Module 2 The climate science behind climate change

Module The climate science behind climate change

module 19 All these reports are available on the IPCC homepage at: http://www.ipcc.ch/ publications_and_data/publications_and_data_reports. shtml#1. 1 Introduction Module 1 showed that human actions can influence the climate because the economy and the environment are interdependent. This module focuses specifically on the climate system and the climate science behind climate change. By reviewing key points, the module enables readers to understand how the climate system works, why climate change occurs, and how humans induce climate change. The module highlights recent empirical evidence pointing towards the existence of human-induced climate change, and discusses climate-change-related impacts that can already be observed today. To conclude, the module outlines different scenarios for the future and discusses the anticipated impacts of climate change. To this end, this module draws on the work of the Intergovernmental Panel on Climate Change (IPCC) Working Group I, which deals with the physical science basis of climate change. As mentioned in Module 1, the IPCC is the leading body collecting, reviewing, and assessing recent state-of-the-art scientific work related to climate change. The five IPCC assessment reports and the different IPCC special reports 19 together form the most comprehensive and reliable source of scientific work today on climate change. The module therefore uses some of the terminology introduced by the IPCC (see Box 9). Box 9 The IPCC s terminology to report findings to the public The Intergovernmental Panel on Climate Change (IPCC) developed specific terminology to report its findings to the public. In the fifth assessment report (IPCC, 013a), a finding is assessed in terms of the underlying evidence and the agreement among scientists regarding that finding. In addition, the IPCC often assigns a level of confidence to the different findings that is based on evidence and agreement (see Figure 11). Figure 11 Relationship between agreement, evidence and confidence levels Confidence scale High agreement Limited evidence High agreement Medium evidence High agreement Robust evidence Agreement Medium agreement Limited evidence Low agreement Limited evidence Medium agreement Medium evidence Low agreement Medium evidence Medium agreement Robust evidence Low agreement Robust evidence Evidence (type, amount, quality, consistency) Source: Mastrandera et al. (010: 3). To rate the degree of agreement among scientists regarding the evidence, it uses the terms low, medium, and high. Confidence levels are expressed using the terms very low, low, medium, high, and very high. If the likelihood of an outcome or a result has been assessed using statistical techniques, the IPCC reports probability values using the following terminology: virtually certain (99 100 per cent probability); very likely (90 100 per cent probability); likely (66 100 per cent probability); about as likely as not (33 66 per cent probability); unlikely (0 33 per cent probability); very unlikely (0 10 per cent probability); and exceptionally unlikely (0 1 per cent probability). Sometimes it also uses the terms extremely likely (95 100 per cent probability); more likely than not (>50 100 per cent probability); more unlikely than likely (0 <50 per cent probability); and extremely unlikely (0 5 per cent probability). For more details, see Mastrandrea et al. (010). Source: Author's elaboration based on Mastrandera et al. (010). Section introduces the theoretical basis of the climate system and climate change. Section.1 familiarizes the reader with the concepts of weather, climate, and climate change, and introduces the five components of the climate system (i.e. atmosphere, hydrosphere, cryosphere, land surface, and biosphere). Section. focuses on the planet s energy balance, which influences all five components of the climate system and is therefore of crucial importance for the climate. The section stresses that the natural greenhouse effect plays an integral role in the planet s energy balance and is generally responsible for the relatively warm average temperature on the surface. With the module having outlined how the climate system works, Section.3 then explains that the climate not only changes in response to factors within the climate system but also in re- 34

sponse to some factors that are external to the climate system. In fact, some external factors (e.g. human activities) affect climate change drivers (e.g. greenhouse gases), which in turn affect the energy balance of the planet. Because the earth s energy balance affects all five components of the climate system, external factors can thus affect the climate. Section.4 introduces two important concepts that allow for assessing how the planet s energy balance changes as a result of externally induced variations in climate change drivers: radiative forcing and effective radiative forcing. These two important concepts allow for quantifying the extent to which different external factors influence the climate. Finally, Section.5 specifically focuses on the different ways in which humans affect the climate, altering atmospheric concentrations of greenhouse gases and aerosols, and influencing properties of the land surface. It concludes by assessing the relative strength of these different human-induced perturbations of the climate system, and highlights the importance of feedback effects. Section 3 then provides an overview of the main changes that have occurred in the climate system and highlights to what extent human activities have contributed to these observed changes. To this end, the section discusses observed changes in the means of temperature, precipitation, ice and snow cover, and sea levels. It then shows that not only the mean state of climate variables has changed, but that the frequency of extreme climate events has also been affected. The section concludes with a short discussion on the impact of these observed changes on human and natural systems. To conclude Module, Section 4 looks at the future of the planet s climate system. To this end, it introduces different scenarios describing possible pathways of climate change drivers that are used by the IPCC s climate models to project future changes in the climate system. After discussing these scenarios, Section 4 highlights the most important predicted changes in the main climate variables (temperature, precipitation, snow and ice cover, and sea levels) that are likely to occur until 100. The section ends by providing selected examples of anticipated future risks for humans and natural systems that might result from the simulated changes in the climate system. At the end of this module, readers should be able to: Distinguish the five components of the climate system; Understand the radiative balance of the planet; Explain how the natural greenhouse effect operates; Define and understand the concept of radiative forcing; Explain how economic activities can alter the climate; List major observed human-induced changes of the climate system; Discuss anticipated future impacts of climate change. To support the learning process, readers will find several exercises and discussion questions in Section 5 covering the issues introduced in Module. Additional reading material can be found in Annex. The theoretical basis of the climate system and climate change 0 When speaking about climate change, it is important to clearly distinguish three distinct but related concepts: weather, climate, and climate change. Weather can be defined as the changing state of the atmosphere, which is characterized by temperature, precipitation, wind, clouds, etc. (Baede et al. 001). Weather fluctuates frequently as a result of fast-changing weather systems. Weather systems, and hence the weather, can only be predicted with some degree of reliability for a very short period of time (one or two weeks). They are unpredictable over longer time horizons. Climate, on the other hand, is, loosely speaking, long-term average weather. IPCC (001a: 788) defines climate as the statistical description in terms of mean and variability of relevant quantities [such as temperature, precipitation, and wind] over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization. Climate not only varies from location to location (depending on a variety of factors such as distance to the sea, latitude and longitude, the presence of mountains, etc.) but also over time (e.g. from season to season, year to year, century to century, etc.). Based on this definition, IPCC (001a: 788) defines climate change as a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Understanding the interactions of the variety of factors that influence the climate is complicated. Climate, climate change, and the role of human activities in changing the climate can only be understood if one has an understanding of the whole climate system. The following section 0 This section draws on the work of the IPCC, in particular Chapter 1 of the contribution of Working Group I to the third IPCC assessment report (Baede et al., 001), as well as several chapters of the fifth assessment report (especially Boucher et al., 013, Cubasch et al., 013, Masson-Delmotte et al., 013, and Myhre et al., 013). module 35

module looks briefly at the components of the planet s climate system and shows how they interact..1 The five components of the climate system The climate system, schematically displayed in Figure 1, is part of the environmental system discussed in Module 1. It consists of five main components (Baede et al., 001): the atmosphere, hydrosphere, cryosphere, land surface, and biosphere (in bold in Figure 1). These five components interact with each other (thin arrows in Figure 1) and are all affected by the planet s energy balance, which will be discussed in Section.. Figure 1 The climate system Changes in solar inputs Changes in the atmosphere: composition, circulation Changes in the hydrological cycle Atmosphere Clouds Atmosphere-ice interaction Sea ice Heat exchange Ice-ocean coupling text Wind stress Hydrosphere: ocean Precipitation - evaporation N, O, Ar, H O, CO,CH 4, N O, O 3, etc. Aerosols Terrestrial radiation Hydrosphere: rivers & lakes Human influences Volcanic activity Glacier Soil-biosphere interaction Atmosphere-biosphere interaction Biosphere Cryosphere: sea ice, ice sheets, glaciers Ice sheet Landatmosphere interaction Land surface Changes in the ocean: circulation, sea level, biogeochemistry Changes in/on the land surface: orography, land use, vegetation, ecosystems 1 We exclude here the exosphere and consider that the atmosphere stops at the top of the thermosphere. Many consider the thermosphere as the boundary of the atmosphere. But strictly speaking the thermosphere is followed by the exosphere (500 10,000 km above surface), which is considered by some to mark the actual boundary of the atmosphere and thus the environmental system (discussed in Module 1) with the rest of the universe. Source: Baede et al. (001: 88). The most variable and rapidly changing part of the climate system is the atmosphere (Baede et al., 001), which marks the boundary of our environmental system as defined in Module 1. The entire atmosphere lies within 500 km from the surface of the planet, 1 with 99 per cent of its total mass within 50 km from the surface (Common and Stagl, 004). The atmosphere can be subdivided (or stratified) into the five layers displayed in Figure 13. The troposphere is the lowest layer of the atmosphere and extends up to approximately 11 km from the planet s surface. It contains most of the atmosphere s mass and plays a key role in determining the planet s climate. Mean temperature in the troposphere decreases with distance from the surface. In the text that follows, we will often refer to the so-called surface-troposphere system, which encompasses the planet s surface and the troposphere. The next layer is called the stratosphere and extends to roughly 50 km above the surface. Most of the incoming ultraviolet radiation is absorbed by ozone that is concentrated in the stratosphere. The boundary between the troposphere and the stratosphere is called tropopause. The stratosphere is followed by the mesosphere (50 90 km above the surface) and the thermosphere (90 500 km above the surface). 36

Figure 13 Layers of the atmosphere Exosphere 50 510 module 500 490 Thermosphere Mesosphere Stratosphere Troposphere 170 160 150 140 130 10 110 1oo 9o 8o 7o 6o 5o 4o 3o o 1o Altitude (km) -100-50 0 50 100 150 00 500/1500 Temperature C Source: National Aeronautics and Space Administration (NASA), Climate Science Investigations, available at: http://www.ces.fau.edu/nasa/module-/atmosphere/earth.php. Note: The average temperature varies with altitude and is indicated by the red line. The atmosphere is mainly a mixture of different gases but also contains some solid and liquid matter, namely aerosols and clouds. The composition of the atmosphere has been changing throughout the history of the planet. Today the main bulk of the volume of the earth s atmosphere is composed of approximately 78.1 per cent nitrogen (N ), 0.9 per cent oxygen (O ), and 0.93 per cent argon (Ar). Additionally, the atmosphere contains several trace gases such as carbon dioxide (CO ), methane (CH 4 ), nitrous oxide (N O), and ozone (O 3 ). These trace gases, which constitute less than 0.1 per cent of the atmosphere s volume, are called greenhouse gases (GHG). Water vapour (H O) is also a greenhouse gas whose volume in the atmosphere is volatile depending on the hydrosphere s hydrological cycle. Despite their small share in the total volume of the atmosphere, greenhouse gases are of crucial importance for the earth s climate. While the three main gases (N, O, and Ar) do not absorb or reflect the infrared radiation emitted by the planet, greenhouse gases are different they absorb infrared radiation coming from the planet s surface and emit this radiation towards space and back towards the earth s surface, thereby increasing the temperature at the surface (see Section. for a detailed discussion). Consequently, greenhouse gases are crucial for our planet s climate (Baede et al., 001). The hydrosphere consists of all forms of liquid water and includes oceans, rivers, and lakes. Approximately 70 per cent of the total surface of the planet is covered by water. Oceans alone store roughly 97 per cent of all forms of water (liquid, solid, and gas) available on earth, while rivers and lakes store roughly 0.009 per cent (Common and Stagl, 004). The basic process that takes place in the hydrosphere is called the hydrological cycle. The cycle starts with the water evaporating from the oceans, lakes, and rivers and being released into the atmosphere, which leads to an exchange of heat between the hydrosphere and the atmosphere. The water then returns from the atmosphere to the surface in the form of precipitation, either directly into the oceans or indirectly on the land from where it reaches the oceans through rivers. Water that returns from land to oceans then in turn influences the composition and circulation of oceans. Through this cycle, oceans not only exchange water but also heat energy, carbon dioxide, and aerosols with the atmosphere. As oceans are able to gradually store and release large quantities of heat, carbon dioxide, and aerosols over a long time period, they act as the planet s climate regulator and thus are an important source of longterm natural climate variability (Baede et al., 001). The cryosphere, consisting of solid water, includes continental glaciers, snow fields, sea ice, perma- 37

module frost, and the large ice sheets of Antarctica and Greenland. The cryosphere is relevant for the planet s climate because of its capacity to store heat, its low capacity to transfer heat (in other words, its low thermal conductivity), its high reflectivity of incoming solar radiation, and its influence on ocean circulation and sea levels (Baede et al., 001). The land surface, made up of soils and vegetation, encompasses all parts of the planet that are not covered by oceans. Land surface matters for the climate because it determines how energy from solar radiation is returned to the atmosphere. Part of the energy from the sun is directly returned to the atmosphere as longwave infrared radiation and part of the energy is used to evaporate water, which then returns as water vapour to the atmosphere. The texture of the land surface, which depends on the type of soil and/or vegetation covering the surface, also influences the atmosphere indirectly, as different textures influence winds in different ways (Baede et al., 001). The biosphere is composed of all living organisms (also called biota). While it only represents a very thin layer of the planet (roughly 0.4 per cent of the planet s radius (Common and Stagl, 004), biota plays a key role in influencing the composition of the atmosphere and thus the climate of the planet. Plants perform photosynthesis, a process by which they use energy from solar radiation and specific enzymes to transform carbon dioxide from the atmosphere and water from the hydrosphere into glucose (a carbohydrate) and oxygen (Figure 14). During this process, they extract large amounts of carbon from carbon dioxide present in the atmosphere and store it in the form of glucose, and release oxygen, a by-product of photosynthesis, into the atmosphere. The biosphere also has an impact on atmospheric concentrations of other greenhouse gases like methane or nitrous oxide (Baede et al., 001). Some digestive processes of animal species release for instance methane as a by-product. Figure 14 Photosynthesis the chemical reaction Solar radiation 6 CO + 6 H O C 6 H 1 O 6 + 6 O Enzymes Carbon dioxide Water Glucose Oxygen Source: Author. 3 In this material, we also use the term radiative balance as a synonym for energy balance. The five components of the climate system interact in numerous ways, as schematically illustrated by the thin arrows in Figure 1. For instance, water vapour is exchanged between the atmosphere and the hydrosphere, carbon dioxide is constantly extracted from the atmosphere by plants from the biosphere, and ice sheets from the cryosphere influence the hydrosphere s ocean circulations and levels. These are just a few examples of the physical, chemical, and biological interactions that make the climate system extremely complex. Some of these processes are still only partly known, and there may be processes that are still completely unknown (Baede et al., 001). All five components of the system and all processes happening within the system use energy. The balance between energy flowing into the system and energy leaving the system is called the energy balance. Changes in the energy balance have a profound impact on all the components and processes of the system. Given the crucial importance of the energy balance, Section. familiarizes readers with this term and discusses the natural greenhouse effect that is an integral part of the balance.. The earth s energy balance and the natural greenhouse effect The planet s energy balance influences all five components of the climate system that were introduced in Section.1. 3 Understanding the energy balance and how it can be altered is therefore of crucial importance for understanding the mechanisms behind climate change. Solar radiation is the energy source that powers the entire climate system. Roughly 50 per cent of solar radiation consists of visible light, with the rest consisting mostly of infrared and ultraviolet light (Baede et al., 001). New satellite-based data allow for accurately quantifying the exchange of radiative energy between the sun, the earth, and space. However, it is more difficult to quantify energy flows within the climate system because 38

those flows cannot be directly measured. Consequently, it is not surprising that estimates of the global energy balance differ considerably (Wild, 01). IPCC (013a) updated its energy balance diagram in the fifth assessment report building on the newly available estimates of Wild et al. (013), 4 which use satellite and ground-based radiation network data combined with models from the fifth IPCC assessment report. Figure 15 provides a schematic illustration of the planet s energy balance as outlined by Wild et al. (013) and used in the fifth IPCC report. Arrows represent radiation flows, numbers indicate best estimates of the magnitude of these flows, and numbers in parentheses provide the uncertainty range of these magnitudes, representing present-day climate conditions at the beginning of the 1st century. Accounting for day and night as well as for different yearly seasons, an average amount of energy equivalent to 340 watts per square meter (Wm - ) enters the earth s atmosphere, and hence the environmental system, each second. 4 In the third and fourth assessment reports, the IPCC used the energy balance diagram of Kiehl and Trenberth (1997). module Figure 15 The global mean energy balance of the earth Source: Wild et al. (013: 3108). Note: TOA: top of the atmosphere. Wm - : watts per square meter. Of these 340 Wm -, roughly 76 Wm - are directly reflected back to space by clouds, atmospheric gases, and aerosols. Another 4 Wm - reach the earth s surface and are directly reflected back to space by the surface (due to surface reflectivity, technically referred to as surface albedo). As white light-coloured surfaces reflect more light compared to dark-coloured surfaces, most of these 4 Wm - are reflected back to space by snow fields, glaciers, ice sheets, and deserts. Of the remaining 40 Wm -, roughly 79 Wm - are absorbed by the atmosphere. This leaves 161 Wm - that warm the planet s land surface and oceans (see the lower left-hand side of Figure 15). The planet s land surface and oceans subsequently return this energy towards the atmosphere and space as sensible heat, water vapour, and longwave infrared radiation. In order to have a stable climate, there needs to be a balance between incoming solar radiation and outgoing radiation emitted by the earth. Thus, the 40 Wm - of incoming radiation absorbed by the planet s surface and atmosphere should be returned back to space. If this does not happen, the radiative balance of the planet is not in equilibrium. If significantly more radiation were to enter the planet than leave, the planet would become too hot for life; if significantly more radiation were to leave than enter, the planet would become too cold for life. Note that our planet s energy balance is currently not in a complete equilibrium: Wild et al. (013) as well as other recent studies (Hansen et al., 011; Murphy et al., 009; Trenberth et al., 009) find a small positive imbalance of the earth s radiative balance. In fact, instead of 40 Wm -, only 39 Wm- leave the planet (see the upper right-hand side of Figure 15). 39

module 5 In the text that follows we use the terms longwave radiation, infrared radiation, and longwave infrared radiation synonymously. 6 In other words, these greenhouse gases make the atmosphere opaque (i.e. impenetrable) to a lot of the longwave radiation emitted from the planet s surface, but not to the incoming shortwave radiation, which explains why much of this incoming radiation can directly reach the surface. 7 Note that clouds also act as such a blanket. At the same time, due to their brightness, they reflect incoming solar radiation. As a net impact, clouds tend to have a slight cooling effect on the climate system (Baede et al., 001). 8 With the exception of a small share of infrared radiation that is directly radiated from the surface through the so-called atmospheric window towards space. 9 Drivers of climate change are substances and processes that alter the planet s energy balance. The earth returns the incoming solar radiation as longwave infrared radiation, 5 which is the heat energy you can feel, for instance, emanating out from a fire. The quantity and wavelengths of energy radiated by physical objects are specific to the temperature of the object. Hotter objects radiate more longwave infrared radiation (in terms of magnitude and energy) than colder objects. In order to radiate 39 Wm - in longwave infrared radiation, the radiating surface should have an average temperature of roughly -19 C (Baede et al., 001). However, the average temperature of the earth s surface is not -19 C but 14 C. At this temperature, the surface alone radiates on average 397 Wm - (Figure 15), which is a considerable higher amount than 39 Wm -. So how is the planet able to only radiate the 39 Wm - required to (almost) maintain its radiative balance and have at the same time a relatively high average temperature on the surface? As discussed in Section.1, the atmosphere contains several trace gases such as water vapour (H O), carbon dioxide (CO ), methane (CH 4 ), nitrous oxide (N O), and ozone (O 3 ). These gases are able to absorb longwave infrared radiation from the surface of the planet and from the atmosphere itself. 6 Greenhouse gases then emit infrared radiation in all directions. This means that they emit radiation towards space but also back towards the surface (see the right-hand side of Figure 15). The downward-directed flux, currently estimated at 34 Wm -, heats up the lower layers of the atmosphere and the surface, and thus maintains the relatively high surface temperature of 14 C. Hence, greenhouse gases act like a blanket that traps heat in the lower layer of the atmosphere. 7 This effect, known as the natural greenhouse effect, results in a net transfer of infrared radiation from warm areas near the surface to higher levels of the atmosphere (Baede et al., 001). The main part 8 of the 39 Wm - of outgoing longwave radiation that are needed to balance the incoming solar radiation is subsequently radiated back towards space from relatively high altitudes and not directly from the surface. These areas in the higher level of the troposphere are approximately five km above the surface at mid-latitudes and have an average temperature of roughly -19 C (see the upper right-hand part of Figure 15). Thus, the natural greenhouse effect is an integral part of the planet s energy balance system that is responsible for the relatively warm average temperature on the planet s surface. Having covered the components of the climate system as well as the radiative balance that influences all these components, we can now turn our attention towards climate change. Section.3 provides an overview and a classification of factors that can alter the climate system..3 Internally and externally induced climate change Sections.1 and. showed that the earth s climate is shaped by factors that are internal to the climate system (i.e. processes within and between components of the climate system, such as interactions between the atmosphere and oceans). It is important to understand that in addition to these internal factors, some external factors are also able to shape the climate. Why is this so? Section. showed that the climate system is in its equilibrium if the net incoming solar radiation is balanced by the outgoing longwave radiation. Such an equilibrium is marked by a stable climate (e.g. stable mean temperature, mean precipitation, etc.). If the radiative balance of the planet changes, the climate is likely to change as well because through various interactions and feedback mechanisms a change in the radiative balance affects virtually all components of the climate system. For example, a change of the radiative balance can affect means or variances of climate variables but also other statistics such as the occurrence of extreme events (Baede et al., 001; see Box 10 for a definition of extreme events). Hence, factors that are external to the climate system but somehow influence the radiative balance of the planet can also shape the climate. External factors can be further subdivided into natural external factors and humaninduced (i.e. anthropogenic) external factors. The most obvious example of a natural external factor is solar activity whose variations result in a changing amount of incoming solar radiation. Volcanic activity is another example: volcanic eruptions emit aerosol particles into the atmosphere, which can influence the amount of incoming solar radiation reflected back towards space. Among the human-induced external factors, human industrial activity influences greenhouse gas concentrations in the atmosphere and thereby affects the amount of longwave radiation that is being radiated from earth back towards space. In short, external factors (e.g. solar activity, volcanic activities, or human activities) have an influence on so-called climate change drivers (e.g. solar radiation, aerosol particles, greenhouse gases), 9 which in turn affect the radiative balance and thereby shape the climate (see Figure 18 for a schematic illustration of externally induced climate changes). 40

Box 10 Extreme events Climate change does not only affect the means of climate variables such as temperature or precipitation, but can also affect the likelihood of the occurrence of extreme weather and climate events (Cubasch et al., 013). Examples of such extreme events are droughts, cyclones, or heat waves. The Intergovernmental Panel on Climate Change (IPCC) defines an extreme weather event as an event that is rare at a particular place and/ or time of year. Definitions of rare vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations (Cubasch et al., 013: 134). Extreme climate events can be defined as extreme weather events that persist for some time (Cubasch et al., 013). module Cubasch et al. (013) show that statistical reasoning can illustrate that increases or decreases of the frequency of extreme weather events (e.g. an increase of extremely hot days) can result from small changes in the distribution of climate variables (e.g. an increase in the mean temperature). For example, Figure 16 displays the probability density function of temperature. Note that temperature is almost normally distributed (other climate variables such as precipitation are not normally distributed but have skewed distributions). Now suppose that climate change increases the mean temperature. As a result, the probability density function of temperature shifts to the right (i.e. average temperature increases) as illustrated by the solid curve in Figure 16. This shift of the average temperature affects the frequency of extreme events. On the one hand, one observes more hot extremes; on the other, one observes fewer cold extremes. Changes in the variance, skewness, or shape of distributions can also affect the frequency of extreme events (see Cubasch et al., 013: 134 35, for a more detailed discussion). Figure 16 Extreme events - schematic presentation Temperature (a) Increase in mean Fewer cold extremes More hot extremes Cold Average Hot Source: Author's elaboration based on Cubasch et al. (013:134). The climate of our planet is thus shaped by factors that are internal to the climate system and by factors that are external to the climate system (Baede et al., 001). This implies that climate change, which is defined as a persistent variation in either the mean state of the climate or in its variability (see the introduction to Section ), can be induced internally or externally. 30 The El Niño- Southern Oscillation (ENSO), described in Box 11, is an example of an internally induced climate variability (Baede et al., 001). The ENSO is the result of an interaction between the atmosphere and the Pacific ocean, and affects different climate variables such as precipitation and temperature in many parts of the world. As this teaching material focuses on human-induced (i.e. externally induced) climate change, we will not address internally induced climate variability further, 31 and limit ourselves to a discussion of externally induced climate change. To do so, Section.4 will introduce two concepts radiative forcing and effective radiative forcing that allow for measuring the influence of natural and human-induced external factors on climate change. 3 30 Internally induced climate change means that factors internal to the climate system affect the mean or the variability of the climate, while externally induced climate change means that factors external to the climate system affect the mean or the variability of the climate. 31 See the IPCC reports listed in Annex for additional readings on internally induced climate variability. 3 Note that these concepts can a priori also be used to measure the influence of most internal factors on climate change. 41

module Box 11 The El Niño-Southern Oscillation an example of an internal interaction among components of the climate system affecting the means and variability of different climate variables El Niño-Southern Oscillation (ENSO) events are naturally occurring phenomena that result from an interaction between the atmosphere and the hydrosphere. According to the Intergovernmental Panel on Climate Change (IPCC), El Niño involves warming of tropical Pacific surface waters from near the International Date Line to the west coast of South America, weakening the usually strong sea surface temperature (SST) gradient across the equatorial Pacific, with associated changes in ocean circulation. Its closely linked atmospheric counterpart, the Southern Oscillation (SO), involves changes in trade winds, tropical circulation and precipitation (Trenberth et al., 007: 87). Historically, the ENSO alternates between two states: El Niño and La Niña, each of which has specific regional impacts on climate variables such as temperature or precipitation (Figure 17). For example, surface temperature is above average during El Niño events and below average during La Niña events in the eastern tropical Pacific region. El Niño events occur every 3 to 7 years and alternate with their counterpart La Niña (Trenberth et al., 007). 1 Figure 17 Correlations of surface temperature, precipitation and mean sea level pressure with the Southern Oscillation Index 3 Darwin southern oscillation index 1 0-1 - -3 1860 1890 190 1950 1980 010 Year Source: Trenberth et al. (007). Note: Correlations with the Southern Oscillation Index (SOI), based on standardized Tahiti minus Darwin sea level air pressure, for annual (May to April) means of sea level air pressure (top left), surface temperature (top right) for 1958 to 004, and precipitation for 1979 to 003 (bottom left). In the SOI graph (bottom right), red (blue) values indicate El Niño (La Niña) conditions. The graph shows the long-term periodic fluctuation between these conditions since 1850. Standard deviations The ENSO influences regional climate patterns in several parts of the globe and has a global impact on climate variables such as surface temperature and precipitation. Figure 17 illustrates these effects based on annual mean correlations between the climate variables and the Southern Oscillation Index (SOI). The SOI (bottom right panel) uses observed atmospheric pressure at sea level to infer the presence of El Niño and La Niña events. It is calculated as the standardized air pressure in Tahiti (Eastern Pacific) minus the standardized air pressure in Darwin, Australia (Western Pacific). Positive values (higher pressure in Tahiti) indicate a La Niña event; negative values (higher pressure in Darwin) indicate an El Niño event. The bottom left panel of Figure 17 shows the correlation between SOI and precipitation: one observes, for example, a strong positive correlation among the variables over the Western Pacific, indicating that this region experiences above (below) average precipitation during La Niña (El Niño) events. The upper right panel displays the correlation between SOI and surface temperature. One observes, for example, that there is a strong negative correlation between the two variables over the eastern tropical Pacific region, indicating that in this region, surface temperature is above (below) average during El Niño (La Niña) events. Thus, as shown in Figure 17, internal interactions such as the El Niño-Southern Oscillation can have significant effects on the climate. Source: Author's elaboration based on Trenberth et al. (007). 1 An intuitive explanation of the El Niño-Southern Oscillation can be found in geoscientist Keith Meldahl s video (available at https://www.youtube.com/watch?v=gtgz6ieesy). A more extensive explanation of the phenomenon is provided by a Yale University open course given by Ronald Smith, a professor of geoscience, geophysics and mechanical engineering, available at https://www.youtube.com/watch?v=bk-n0cefwtk. 4

.4 Measuring the importance of factors driving climate change: radiative forcing and effective radiative forcing In theory, there are many ways to assess how strongly different external factors change the climate. One could, for instance, try to directly measure the effect of a change in a single climate change driver (e.g. greenhouse gases) that has been induced by an external factor (e.g. human industrial activity) on different climate variables (e.g. temperature and precipitation). However, identifying and isolating such effects is extremely difficult. Climate scientists therefore rely on intermediate measures that quantify the influence of external factors on climate change indirectly. The basic idea behind these measures is rather intuitive. First, climate scientists measure how strongly the radiative balance is affected by an externally induced variation in a climate change driver. Then they estimate how this change of the radiative balance affects the climate (Figure 18). To do so, they use a measure called radiative forcing. Radiative forcing is the most commonly used indicator that allows for capturing how externally induced changes in climate change drivers affect the radiative balance and subsequently change the climate (Myhre et al. 013). The term forcing indicates that the radiative balance of the earth is forced away from its equilibrium state by an externally induced variation in a climate change driver (Perman et al., 011). 33 Intuitively, radiative forcing measures the radiative imbalance that occurs from an externally induced change in a climate change driver. IPCC (001a: 795) defines radiative forcing as the change in the net vertical irradiance (expressed in Watts per square metre: Wm - ) at the tropopause 34 due to a change in the external forcing of the climate system, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun. While the radiative forcing concept is the most widely used measure to assess and compare the size of the radiative imbalance created by externally induced variations in climate change drivers, it has some weaknesses. The main one is that the concept keeps all surface and tropospheric properties fixed and does not allow them to respond to the changes induced by the variations in climate change drivers. In the fifth assessment report, the IPCC therefore introduced a new, complementary concept called effective radiative forcing. Radiative forcing and effective radiative forcing are very similar, with the exception that effective radiative forcing allows some surface and tropospheric properties to respond to perturbations in the short term. Effective radiative forcing is defined as the change in [the] net top of atmosphere downward radiative flux after allowing for atmospheric temperatures, water vapour and clouds to adjust, but with surface temperature or a portion of surface conditions unchanged Hence effective radiative forcing includes both the effects of the forcing agent itself and the rapid adjustments to that agent (as does radiative forcing, though stratospheric temperature is the only adjustment for the latter) (Myhre et al., 013: 665). Due to the inclusion of short-term adjustments of some surface and tropospheric properties, the effective radiative forcing concept is believed to be a better indicator of potential temperature responses (Myhre et al., 013). Radiative forcing (and effective radiative forcing) can be negative or positive. Positive radiative forcing implies that incoming radiation is larger than outgoing radiation, leading to an energy increase in the environmental system (i.e. a positive energy imbalance). To rebalance the system, temperatures in the surface-troposphere system have to increase. Negative radiative forcing implies that incoming radiation is smaller than outgoing radiation (i.e. a negative imbalance), leading to an energy decrease in the environmental system. To rebalance the system, temperatures in the surface-troposphere system have to decrease. 33 Climate change drivers are subsequently also called forcing agents, while externally induced variations in climate change drivers are also called external forcings or sometimes simply forcings. For the sake of simplicity, we do not adopt this terminology in this teaching material and continue to use the terms climate change driver and externally induced variations in climate change drivers. However, the terms external forcings, forcings, and forcing agents do appear in direct citations from the IPCC. 34 The tropopause is the boundary between the troposphere and the stratosphere (see Figure 13). For practical reasons, the tropopause is defined as the top of the atmosphere (see Ramaswamy et al., 001, for a detailed discussion). module Figure 18 Externally induced climate changes Natural external factors (e.g. solar activity, volcanic activity) Direct and indirect changes in climate change drivers (e.g. greenhouse gases, aerosols, solar irradiance) Radiative forcing Human-induced external factors (e.g. industrial activity) Climate perturbations and responses (e.g. temperature, precipitation, extreme weather events) Feedback effects Source: Author's elaboration based on Forster et al. (007: 134). Note: Non-initial radiative forcing effects have been omitted from the figure as they are not addressed by this teaching material. Feedback effects are discussed in Section.5. 43

module Before we focus entirely on human influences on the climate in Section.5, let us briefly illustrate the radiative forcing concept by listing some examples of radiative forcing induced by certain natural external factors. First, let us consider solar activity. Solar activity, and hence solar output, is not constant but fluctuates over time at various time scales, including centennial and millennial scales (Myhre et al., 013). Solar output has increased gradually during the industrial era (from 1750 up until today), which has led to an increase in incoming solar radiation and thereby caused a small amount of positive radiative forcing. This has had a small warming effect on the surface-troposphere system (Forster et al., 007). Another natural external factor, the astronomical alignment of the sun and the earth, also varies and induces cyclical changes in radiative forcing. However, these changes are only substantial over very long time horizons, and partially explain, for instance, different climatic periods such as ice ages (Myhre et al., 013). Volcanic eruptions are another natural external factor that can, over a short period of time lasting from several months up to a year, increase the concentration of sulphate aerosol particles in the stratosphere that block parts of incoming solar radiation, inducing short-term negative radiative forcing, which tends to have a cooling effect on the surfacetroposphere system (Forster et al., 007). While such natural external factors are important, they have played a relatively small role during the industrial era. In its fifth assessment report, the IPCC states that there is a very high confidence that industrial-era natural forcing is a small fraction of the anthropogenic forcing except for brief periods following large volcanic eruptions. In particular, robust evidence from satellite observations of the solar irradiance and volcanic aerosols demonstrates a near-zero (-0.1 to +0.1) Wm - change in the natural forcing compared to the anthropogenic effective radiative forcing increase of 1.0 (0.7 to 1.3) Wm- from 1980 to 011. The natural forcing over the last 15 years has likely offset a substantial fraction (at least 30 per cent) of the anthropogenic forcing (Myhre et al., 013: 66)..5 Human-induced climate change After outlining basic mechanisms that influence the climate system and introducing concepts that allow for measuring climate change, we are now equipped with the necessary tools to analyse human-induced climate change. Human activities cause changes in the amounts of greenhouse gases, aerosols, and clouds in the earth s atmosphere. These human-induced changes in climate change drivers influence the planet s radiative balance and hence the climate system. Human activities also change the land surface of the planet, which can effect, for instance, surface reflectivity (albedo), influencing the radiative balance and thus also the climate system (IPCC 001b, 007, 013a). The sections that follow first discuss these human-induced changes in climate change drivers separately, then assess their respective impact on the radiative balance, and finally take a look at feedback effects that can amplify or reduce the impacts of the different climate change drivers..5.1 Human-induced greenhouse gas emissions and the enhanced greenhouse effect The main greenhouse gases emitted by human activities are carbon dioxide (CO ), methane (CH 4 ), nitrous oxide (N O), and halocarbons (Forster et al., 007). Carbon dioxide, methane, nitrous oxide, and many halocarbons are called well-mixed greenhouse gases because they mix sufficiently in the troposphere for reliable concentration measurements to be made from only a few remote observations (Myhre et al., 013). Anthropogenic carbon dioxide emissions mainly result from human use of fossil fuels in sectors such as transportation, energy, and cement production. Anthropogenic methane is mostly emitted during agricultural activities and natural gas distribution, and from landfills. Anthropogenic nitrous oxide emissions stem mostly from burning of fossil fuels and the use of fertilizers in agricultural soil. Anthropogenic halocarbons, which include chlorofluorocarbons (see also Box 14 in Module 3), are emitted by diverse industrial activities and have in the past also been released by refrigeration processes. In addition to the four main greenhouse gases, human activities also emit other pollutants such as carbon monoxide (CO), volatile organic compounds, nitrogen oxides (NO x ), and sulphur dioxide (SO ). While these gases are negligible greenhouse gases, they indirectly influence concentrations of other greenhouse gases such as methane or ozone through chemical reactions (Cubasch et al., 013). So how strongly have humans influenced atmospheric concentrations of greenhouse gases? Instrumental measurements provide accurate atmospheric GHG concentrations back to 1950, while indirect measures are used for dates prior to 1950: ice core data allow for analysing air bubbles enclosed in ice and thus provide an indirect record of past atmospheric concentrations of 44

well-mixed greenhouse gases (Masson-Delmotte et al., 013). Figure 19 combines data based on instrumental measurements and ice core data to show that while atmospheric carbon dioxide, methane, and nitrous oxide concentrations were fairly stable for more than a thousand years before the industrial revolution (starting around 1750), they have increased rapidly since then. module Figure 19 Atmospheric carbon dioxide, methane and nitrous oxide concentrations from year 0 to 005 400 000 1800 Carbon dioxide (CO ) CO (ppm), N O (ppb) 350 300 1600 1400 100 1000 CH (ppb) Methane (CH 4 ) Nitrous oxide (N O) 600 50 0 500 1000 1500 000 Year 800 Source: Forster et al. (007: 135). Note: Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. Ice core data also allow us to look further back in history and track atmospheric GHG concentrations dating several hundreds of thousands of years. The fifth IPCC assessment report provides information covering the past 800,000 years (IPCC, 013a). Data indicate that pre-industrial ice core GHG concentrations stayed within natural limits. For carbon dioxide, maximum concentrations of 300 parts per million (ppm) and minimum concentrations of 180 ppm have been found. For methane, data indicate maximum concentrations of 800 parts per billion (ppb) and minimum concentrations of 350 ppb. And for nitrous oxide, ice core data show maximum concentrations of 300 ppb and minimum concentrations of 00 ppb (Masson-Delmotte et al., 013). The fifth IPCC assessment report reaches the conclusion that it is a fact that present-day (011) concentrations of CO (390.5 ppm), CH 4 (1803 ppb) and N O (34 ppm) exceed the range of concentrations recorded in the ice core records during the past 800 ka. 35 With very high confidence, the rate of change of the observed anthropogenic WMGHG [well-mixed greenhouse gases] rise and its RF [radiative forcing] is unprecedented with respect to the highest resolution ice core record back to ka for CO, CH 4 and N O, accounting for the smoothing due to ice core enclosure processes. There is medium confidence that the rate of change of the observed anthropogenic WMGHG rise is also unprecedented with respect to the lower resolution records of the past 800 ka (Masson-Delmotte et al., 013: 391). In short, given the data and the methods to determine the origin of GHG emissions, 36 the scientific community has reached a consensus: humans have substantially altered the composition of the atmosphere and continue to do so. Since the industrial revolution, anthropogenic GHG emissions have substantially increased GHG concentrations in the atmosphere (IPCC 001b, 007, 013a). The rate of this increase is unprecedented over the last 800,000 years, and the concentrations are currently higher than all concentrations recorded in ice cores over those years (IPCC, 013a). So how does this human-induced change of the atmosphere affect the climate? The answer is relatively straightforward: the anthropogenic increase in GHG concentrations amplifies the natural greenhouse effect (see Section.). Increased GHG concentrations lead to an increased rate of absorption and subsequent emissions of infrared radiation coming from the surface. In other words, increased GHG concentrations increase the atmosphere s opacity to longwave radiation, but more so at lower altitudes where air density and GHG concentrations are higher than at higher altitudes where they are both lower. Thus the share of upward flux longwave radiation leaving the atmosphere from higher, relative to lower altitudes, increases. As a result of the increase in GHG concentrations, the altitudes from where earth s radiation is emitted towards space thus become higher. At these higher altitudes, the troposphere is colder (Figure 13) and therefore, less energy is emitted towards space, causing a positive radiative forcing (Baede et al., 001). This effect is called the enhanced greenhouse effect. 35 ka is a unit of time indicating a thousand years. 36 For instance, one can analyse the changing isotopic composition of atmospheric CO. By doing so, it can be shown that the observed increase in the atmospheric CO concentration is of anthropogenic origin, because the changing isotopic composition of atmospheric CO betrays the fossil origin of the increase (Baede et al., 001). 45