Laura Nielsen for Frontier Scientists –
Cyclical changes in the way the Earth circumnavigates the Sun can influence Earth’s climate. Last week, we looked at Milankovitch’s assessments of orbital dynamics, including: orbital eccentricity, Earth’s tilt or obliquity, and the precession or change in orientation of the Earth's axis of rotation which determines what direction each hemisphere points. Now, let’s examine how Milankovitch’s observations can lend themselves to an astronomical theory of paleoclimates. How have these forces shaped Earth’s climate history, and what can they tell us about Earth’s climate future?
At the heart of grasping Milankovitch's theory is an understanding of insolation. Insolation describes how much sunlight, or solar radiation, reaches a given area of the Earth. How much sunlight strikes the ground is part of what determines the energy balance of a region. An eccentric (oval) orbit takes our planet closer or further from the Sun during different seasons, affecting the seasonal energy balance. A highly oblique (tilted) planet exposes more sunlight to the far poles, while a lesser tilt keeps light shining more on the equator and less directly at the poles. And the precession of the equinoxes determines which hemisphere is bathed in sunlight during perihelion, our closest approach to the Sun. These Milankovitch cycles govern the location of intense solar energy meeting the surface of the earth, and thus impact the severity of the seasons.
Early in his studies, Milankovitch supposed that severe seasonality (extremely hot summers and cold winters) must encourage ice and snow to build up and thus encourage glaciation. Later, he and other scientists realized that less severe seasonality (temperate mild summers and cool winters) favor glaciation. Warm but not scorching summer temperatures allowed more water vapor to sit in the air, helping to encourage snowfall in the winter. The temperate summers failed to fully melt away the snowpack, permitting layers of snow to build up... the building blocks of glaciers.
Currently, we are living in a geological epoch known as the Holocene. The geologic time period which preceded the Holocene was called the Pleistocene (spanning from about 2.5 million years ago to 11,700 years ago). The Pleistocene was characterized by a string of ice ages, or glacial periods, interrupted by warmer interglacial periods. The Holocene encompasses our present era of human global impacts. That doesn’t mean, though, that we have escaped the cycle of the ice ages. Though we do not know for certain, many believe we currently live in an interglacial period, and there will be another glacial period in the far future.
We know historical ice ages happened, and that orbital dynamics impacted them, because of climate records. Scientists drill ocean sediment cores and examine the remains of long-dead foraminifera that have been deposited in layers on the seabed. The chemical composition of these microscopic organisms give clues about the climate they lived in. Bubbles of atmospheric gasses trapped inside ancient ice recovered as ice cores also tell of Earth’s climate past. Milankovitch supposed that variations in the three cycles of orbital dynamics he focused on would define trends in insolation and seasonality, thus affecting the formation of glaciers, their advance and retreat. And he was right: comparing Milankovitch’s mathematical modes and -more tellingly- modern-day climate models run on supercomputers with paleoclimate records shows that orbital dynamics do indeed influence Earth’s climate system. Our climate records also show that carbon dioxide levels move in close (near-) lockstep with temperature trends.
We can take those models a step further to look at our potential future. A model prediction using orbital dynamics but leaving out anthropogenic (human-caused) climate disturbance illustrates that we are indeed in an interglacial period, and heading towards a distant future ice age. It predicts that the long-term cooling trend which began some 6,000 years ago will continue for the next 5,000 years. Then, temperatures will become milder temporarily in a lull centered around 15,000 years after present; however, that won’t last long: a cold interval centered around 23,000 years after present will freeze things back up, setting the stage for a major glaciation, a new ice age, around 60,000 years after present.
Let’s look at climate today, again setting aside anthropogenic climate disturbance. Orbital dynamics are actually asserting little influence at present. We examined how our planet is orbiting the Sun. The eccentricity of our orbit is nearly zero, describing a very circular orbit. There is only about a 3 percent difference between perihelion (our closest approach to the Sun) and our furthest distance from the Sun. Because of this, precession has little impact. Earth's obliquity or tilt (its angle away from a hypothetical vertical line) is 23.45 degrees out of a possible 24.5 degrees ...We are near the middle of the range. As we move toward the minimum tilt of 22.1 degrees over the next 20,000 or so years, seasons will become less intensely varied, which hypothetically promotes the growth of ice sheets.
Climate, though, is a tricky thing. Orbital dynamics are not the only forces influencing our planet’s climate future. The Sun itself experiences cyclical changes governing how much energy it emits. On Earth, ocean temperature and circulation alters the climate signal. The contents of the atmosphere matter. Cloud-cover and aerosols like volcanic particulates change how much sunlight reaches Earth, and how much reflects back into space. And -as you know- greenhouse gasses act to keep heat trapped inside the atmosphere. The long-term Milankovitch-based climate model suggests a cooling trend, but it does not include human-caused emissions of carbon dioxide, methane, and other greenhouse gasses.
Eccentricity, obliquity, and precession are orbital trends that change over tens of thousands of years. Nothing in the natural world can explain the abrupt increase of carbon dioxide in the atmosphere following the advent of the Industrial Revolution. It is a human-initiated trend that will have serious consequences for the future of all living species; there’s a reason the Holocene includes the era of human civilization. This study of orbital dynamics shows that although the motion of our planet around the Sun does impact climate, it cannot explain away our self-inflicted rapid modern day trends.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
- 'In Defense of Milankovitch' Department of Earth and Space Sciences, University of Washington, Gerard Roe (accessed 2013)
- ‘Milankovitch Theory and climate’ American Geophysical Union, Reviews of Geophysics, Berger, A. (2010)
- 'Variations in the Earth's Orbit: Pacemaker of the Ice Ages' Science, the American Association for the Advancement of Science, Hayes, J.D. (1986)