Chapter 2: Radiative Forcing of Climate Change2.1. RADIATIVE FORCING
2.2.A. Carbon Budget
2.2.B. Atmospheric Carbon Dioxide
2.2.C. Recent Anomalies in the Global Carbon Cycle
2.2.D. Carbon Doxide Record Over the Last Climatic Cycle
2.2.E. Carbon Dioxide Concentrations in the Future
2.2.F. Summary from IPCC 2001 Assessment
2.4. NITROUS OXIDE
In an unperturbed state, the net incoming solar radiation at the top of the atmosphere (Sn) must be balanced by the outgoing longwave radiation (Lo), on an annual basis globally. While this may not be exactly so in any given year, but when a long time series of the annual differences are viewed, Sn must equal Lo.
A true change in either Sn or Lo is called Radiative Forcing.
A positive radiative forcing, that is, when Lo is less than Sn, results in warming of the Earth surface. And, vice versa. For instance, increasing the concentration of greenhouse gases in the atmosphere, such as carbon dioxide, decreases the outgoing longwave radiation, and therefore, results in warming of the Earth's surface. On the otherhand, increasing the aerosol loads in the atmosphere, increases the direct effect of reflection of solar radiation by these particulates, and therefore, results in a negative radiative forcing or cooling of the Earth's surface.
Human activities can result in changes of the various components of the climate system, the climatic effects of which can be estimated by investigating the magnitude of radiative forcing. Estimates of the globally averaged radiative forcing due to changes in greenhouse gases and aerosols from pre-industrial time to the present, and changes in solar variability from 1850 to the present are shown in Figure2.1. The height of the bar indicates a mid-range estimate of the forcing, whilst the lines above show the possible range of values. An indication of the relative confidence in the estimate is given below each bar.
The increase in CO2 concentration from 280 ppmv (preindustrial) to 360 ppmv (1995) resulted in a positive radiative forcing of the climatic system of about 1.5 W/m2. A further 0.5 W/m2 forcing has resulted from increase in methane concentration (700 ppbv preindustrial to 1700 ppbv in 1995). The total radiative forcing due to greenhouse gases is thus estimated to be about 2.5 W/m2. A general rule of thumb is warming of about 0.6 C results from 1 W/m2 forcing of the present climate system.
The negative values for aerosols should not necessarily be seen as an offset against the greenhouse gas forcing because of uncertainities over the applicability of global mean radiative forcing in the case of non-homogeneously distributed species such as aerosols and ozone. It is especially important to note the confidence level associated with tropospheric aersol indirect effects.
A summary of key greenhouse gases affected by human activities is shown below in Table 2.1. From this table and Figure 2.1, you can note that the concentration of CO2 increased by about 80 ppmv since the preindustrial times and the resulting radiative forcing is about 1.5 W/m2. But, during the same period, the concentration of methane increased by about 1 ppmv, but the consequent radiative forcing is 0.5 W/m2, which clearly shows that methane is a much more powerful greenhouse gas than CO2. It is also important to note the lifetimes given for each of the gases. Note that the growth rates are for the decade of 1980s.
ppbv: parts per billion by volume
pptv: parts per trillion by volume
No single lifetime for carbon dioxide can be defined because of the different rates of uptake by various sinks.
2.2.A. Carbon Budget
The global carbon budget during the 1990s is shown schematically in Figure2.2. Our understanding of the global carbon cycle has improved considerably in the recent years, particularly in how the carbon from the atmosphere is removed and stored in sinks on land and ocean. However, considerable uncertainity exists regarding the processes which contribute to the sinks on land.
The main anthropogenic sources of carbon dioxide are the burning of fossil fuels (with additional contributions from cement production), and land-use changes. During the 1990s, the average emissions from fossil fuel burning and cement production were 6.5 GtC/yr. A GtC is a Giga Tonne of Carbon or 10^9 (billion) tonnes of carbon or 10^15 (Peta) grams of carbon.
Land-use changes cause both release and uptake of CO2. On average CO2 will be released to the atmosphere if the original ecosystem stored more carbon than the modified ecosystem which replaces it. Thus, deforestation acts as a CO2 source. During the 1990s, tropical deforestation is estimated to have resulted in an average emission to the atmosphere of 1.0 to 1.5 GtC/yr.
Carbon exchanges between vegetation on land and the atmosphere are considerable, at annual scales. Global vegetation fixes about 100-120 GtC/yr through the process of photosynthesis (Gross Primary Production, or GPP). About 40-50% of this carbon is respired by the vegetation (Autotrophic or plant Respiration, AR) , with the result Net Primary Production of global vegetation is about 50 to 60 GtC/yr. Heterotrophs in the soil constribute a similar amout to the atmosphere (Heterotrophic or soil Respiration, HR). The difference between NPP and HR is called Net Ecosystem Exchange or NEE). Carbon from land is also lost through non-respiratory processes, for example, fire, herbivory, harvests, insect damage, river run-off, etc. The resulting balance between carbon gains and losses is called Net Biome Production, NBP. Currently, it is estimated that NBP is about 1-3 GtC/yr, that is, there is a carbon sink on land.
There are also considerable exchanges of CO2 between the atmosphere and the oceans, of the order of about 90 GtC/yr, with a net sink of about 2 GtC/yr. Carbon in the surface ocean layers is used by the marine biota, and more is exchanged with the intermediate to deep layers in the ocean. The carbon stocks in the ocean are significantly larger than on land (cf. Figure2.2 ).
Accurate and direct measurements of the concentration of CO2 in the atmosphere began in 1957 at the South Pole and in 1958 at Mauna Loa, Hawaii. The Mauna Loa record is shown below in Figure2.3.
In 1958, the concentration of CO2 was about 315 ppmv, and the growth rate was about 0.6 ppmv/yr. This growth rate has generally been increasing since then; it averaged 0.83 ppmv/yr in the 1960s, 1.28 ppmv/yr during the 1970s, amd 1.53 ppmv/yr during the 1980s. The concentration in the spring of 2002 was about 375 ppmv, the most recent month for the which the data were available. Data from Mauna Loa are close to, but are not precisely the global mean value. The Mauna Loa record is due to Prof. Keeling of the Scripps Institution of Oceanography.
The annual cycle in the Mauna Loa record (Figure 2.3) is due to the seasonality of vegetation. In early spring, the concentration of CO2 is at its maximum, and as the plants greenup, the concentration drops, reaching a minimum value towards the end of the summer, and when it starts to build up again. This swing in the amplitude is most pronounced in the records from the northern high latitudes, where it can be as large as 15 ppmv.
The atmospheric CO2 record prior to 1957 comes mainly from air bubbles in ice cores, which is reasonably accurate (Figure 2.4). This record extends back almost a 1000 years. Data from the period before that is of poor quality because of the defects in the ice. Over the last 1000 years, CO2 concentration in the atmosphere has fluctuated at about +/- 10 ppmv around 280 ppmv. Therefore, the 20th century increase in CO2 concentration is unparalleled in the past 1000 years.
There are at least three arguments to be made for the case that the observed increase in atmospheric CO2 concentration is due to emissions related to human activity.
The CO2 record exhibits a seasonal cycle, with small peak-to-trough amplitude (about 1 ppmv) in the Southern Hemisphere, but increasing northward to about 15 ppmv in the boreal forest zone (55-65 degrees North). This cycle is caused by the seasonal uptake of atmospheric CO2 by terrestrial ecosystems.
Recently, an increase in amplitude of the seasonal cycle of atmospheric carbon dioxide exceeding 20% since the early 1970s,and an advance in the timing of the drawdown of CO2 in spring and early summer of up to 7 days has been reported. This suggests that plants in the northern high latitudes have been growing vigorously, and that springtime greening is happening earlier. This has also been confirmed with analysis of satellite data for the 1980s.
The close association between CO2 and temperature changes during the last climatic cycle was revealed by data from the ice core record ( Figure 2.6). Samples from Greenland and Antarctica representing the last glacial maximum (about 18,000 years before present or 18K BP), indicate that CO2 concentration at that time was about 190-200 ppmv, that is, about 70% of the pre-industrial level. Thus, an increase of about 80 ppmv occurred in parallel with the warming at the end of the glacial period, when the estimated rise in mean surface temperature of the Earth was about 4C over about 10,000 years. The Antarctic record shows a marked correlation between temperature and CO2 and CH4 concentration.
The Antarctic record provides an important perspective for recent and potential future changes, although the magnitudes and rates of changes in the paleo record may differ from those due to greenhouse warming.
In order to predict future concentrations of CO2 in the atmosphere, an understanding of the relationship, and its quantification, between emissions and concentrations is required. Models have been used towards this purpose, and these do have well recognised limitations, for example, they do not include the climatic feedback on the CO2 cycle.
Two important questions are considered here -
For this purpose, certain standard emission scenarios have been developed by the Intergovernmental Panel on Climate Change (IPCC) in 1992; these are called IS92a, b, c, d, e, and f (Figure 2.7a).
Now, we consider the first question, that is, for a given CO2 emission scenario, how might CO2 concentrations in the future might change? Model results are shown in Figure 2.7b.
Let us now consider the second question, that is, for a given CO2 concentration profile leading to stabilization, what anthropogenic emissions are implied? Towards this purpose, concentration profiles have been devised leading to stabilization from 350 ppmv to 750 ppmv; for comparision, the preindustrial concentration was about 280 ppmv and 2002 concentration was above 370 ppmv. These profiles are shown in Figure 2.8a. These profiles have been devised such that stabilization at 350, 450, 550, 650 and 750 ppmv is achieved in 2150, 2100, 2150, 2175, 2250, respectively. The selection of the range 350 to 750 ppmv is quite arbitrary and has no policy implications. These profiles are illustrative and give a smooth transistion from the present levels to the levels of stabilization. The model derived profiles of total anthropogenic emissions from fossil fuel burning, changes in land-use and cement production, are shown in Figure 2.8b.
Stabilization of CO2 at or below 750 ppmv (the highest level studied) would require accumulated emissions from 1900 to 2100 lower than those occurring under IS92a, IS92b, IS92e and IS92f scenarios, and even lower in the next two centuries. In order to achieve stabilization at 350 ppmv (the lowest level), even the lowest emission scenario IS92c leads to emissions well above 300-430 GtC required.
The previously shown model results on future CO2 emissions and atmospheric concentrations did not take into account any feedbacks on the carbon cycle, especially those related to global vegetation. Higher temperatures and increased precipitation, thought to result from the greenhouse effect, can increase photosynthesis and plant growth, and hence, carbon storage in vegetation and litter (a negative feedback). Also, experiments have shown that plants grown in CO2 rich air tend to grow more vigorously, as CO2 is a nutrient (the fertilization effect).
The storage of carbon in the soils tends to decrease with warming due to increased rates of decomposition (a positive feedback). On the otherhand, soils which are flooded tend to store large amounts of carbon in the form of peat. In anycase, it is illustrative to see how one such feedback can alter the previously reported results on emission and concentration profiles. Figure 2.9 shows modifications to the business-as-usual scenario (IS92a) at different rates of vegetation net primary productivity (NPP) enhancement.
Indeed some reasonably significant changes in atmospheric CO2 can occur depending on the NPP enhancement factor (about 14 percent or about 50-100 ppmv in 110 years or so). However, it is also abundantly clear that vegetation growth enhancement cannot be assumed to offset anthropogenic carbon emissions. Forty percent NPP enhancement has been observed in some plants, under closed chamber and optimal water and nutrient situations. In general, there is no reason to believe that this can achieved globally under all conditions, and the real number may be half of this or even less.
Methane (CH4) is another naturally occuring greenhouse gas whose concentration in the atmosphere has been increasing as a result of human activities (rice paddies, animal husbandry, landfills, biomass burning, and fossil fuel production and use). Ice core measurements of methane concentration indicate that its concentration has increased from a preindustrial level of about 750 ppbv to about 1750 ppbv in early 2001 (Figure 2.10). Figure 2.11).
Globally methane increased by about 7% over the decade from 1983. However, the decade of 1980s and 90s were characterized by declining methane growth rates, dropping from about 15 ppbv/yr in the early 1980s to under 10 ppbv/yr by 1990. Growth rates slowed dramatically in 1991 and 1992, and more recently in 1999 and early 2000.
Note that although methane concentration is in ppbv, compared to carbon dioxide, it is a highly effective radiative gas. The increase in methane since preindustrial times is estimated to have resulted in a radiative forcing of about 0.5 W/m2, which is quite significant when compared to the increase in carbon dioxide concentration during the same time period and its radiative forcing (1.5 Watts/m2) (Figure 2.1).
Analysis of the global methane budget indicate that about 20% of the total annual methane emissions are related to fossil fuel use (for example, combustion, coal mines, natural gas production, petroleum industry operations, etc). In total, it has been estimated that about 60-80% of current methane emissions are due to anthropogenic activities. About 20% of current methane emissions are related to natural sources such as wetlands, termites, oceans etc.Climate change can either -
There are many sources of Nitrous Oxide (N2O), both natural and anthropogenic, which are difficult to identify and quantify. The main anthropogenic sources are from agriculture, biomass burning and industrial processes. The best estimate of 1980s anthropogenic emission of N2O is 3 to 8 TgN/yr (terra grams of Nitrogen or 10^12 grams of Nitrogen). Natural sources are probably twice as large as anthropogenic sources.
Ice core measurements of nitrous oxide concentration indicate that its concentration has increased from a preindustrial level of about 275 ppbv to over 310 ppbv in early 1990s.
The average growth rate over the past four decades was about 0.25%/yr (0.8 ppbv/yr). The radiative forcing due to change in nitrous oxide concentration from preindustrial level to the presenthas been estimated at about 0.14 W/m2 (Figure 2.12). Because of its long life-time, about 120 years, if emissions were held constant at the present level, atmospheric N2O abundance would still climb from 311 ppbv to about 400 ppbv over the next several hundred years.
Aerosols are suspensions of particles in the atmosphere, with diameters in the range 10-3 to 10-6 meters. Tropospheric aerosols are formed by -
The addition of tropospheric aerosols from anthropogenic sources can influence the radiative balance in two major ways -
There are many uncertainities associated with how aerosols influence the climate. Aerosols are highly variable spatially, both in their concentration and chemical composition, and direct observations of their amount and kind are begining to emerge just now. Nevertheless, some attempts have been to estimate the globally averaged radiative forcing due to aerosols from anthropogenic sources. The direct radiative forcing due to increases in sulphate aerosol since 1850, averaged globally, is estimated to lie between -0.25 to -0.9 W/m2. And, the direct effect of aerosol from biomass burning is estimated to be in the range of -0.05 to -0.6 W/m^2.
The spatial distribution of direct radiative forcing due to sulphate aerosols derived from a radiative transfer model is shown in Figure 2.13. The largest forcing occurs over or close to industrial activity. Over the eastern USA, central Europe, and eastern China, sulphate forcing may have offset much of, or in places been greater than, the greenhouse gas forcing. However, it must be remembered than local forcing does not necessarily govern local response, for changes can still happen locally due to circulation as the atmosphere may respond to forcings elsewhere.