Chapter 16 Summary

The topics covered in this chapter can be summarized as follows:

16.1 What Is the Earth System?

Viewing Earth as a system allows us to take into account the complex ways in which the atmosphere, hydrosphere, biosphere, and lithosphere interact. Positive feedbacks amplify changes in the Earth system, and negative feedbacks reduce them. The stability of the Earth system will depend on what feedbacks are available. The presence of ice sheets makes the Earth system less stable.

16.2 Causes of Climate Change

Weather describes day-to-day conditions, but climate refers to the long-term average conditions over decades or longer. Climate forcings alter climate. They include processes that change the rate and location of solar energy reaching Earth’s surface; processes that alter how ocean currents move heat around Earth’s surface; and processes that affect how heat moves into and out of the atmosphere. Climate forcings operate on a range of timescales, from billions of years to less than a decade. Changes in greenhouse gas concentrations and albedo are two climate forcings affected by human activities.

16.3 Methods for Studying Past Climate

Climate conditions for some of human history can be determined from direct measurements that have been recorded, but for studying paleoclimate it is often necessary to use proxy data. Proxy data come from natural materials that behave in a systematic way in response to climate conditions like temperature or precipitation. Proxies include tree ring data, stable isotopes, measurements of gas bubbles trapped in ice, and the geographic distribution of rocks and fossils.

16.4 Computer Models of the Earth System

Earth-system models use mathematical equations to simulate Earth-system processes. Models are set up and checked using real-life measurements. Model uncertainty is a number that tells us the likelihood that a particular model result falls within a certain range of values. It is a way to evaluate whether results can be used to draw meaningful conclusions.

16.5 Humans in the Earth System

Data show recognizable anthropogenic influence on the Earth system beginning when humans began to use fossil fuels for industrial purposes. CO₂ in the atmosphere has the isotopic fingerprints of fossil fuels. The flow of anthropogenic carbon into the Earth system is relatively small compared to some natural flows, but natural processes do not remove all of what humans put in, causing CO₂ to accumulate in the atmosphere.

16.6 Welcome to the Anthropocene

Humans today are experiencing the results of past human influence on the Earth system, and humans in the future will experience the results of decisions made today. The main source of radiative forcing is anthropogenic CO₂. Humans are already experiencing extreme climate events related to warming. The best-case projected warming scenario will stabilize global average temperatures at ~1ºC above 1996 temperatures by the end of the century, but this will require a peak in anthropogenic CO₂ emissions by 2020, and an end to anthropogenic CO₂ emissions by 2080.

Review Questions

1. If you receive positive feedback on a project, this means someone says you did a good job. Does this mean a positive feedback in the climate system is also something good? Explain.
2. Why does the presence of ice sheets cause the Earth system to be less stable?
3. Using the orbital information on eccentricity, tilt, and precession, we could calculate variations in insolation for any latitude on Earth and for any month of the year. Why do we focus on a latitude of 65° N?
4. Explain how the positioning of Gondwana at the South Pole contributed to glaciation during the Paleozoic.
5. If the major currents in the oceans were to slow down or stop, how would that affect the distribution of heat on Earth, and what effect might that have on glaciation?
6. Most volcanic eruptions lead to short-term cooling, but long-term sustained volcanism can lead to warming. Describe the mechanisms for these two different consequences.
7. What property of greenhouse gases allows them to absorb infrared radiation and thus trap heat within the atmosphere?
8. What are the advantages of proxy data over direct measurements of climate? What are the disadvantages of using proxy data?
9. What are some steps that scientists take to help ensure that meaningful conclusions can be drawn from Earth-system computer models?
10. What evidence tells us that rising atmospheric CO₂ levels are primarily the result of humans burning fossil fuels?
11. The flows of carbon through the carbon cycle that are related to human activities are actually much smaller than some natural flows. Why do human activities have such a large impact on the carbon cycle?
12. Some people argue that climate has always changed, and present-day climate change is not significant compared to the long-term record of Earth’s climate. Is it true that present-day climate signals show that changes are not significant compared to Earth’s long-term climate history? Explain your answer.
13. What climate mechanism links both extreme heat waves and droughts, and extreme flooding and rainfall events to climate change?
14. What evidence suggests that increasingly intense tropical storms can be expected as climate warms?
15. What are the potential impacts of thawing permafrost?