• Global average temperatures have increased about 1°C(1.7°F) since mid-20th century.

This may not seem much, but extreme hot weathers will be more common.

For comparison, at the end of the last ice age 10,000 years ago, global average temperatures were only 3°C(5.5°F) colder and Chicago was covered in ice 1 mile thick.

• Over any time span, there are local anomalies of warmer or colder than normal temperatures.

This anomaly diagram shows that the month of August 2011 was warmer than Augusts in the past.

This was the 142th consecutive month that the global land temperature has shown a warm anomaly.

• This anomaly chart shows a global warming trend over the past 130 years.

Though there were short-term fluctuations, temperatures over the last 30 years have been above the 20th-century average.

Human activity that increases atmospheric greenhouse gases is a major cause of such climate change.

• CO₂ acts as a greenhouse gas to trap heat from the sun.

Natural amounts of CO₂ has kept the earth warm to support life - as we know it.

Burning fossil fuels emits excess CO₂ to the atmosphere.

This enhanced greenhouse effect traps more heat and leads to global warming.

Methane is also a greenhouse gas and can trap heat 25 times as much as CO₂, but since much less methane is being released to the atmosphere, its current CO₂ equivalent contribution to global warming is only 14.3%.

• Albedo is a measure of the reflectivity of a surface, and varies among different materials.

Light-colored surfaces have higher albedo, and tend to reflect heat away from earth.

Dark-colored surfaces have lower albedo, absorbing more heat and releasing the heat into the atmosphere.

Melting glaciers reduces high-albedo ice surfaces, exposing darker water and ground.

This becomes a positive feedback loop: the lower-albedo surfaces absorb more heat, and ice melt accelerates.

• Earth's orbit around the Sun undergo periodic Milankovitch cycles.

• Orbit eccentricity: the shape of Earth's orbit varies between round and elliptical every 100,000 years.

  The current rounder orbit yields less variation in solar radiation as the Earth moves around the Sun; the Northern Hemisphere is closer to the Sun during winter.

• Axial tilt: the angle of the tilt as the Earth spins on its axis varies between 22.1° and 24.5° every 41,000 years.

  Larger tilt results in greater extremes between seasons - hotter summers and colder winters.

• Axial precession: the tilt of the Earth's axis of rotation "wobbles" in 360° cycles of 20,000 years.

  Our current tilt toward the North Star points the Northern Hemisphere toward the Sun during calendar summer to receive greater solar radiation.

These alterations of solar radiation affect climate on Earth over geologic time, triggering periods of cold glaciation and warm interglacial times.

The current global warming is not part of any of these cycles.

• Charles Keeling began collecting CO₂ data at Mauna Loa Observatory since 1958.

The atmospheric CO₂ concentrations correlate well with global air temperatures over the past half century.

Ice cores drilled in Antarctica contain air bubbles that can be analyzed for climate data.

The correlation between CO₂ levels and temperatures suggests a major role played by human activity.

• Ice core data from Vostok in Antarctica extend back 400,000 years.

These data reveal a correlation between CO₂ concentrations and temperatures spanning 4 glacial cycles.

The historical warming seemed to occur first, triggered by Milankovitch cycles.

The resulting ice melt started warming positive feedback loops with atmospheric CO₂ concentrations.

• Climate change may have global environmental consequences.

• melting of glacial ice and sea level rise

• changes in precipitation patterns and more severe storms

• The minimum sea ice extent in the Arctic Ocean for 2011 was much smaller than the past 30-year average.

This melting sea ice has opened up the Northwest Passage shipping lanes.

• Warmer temperatures will melt ice now locked up in glaciers.

As ice turns to water, sea levels rise, endangering coastal areas.

• Climate change can cause both drier and wetter weather - precipitation anomalies of drought and flood conditions within the same time span in different areas.

Rise in sea surface temperatures will also lead to more severe hurricanes in the Atlantic.