Teaching Essential Principle Three
Life on Earth depends on, is shaped by, and affects climate.
The essence of this principle is that life affects the climate system and in turn, the climate dictates where and how species can survive. Life affects the composition of the atmosphere and therefore the climate because different life forms take in and release gases like carbon dioxide, methane and oxygen at different rates. Climatic conditions help to shape various ecosystems and habitats around the globe. A particular climate can be a boon to one species and a devastation to another. As the climate changes, species and ecosystems respond by adapting, migrating or reducing their population. Gradual shifts in the climate are easier to adapt to than abrupt swings, and this is certainly true for humans as well as other species. Studies of Earth's climatic history indicate that climates have changed in the past and resulted in dramatic shifts in ecosystems. The most recent geological period the Holocene (about last 10,000 years), however, has been unusually stable.
Teaching this principle is supported by five key concepts.
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- Individual organisms survive within specific ranges of temperature, precipitation, humidity, and sunlight. Organisms exposed to climate conditions outside their normal range must adapt or migrate, or they will perish.
- The presence of small amounts of heat-trapping greenhouse gases in the atmosphere warms Earth's surface, resulting in a planet that sustains liquid water and life.
- Changes in climate conditions can affect the health and function of ecosystems and the survival of entire species. The distribution patterns of fossils show evidence of gradual as well as abrupt extinctions related to climate change in the past.
- A range of natural records shows that the last 10,000 years have been an unusually stable period in Earth's climate history. Modern human societies developed during this time. The agricultural, economic, and transportation systems we rely upon are vulnerable if the climate changes significantly.
- Life—including microbes, plants, animals and humans—is a major driver of the global carbon cycle and can influence global climate by modifying the chemical makeup of the atmosphere. The geologic record shows that life has significantly altered the atmosphere during Earth's history.
Why are these topics important?
The manner in which the earth sustains life is of vital importance on many levels.
- Throughout geologic history, life has affected the climate system and vice versa.
- Extinctions of species, both in the geologic past and in the present day, can be linked to changes in climate.
- Unraveling past climatic changes is key to understanding present and future shifts in the climate.
- Changes in climate will result in shifting ecosystems. It is not possible to predict the specific effects of climate change on each of the world's ecosystems.
- Although the concentrations of greenhouse gases have changed throughout Earth's history, there is no natural analog to today's rapid increases in human-created greenhouse gas emissions.
What makes these topics challenging to teach?
This principle ties in understanding of different scientific disciplines. Understanding the feedback between climate and life on Earth requires an understanding of biochemical processes like photosynthesis as well as basic climate science (eg. greenhouse effect). Students may confuse the natural greenhouse effect that makes life on Earth possible with the enhancement of the effect by the emission of greenhouse gases from fossil fuel burning (McCaffrey & Buhr, 2008). Students might find it counter-intuitive that plants get their mass from the carbon in the air through photosynthesis (Private Universe study), which is one of the key concepts of the carbon cycle.
Teaching about the limited ability of organisms to adapt to climate change (eg. mass extinctions in geological past) should not lead to gloom-and-doom scenarios. Instead it can lead to an understanding that humans have a responsibility to stabilize the natural climatic conditions in order to preserve the environments in which humans and the surrounding ecosystems thrive.
When teaching about the feedback loops between climate and life and the effects of climate change, the differences between natural and human caused changes should be emphasized. It is likely that questions arise such as: Are all natural changes good? Are all human-caused effects bad? Is our current climate the "right" climate? It will be important to emphasize that the recent increases in greenhouse gas concentrations in the atmosphere are unprecedented in the geologic past and to build an understanding of stewardship of the planet among students.
How can I use this principle in my teaching?
Most ideas that are highlighted in this principle are part of the life science curriculum but integrate concepts previously introduced in physical sciences, geography and other disciplines. The key ideas in this principle build on the comprehension of the carbon cycle to understand the relationship between climate cycles, such as seasons, and biologic systems. Possible topics to teach an understanding of this principle are:
- Annual seasonal migrations of species.
- The effects of the spring "green up" in the northern hemisphere and the resulting seesaw pattern in atmospheric CO2 concentrations as as illustrated by the Carbon Dioxide Exercise
- Decade-scale events such as insect outbreaks, forest succession or drought.
- The 100,000 year cycle of ice ages, the role of CO2 in enhancing the temperature swings, and the response of biologic systems to these dramatic shifts in climate.
- Periodic mass extinction events that punctuate the geologic record and were likely related to climatic changes.
Another approach that may be engaging for older students is the delicate relationship between life and the climate. Many students will be surprised to learn of past mass extinction events and other sharp swings in the balance of the biosphere. This is a key place to discuss the role of humans in changing our environment and climate.
- Middle school students in many parts of the world can observe the "green-up, green-down" seasonal process and participate in citizen science programs that encourage seasonal observations of migrating birds or butterflies, or when buds burst. See Blooming Thermometers for an example. Students can also learn about the impact of climate change on ecosystems and on animal habitats, as in Climate Change and Arctic Ecosystems.
- In high school, students can learn about the global carbon cycle. This theme touches on many different processes such as photosynthesis, the formation of fossil fuels and the role of carbon dioxide in keeping Earth warm enough to sustain life. The Global patterns in Green-up and Green-down activity illustrates the global carbon cycle.
- In the introductory undergraduate curriculum, students can use an Earth systems approach to learn how Earth operates in several "spheres." The evolution of the atmosphere and its relationship to early life is a suitable topic, as are group projects such as Understanding the Carbon Cycle: A Jigsaw Approach.
- Upper-level college students can examine correlations between climate and life in a number of settings. For example, how did early humans adapt to the ice ages? How are climate perturbations expressed in various climate proxy data? What types of adaptations, migrations or population shifts are already being observed due to our warming climate?