SAJC Cluster 3.2 Lecture 10 notes (The Science of Climate Change (III))

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 10_2025/pg1 Lecture 10 The Science of Climate Change (III): Human Activities and Contemporary Climate Change KEY QUESTION: How significant is the influence of human activities on Earth’s climate compared to natural factors? With the completion of this lecture, attached readings and tutorial, you should be able to understand that: • The consensus with the scientific community, represented by the IPCC, is that climate change in the last two centuries is unequivocal, and it is very likely contributed by human activities • Human activities influence the global carbon cycle by increasing carbon emissions and reducing carbon sinks • The enhanced greenhouse effect is due to the increased concentration of greenhouse gases by human activities • Human activities have a role in accelerating warming through positive feedback and suppressing warming through negative feedback Lecture Outline 10.1 Contemporary Climate Change: Global Warming 10.2 Greenhouse Effect versus Enhanced Greenhouse Effect (a) How the Earth is heated up: The Greenhouse Effect (b) Global Warming: The Enhanced Greenhouse Effect 10.2.1 Per Capita GHG Emissions by Sector 10.2.2 Relative Contribution of Countries to GHG Emissions 10.3 Role of Carbon Cycle and Carbon Sinks in Contemporary Climate Change 10.4 Influence of Human Activities on the Global Carbon Cycle 10.4.1 Increasing Carbon Emissions 10.4.2 Reducing Carbon Sinks 10.5 The Impact of Human Activities on the Feedback Mechanism 10.5.1 Accelerating Warming through Positive Feedback 10.5.2 Suppressing Warming through Negative Feedback 10.6 The IPCC: Contemporary Climate Change is Unequivocal 10.6.1 What is the IPCC? 10.6.2 What is IPCC’s assessment about our climate today? 10.6.3 Significance of Anthropogenic Activities in Contemporary Climate Change 10.7 Conclusion

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 10_2025/pg2 10.1 Contemporary Climate Change: Global Warming • Climate variability refers to variations in the mean state and other statistics such as standard deviations and the occurrence of extremes of the climate on all spatial and temporal scales beyond that of individual weather events . Climate variability may be due to natural internal processes within the climate system or to variations in natural or anthropogenic external forcing. • We have learnt that climate variability is not a new phenomenon. In fact, the climate of the Earth has been varying over a long period of time. We have studied the patterns of climate variability from the past (that is, during the Pleistocene and the Holocene) till the present. This knowledge is important to help us understand the current or contemporary phase of climate variability, and to examine the role of human activities in this change. • The terms – climate change and contemporary climate change (global warming) – are sometimes used interchangeably, although technically they are not the same. Climate change is more accurately described as ‘climate variability’. The term contemporary climate change (global warming) refers to the changes in the state of the climate that persists for an extended period, typically decades or longer in more recent times. The IPCC uses the pre-industrial period – the period prior to the onset of large -scale industrial activity around 1750 – as a reference. The reference period 1850 –1900 is used to approximate pre-industrial global mean surface temperature. (see Fig. 1). Fig. 1

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 10_2025/pg3 10.2 Greenhouse Effect versus Enhanced Greenhouse Effect • What is the science behind contemporary climate change? o The fundamental source of heat for the Earth’s atmosphere is the sun. o While the Earth absorbs heat from the sun, it also loses heat to space. o The difference between what it receives and what it loses will determine the Earth’s climate. • The concerns for global warming arise because it is perceived that the Earth is not losing enough heat to space in comparison to what it receives. o While the Sun’s output has increased only very slightly, about 0.007% over the last one million year or so, it does not seem to account for the reported trends in temperature increase. o Most scientists attribute this warming to the enhanced greenhouse effect. To understand this, we first need to be familiar with the greenhouse effect. (a) How the Earth is heated up: The Greenhouse Effect • The sun is the only source of energy on earth. Atmospheric temperature is determined by the balance between insolation received by the earth and terrestrial radiation released by the earth into the atmosphere. o The term “insolation” refers to incoming solar radiation . This input of shortwave solar radiation heats up only the earth’s surface. o The earth’s surface releases the heat in the form of longwave terrestrial radiation . Terrestrial radiation heats up the atmosphere as greenhouse gases like water vapour, carbon dioxide, methane, nitrous oxides absorb the heat (greenhouse effect; see Fig. 3). This increases the temperature of the atmosphere. Table 1 shows the location and sources of major greenhouse gases, and their influence on global warming. Note: In our syllabus, we focus on carbon emissions as carbon dioxide has contributed the most to contemporary climate change. Table 1: Location and sources of major greenhouse gases, and their influence on global warming Greenhouse gas Where it is found Where it comes from Function Water vapour Concentrated in lowest 10 km of troposphere. Surface water e.g. lakes, rivers, sea, oceans. Moisture loss from plants and other organisms. Largely constant in amount with spatial variations. Not significantly affected by human activities. Water vapour only absorbs a small amount of outgoing radiation. Water droplets absorb wavelengths not absorbed by gases/vapour and radiate this heat back to the ground. Carbon dioxide (CO2) Concentrated in lower troposphere. Released by animals during respiration; burning of fossil fuels and vegetation e.g. rainforests. Annual fluctuations reflect seasonal vegetation growth, especially in northern hemisphere in summer huge amounts of CO2 are absorbed, while in winter the process slows. The southern hemisphere does Absorbs long-wave radiation from the earth.

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 10_2025/pg4 not feature strongly as there is less land area. Methane (CH4) Concentrated in lower atmosphere. Released as bacteria break down organic matter: Marshes (also known as ‘marsh gas’), swamps, tundra, wetlands (used for rice-growing), disposal of organic waste, waste from digestion in mammals. An increase in ruminating animals e.g. sheep, cows has caused levels to rise over the last 200 years. Absorbs long-wave radiation from the earth. Nitrous oxide (N2O) Concentrated in lower troposphere. Combustion of fossil fuels in power stations and transport. Also, from denitrifying bacteria which break down nitrate and nitrites – increasing due to use of nitrate fertilisers. Absorbs long-wave radiation from the earth. Concentration of N2O is rising at approx. 0.3% per year. CFCs Ascends to the ozone layer: 15-50km above earth’s surface. Released from aerosol sprays, 1960s- 1970s. Used in refrig