Jessica Gurevitch

Earth’s climates are set in motion by the radiant energy from the sun shining directly on the tropics, setting up currents of wind and ocean water that move heat and water around the planet. The energy from that intense direct light there heats Earth’s surface, causing air to rise. The rising air cools and loses moisture, causing the tropics to be warm and wet. The rising air eventually falls at about 30 degrees north and south of the equator, warming and drying and creating hot, arid deserts around the globe. At Earth’s surface, the air rushes back to the tropics. This creates the trade winds on which sailing ships traveled from Europe to the Americas from the sixteenth to the nineteenth centuries, carrying cargo and millions of enslaved people. The Coriolis effect curves the winds and the ocean currents westward when they are moving toward the equator, and eastward when moving toward the poles. Air currents move ocean currents, which move heat as well as water around the globe, altering terrestrial climates. Rain shadows and urban heat islands wrap up the story for this chapter.

This last chapter considers climates of the deep and more recent past, and briefly looks at how climates are changing at present; these breathtakingly fast changes are getting a huge push from humans. Our planet has experienced enormous changes over the billions of years of its history. Continents have moved, merged, and split in an epochal dance. Vast seas have dominated large regions and then disappeared; the oceans have changed their extent and how they move water and heat around the planet. Organisms have originated, evolved, and left descendants, or not, as lineages have flourished or disappeared. The composition of the atmosphere has changed dramatically, and climates have changed regionally and globally. The chapter examines how scientists use different kinds of evidence to understand past and current changes in climate, and introduces some of the proposed ideas and approaches to dealing with current climate change. It also considers some of the ways science progresses, sometimes making errors or failing to recognize new insights, then rising to correct past misunderstandings. The book concludes with the hope that readers will be stimulated to think more about our magnificent planetary system of land, ocean, atmosphere and living things, to learn more about them, and to be aware of their interconnectedness.

This chapter explains why seasons differ across Earth and introduces a major theme of the book: how these contrasts shape Earth’s environments, the human experience, and the living world. Earth is tilted on its axis, and because of that, different places on Earth’s surface receive different amounts of radiant energy from the sun, and different amounts of radiant energy at different times of the year. This is the foundation for the seasons, but there is a lot more to the story. For example, Earth is closer to the sun during the northern hemisphere winters, perhaps surprisingly. And the North Star has not always been the North Star, because Earth has tilted and pointed differently during its four billion plus years of existence! Understanding the foundations for seasons on Earth will set the stage for the remainder of the book.

Carbon connects us to all life on Earth and to our non-living environment. This chapter introduces the nature of the element carbon, and explains why it plays such a central role for our planet and its inhabitants. We humans, and all of the other things that live, are made of carbon atoms hooked together in different configurations. These atoms are the foundation for our structure and our ability to function. The carbon that we are made of links us to the green plants of our planet, to the soils we walk on and grow our food in, and to the atmosphere that we breathe. It connects us intimately to our climate, and is a major reason that life on Earth exists. Carbon links us to ancient organisms, whose DNA we inherited and whose remains we burn for energy, and to the changes we are forcing in Earth’s climate. This chapter elaborates on the nature of carbon, how it moves between the living and non-living parts of our planet, and why it has the unique and amazing properties it demonstrates.

The broad patterns of global life—the biodiversity that exists on Earth—are the focus of this chapter. Previous chapters have outlined the basic patterns of Earth’s climates. The book now turns to look how climate shapes what lives where, and why. Over two million species currently exist on Earth that we know of; doubtless there remain many to be discovered. Earlier chapters have outlined why rainforests are warm and wet, why deserts are dry, and why it is cold near the poles. The grand distribution of Earth’s biomes—the distinctive, naturally occurring and interconnected collections of living organisms that cover large areas of our terrestrial environment are illustrated here, with some of the organisms that define them. The remarkable fact that species with very different evolutionary histories converge in their forms and functions in similar environments is highlighted. The chapter also considers how humans have transformed the biomes, sometimes compromising the benefits or “ecosystem services” we take from them.

All organisms possess adaptations in physical form, physiological function, behavior, and many other traits to their physical and living environments. This chapter highlights some of these adaptations, focusing on examples of adaptation to the thermal environment. The amazing dance of organisms with the abiotic world is due in part to the myriad of adaptations on the different branches of the tree of life, many of which are the product of evolution by natural selection. Earth’s climates range from bitter cold to baking heat, from searing aridity to rainy skies and swampy ground, while the oceans also range from sunlit shallows to the deepest, darkest, coldest abysses. And yet, Earth is teeming with life, and organisms range from the enormity of blue whales to the tiniest tardigrades and bacteria. The rich biodiversity of life on Earth is, in part, due to the adaptations evolved by different evolutionary lineages to the different conditions in which they survive, grow, and reproduce. These adaptations reflect both the challenges their ancestors have faced and overcome and the opportunities they have made the most of. To highlight and explain contrasting adaptations, this chapter examines the specific adaptations of arctic polar bears, Antarctic penguins, Sonoran desert kangaroo rats, creosote bushes, and desert annual plants to the challenges posed by their contrasting environments. The chapter ends with a ridiculous joke that highlights an important principle of evolution by natural selection.

Humans are exquisitely sensitive to our thermal environment from the moment we are born, whether we are conscious or unconscious, till the moment we die. This chapter first considers what heat and temperature are, and then examines how we and other organisms exchange heat with our surroundings. It is essential for living things to regulate our temperature so that we can function—maintaining our thermal energy balance—and to stay alive. The same concepts and principles of thermal energy balances apply to a mouse, your house, the ocean, and the planet. These concepts are explained in terms of how a meerkat—a small animal living in an arid landscape in South Africa—experiences the thermal world in which it lives.

This chapter introduces the scope of the book and makes the case that understanding the basics of Earth’s climates is interesting and rewarding. Some surprising facts are introduced and used to illustrate how Earth’s systems come together. For example, a person sitting right now in Kampala, Uganda or Palu, Sulawesi, Indonesia, would be moving with Earth’s rotation eastward at the very rapid clip of approximately 1040 mph (1674 km/h), but someone dodging polar bears on Svalbard, Norway, or penguins on the Ross Ice Shelf in Antarctica, would be sauntering eastward almost five times more slowly, at a mere 216 mph (348 km/h). Climates in different parts of our planet, and the explanation for why they are different, are a matter of daily existence and a concern for life, death, and livelihood for billions of our fellow humans. They set the stage for life itself on Earth. The trillions of organisms that we share our planet with live where they live and function the way they function largely because of the climates where they exist.

This chapter brings together many of the concepts discussed earlier in the book, with a focus on moving air at very large spatial scales. The role of the jet streams is highlighted, those gargantuan wind belts that blow around circumference of the entire Earth with large impacts on weather and climate. As has been shown throughout this book, the physical components of the atmosphere and oceans interact continuously with each other and with living systems. We may experience these interactions as the movement of air—that is, wind. One of the most dramatic, dominant, and important manifestations of atmospheric circulation are the jet streams. The jet streams are rivers of strong winds—some of the strongest winds near Earth’s surface. The jet streams exist high in the troposphere, the layer of the atmosphere closest to Earth’s surface, and they have profound effects on weather, particularly in the northern temperate regions. Huge rotating clusters of clouds, called cyclones, with horizontal scales on the order of thousands of kilometers, are associated with the jet streams. The waviness of the jet streams and role of cyclones play an important role in energy transport in the Earth system. Fronts are the intersections between air masses with different temperatures and humidity, and fronts are one of the main mechanisms for vertical movement of air in the atmosphere, leading to precipitation. The warm and cold fronts associated with cyclones are responsible for much of the weather experienced by people, animals, and ecosystems, particularly those found in the mid-latitudes.

Soils clothe the non-glaciated terrestrial world, and they are the foundation for life on land. This chapter describes how they are created, their great variety in form and function, and, their conservation—and why they are so important to life on land. Soils are an essential component of Earth’s biogeochemical system and are intimately tied to the atmosphere, ocean, and climate. There is a lot more to soil than “dirt,” and it is a lot more interesting as a topic and as a system than you might be familiar with. Soils have structure, complexity, and great variation, and they even have names. It takes a long, long time to make a soil, from very young soils that may be 10,000 years old to old soils over 100,000 years or more. Soils consist of air space, minerals, and decomposed organic matter. Soil organic matter comes from living things and is a vast reservoir for carbon, gaining that carbon from living and once-living things. Soil takes up and releases carbon to the atmosphere and is a central player in Earth’s carbon balance. Protecting soils is a major and largely overlooked conservation challenge, as most human activities cause soil erosion and loss with cascading consequences for the planet and for life on Earth.