Astronomical Influences on Earth's Climate

The Earth's climate is an extremely complex non-linear system with a diverse array of factors that contribute to changes over a very large range of time scales. We know that Earth's climate has gone from ice ages to more temperate epochs many times in past. There is good evidence to support the conjecture that some of this change is due to astronomical influences.27-29

Climate and Weather

If someone told that next July 3rd at 2:30 pm the temperature would be 18.6 C, overcast with a breeze of 3 km/hour from the west you would greet this news with a great deal of skepticism. On the other hand, if that same person told you that it will be above freezing on July 3rd you would likely not be too surprised. The first example speaks of weather - the atmospheric conditions we experience at a given moment. The second one speaks of climate or the averaged affects of weather over many decades. In trying to understand Earth's climate and especially the science of climate change it is crucial to understand the distinction between weather and climate. While the modern mathematics of chaos theory tells us there are very distinct limits to how far in the future we can predict weather the mathematics of statistics and the underlying physics and chemistry of the atmosphere assures us that we can speak with some precision about climate and climate change.

Anthropogenic versus Natural Climate Change

Before exploring the idea that there are astronomical influences that can cause climate change it is important to acknowledge that our current understanding of global climate change and the extreme challenge it poses for the survival of our planet is more aptly focused on human caused or anthropogenic climate change. In a future unit we will explore the role of greenhouse gases in warming planetary atmospheres. At that point we will discuss the serious affect that the anthropogenic production of greenhouse gases is having. We are not at all putting forward the notion that the current alarming signs of global climate change are due to astronomical effects. But you will see that there is very good evidence that over much longer time scales (measured in hundreds of thousands to millions of years) astronomical phenomena do have a profound influence on Earth's climate.

The Evidence - How do we Know that the Climate has Changed?

Ice core data from Greenland and Antarctica tell us clearly that Earth's climate has undergone enormous and at times very rapid change. Figure 3.29 shows the correlation between air temperature and the amount of CO2 in the Earth's atmosphere. The low points in the temperature graph would imply conditions consistent with ice ages.

Figure 3.29 Variation in mean Earth surface air temperature and atmospheric Carbon Dioxide levels over the past 500 000 years.

The data tells us that the last ice age ended 12 000 years ago and we have enjoyed a relatively stable and benign climate since then. The temperature graph also reveals very significant temperature variations that appear to be periodic or nearly periodic. Very large changes seem to occur on time scales on 100 000 years or so with smaller but still very significant changes occurring on time scales of 30 000 to 40 000 years. What could have caused such "quasi-periodic" temperature variations?

(Click here to go to a tool that will allow you to explore tempertature and CO2 levels over the past 800 000 years)

The Milankovitch Hypothesis

In 1920 Yugoslavian meteorologist Milutin Milankovitch proposed that a number of small changes in Earth's orbit and inclination, acting over periods of tens of thousands of years could produce changes in Earth's climate of the magnitude revealed in the data shown in Figure 3.29. Table 3.6 summarizes these changes.

Variation in Orbital Eccentricity: Currently the Earth's orbit is only slightly elliptical with less than 2% variation between our nearest and farthest distance from the Sun. This produces a minor effect on climate. However, interactions with other bodies in the solar system could increase the Earth's orbital eccentricity. If the eccentricity was large enough it could lead to pronounced effects in which northern summers could be too cold or winters too harsh and lead to the onset of an ice age.  
  Figure 3.30 The eccentricity of Earth's orbit undergoes a mild change from very nearly circular to slightly oval. This occurs with a period of approximately 400 thousand years. The effect is greatly exaggerated in this figure.
Precession: Currently northern summers occur when the Earth is farthest from the sun. In approximately 13 000 years northern summers will occur at closest approach and this could lead to warmer summer which would reduce the extend of the Northern Polar Cap and could prevent or lead to the end of an ice age.  
Variation in Inclination of Earth's Axis: The rotational axis of the Earth also slowly changes its inclination or obliquity from 22.1o to 24.5o with a period of about 41 000 years. A change of a few degrees will a significant effect on the amount of sunlight polar regions receive. A drop of a few percent in solar energy received at the pole could result in cooling conditions that could trigger the onset of an ice age. A decrease in obliquity would have an opposite effect with slightly more solar energy received at the poles.  
Table 3.6 Summary of changes in the Earth's orbit and implications for climate. Figure 3.31 The inclination or obliquity of Earth's rotational axis slowly varies between 22 degrees and 24.5 degrees. This occurs with a period of about 40 thousand years.

Figure 3.32 summarizes the various effects that Milankovitch speculated about.

Figure 3.32 Variations in precession, obliquity and orbital eccentricity with correlation to global mean temperature over the past 1 million years. (image is in the Public Domain and is produced by Robert A. Rohde from publicly available data)

In Figure 3.32 the solar forcing is shown for a latitude 65o N during the summer. The solar forcing (energy received from the sun and influencing climate) has been modeled using a sophisticated climate model that has used the changing eccentricity, precession and changing obliquity to calculate the amount of energy received in the northern polar regions. All of this lends credibility to the hypothesis that gradual changes in Earth's orbital characteristics can influence climate.

So Why is it called the Milankovitch Hypothesis and not "Milankovitch Theory"?

The Milankovitch Hypothesis rests on a number of generally known effects and some strong correlations. However, at the present time it lacks the ability to make precise and testable predictions (or retro-dictions). In science a hypothesis is considered to be a much weaker claim than a theory. Although the distinction is somewhat arbitrary a theory must make predictions (ie be testable). Some would even argue that in order to be a theory it must make statements that, if proven false, would refute the theory. At this stage the Milankovitch Hypothesis cannot clearly show a link direct between orbital variation and global temperature. Perhaps improvements in mathematical modeling of climate will change this in the near future. For the time being it is safe to conclude that orbital changes can produce measurable changes in climate over very long periods of time.

Example 3.8 Why would it be incorrect to conclude that currently observed global climate change is the "Milankovitch Effect" and not due to anthropogenic production of greenhouse gases (GHG's)?

Solution The most obvious difference is the rate at which change is occurring. Changes in orbital "forcings" of the type described by Milankovitch are very gradual and we do expect them to produce long term climate changes. The changes in climate can happen very rapidly, however. The problem is that in the span of 200 years (roughly the time scale of the present rapid change in which climate is occurring) any changes in orbital characteristics are negligible. On the other hand, there is a very clear correlation between the increased levels of human produced Carbon Dioxide and other greenhouse gases in our atmosphere. Beyond a "mere" correlation, however, we understand very well how greenhouse gases cause atmospheric warming. Figure 3.32 shows how the mean global temperature and CO2 levels have changed since the time of the industrial revolution (late 18th century). It would be completely irresponsible to argue that the present climate change is due to the Milankovitch Effect and hence we should not limit our production of GHG's.

Figure 3.32 Correlation between CO2 levels and global mean temperature.



To understand how astronomical effects can influence the Earth's climate