitude. These astronomic parameters vary, as we have seen, very slowly, but another factor must be taken into account. This is the intensity of the heating. In fact, the solar constant varies slightly with the rhythm of the fluctuations of the activity of the Sun, which is seen, for example, in the number of sunspots.
The variations in solar activity have probably been great since the beginning of the history of the Earth, a period when the power of the Sun was around 30% weaker than it is today. Those variations do not, however, have any influence on the time scales of the great climatic cycles characteristic of the Quaternary and are thus legitimately ignored. But solar activity also varies over short periods of time with a very marked cycle of eleven years over which are superimposed periodic variations such as the one characteristic of the period of the Maunder Minimum, which corresponds to a near absence of sunspots during the second part of the seventeenth century. These variations in solar activity are weak, on the order of 1‰ (or 1 per mil) for the eleven-year cycles and a bit more than 3‰ (or 3 per mil) at the most, since that Maunder Minimum. Their influence on our climate nevertheless is the object of an intense debate that we will discuss in the next chapter.
Up to now, we have looked at the energy that arrives at the top of the atmosphere. But as we see in figure 11.1, which is very simplified, many things occur within the atmosphere itself, most of the mass of which is concentrated over approximately twenty kilometers of thickness. Around 107 Wm–2—or approximately 30% of the 342 Wm–2 that, on average, arrive at the top of the atmosphere—are reflected back to space. Two-thirds of this is due to the presence of clouds and the rest to the diffusion through air molecules, through particles that are suspended in the atmosphere, and by the surface. This reflective power is called, a bit mysteriously for the uninitiated, albedo. This varies between zero and one and the higher it is, the more reflective the surface; an albedo of 0.9 (a level that new snow can reach) means that this surface reflects 90% of the incoming solar radiation. The net result of this albedo, whose level on a planetary scale is close to 0.3 (30%), is that the energy truly absorbed is 235 Wm–2.
Figure 11.1. How the greenhouse effect works. Source: IPCC, Climate Change 2007: Fourth Assessment Report (Cambridge University Press, 2007). Note: All numbers correspond to Wm–2.
From one year to another, our planet holds a relatively constant temperature, which indicates that it is in thermodynamic equilibrium. In other words, the 235 Wm–2 of solar energy must be compensated for by an equivalent flux emitted toward space. In fact, every celestial body emits a radiation whose wavelength becomes smaller as its temperature rises. This is the case for the Sun and the Earth. But whereas the Sun, whose average surface temperature is close to 6,000°C, emits in the visible around 0.6 microns (μm) and in a range covering the ultraviolet up to the close infrared of 0.2 to 4 μm, the Earth emits in the infrared with a maximum in
tensity centered around 12 μm. If the atmosphere were perfectly transparent to infrared radiation, that radiation would be emitted by the Earth’s surface, whose mean annual temperature would be –18°C. This would be the case if air were formed of only its three major components: nitrogen (78%), oxygen (21%), and argon and other rare gases (0.9%).
Fortunately this doesn’t happen, thanks to a series of minor compounds. Formed of at least three molecules, they have a more complex structure than that of oxygen, nitrogen, and argon. It is thanks to them that infrared radiation emitted on the surface is absorbed by the atmosphere. The atmosphere reemits it into space but in a weaker quantity because the atmosphere is colder than the surface. The difference, around 150 Wm–2, is sent back to the ground and serves to heat the lower layers of the atmosphere in which it is in some way trapped by the action of those minor compounds. Analogous to the gardener’s greenhouse, those compounds are designated by the generic name of greenhouse gases. Water vapor (H2O) is the primary greenhouse gas; it makes up 60% of the greenhouse effect. Next come carbon dioxide (CO2) and ozone (O3), which, respectively, represent 26% and 8% of the greenhouse effect. Methane (CH4) and nitrous oxide (N2O) account for the remaining 6%. The other contributors, some of which are pure products of human activity, such as the halogen compounds (CFC, etc.), encompass only around 0.2% of the overall greenhouse effect. However, they contribute to more than 10% of its current increase, which we call the additional greenhouse effect. Water vapor, carbon dioxide, ozone, methane, nitrous oxide, and so forth have been present in the atmosphere throughout time: this greenhouse effect is thus a completely natural occurrence. It is extremely beneficial, moreover, because it is hard to imagine how life could have developed in a mean temperature of –18°C. The temperature of +14°C reached thanks to this natural greenhouse effect is much more favorable.
How are the 235 Wm–2 that the Sun brings to the atmosphere used? A bit less than 30%, or 67 Wm–2, are absorbed by the atmosphere and, for a large part, by the ozone in the ultraviolet, protecting us from that radiation and thereby making it possible for life outside the oceans to develop. This absorption takes place mainly in the stratosphere, the upper part of the atmosphere in which concentrations of ozone are highest and thus participate in the warming of that region; this is why the temperature in the troposphere (in the lower part of the atmosphere) decreases with altitude and increases in the stratosphere. But the greatest part of the Sun’s radiation, 168 Wm–2, traverses the atmosphere. About 100 Wm–2 is used to heat the surrounding air, of which most, 78 Wm–2, enables the evaporation of water on the surface of oceans and continents, a process that plays a major role in the cycle of water and the redistribution of energy. In fact, this “latent heat” is used to evaporate water molecules and is restored to the atmosphere when those molecules are condensed, thereby redistributing energy depending on the formation of precipitations.
The redistribution of energy and the water cycle are at the heart of the functioning of the climatic machine. The energy budget that we have just presented is balanced on average over the entire planet, but it is not balanced locally. The reason for this is simple. On average, the Sun’s radiation decreases annually by more than a factor of 2 between the equator and the poles, whereas the infrared radiation varies little (on the order of 10%). This isn’t surprising that it is warmer at the equator. The excess is transported by the atmosphere and the ocean to the poles, which would be even colder without that contribution. About half of that contribution is carried out by the atmosphere, thanks to the winds and the water vapor that accompanies the air masses; the other half is brought by the ocean and its currents.
The Greenhouse Effect Due to Human Activity: A Slow Awareness
In the first two parts of this book we looked at the greenhouse effect through the work of Joseph Fourier and Svante Arrhenius in the nineteenth century. We might have added the Swiss Horace Bénédict de Saussure and his heliothermometer, which he created to record the intensity of the solar flux at different altitudes, the French scientist Claude Pouillet, who demonstrated that thanks to the greenhouse effect the planet is warmer by several dozen degrees, or the Irish scientist John Tyndall, who explained that certain gases absorb infrared radiation and retain it within the atmosphere.
Then without saying anything—or almost nothing—about the twentieth century, we looked at Antarctica and the great ice core drillings of Vostok and Dôme C, which revealed that the great glaciations of the Quaternary were characterized by a close interaction between climate and greenhouse effect.
Arrhenius’s calculations estimated a decrease of 40% in the concentration of CO2 in a glacial period, an estimate which, somewhat by luck, proved to be correct. Those that were connected to a warming associated with a doubling in concentrations of carbon dioxide, estimated at around 5°C, appear a bit high in view of the current estimates, but they remain in the realm of the plausible. However, that prediction was largely ignored because Arrhenius foresaw that this doubling would only occur in the next 3,000 years. Granted, given the increase in emissions due to an increased use of carbon, he revised that number to a few centuries some dozen years later. But at the beginning of the twentieth century, climatologists were concerned more with explaining the glacial ages than with speculating on the future of our climate.
There was almost universal conviction at the time that the ocean or, by default, the vegetation would be able to absorb that excess carbon dioxide. However, just before World War II, the Englishman Guy Callendar deduced from the data available at the time that concentrations of CO2 had already increased by 10%. He drew attention to the ocean’s limited capacity for absorption, noting that the surface waters could rapidly become saturated. Callendar saw rightly, at least qualitatively. It was, however, necessary to wait until the 1950s for the idea that human activity was modifying the composition of the CO2 in the atmosphere to be put forth, and thus the greenhouse effect took hold within the scientific community.
Three American researchers, Hans Suess, Roger Revelle, and Charles Keeling, played a major role. From an analysis of carbon 14 in tree rings, dated year by year, Suess showed that the concentration of that carbon isotope in the atmosphere had gradually decreased in the preceding years, which one would expect due to the emission of CO2 coming from very old fossil fuel without carbon 14. Revelle was interested in the absorption of CO2 by the oceans. Two years later, in 1957, Revelle and Suess explained why a part of the CO2 remained in the atmosphere rather than being absorbed by the oceans. Persuaded of the need to measure very precisely the evolution of atmospheric CO2, Keeling took the bull by the horns: in 1958 during the International Geophysical Year, he established the first station to measure the concentration of CO2 near the summit of Mauna Loa in Hawaii.1
Others would follow, henceforth establishing a network of relatively dense stations, but they are still not enough. Let’s first mention the precise measurements taken at South Pole Station. They began to be taken simultaneously with those of Mauna Loa and quickly confirmed the global nature of what was henceforth known as the Keeling curve, due to the position of the South Pole in the middle of Antarctica and thus beyond any significant local source of CO2. Thanks to the initiative of Gérard Lambert, who, at the beginning of the 1980s, proposed building a station on the island of Amsterdam in the southern part of the Indian Ocean, France contributed significantly to this network through a team from the LSCE.
The results were not long in coming. Using monthly averages, Keeling revealed the existence of regular seasonal variations in CO2 linked to the rhythm of development of vegetation. In the spring, vegetation grows while assimilating atmospheric CO2, which consequently diminishes, then increases again in the winter; vegetation no longer plays its role as a CO2 pump, and the decomposition of the dead leaves releases their carbon in the form of CO2. These seasonal variations, which are very marked in the Northern Hemisphere, are much less pronounced in the Southern Hemisphere where the continents are much smaller. But regardless
of the site, these variations are characterized by a huge increase in their mean annual levels and are practically identical from one place to another: 315 ppmv in 1958, 355 in 1992, and 387 in 2009 (Figure 11.2)—levels that the results from the ice cores of Vostok and Dôme C tell us have not been observed in at least 800,000 years.
This increase in concentrations of CO2 has been shown in an increasingly detailed way for fifty years now. Carbon 13, another isotope of carbon, represents less than 0.1% of the carbon present on Earth (carbon 12, which amounts for more than 99.9% of the carbon on the Earth, is the main isotope of carbon). Analysis of it has shown that the CO2 increase is indeed linked to fossil fuels. Compared to the CO2 present in the oceans and in the volcanic or geothermal emissions, vegetation is indeed slightly lacking in carbon 13 because it has some difficulty using this heavy isotope during photosynthesis. The slight decrease in carbon 13 measured in the CO2 of the atmosphere proves the contribution of fossil fuels, since these are of vegetal origin. In addition, the oxygen-nitrogen relationship decreases as the quantity of CO2 increases, as we might expect when it comes from the combustion of oil, gas, and coal.
