CHAPTER 6 Focus Points

• The climate system: The climate system is composed of Earth’s physical systems, mainly the atmosphere, biosphere, lithosphere, hydrosphere, and cryosphere.

  Temperature increase: The average global atmospheric temperature has risen by 0.83°C (1.5°F) since 1880.

  Climate change: Climate change occurs when the long-term trend of meteorological variables or weather extremes, such as the average temperature or the number of heat waves, changes.

  Extreme events: The occurrence of extreme events cannot be scientifically attributed to climate change, but the intensity of those events can.

  The ice-albedo positive feedback: The ice-albedo positive feedback destabilizes the climate system and causes climate change by enhancing whatever trend is already taking place.

• Modes of climate change: Climate change occurs through long-term trends, repeating cycles, and random anomalies. These changes are caused by climate forcing factors and climate feedbacks.

  The Holocene: The Holocene (the last 10,000 years) is Earth’s most recent interglacial. Climate has been unusually stable during the Holocene.

  Rapid climate change: Climate can rapidly switch between different states on a time scale of decades.

  Reconstructing past climates: Earth’s past climates are recorded in various natural materials, including tree rings, glaciers, cave deposits, and ocean sediments.

• Carbon cycles: Carbon is removed from the atmosphere, oceans, and biosphere and stored for many millions of years in the lithosphere in the long-term carbon cycle. Carbon moves relatively quickly among the atmosphere, biosphere, and oceans in the short-term carbon cycle.

  Human activity and carbon cycling: Each year, burning of fossil fuels and deforestation are moving about 35 billion metric tons of carbon from the lithosphere and biosphere into the atmosphere.

• Anthropogenic greenhouse effect: Natural atmospheric CO₂ concentrations have not exceeded 300 ppm over the last 800,000 years. Because of human activities, atmospheric CO₂ concentrations are now 400 ppm and rising 2.5 ppm per year. As a result, atmospheric temperatures are rising.

  Carbon dioxide and temperature: Atmospheric CO₂ concentrations and temperatures have risen and fallen together during the last 800,000 years.

  The Arctic: The ice-albedo positive feedback is warming the Arctic at about twice the rate of the rest of the world.

  Causes of the current warming trend: No known natural climate forcing factor, such as volcanic activity or sunspot activity, can explain the current warming trend.

  Effects of warming: The negative aspects of warming far outweigh any positive aspects.

  Changes in Earth’s physical systems: Earth’s physical systems are changing because of atmospheric warming.

  Climate projections: Climate projections based on computer modeling vary depending on assumptions about how much CO₂ will be emitted into the atmosphere.

• 500 ppm goal: Most climate scientists conclude that limiting atmospheric CO₂ concentrations to less than 500 ppm could avoid significant climate change. To meet this goal, annual global CO₂ emissions must be cut by about 25 billion metric tons, or 80%, in the next few decades.

  How to reduce carbon: Doubling the efficiency of cars, increased use of carbon-free energy, carbon taxes, and personal choices are some of the options available to address climate change.