The earth is unique - the only planet where complex life (like us) could survive. We now look at how that life arose and whether something similar could happen on planets around other stars.

Long Term Climate

"Climate is what we expect, weather is what we get." - Mark Twain

Key points: Multiple factors that cause climate change; current conditions in long term perspective

The atmosphere and climate of Earth are the key to life here. To set the stage for understanding the origins of life on our planet, we discuss its climate in more detail.

The energy output of the Sun is incredibly constant and thus any changes are too small to have a significant effect on the climate.

The total output of the sun (= "solar irradiance") is so constant, it was not possible to measure variations until recently (notice how expanded the scale is to the left -- the total change in the average shown by the black line is only about 0.1%). The largest changes are associated with the solar cycle, with abrupt (still very small) changes due to sunspots. The steadiness of the sun contrasts with the long-term trends we see in climate change. http://serc.carleton.edu/images/introgeo/teachingwdata/Solar.jpg

Sunspot numbers for the last 400 years document the solar cycle, including the minimum from 1640 to 1700.

There are climate effects associated with the solar cycle that are not directly from a change in the output of the sun, but which we do not understand well. For example, the minimum in solar activity between 1640 and 1700 coincides with a widespread global cooling, the "Little Ice Age" . As we shall see, the Little Ice Age was a relatively small change.

In comparison, huge changes in average temperature have occurred over time scales of tens of millions of years. These major changes appear to be in response to large changes in the amount of carbon dioxide in the atmosphere, which at some times was two to three times the current level. We are currently coming out of a cool period, one of the major glacial ages.

from Page Paleontological Science Center, http://www.lakepowell.net/sciencecenter/paleoclimate.htm:

There are four general categories of significant global effects on the weather:

1. Evolution of the sun - Although the sun has been remarkably constant, we cannot forget that over billions of years, its output changes significantly. Over the last 4 billion years, it has gotten about 25% more luminous due to normal stellar evolution, as it ages on the main sequence. Although that change might indicate that it was substantially colder on Earth 2 - 4 billion years ago, most scientists think that the effect was counteracted by a stronger greenhouse effect than now caused by greater volcanic activity and release of more carbon dioxide. It appears that the surface temperature has been very constant on average, with the fluctuations discussed below around that average.

2. Long Term Climate Changes (millions of years) - Possibly associated with plate tectonics and its influence on the atmospheric greenhouse effect. Over the time scale of 300 million years (back to the last known Great Ice Age - the Gondwanan), the continental plates have moved greatly. Of course, these motions affect the climate in any one "place" that may have been moved to a new location. In addition, overall temperatures changed because of changes in the natural production rate of carbon dioxide. CO₂ is produced in volcanoes and in the mid-ocean trenches. It is lost by being slowly absorbed in the oceans. The periods with a high overall temperature probably had a lot of greenhouse warming due to volcanic carbon dioxide!

During periods of very active plate movements, carbon dioxide may be produced by a faster rate of sea-floor spreading and a subsequent increase in volcanism.   The enriched CO2 content of the atmosphere may promote global warming. Similarly, global cooling may result from stalled or slowed spreading from Page Paleontological Science Center, http://www.lakepowell.net/sciencecenter/paleoclimate.htm

In addition, the placement of the continents affects ocean currents. The current placement of largely north/south continents causes currents to flow from the warm equatorial regions to the north and moderate the weather in the north.

3. Medium-Term Climate Changes (many thousands of years) -

The best clue for explaining these changes comes from a consideration of the Milankovitch cycles, changes in the orbital characteristics of the Earth. The basic premise of the theory is that, as the Earth travels through space, three separate cyclic movements combine to produce variations in the amount of solar energy falling on the Earth. (From Scott Rutherford, http://deschutes.gso.uri.edu/~rutherfo/milankovitch.html) Due to the interactions with other planets, the eccentricity of the earth's orbit, the tilt of its axis, and the direction of that tilt relative to its place on the orbit all change over periods of 19 to 100 thousand years (k.y. in the illustrations to the left). For example, at some times the northern hemisphere has summer when it is closest to the sun because of the ellipticity of the orbit, and if the tilt of the axis is also at a maximum, all the effects that make the summers hot add up. At other times, northern summer can be when the earth is farthest from the sun on the orbit and the eccentricity of the orbit can be large, making summers cooler.

Predictions of these cycles still vary a bit due to the mathematical complexity. Here is one that shows changes of about 20% in the sunlight received on the same time and same day at 65^o north latitude. The plot begins about 260,000 years ago and extends to 700,000 years in the future. It appears that we can look forward to 50,000 years of very steady weather, at least from the perspective of Milankovitch cycles (as shown by the horizontal arrow)!

from Jan Hollan, http://amper.ped.muni.cz/gw/articles/html.format/orb_forc.html

The Milankovitch cycles may help explain the advance and retreat of ice over periods of 10,000 to 100,000 years. They do not explain what caused the Ice Age in the first place; that appears to depend on the long-term changes.

4. Shorter-term climate fluctuations in our present-day interglacial period

So far in the past 18,000 years, the earth's temperature has risen approximately 9 degrees F (5 degrees C) and the sea level has risen 300 feet:

18,000 years ago - The climate begins to warm 15,000 years ago - Advance of glaciers halts and sea levels begin to rise 10,000 years ago - Ice Age megafauna goes extinct 8,000 years ago - Bering Strait land bridge becomes drowned, cutting off migration of men and animals. 6,000 years ago - The Halocene Maximum warm period from Page Paleontological Science Center, http://www.lakepowell.net/sciencecenter/paleoclimate.htm:

The changes are thought to arise from effects like:

• The amount of dust in the atmosphere
• The reflectivity of the ice sheets
• The concentration of greenhouse gases
• The changing characteristics of clouds

From this perspective, the Little Ice Age is totally insignificant, although it was enough to cause substantial disruption of human activity.
In summary, what drives the climate

Although the problem is very complex, here is a prediction of where we may be going, compared with where we have been. On top of the warming trend driven by carbon dioxide, some look at the recent tendency for ice ages and predict in the long run we may go into another one  The earth will survive -- but we may not, since climate change is one of the major drivers of mass extinctions and biological change. from Page Paleontological Science Center, http://www.lakepowell.net/sciencecenter/paleoclimate.htm:

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