SAJC Cluster 3.2 Lecture 8 Notes (The Science of Climate Change)

St Andrew’s Junior College H2Cluster3: Sustainable Future and Climate Change H2_Cluster3 Lecture 8_2025/pg1 Lecture 8 The Science of Climate Change (I): Past Climates KEY QUESTION: Is climate variability a new phenomenon in Earth’s history? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Episodes of warming and cooling of Earth during the Quaternary period • Evidence of past climate variability derived from proxy indicators through the study of ice and ocean cores Lecture Outline 8.1 Climate Variability Is Nothing New 8.1.1 The Pleistocene 8.1.2 The Holocene 8.2 Evidence of Past Climate Variability 8.2.1 Ice Core Records (a) Greenhouse gas concentrations (b) Oxygen isotopes 8.2.2 Ocean Core Records (a) Shells of marine microorganisms (b) Materials removed from land 8.3 Conclusion In a project called Climate: Long -Range Investigation, Mapping, and Prediction (CLIMAP) in the 1970s, researchers reconstructed the climate of the Earth about 20,000 years ago, during a time many refer to as the Ice Age. This map shows the difference in temperatures between then and now (blue areas were colder, red areas warmer). The most obvious feature is perhaps the presence of much colder water in the North Atlantic. Ocean cores proved invaluable in this project. What are ocean cores and how did they help? What else do we know about Earth’s climate in the past?

St Andrew’s Junior College H2Cluster3: Sustainable Future and Climate Change H2_Cluster3 Lecture 8_2025/pg2 8.1 Climate Variability Is Nothing New • 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 b e due to natural internal processes within the climate system or to variations in natural or anthropogenic external forcing. • Contemporary climate change in recent years is deemed as a global challenge, but it is important to note that climate variability itself is not a new phenomenon. Climate of the Earth has been varying over a long period of time. • In this section of the syllabus, we shall first outline the changes in global climatic patterns as far back as 2.6 million years ago (mya). We then proceed to examine the evidence that suggest that these changes did occur (Section 8.2) and find out why these changes occur (Lect. 9). • The Earth is more than 4.5 billion years old. This vast span of time is called geologic time . Geologists have divided the geologic time into a series of time intervals. Of most relevance to us is the time interval known as the Quaternary Period. The Quaternary period extends from 2.6 million years ago till present. There are two epochs within the Quaternary period - the Pleistocene and the Holocene. o The Pleistocene is the period of Earth’s history covering approximately 2.6 million years ago until 11,700 years ago. o The second epoch of the Quaternary, the Holocene, is the last 11,700 years of earth’s history. • Fig. 1 shows the past climate variability over the Quaternary Period. Epoch Fig. 1 Climate during the Quaternary Period

St Andrew’s Junior College H2Cluster3: Sustainable Future and Climate Change H2_Cluster3 Lecture 8_2025/pg3 8.1.1 The Pleistocene • Although sometimes called the Ice Age , the Pleistocene was not one long, uninterrupted deep freeze. Rather it was marked by a recurring cycle of climate oscillations or changes in average temperature and rainfall . Such oscillations or changes are referred to as glacials and interglacials. o Glacials – time during the Ice Age when ice sheets expanded, and average global climate was colder and drier than during the intervening interglacials (see next line). For example, the Maunder Minimum is marked by a period of significantly reduced sunspot activity, which correlates with cooler global temperatures. During this time, the sun's radiation output was lower, and this lack of solar activity is thought to have contributed to the cooling observed on Earth. o Interglacials – phase of warmth between glacials when the great ice sheets melt and retreat. • During the long and cold periods , global temperatures were much lower, perhaps as much as 10°C in higher latitudes, and at least 5°C in the tropics. When the Pleistocene ended 11,700 years ago, temperatures were 4°C lower than present day. • From Fig. 1, note in particular: the beginning of the decline in temperatures at around 2.6mya, the four major climatic fluctuations commencing 500,000ya (giving the 4 major glacial periods we also refer to as ice ages), and the relative stability of temperatures during the Holocene. 8.1.2 The Holocene • This began 11,7 00 years a go (see Fig. 1), at the end of the last glacial period, when average temperatures increased rapidly by 6°C and have stayed at that higher level through to the present day, with oscillations limited t o around 0.5 degrees centigrade . (Note: The Holocene period is considered by some to be an interglacial period, not the end of the Ice A ge. This may help to explain the views of climate change deniers that we hear of today. See Lect. 9 also.) • The postglacial warming trend culminated in the so -called Climatic Optimum about 8 000 to 5000 years ago, a time when the mean global temperature was somewhat higher (perhaps 1°C) than at present. (There is evidence that indicates that 6000 years ago, over most of Europe, July temperatures were about 2°C higher than now.) • From Fig. 1 , n otice also that beginning 8000 years ago, the overall trend in temperatures is downward (approximately 0.75 °C). That is until the onset of the industrial revolution, when temperatures again started to rise . The average global temperature has risen by a little more than 1°C since 1880. This is known as the contemporary phase of climate change (or global warming as is often referred to today). 8.2 Evidence of Past Climate Variability • Instrumental records (from thermometers, rain gauges, etc.) only exist ed for the last 150 years. Hence, we have to rely on proxy indicators to know how climate has varied over the past 4.5 billion years or so. • Proxy indicators are indirect evidence of past climate change. In the study of past climates, the information obtained from geological and biological records are proxy indicators – evidence

St Andrew’s Junior College H2Cluster3: Sustainable Future and Climate Change H2_Cluster3 Lecture 8_2025/pg4 derived from sources other than human measurements. In the syllabus, you are required to understand evidence that is obtained from ice cores (see Section 8.2.1) and ocean cores (see Section 8.2.2). 8.2.1 Ice Core Records • As snow accumulates on ice caps and sheets where temperature usually remains below freezing point year-round, it lays down a record of the environmental conditions at the time of its formation. • Over time the snow, buried under further accumulations, is compacted to ice, preserving the climatic information. Ice-core (see Fig. 2) records are therefore important because bubbles of air that were trapped in the ice as it became compressed provide tiny ‘ time capsules ’ of the atmosphere over time. • Analysing microscopic air bubbles in ice cores helps to reveal past climates. Temperatures of the past is inferred from the isotopic composition of the water molecules released by melting the ice cores. • However, generally the changes in gas concentrati