Page 355 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 11 Climate Change 319
(see discussion in Chapter 6). After the Little Ice Age, temperatures steadily warmed, and with growing human population and the onset of the Industrial Revolution, this warming has continued—a trend that is accelerating today.
Mechanisms of Natural Climate Fluctuation
In reviewing climate records on various scales, we see that Earth’s climate cycles between warmer and colder periods. When temperature is viewed over certain time- scales, such as over periods of about 650000 years (illus- trated in Figure 11.11), patterns are apparent that appear to follow cycles of about 100000 years, 40000 years, and 20000 years. Scientists have evaluated a number of nat- ural mechanisms that affect Earth’s climate and might cause these long-term cyclical climate variations.
Solar Variability
As we learned in earlier chapters, energy from the Sun is the most important driver of the Earth–atmosphere climate system. The Sun’s output of energy toward Earth, known as solar irradiance, varies over several timescales, and these natural variations can affect cli- mate. Over billions of years, solar output has generally increased; overall, it has increased by about one-third since the formation of the solar system. Within this time frame, variations on the scale of thousands of years are linked to changes in the solar magnetic field. Over recent decades, scientists have measured slight variations in the amount of radiation received at the top of the atmosphere using satellite data and have correlated these variations to sunspot activity.
As discussed in Chapter 2, the number of sunspots varies over an 11-year solar cycle. When sunspot abun- dance is high, solar activity and output increase; when sunspot abundance is low, solar output decreases. Sci- entists have determined that these relationships are re- flected in climatic indicators such as temperature. For example, the record of sunspot occurrences shows a prolonged solar minimum (a period with little sunspot activity) from about 1645 to 1715, during one of the cold- est periods of the Little Ice Age. Known as the Maunder Minimum, this 70-year period would suggest a causal ef- fect between decreased sunspot abundance and cooling in the North Atlantic region. However, recent tempera- ture increases have occurred during a prolonged solar minimum (from 2005 to 2010), which corresponds with a period of reduced solar irradiance. Thus, the causal ef- fect is not definitive. The IPCC Fifth Assessment Report includes solar irradiance as a climate forcing (discussed later in the chapter); however, scientists agree that solar irradiance does not appear to be the primary driver of re- cent global warming trends (as an example, see www.giss .nasa.gov/research/news/20120130b/).
To explain the apparent correlation between solar output and cooler temperatures during the Maunder Minimum, many scientists think that reduced solar output, while not the cause of cooling, did serve to re- inforce the colder temperatures through feedback mechanisms such as the ice–albedo feedback (dis- cussed ahead). Recent research suggests that solar out- put may have some effects on regional climate, such as occurred during the Maunder Minimum, without af- fecting overall global climate trends (see science.nasa .gov/science-news/science-at-nasa/2013/08jan_sunclimate/).
Earth’s Orbital Cycles
If the overall amount of energy from the Sun does not drive climate change, then another logical hypothesis is that Earth–Sun relationships affect climate, a reasonable connection given that orbital relationships affect energy receipts and seasonality on Earth. These relationships in- clude Earth’s distance from the Sun, which varies within its orbital path, and Earth’s orientation to the Sun, which varies as a result of the “wobble” of Earth on its axis, and because of Earth’s varying axial tilt (please review Chapter 2, where we discussed Earth–Sun relations and the seasons).
Milutin Milankovitch (1879–1958), a Serbian astron- omer, studied the irregularities in Earth’s orbit around the Sun, its rotation on its axis, and its axial tilt, and identified regular cycles that relate to climatic patterns (Figure 11.16).
▲Figure 11.16 Astronomical factors that may affect broad climatic cycles. Figures are an exaggeration of actual orbital paths, axis wob- ble, and axial tilt.
Earth Earth
Sun
(a) Earth’s elliptical orbit varies widely during a 100 000-year cycle.
24.5° 21.5°
(b) Earth's axis “wobbles” (c) Earth’s axial tilt varies over a 26 000-year cycle. over a 41 000-year cycle.