The Cold of Space

The Cold of Space

During the 1920s and 1930s, a Serbian mathematician named Milutin Milankovitch, building on work by earlier scientists, calculated how the sunshine received at different places and seasons on the Earth has changed over a long time in response to features of Earth's orbit. As the sun, moon, Jupiter and other planets tug on the Earth, the orbit changes a bit. Earth wobbles with a 19,000-year periodicity, the pole tilts a little more and then a little less with a 41,000-year periodicity, and the orbit changes from nearly round to more squashed or elliptical and back with a 100,000-year periodicity. NASA animations of these are shown below. With modern computers, these changes are relatively easy to calculate for many millions of years; for Milankovitch, the calculation was the labor of a lifetime. (He did it very well, though, even correctly noting that the 19,000-year periodicity goes from 19,000 to 23,000 years and back, a pattern that is indeed observed in the data testing his prediction! Also note that the NASA animation for precession says that it is a 26,000-year cycle, which is correct, but it interacts with other cycles to cause variation in sunshine with periodicities of 19,000 to 23,000 years. The ability to calculate these variations accurately really is amazing.)

Video: Changes in Eccentricity (Orbit Shape) (0:07 seconds)

Video: Changes in Obliquity (Tilt) (0:01 second)

Video: Axial Precession (Wobble) (0:04 seconds)

These orbital wiggles have little effect on the total sunshine received by the planet, but they do move the sunshine from north to south and back, poles to the equator and back, or summer to winter and back in various ways. For example, today the Northern Hemisphere is farther from the sun in northern summer than in northern winter. (Remember that summer is controlled by the tilt of the planet’s spin axis relative to the plane in which the planet orbits, not by the distance from the sun!) In the few millennia centered 9000 years ago, the Northern Hemisphere had slightly warmer summers and cooler winters than recently because the Earth was closer to the sun during northern summers and farther from the sun during northern winters than today. (Note that this was reversed in the south.) The drop in summer sunshine in the north over the last 9000 years allowed mountain glaciers to slowly expand a little, culminating in the so-called "Little Ice Age" of the 1600s to 1800s; the strong melting of glaciers since then is mostly the result of human-caused warming. (We will discuss this later in the course.)

Summer in the Northern Hemisphere is most important in controlling ice ages, because the Northern Hemisphere is mostly land and can grow big ice sheets, but the Southern Hemisphere is mostly water, and already has ice on Antarctica, so can’t change its land ice much more. In the north, even during warm winters, the highlands around Hudson Bay are cold enough to have snow rather than rain. During times when features of the Earth's orbit gave reduced sunshine in the north, ice has survived summers and grown; increasing summer sunshine has melted the ice. The way the various cycles interacted led to larger or smaller changes, and thus to the ice ages we know.

You may guess that this is slightly oversimplified so far. For example, during times when Canada has received more summer sunshine, allowing its ice to melt, the Southern Hemisphere or the tropics often received less sunshine, yet they warmed too! How Canada told the glaciers of Patagonia and Antarctica to shrink was a great puzzle for a long time. The answer involves global warming from atmospheric carbon dioxide. The growth and shrinkage of the vast ice sheets, the changes in sea level, and other changes had the effect of shifting some carbon dioxide (CO₂) from the air into the deep ocean during ice ages and bringing the CO₂ back out into the air during times when the northern ice was melting. The orbits affected the ice, which affected currents and sea level, and plants and other things, which affected CO₂. But, as we will discuss later in the semester, CO₂ in the air warms the Earth's surface no matter how CO₂ gets into the air. And, the changing CO₂ explains why, when the ice was growing, places getting more sunshine still got colder, and why, when the ice was shrinking, places getting less sun still got warmer.

Climate records show many other types of changes. Very large, rapid changes have been caused by sudden surges of ice sheets, and by jumps in the way the ocean circulates. We do not understand these faster changes well enough to know whether they could happen again. We're cautiously optimistic that we will avoid crazy climate jumps, but we're more worried about Antarctic ice-sheet surges raising sea level. Naturally, the Earth’s orbit right now is in an intermediate state, orbital changes are causing almost no change in the climate, and we should be looking forward to another 50,000 years or more with little change in the climate from orbits before we begin the natural slide into a new ice age. (See the Enrichment for a little more on this.) However, humans almost certainly are now more important to the climate than are such slow changes, as we will see later, and we probably have already stopped the next natural ice age.