by Jean Jouzel
Faced with this increase, it is important to know how the concentration of CO2 has evolved throughout the first part of the twentieth century, before 1958, and during the last centuries and millennia. Here, too, the polar ice proves to be extremely precious. This time, the objective is not to go back far in time; it is to have the most detailed records possible of the recent periods. To do this, we must use sites where there is a lot of accumulation. Situated in the coastal regions of Antarctica, they must also be cold enough: a mean average temperature below –20°C indeed avoids any summer melting of the ice that would modify its content of CO2. Such sites have been identified: for example, at Law Dome, the high accumulation (70 centimeters per year) has enabled our Australian colleagues to use an overlap of more than twenty years between the record obtained from the air bubbles trapped in the ice and the direct measurements achieved through the atmospheric network. The correlation is excellent and provides proof of the viability of the measurements taken from the ice. The increase in CO2 is well documented since the beginning of the industrial age. Before then, the concentrations were close to 280 ppmv and were even at 260 ppmv at around the beginning of the Holocene (around 10,000 years ago).
Figure 11.2. The atmospheric carbon dioxide record in Mauna Loa, Hawaii. The Keeling curve: variations in atmospheric concentrations of carbon dioxide shown in parts per million in volume (ppmv) measured at Mauna Loa. Seasonal variations are superimposed on the curve indicating the increase in the mean annual level, showing a typical annual cycle in the insert.
We had to wait until the 1970s for regular and precise measurements in these observation networks, which were initially put into place to follow carbon dioxide, of other greenhouse gases (methane, nitrous oxide, and halogen compounds).
How Did We Get to This Point?
We must face the facts. For two hundred years our various activities have rapidly and greatly modified the composition of our atmosphere. Using the most recent IPCC report2 and the Carbon Budget 2009 established by the Global Carbon Project,3 one can describe how this happened.
Let’s begin with carbon dioxide, the gas most responsible for an anthropogenic increase in the greenhouse effect. Each time we burn coal, gasoline, natural gas, or wood, we produce CO2. This is also the case when we produce cement. Since 1750 the concentration of this gas in the atmosphere has gone from 280 to 390 ppmv (at the end of 2010) (see Figure 11.3). Its increase comes on the one hand from the modification of the land use, in particular from deforestation, and on the other from fossil fuels and cement plants. The latter contribution has gradually taken over from the former, which was dominant during the nineteenth century. At the end of 2007, the cumulative emissions linked to fossil fuels went beyond 330 billion tons of carbon (gtC). These emissions are constantly increasing; they have more than doubled since 1970. An exception: in 2009, they were the second highest in human history; they decreased from the previous year by 1.3% due to the global financial crisis, but in 2010 they increased again by 5.9%.4 They are now close to 9.2 gtC/yr (8 in 2005; 8.7 in 2008; 8.4 in 2009), a bit less than 4% of which is due to cement factories and the rest to fossil fuels. The emissions linked to deforestation, which are difficult to calculate, are more than 1.5 gtC/yr and thus contribute less than 20% of the emissions that globally are close to 10 gtC/yr. Fortunately not everything remains in the atmosphere. Vegetation (despite the deforestation) and oceans absorb CO2 roughly equally (each around 2 gtC/yr). The result: each year 45% of total CO2 emissions remains in the atmosphere. The increase in the concentration of CO2 has never been so high: 1.9 ppmv each year from 2000 to 2009.
Looking at the other greenhouse gases, we will distinguish between those that stay in the atmosphere for a long time (methane, nitrous oxide, halogen compounds) and those that react rapidly, such as ozone. Each of those molecules has a very different heating power: it is 23 times greater for a methane molecule than for a CO2 molecule, and more than 10,000 times greater for certain halogen compounds. We compare these different compounds by measuring the quantity of additional energy available in the atmosphere due to the increase in their concentration since 1750, the year of reference: we call this “radiative forcing.” Its increase thus corresponds to an increase in the greenhouse effect. That of CO2 is estimated at 1.66 Wm–2.
Figure 11.3. Atmospheric concentrations of the principal greenhouse gases in the past 2,000 years, showing the increase in those of human origin since the beginning of the industrial era, around 1750. Source: IPCC, Climate Change 2007: Fourth Assessment Report (Cambridge: Cambridge University Press, 2007).
The second gas in importance that contributes to an increase in the greenhouse effect linked to human activity is methane, which is produced when organic matter decomposes in an anaerobic way, that is, outside the presence of oxygen. This occurs naturally in swamps and humid soils, sources to which are added termites, oceans, vegetation, and (we will come back to this) carbon hydrates. Human activities release CH4 through the production of energy from natural gases and coal, the treatment of garbage and its burial, raising livestock, growing rice, and the combustion of biomass. Anthropogenic emissions represent close to 70% of the 600 million tons emitted, on average, every year. We should note that these emissions have nothing to do with those of CO2; we speak in millions and not in billions of tons. Concentrations are also much weaker, more than 200 times less, or 1,774 ppbv (1.774 ppmv) in 2005. But they have greatly increased since 1750 since they have been multiplied by 2.5 (Figure 11.3), which corresponds to a radiative forcing of 0.48 Wm–2. Let’s add that once emitted, methane, which is eliminated through chemical oxidation, has a lifetime in the atmosphere of around ten years. This oxidation produces carbon monoxide (CO), CO2, and water vapor. The oxidation of methane is, moreover, at the origin of the increase in the quantity of water observed in the stratosphere, an increase responsible for a radiative forcing estimated at 0.07 Wm–2.
The question is often asked: do human activities contribute to an increase in the quantity of water vapor in the atmosphere? Their direct contribution is completely negligible because the recycling of water in the troposphere occurs over a very short amount of time, a few weeks at most. The quantity of water vapor is indeed increasing in the atmosphere but is doing so in reaction to the climate warming, which gradually affects the oceans, which consequently evaporate more.
The third contributor in importance to an increase in the greenhouse effect linked to human activities is the halogen compounds, which have contributed to a radiative forcing of 0.34 Wm–2. The concentration of these constituents of purely anthropogenic origin, as is the case with the chlorofluorocarbons (CFC), has rapidly increased since World War II when their production and use became universal, whether as propellant in aerosols or as refrigerants. These compounds, which free chloride or bromide atoms into the upper atmosphere, are doubly toxic: they participate in the destruction of the ozone layer and are powerful greenhouse gases. Since the signing of the Montreal Protocol and its amendments, the production of the CFCs and of certain other compounds have been stopped. This has enabled a decrease in atmospheric concentrations of those compounds that have a short life and has slowed down the rate of increase for those that remain in the atmosphere a long time. By contrast, the substitute products, hydrochlorofluorocarbons (HCFC) and hydrofluorocarbons (HFC), less dangerous vis-à-vis the ozone layer, are largely contributing to the increase in the greenhouse effect linked to human activities and their concentration is increasing, just like that of the perfluoride compounds (PFC), also a greenhouse gas.
Nitrous oxide (N2O), whose natural sources are on the same order as those connected to human activities, has also increased by close to 18% since 1750 (Figure 11.3). The natural sources come from the ocean and from vegetation, while human activities contribute to emissions through the transformation of nitrogen fertilizers, the combustion of the biomass, raising livestock, and industrial activities such as nylon production. Once emitted, nitrous oxide remains in the atmosphere for a long time—
more than 110 years—before being destroyed in the stratosphere. Its contribution to radiative forcing is 0.14 Wm–2.
At this point it is interesting to evaluate the contributions to an increase in the greenhouse effect by large sector and by country, once the contribution of each gas has been taken into account. Thus in 2004 on a global level the production of energy contributed to 26% of the increase; industry to 19%; land use change and deforestation to 17%; agriculture to 14%; transportation to 13%; residential, commercial, and tertiary sectors to 8%; and waste to 3%. In that same year the situation was very different in France. The greatest contribution to the greenhouse effect there, 26%, was linked to transportation. It was followed by industry at 20%, then in equal parts at 19% by the residential, commercial, and tertiary sectors and those of agriculture and forestry; the production of energy and waste contributed, respectively, 13% and 3%.
These assessments have enabled us to identify the contributions of each country—and this is a crucial point in international discussions—which has revealed enormous disparities per capita among developed and developing countries. Let’s look at the CO2 emissions due to fossil fuels for 2005. The United States, with 21% of total emissions, and China, with 19%, were the largest contributors (China has now surpassed the United States). But the scene is different when we use these figures to calculate each country’s per capita carbon emission; in the United States this is nearly 5.5 tons of carbon per inhabitant, five times more than in China (1.11). But emissions do not exceed a few hundred kilograms per inhabitant in some Asian countries (such as Vietnam) and in Central America (such as El Salvador) and are only a few dozen kilograms in Haiti and in many African countries; 100 times less than that of an American. A French person emits three times less than does an American. With 1.8 tons of carbon emitted per inhabitant, a French person emits less, by around a third, than a German or an Englishman because close to 90% of France’s electricity comes from sources that do not produce CO2—a bit less than 80% is nuclear, around 10% is hydraulic, and less than 1% is wind.
Finally, ozone (to which we will return in chapter 17) is also a climatic gas. It enables the maintenance of life on Earth by preventing the ultraviolet solar rays with short wavelengths from reaching the surface, but it also absorbs the Earth’s infrared radiation and in so doing participates in the greenhouse effect and in its variation. It does so in two ways. First, the decrease of ozone in the stratosphere decreases the corresponding greenhouse effect by around 0.05 Wm–2. Second, various gaseous species contribute to the production of ozone in the troposphere; this is the case of the CO already cited, nitrogen oxides (NOx, not to be confused with N2O), methane, and other hydrocarbons. These are called “indirect greenhouse gases,” the production of which is linked, in one way or another, to human activities. As a result, since the preindustrial period there has been an estimated increase of 30% of ozone in the troposphere with a radiative forcing of 0.35 Wm–2.
It is thus incontestable: through our activities we have been rapidly and greatly modifying the composition of our atmosphere for two hundred years. What is more, we know precisely how the greenhouse effect has increased as a result of human activity, let’s say by about 10%. The assessment is simple: the observed variations in concentrations of carbon dioxide, methane, halogen compounds, nitrogen protoxide, and ozone have increased the greenhouse effect by around 3 Wm–2, approximately 55% of which is linked to an increase in CO2—an increase of more than 1% compared to the 235 Wm–2 available to heat the lower layers of the atmosphere and of which a large part results from human activity post–World War II. The questions then become: Is the climate in the process of warming? If so, are we responsible for this?
CHAPTER 12
Have Humans Already Changed the Climate?
In 1896 Svante Arrhenius brought attention to the fact that humans were in the process of changing the amount of carbon dioxide in the atmosphere and that as a consequence our planet would warm up by 5°C from that point to the end of the twentieth century, according to his estimates. It was only eighty years later that this risk of warming and its potential consequences were taken seriously. This awareness quickly led to the establishment of the Intergovernmental Panel on Climate Change (IPCC) in 1988, which in 2007 concluded that our activities are very likely at the origin of a marked warming that we have been experiencing since the mid-twentieth century.
The Time of the Pioneers
Until the 1970s very few research teams were looking at the impact of human activity on the climate. It is true that after an initial period of warming up to the beginning of the 1940s, temperatures stabilized, then slightly decreased. Moreover, in 1975 Willi Dansgaard,1 the Danish glaciologist, a pioneer in the reconstruction of past climates from the ice of Greenland, extrapolated one of the climatic records he had obtained at Camp Century northwest of that ice cap and predicted a future cooling of the planet. The idea at that time was that the length of the interglacial period in which we are living, the Holocene, would end quickly. That fear was in fact unfounded since the Earth’s orbit is such that a return to a glacial period is not anticipated for another 10,000 years or even more. But thirty years ago, we were far from the cry of alarm that we are sounding today regarding the effect of human activity on the climate.
A few pioneers did, however, attempt to have their voices heard. In the 1930s Guy Callendar, who had correctly seen an increase in CO2 in the atmosphere, sought to establish a connection between the warming that had been occurring since the beginning of the century and an increase in the greenhouse effect that resulted from an increase in CO2. With this idea, he established a curve showing the average temperature over all the continents from series of temperatures measured in around two hundred meteorological stations. Wladimir Köppen, already cited for his work in paleoclimatology, had undertaken the same study at the end of the nineteenth century, but Callendar had much longer records that enabled him to show and to correctly document the warming that had begun since the beginning of the twentieth century. He postulated that this warming might be a result of the use of coal and estimated that the average warming linked to a doubling of CO2 was 2°C, with more elevated levels in polar regions. Like Arrhenius, he saw only beneficial effects in this warming that would make northern climates milder and would delay a return to a glacial period.
The Awareness
Modelers were really the first to become aware of the potential importance of the influence of human activity on the climate. The postwar period saw the appearance of the first calculators, which at the time were quite rudimentary. Forecasting the weather had been within the realms explored by the pioneers of those machines that would revolutionize our world and, in particular, that of scientific research. These models rested on a system of physical equations that enabled scientists to describe movements of the atmosphere and the water cycle from evaporation on the surface of the ocean to the formation of precipitation. These equations also took into account exchanges of energy—that provided by the Sun, a part of which is used during evaporation, and that freed when the water vapor is condensed in the atmosphere and forms the fuel of the climatic machine. The first applications were in the realm of meteorology, which involves the prediction of weather disturbances whose individual evolution we can follow only over a few days; beyond a dozen days the atmosphere loses its memory.
Climatologists are interested in longer time scales—a month and beyond—and their forecasts bear only on the average temperature values, quantities of precipitations, or characteristics of the atmospheric circulation, not on following specific disturbances. We are all well aware of the progress made in forecasting the weather, progress that went hand in hand with the increase in the capacity of the means of calculation, but also of those dedicated to observation. The climatic models were also much improved even if, as we will see, much uncertainty remains. They are, we emphasize, based on equations that are identical to those of meteorological models.
The first experiments, carried out in
the 1960s, enabled us to verify that the models could simulate the great characteristics of the climate: mean annual levels of temperatures and precipitations, winds, seasonal cycles, and geographic distribution of climatic zones. But climatologists quickly became interested in analyzing the ability of these models to account for climates different from the one in which we are currently living. In principle, it was easy for a climate modeler to modify the composition of the atmosphere and to make a new simulation even if the number of simulations was at the time fairly limited due to the lack of computing power. The first experiments of this type, carried out in the 1970s, looked at the effect of a doubling of the quantity of carbon dioxide compared to its current value.
Granted, the results differed noticeably from one model to another; thus the warming predicted by four different models (three American models, one English) in the event of instantaneous doubling of the concentration of carbon dioxide varied from 1.5 to 4.5°C. This factor of three, in the value of what we call “climate sensitivity,” results for the most part from the way in which the formation of clouds was dealt with. Their optical properties cause them to both absorb and reflect solar radiation. They are, moreover, affected in different ways depending on whether they are “high clouds” or “low clouds.” This complexity means that the behavior of cloud systems is difficult to account for in models. Furthermore, two versions of the same model (that of the UK Meteorological Office), between which only this taking into account of clouds has been modified, forecasted respective warming of 1.9 and 5.2°C.2