Climatic Changes Since 1700

Climate is all about variability. Imagine a world in which temperature and other meteorological elements would not vary in time and space. There would be no need for the term “climate”. In reality, it may be cool today but warm tomorrow (Fig. 1.1, left). Travelling a few hundred kilometres away, one may find a very different weather situation upon arrival (Fig. 1.1, right). These variations may not be predictable more than a few days ahead, but there are typical spatial patterns and temporal evolutions in this variability. The term climate describes these typical variations and aims at bringing an order to them. Climatic changes, then, are changes in these typical variations over time.

If you are worried about a thunderstorm forming near your town, if snowfall is imminent, if an El Niño event builds up in the Pacific, or if media reports that a heatwave strikes Australia, you can find a large amount of real-time information on weather and climate on the Internet. Just a few mouse clicks away, you will find observations, analyses, model simulations, satellite images, Radar data, and many other products (Fig. 2.1). Where does information on the atmosphere come from, what does it really tell us, and how can we today explore the weather patterns of the 18th century? In this chapter, we will cover these questions, starting with some general considerations of weather observations and measurements, the present day observing system, and historical climate observations. We cover uncertainties and problems in climate data and see how models can be used to learn about past climate and how they can be combined with observations. This chapter also covers climate proxies and the methods used to derive climate information from these proxies. Finally, this chapter concludes with a more detailed description of those datasets that form the basis of many of the analyses that follow in Chaps. 3 and 4.

Brückner’s depiction of climate as a clockwork is remarkable. During the last 125 years, science has unravelled many of the hidden mechanisms. Before discussing climatic changes since 1700, it is useful to start with a tour of the “machinery” of the climate system. Basic physical concepts and mechanisms operating in the climate system will be briefly introduced—the motor, transmission, and basic mode of the operation of the machinery. Then, we will analyse how these mechanisms produce variations over time. We will look at changes in the general circulation, how they are expressed in systematic form in circulation variability modes, and how they affect local climate in the form of teleconnections. Special attention is devoted to how the machinery reacts to external forces and how feedback processes evolve in the climate system.

Over the last 300 years, countless climatic variations at different places and times have been witnessed, some affecting millions of people and changing the course of history, some going largely unnoticed. In this chapter, I discuss selected climatic changes in European and global climate history from about 1700, the end of the Little Ice Age, to the present. The 20 events start with the cold Maunder Minimum and end with the global warming hiatus, covering different aspects of the climate system. We will encounter continental-scale temperature dips, hydroclimatic anomalies, perturbations of the stratosphere, and changes in atmospheric composition. Thereby, we will see the mechanisms discussed in Chap. 3 at work: oceanic modes and atmospheric variability, volcanic eruptions, and humans changing their environment.

Eduard Brückner, an active climate scientist 125 years ago, studied climatic changes back to 1700. He found that the climate system exhibits variations on multidecadal time scales—a bold statement to make at that time. However, he could not see inside the machinery; the mechanisms remained hidden. Considerable progress has been made since Brückner’s time, particularly in recent years. Thanks to new analytical and sampling techniques, new proxies have emerged that provide information on poorly understood aspects of the climate state. Modelling capabilities have improved and the use of simulations to study past climate has become commonplace. Furthermore, the increasing use of powerful numerical techniques such as data assimilation has triggered new data recovery efforts (see Chap. 2). If this momentum continues, we may soon be able to study continental- to hemispheric-scale weather back to the 1780s.