Climate change is not new to the 20th century. The holocene maximum and little ice age feature prominently in some proxy data of the past 10,000yrs. The little ice age is sometimes associated with a dip in the sun spot frequency, known as the Maunder minimum. The warming in the earlier part of the 20th century is also sometimes linked to the sun spot record. There is not widespread agreement about either of these linkages though. 

If we go even further back in time, we must consider other factors that alter Earth's climate such as the location of the continents. Solar luminosity has grown significantly on timescales of billions and hundreds of millions of years. I believe I forgot to say that natural variations in Earth's orbit about the sun also influence the solar flux on Earth. We will come back to the issue of changes in the solar flux later in the course.

I covered the mass extinction (including dinasours) that occured 65million yearago. It is well described in the text.

Contemporary global changes caused by humans

ecosystem destruction - examples on a global scale are deforestation and fish population decline

Some facts about deforestaion:

• Deforestion occurs at a rate of 150,000 square km per year worldwide
• Tropical forests once occupied 16 million square km on Earth's surface but now only cover 9 million.
• Latin American and Asia have already lost about 40% of ther original forests, Africa a little more than half.
• In many countries the rate of deforestation is accelerating. Most of the forested areas of Bangladesh, India, the Phillippines, Sri Lanka and parts of Barazil's rain forests could be gone by the end of the century. 
• Only in the Congo Basin and some of the more isolated areas of the Amason Basin does the rain forest remain largely intact.

• Species loss - The majority of Earth's species live in the tropics. The majority of the species in the tropics have very limited ranges, increasing the importance of small patches of land for overall biodiversity.
• Local climate change - The loss of forests alters precipitation, cloud patterns, and surface winds
• Global carbon cycle - Burning forests releases carbon dioxide into the atmosphere, contributing to the increase in greenhouse gas concentrations.
  

Declining fish populations are largely attributed to fishing practices, although some evidence indicates that climate is a factor too. Fishing in the last half a century almost steadily increased. In the past decade about 20% of fish consumed were raised in farms. While this may hold promise for reducing the loss of wild fish, the practise is sometimes controversial.

ozone destruction - ozone exists in the troposphere (roughly the lowest 10-20 km of the atmosphere) and the stratosphere (just above the troposphere). Ozone in the stratosphere shields the surface from ultraviolet radiation. Ozone in the troposphere is a pollutant coming from industrial and automobile emissions (and some natural sources). Ozone is highly reactive and it damages humans when it reacts with our bodies. Ozone in the stratosphere, where the air is less dense and particulate matter is rare, has a longer lifetime. There are special conditions that occur in the stratosphere over Antarctica that give rise to efficient destruction. Ozone concentrations over Antarctica are naturally lower than elsewhere on the planet.

The presence of CFCs in the atmosphere enhance the natural destruction of ozone over Antarctica. in the last few decades, the ozone hole over Antarctica has grown to the size of North America. Concentrations of ozone in the hole are about 1/3 of normal for several months each year. When the hole collapses each year, ozone from the rest of the globe fills in the hole and leaves the rest of the globe slightly depleted. Scientists think concentrations outside of Anarctica (and the Arctic) are only about 10% lower than normal.

CFC have a very long lifetime in the atmosphere, so even though CFCs are severely restricted in many countries, like the US, the ozone hole will be with us for decades to come.

global warming - A definition was given in the notes from lecture yesterday. The most important greenhouse gas that humans alter appreciably at present is carbon dioxide.

Carbon dioxide concentrations in Earth's atmosphere have been measured since 1958 in Hawaii. They were begun by Richard Keeling and now the measurements from that site form a curve that is commonly called the Keeling Curve. Concentrations have risen by about 50 ppm (parts per mass) from an original 315 ppm in 1958. We can compare this record to a much longer record of CO2 trapped in gas bubbles locked in the ice sheets of Antartica and Greenland. These bubbles give a high quality measurement of the concentration, although identifying the point in time (or range of time) that the data correspond to is subject to some uncertainty. Nonetheless, the gas trapped in bubbles show that the recent concentrations are considerably higher than any time in the past nearly 100 thousand years.