Millions of Years Ago

3.3. The combined effect of changes in the eccentricity of Earth's orbit and in its slow wobbling motion cause changes in the amount of solar radiation received at low and middle latitudes at a 22,000-year cycle, with the size of the cycles varying at 100,000 years.

6 percent farther away at the other extreme compared to a perfectly circular orbit. Precession makes these inequalities in Earth-Sun distance shift slowly around the eccentric orbit so that over thousands of years they occur during different seasons. As a result, the solar radiation received during each season varies through time.

Fortunately, all of these complicated interactions of eccentricity and precession are neatly summarized in the single curve shown in figure 3.3. The effects of this signal on solar radiation are felt at all latitudes, except for regions lying in the darkness of the winter polar night. So, in the end, it all comes down to two wiggly lines. Changes in tilt at a cycle of 41,000 years affect solar radiation at high latitudes (fig. 3.1). And all latitudes on Earth are affected by the 22,000-year cycle of precession, with its eccentricity multiplier effect at 100,000 years (fig. 3.3).

Because these changes in Earth's orbit are relatively small, they might seem unlikely to have much effect on the amount of solar radiation received at any location, but this is not so. For any particular season of the year and location, changes in solar radiation can vary by more than 10% around the average value. This range of variation is larger than the present-day difference in radiation between Quebec and Atlanta, or between London and Lisbon.

For decades many scientists resisted the hypothesis of orbital control of climate. One counterargument was that changes in solar radiation at any location on Earth in any particular season are invariably opposed by the opposite sense of change in the opposite season. If extra radiation arrives in summer, then winters at that same spot will have a deficit of about the same size. The same thing is true for the hemispheres. When Northern Hemisphere summers are receiving extra radiation, Southern Hemisphere summers are receiving a deficit of about the same size. This opposing sense of seasonal and hemispheric radiation trends results from the way our planet moves between the opposite extremes in its orbit through the year. If Earth is unusually close to the Sun in one season, it will inevitably be unusually far away during the opposite season on the other side of its eccentric orbit. And if Earth leans toward the Sun in one season, it must lean away from the Sun six months later, during the opposite season. Over the course of a full year, and over the entire planet, these changes cancel out and leave no significant difference in the amount of radiation received by the planet as a whole. So, for seemingly valid reasons, the critics concluded early on that Earth's net response to long-term differences in solar radiation would be trivial.

But these early critics overlooked something important about the climate system. They were thinking of a homogeneous Earth that reacts in exactly the same way everywhere. This viewpoint made no allowance for the complexity of the climate system, or for the different ways its many components might respond. An obvious example is the distribution of land and ocean. Most of the Southern Hemisphere is ocean, and oceans tend to moderate, rather than amplify, changes in climate. Large amounts of incoming solar radiation are stored in the oceans, but in a layer thick enough that the changes in temperature right at the surface of the ocean are actually modest. In contrast, the Northern Hemisphere has the world's largest continents (Eurasia, North America, and North Africa), and land surfaces are far more reactive to climatic pushes than the ocean. When solar radiation heats land surfaces, the heat can penetrate only a short distance into the sediment or rock, so the land heats up more quickly, and to a far larger extent, than the ocean.

These fundamentally different behaviors are evident in the relative response of oceans and land masses to the changes in solar heating during our present-day seasons. Maximum heating in the Northern Hemisphere occurs at the summer solstice (June 21), and the interiors of the northern continents reach maximum temperatures a month later, during July, yet the Northern Hemisphere oceans do not reach their seasonal maxima until late August or early September. In addition, the land in the center of large continents seasonally warms up 5 to 10 times as much as do the oceans at the same latitude. In winter the land also cools off faster than the ocean, and by a far larger amount. For these reasons, the land-rich Northern Hemisphere does not respond in the same way as the ocean-dominated Southern Hemisphere. The critics failed to allow for differences like these.

Earth's surfaces also vary widely in the amount of flat terrain versus mountains, deserts versus forests, and sea ice versus open (ice-free) water. These differences in surface characteristics also produce different regional responses to changes in solar radiation. As we will see in the next two chapters, the components of the climate system that matter most in a particular region may react more strongly to the extra solar radiation arriving during one season than to the deficit of radiation that arrives during the opposite season. In such cases, the opposing seasonal radiation trends are not canceled out in the net response of the climate system. The critics of the concept that orbital changes could drive climate also failed to allow for this possibility.

Meanwhile, ignoring the critics, a few scientists were working at the task of trying to determine the specific mechanisms by which changes in Earth's orbit might drive changes in its climate. In 1875 James Croll of Scotland made preliminary calculations of changes in the amount of solar radiation received on Earth. In 1904 German mathematician Ludwig Pilgrim published a paper summarizing a decade of tedious work calculating the variations of Earth's orbit over the last three million years. Although Pilgrim made no attempt to tie these calculations to climate changes, his work was essential to efforts that followed.

Beginning in 1911, astronomer Milutin Milankovitch made a series of laborious hand calculations of the amount of solar radiation received by latitude and by season over the entire Earth during the last several hundred thousand years. His calculations continued in a jail cell where he was imprisoned by the Austrians during World War I and then afterward when he was paroled. He took into account the two factors that Isaac Newton had centuries ago shown to be the major controls on solar radiation: (1) the varying angle of incoming solar radiation relative to the surface of the Earth (the effect of tilt), and (2) Earth's distance from the Sun (the combined effects of eccentricity and precession). These laborious calculations, now done with much greater accuracy by computers in just minutes, laid the groundwork for many discoveries, the most important of which are covered in the next two chapters.

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