SAJC Cluster 3.2 Lecture 9 Notes (Natural factors influencing climate change)

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 9_2025/pg1 Lecture 9 The Science of Climate Change (II): Natural Factors influencing Temperature Variability KEY QUESTION: Can natural factors fully account for contemporary climate change? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Natural factors influencing temperature variability in the Quaternary period • Influence of these natural factors on temperatures through feedback mechanisms Lecture Outline 9.1 Introduction 9.2 Natural Factors Influencing Temperature Variability in the Quaternary Period 9.2.1 Changes in Solar Output Box 1: The Maunder Minimum 9.2.2 Changes in Thermohaline Circulation Box 2: The Younger Dryas – A Warning from the Past 9.2.3 Changes in Ice Sheets: Milankovitch Cycles (a) Eccentricity (b) Obliquity (c) Precession Box 3: Can the Milankovitch cycles explain contemporary climate change? 9.3 Feedback Mechanisms 9.3.1 Positive Feedback Loops 9.3.2 Negative Feedback Loops 9.4 Conclusion

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 9_2025/pg2 9.1 Introduction • As we have seen in Lect. 8, Earth’s climate has undergone significant variation over the course of its existence. The Quaternary Period, for instance, consisted of a cold Pleistocene epoch, and a warm Holocene epoch. • Since human activity associated with the modern human ( homo sapiens ) only arose about 200,000 years ago, the factors that caused temperature variability prior to that are likely natural. (But in Lect. 10, we will learn that humans play a bigger role today.) • While examining the list of factors affecting temperature variability, it is important to understand that they do not operate independently but simultaneously. This might therefore mean that while one factor leads to warming, another might counteract or enhance that warming. 9.2 Natural Factors influencing Temperature Variability in the Quaternary period 9.2.1 Changes in Solar Output • The sun is Earth’s only s ource of heat and light, so it is natural to assume that changes in the global temperature must be related to changes in the solar output. • For centuries now, scientists have known that the solar output (irradiance) varies over time. Periods of slightly higher solar output are associated with the appearance of sunspots , and astronomers have been recording the number of sunspots since the 1600s. The abundance of sunspots rises and falls usually in a 11-year cycle. • The so-called “Little Ice Age” (see Lect. 8 Fig. 1) that affected parts of Europe and North America for several centuries ending in the 1800s is thought to have been caused (at least in part) by a decrease in solar output. A more recent example is presented in Box 1. Box 1: The Maunder Minimum • Between 1650 and 1710, a period known as the Maunder Minimum, past climate data showed that Europe was colder than most other areas. • Reduced solar output led to lower temperatures in much of the Northern Hemisphere during the Maunder Minimum. • The map in Fig. 1 represents the temperature difference between 1680 (during the Maunder Minimum) and 1780 (a period of normal solar activity) calculated by a general circulation model. Blue areas were colder than normal, and red areas were warmer. Fig. 1

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 9_2025/pg3 9.2.2 Changes in Thermohaline Circulation • Thermohaline circulation refers to the large-scale circulation of the world’s oceans, involving the localised vertical movement of large bodies of water, driven by variation in temperature (thermo) and salt (halite) content. Thus, the thermohaline circulation is important in transporting heat between the low latitudes and the higher latitudes. • We describe the Thermohaline Circulation in Fig. 2 starting from the North Atlantic: o The Gulf Stream carry warm and less salty water from the Caribbean north -eastward to the seas between Greenland, Iceland and Norway. As this water moves to the higher latitudes it becomes cold, salty and dense . It becomes dense enough to sink in ‘downwelling zones’ particularly at high latitudes in the North Atlantic into the deep ocean. o This descending dense water helps to ‘pull’ the warm waters from the southwest in the lower latitudes, maintaining the flow north -eastwards as well as the transfer of heat from water to the atmosphere. (Note: This is an important stage influencing the Arctic temperature and discouraging the growth of ice sheets there.) o Cold water that sinks to the bottom of the Atlantic Ocean, flows far southward as the North Atlantic Deep Water (NADW), crossing the equator and eventually joining deep waters flowing around Antarctica as Antarctic Bottom Water (ABW) and beyond. Other branches of deep cold water flow northward into the Indian and Pacific oceans, eventually rising in an ‘upwelling zone’ of western South and Central America. o The warm and less saline upwelled water again moves back towards the high latitudes to transfer heat to the atmosphere in the North Atlantic only to sink again. Fig. 2 The Thermohaline Circulation Upwelling zone Upwelling zone Downwelling zone Pacific Ocean Greenland

St Andrew’s Junior College H2 Cluster 3: Sustainable Future and Climate Change H2_Cluster 3 Lecture 9_2025/pg4 • How is the ocean circulation relevant to explaining temperature variability? o This huge, conveyor belt-like circulation system is estimated to take 2,000 years to complete one revolution. Any change in this thermohaline circulation will affect the temperature and thus will cause climate change. o There is now convincing evidence to suggest that the thermohaline circulation in the Atlantic Ocean is not steady. Rates and locations of sinking water changed abruptly, which in turn affected other aspects of the pattern. o Numerous reorganisations of the ocean circulation have been found in marine records running from about 60,000 years ago until about 10,000 years ago. In this way, we have learnt that changes in ocean circulation are implicated in abrupt climate jumps, at l east during the last glacial period (see Box 2). Box 2: The Younger Dryas – A Warning from the Past • About 12,000 years ago, as the Earth was beginning to warm near the end of the Wisconsin glacial, a tremendous cold spell gripped the high latitude regions of the Northern Hemisphere. • For roughly 1,300 years at least some northern regions returned to peak ice age (glacial) conditions. The onset of this return to glacial conditions was relatively sudden; the transition apparently occurred within a century or less. • This sudden return to extremely cold (glacial) conditions is known as the Younger Dryas, otherwise known as the last stage of the Pleistocene epoch (see Fig. 3). Scientists believe a partial or total shutdown of the thermohaline circulation may have been involved in the formation of Younger Dryas. The theory goes like this: o As the vast North American ice sheets began to melt towards the end of the Wisconsin glacial, it suddenly dumped a large supply of fresh water into the North Atlantic Ocean in the high latitudes. o The influx of fresh water in the high latitudes reduces the temperature and/or salinity of the North Atlantic Ocean. This stops the downwelling of ocean water and thus shut down the normal cycle of the thermohaline circulation, effectively turning off that current syste