Climate History & the Cryosphere
Part A: Glacial Ages
Temperatures change all the time. Locally, it's not uncommon for temperatures to drop 5, 10, even 20 degrees or more overnight. Over the course of a year, we see gradual increases in daily and monthly average temperatures as winter eases into spring and summer and watch them fall again as summer turns to autumn and then back to winter. When we look at temperature on a regional or global scale over the course of many years, climatic patterns emerge.
Throughout its history, Earth has experienced several periodic swings in climate. For example, Earth was entirely ice-free and temperatures were hot enough for turtles and palm trees to thrive at the poles during the Early Eocene Climatic Optimum around 49 million years ago. On the other hand, during the Last Glacial Maximum, which occurred between 26,500 and 19,000 years ago, ice sheets covered nearly one third of Earth's surface. Today, we are somewhere in between extremes. Snow and ice exist year round near the poles and seasonally at lower latitudes. Glaciers cover about 10% of Earth's surface and can be found on every continent except Australia.
The term "ice age" typically invokes images of a frozen world, covered in snow and ice, in a time when woolly mammoths and sabre-toothed tigers roamed the Earth. However, scientists use the term ice age or glacial age to describe any geological period in which long-term cooling takes place and ice sheets and glaciers exist. That means we are currently in the midst of an ice age right now! More specifically, we are in an interglacial (warm period) within a glacial age. Cold periods within a glacial age are called glacials or glaciations, and are characterized by cooler temperatures and advancing glaciers.
Glacial ages come and go over millions of years. Interglacial periods, like the one we are in now, are typically spaced apart by hundreds of thousands of years. Based on observed patterns, we should be swinging back to an "icehouse Earth." However, since the industrial revolution, increased carbon dioxide (CO2) levels in the atmosphere (largely due to the burning of fossil fuel), are pushing Earth toward a warmer climate. In fact, we now see that this increase in CO2 is warming Earth at a rate ~100 times faster than Earth has seen through slow natural swings.
Carefully examine the graphs below. Notice that the x-axis shows age, so an age of 0 represents something happening today, and an age of 400,000 would represent something that happened 400,000 years ago. As you move to the right along the x-axis, you are essentially looking back in time. You will need to read the graph from right to left to follow events as they occurred in chronological order. The bottom graph shows just the portion of the top graph from the present back to 150,000 years ago.
Image source: Woods Hole Oceanographic Institution: Dive and Discover
Image source: Woods Hole Oceanographic Institution: Dive and Discover
Answer the following questions based on the two graphs above.
We know that Earth's climate has been highly variable over time, but what processes are behind these climatic swings?
Serbian astrophysicist Milutin Milankovitch is credited for developing one of the most significant theories relating changes in Earth's orbit to long-term changes in climate, including ice ages. Milankovitch's theory is based on cyclical variations in three aspects of Earth's orbit that result in changes to the seasonality and location of solar radiation reaching Earth:
- Changes in the obliquity (tilt) of Earth's axis
Earth is slightly tilted—that's why we have seasons. As Earth orbits the sun, one hemisphere will be tilted toward the sun for a period of time (summer) and tilted away from the sun six months later (winter). Today, Earth's rotational axis is tilted at about 23.5 degrees from vertical. However, this tilt oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. Variations in the obliquity (tilt) of Earth's rotational axis result in changes in the severity of seasonal changes. When the tilt is larger, the extremes between summer and winter temperatures are greatest. When the tilt is smaller, the average temperature difference between winter and summer is less drastic. It is believed that it's actually these periods of smaller tilt that promote the growth of ice sheets. When Earth's axis is less tilted, winters are relatively warmer and summers are relatively cooler. This means that there is more moisture in the air in winter and therefore more snowfall. It also means that there is less summer melting, so more of the winter snow accumulation will last through the warmer months.
- Variations in the shape of Earth's orbit (eccentricity)
The gravitational pull of other planets orbiting the sun causes the shape of Earth's orbit to be elliptical rather than perfectly circular. Eccentricity (e), which ranges from 0 to 1, is a measure of how much an ellipse deviates from a perfect circle (how flattened the circle is). An orbit with an eccentricity of 0 is perfectly circular, and an orbit with an eccentricity of 1.0 is a parabola (no longer a closed orbit). The shape of Earth's orbit ranges from nearly circular (e = 0.005) to slightly elliptical (e = 0.058) and back again about every 100,000-400,000 years. Changes in eccentricity are important to determining periods of glaciation because they determine the distance between the Earth and sun, and therefore how much radiation is received at the Earth's surface during different seasons. When the orbit is nearly circular, the distance between the Earth and sun (and therefore the amount of solar energy reaching Earth) remains relatively constant throughout the year. However, when the orbit is more elliptical, the distance between the Earth and sun (and the amount of energy reaching Earth) fluctuates between seasons, resulting in slightly warmer or cooler temperatures. Today, Earth's orbit has an eccentricity of 0.017.
- Changes in Earth's "Wobble" (Precession)
Earth's axis of rotation behaves like a spinning top that is slowing down, wobbling in a circle over time. Earth's axis wobbles between pointing at Polaris (what we now call the North Star) and pointing at the star Vega (which would then be considered to be the North Star). Every year, this wobble causes Earth to travel slightly farther than one full orbit each year. This means that on today's date next year, Earth will be a little bit further in its orbit than it is right now. This is called precession. Earth's axis completes a full cycle of precession approximately once every 26,000 years. Because Earth's orbit isn't perfectly circular, the distance between the Earth and sun (and the average temperature) will be slightly different each year on the same date. Precession can cause significant changes in climate due to greater contrast in seasons. For example, when Earth's axis is pointed at Vega, the winter solstice in the northern hemisphere coincides with Earth being at its farthest distance from the sun (aphelion), and the summer solstice coincides with Earth being at its closest distance from the sun (perihelion). Just like with variations in obliquity and eccentricity, the more drastic seasons brought on by precession will impact glaciation.
Stop and Think1: Explain how the three Milankovitch cycles combined can cause glacial and interglacial periods to occur.
2: Why is it important to study climate data and glacial ages as far back as hundreds of thousands or even millions of years?
3: How do you think scientists separate human influences on climate from natural variations?