IPCC chapter 2

Summary of Chapter 2 in IPCC AR4

Radiative forcing (RF) is a concept used to quantitatively compare the effects of natural and anthropogenic factors on the climate, or the Earth’s energy balance. RF is a measure of the change in the amount of net radiative energy that reaches the earth’s surface as a result of that particular factor. A change in the energy balance affects the earth’s surface temperature in a linear relationship. RF is measured in Watts per square meter with confidence ranges in brackets and the level of scientific understanding is also noted.
In scientific terms, the IPCC defines RF as “the change in net (down minus up) irradiance (solar plus longwave; in W m-2) at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values.” Radiative forcing agents are sometimes referred to as “climate drivers.”
The factor may increase surface temperatures (positive forcing) or decrease surface temperatures (negative forcing). Given that many other parts of the climate system are not well understood, RFs can be a helpful concept. RFs are easily quantified and they can be ranked, but they are limited as they don’t account for the complex feedback and interactions in the climate system.
Chapter 2 in the IPCC Fourth Assessment Working Group I report summarizes the current understanding of these drivers, followed by a discussion reevaluating the RF concept, synthesizes the global mean RF, and finally discusses other metrics for comparing emissions. In this Wiki page, we will summarize this chapter’s key findings and highlight what’s new, identify areas that need more research, and then offer some comments.
KEY FINDINGS
2.3 CHEMICALLY AND RADIATIVELY IMPORTANT GASES (POSITIVE ANTHROPOGENIC FORCINGS)
What’s new: For the first time, RF values are derived for all anthropogenic climate factors combined, as well as for the individual components. This was facilitated by new global measurement capabilities. These advancements improved the IPCC’s confidence in attributing climate change to human influences.
• Humans are changing the climate. It is extremely likely (≥95% confidence level) that humans have had a significant influence on the climate. This is supported by the derived combined anthropogenic RF value of +1.6 [-1.0, +0.8]^2 W/m2. Furthermore, this RF is likely (≥66%) five times greater than warming caused by solar irradiance (a natural climate driver).
• Long-lived greenhouse gases (LLGHGs) have the biggest effect on the climate. The combined RF of LLGHGs is +2.63 [+/-0.26] W/m2, high level of scientific understanding. LLGHGs include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons and sulphur hexafluoride (SF6). The higher concentration level since 1998 resulted in a 9% increase in this RF since the Third Assessment Report (TAR). Scientific understanding of LLGHGs is considerably better than it is for the other RFs.
• CO2 is the most important man-made greenhouse gas. CO2 RF in 2005 was +1.66 [+/0.17] W/m2. The increase of 20% from 1995-2005 is more than in any decade in at least 200 years. CO2 has increased globally by about 100 ppm (36%) over the last 250 years, from a range of 275 to 285 ppm in the pre-industrial era to 379 ppm in 2005. The primary source of the increase in CO2 is fossil fuel use but land-use changes—such as deforestation and biomass burning—also contribute. The RF estimate depends on the CO2 mixing ratio in the preindustrial base year of 1750. However, new measurements from Antarctic ice show that CO2 mixing ratios vary before 1800, so using 1750 as the base year may result in estimates overestimating the RF. A base year of 1860 results in approximately a 10% reduction in estimated LLGHG RF.
• Emissions of methane are not increasing. The growth rate of methane, the second-largest LLGHG after CO2, has generally decreased over the past two decades, but cause is not well understood. The RF in 2005 was about +half W/m2. The amount of methane in the atmosphere in 2005 (1774 ppb) is unprecedented in at least the last 650 kyr. The primary source of the increase in methane is a combination of human agricultural activities and fossil fuel use. How much each contributes is not well determined.
• N2O on pace to take over the third place in RF ranking. In 2005, N2O contributed a RF of +0.16 [+/0.02] W/m2 and continues to rise approximately linearly. The RF has increased by 11% since the TAR, when N2O had the fourth largest RF among the LLGHGs. At this rate, N2O will outpace CFC-12 for third place. Understanding of regional N2O fluxes has improved since the TAR. Recent studies strengthen the finding that the tropical region influences the spatial distribution of N2O.

• Fluorine-containing Kyoto Protocol gases are increasing by large factors. These gases include hydrofluorocarbons (HFCs), perfluorocarbons, and sulphur hexafluorides (SF6). They have increased between 4.3 and 1.3 times between 1998 and 2005—about 10%/year. In 2005, they contributed a total RF in 2005 of +0.017 [+/-0.002] W/m2.
• The Montreal Protocol gases are beginning to decline. In 2005, the combined RF of the Montreal Protocol (international treaty protecting the ozone layer) gases—chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and chlorocarbons was +0.32 [+/-0.3] W/m2. While their emissions have decreased substantially, their long lifetimes will still have a lasting RF, with their sinks reducing their levels by only about 2% and 1%/ yr.
• Hydroxyl free radical (OH) The hydroxyl free radical (OH) is a major oxidizing chemical in the atmosphere that destroys about 3.7 Gt per year of CH4, HFCs, HCFCs and ozone and thus affecting their RFs. There is no detectable change in global average concentration of OH between 1979 and 2004.
• Ozone increased in the troposphere and may be significantly increasing at low latitudes. In 2005, O3 contributed an RF of +0.35 [-0.1, +0.3] W/m2, medium level of scientific understanding. Since the Montreal Protocol that banned the gases that destroy stratospheric ozone, the updated RF is now -0.05 [+/-0.1] W/m2, medium level of scientific understanding.
• Stratospheric water vapor contributes a positive RF but the level of scientific understanding is poor. The RF for stratospheric water vapor due to oxidation of CH4 is +0.07 [+/0.05] W/m2, low level of scientific understanding. The contribution of other anthropogenic water vapor increase on RF is poorly understood.
2.4 AEROSOLS (NEGATIVE FORCING)
What’s new: Atmospheric models and observation technologies have both improved. Many atmospheric models now include all the important aerosol components and they better model indirect aerosol effects. Satellites and other measuring instruments are also more accurate and help to verify model results. For the first time, RF estimates are given for nitrate and mineral dust aerosols.
Aerosols have direct and indirect effects on RF in the negative direction, but the level of understanding isn’t high. Directly, aerosols scatter and absorb solar radiation, changing the amount of longwave radiation that reaches the earth. Indirectly, aerosols can serve as cloud condensation nuclei; water vapors stick to the cloud nuclei and grow into clouds. This affects the extent and lifetime of cloud cover, and thus how much radiation reaches the surface and the amount of longwave radiation is trapped (cloud lifetime effect). Also, clouds with a lot of aerosols have a higher albedo (cloud albedo effect). The cloud albedo effect was identified by the TAR as a key uncertainty in understanding the climate and considered by the TAR to be an RF.
Models and observations derive an estimate of the total direct aerosol impact on RF to be -0.5 [+/-0.4] W/m2, medium-low level of scientific understanding. Indirect aerosol effects via clouds is -0.7 [-1.1,+0.4] W/m2., low level of scientific understanding. However, estimates of RF for individual aerosol species is less certain. These species include sulphate, fossil fuel organic carbon, fossil fuel black carbon, biomass burning, nitrate, and mineral dust.
2.5 ANTHROPOGENIC CHANGES IN SURFACE ALBEDO AND THE SURFACE ENERGY BUDGET (LAND USE CHANGES)
Land use changes, mainly through conversion of croplands, pastures, and forests alter the albedo of the Earth’s surface and thus the surface energy budget. Deforestation is a human-caused land use change that has a significant effect on surface albedo; the estimated RF is -0.2 [+/0.2] W/m2, medium-low level of scientific understanding. Another example is black carbon on snow, which lowers the surface’s albedo and absorbs more radiation as a result. This effect yields a positive RF value of +0.1 [+/- 0.1] W/m2, low level of scientific understanding. RF values cannot be estimated for other land cover-atmosphere feedbacks so these processes have a very low level of scientific understanding.
2.6 CONTRAILS AND AIRCRAFT-INDUCED CLOUDINESS
Aircraft contribute directly and indirectly to RF through the formation of persistent condensation trails (contrails) in areas of low temperatures and high humidity and the effects of aviation aerosols. Contrails are basically thin cirrus clouds, which reflect solar radiation and trap outgoing longwave radiation. Future changes in atmospheric humidity and temperature distributions in the upper troposphere will have consequences for aircraft-induced cloudiness. The current best estimate for then RF in 2000 is +0.010 W/m2, which is smaller than the estimate from TAR. There is uncertainty in global contrail cover calculations and the fact the extent of supersaturated regions in the atmosphere is poorly known.
Aviation-induced cloudiness (AIC) is defined to be the sum of all changes in cloudiness associated with aviation operations. Thus, an AIC estimate includes persistent contrail cover. The ambiguity of the definition prevented the best estimates of AIC amounts and the associated RF. Global mean RF value for AIC in 2000 is +0.030 W m-2 with a range of +0.01 to +0.08 W m-2. Even though the ambiguity of the definition of AIC and the poorly understood feedback process, the effect of the estimate of RF value could be as large as the observed surface warming over the USA and the absence of AIC has been proposed as the cause of the increased diurnal temperature range (DTR) during the short period when all USA air traffic was grounded starting on 11 September 2001.
Also noted here is the potential role of aviation aerosols in altering the properties of clouds that form later in air containing aircraft emissions which includes aerosols and aerosol precursors into the upper troposphere and lower stratosphere. The aviation aerosol consist sulphate and BC (soot) which are considered could act as nuclei in ice cloud formation and have the potential to alter aviation-induced cloudiness. However, based on different assumptions about the cloud formation process, the effect of aviation aerosol on ice nuclei is uncertain, therefore the exact RF is unknown, even though there may be a correlation between AIC and aviation aerosols.
2.7 NATURAL FORCINGS The two natural forcings assessed in the IPCC are solar irradiance and explosive volcanoes. The increase in solar irradiance since 1750 has a direct RF estimate of +0.12 [-0.06, +0.18] W/m2, low level of scientific understanding. This RF is less than half of the estimate in TAR. Solar radiation also follows an 11-year cycle. Volcanic events have a temporary (2-3 years) cooling effect.
2.8 UTILITY OF RADIATIVE FORCING discusses the several studies since the TAR that have examined the relationship between RF and climate response. RFs are increasingly being diagnosed from complex computational GCM integrations; this assessment is entirely based on climate model simulations.
• Vertical Forcing Patterns and Surface Energy Balance Changes Surface forcings are presented as an important and useful diagnostic tool that aids understanding of the climate response. The recent study on assessing absorbing aerosol finds the surface forcings are arguably a more useful measure of the climate response than the RF.
• Spatial Patterns of RF Each RF agent has a unique spatial pattern. Spatial patterns of RF also affect the pattern of climate response. However, the magnitude of RF is rarely coincident with the magnitude of regional response.
• Alternative Methods of Calculating RF Several studies employed GCMs to diagnose the value of RF based on the assumption of the zero-surface-temperature-change RF definition. And different methodologies are applied with each of their own limitations. The difference between zero-surface-temperature-change RF and other mechanisms is likely to be small, aside from the case of certain aerosol changes. These calculations also remove problems associated with defining the tropopause in the stratospherically adjusted RF definition.
• Linearity of the Forcing-Response Relationship Results from many studies since TAR are consistent with the TAR assumption that the response to individual RFs could be linearly added to gauge the global mean response, except the nonlinearities for large negative RFs where static stability changes in the upper troposphere affect the climate feedback.
• Efficacy and Effective RF The understanding of efficacy is enhanced by calculations based on different mechanisms. The assessment of the efficacy associated with stratospherically adjusted RF is reported. Therefore, cloud aerosol interaction effects beyond the cloud albedo RF are included in the efficacy term in the whole chapter. However, the differences between the assessed values of efficacy from different simulation models are not well understood.
• Efficacy and the Forcing-Response Relationship As the physical understanding of efficacy becoming established, when employing the stratospherically adjusted RF, there is medium confidence that efficacies are within the 0.75 to 1.25 range for most realistic RF mechanisms aside from aerosol and stratospheric ozone changes. Further, zero-surface-temperature-change RFs are very likely to have efficacies significantly closer to 1.0 for all mechanisms.
2.9 SYNTHESIS
2.10 GLOBAL WARMING POTENTIALS AND OTHER METRICS FOR COMPARING EMISSIONS
GWPs provide a metric for comparing the climatic impact of different greenhouse gases, one that integrates both the radiative influence and biogeochemical cycles. The GWPs estimate is the ratio of the cost of abating the same amount of each trace gas, comparing to CO2. The estimation is based on an economic model which is to minimize the cost of abatement subject to certain constraints based on simplifying assumptions about the damage function, marginal cost of abatement, and the baseline scenario.
However, the uncertainty involved with the marginal cost and the poorly known damage function of each trace gas cause the GWP estimates debatable. Further understanding about the damage function and the marginal cost of the abatement of each trace gas is required to better estimate the GWPs and to inform public policy of optimal mitigation.
In addition, the indirect GWPs for NOx, CO and VOCs were proposed. Uncertainties for the indirect GWPs are generally much higher than for the direct GWPs, due to complex nonlinear chemical responses. In terms of sulphate aerosols, both the direct radiative effects and the indirect effects on clouds were acknowledged, but the importance of carbonaceous aerosols from fossil fuel and biomass combustion was not recognized.
In the meantime, there are new alternative index for comparing the impact of different trace gases, including Global Temperature Potential and a revised GWP which includes the efficacy of a forcing agent.

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