The Earth's climate during the past 1000 years
Global warming and cooling during the past 1000 years.

The average temperature of the Earth has varied during the past one thousand years. Grape growing and wine making were possible in England between 1100 and 1300--a time known as the Medieval Warm Period (MWP). The Vikings settled Greenland at this time. There were 3000 farmers in Greenland in 1300, but they died out during a time known as the Little Ice Age (LIA), which ran from 1350 to 1850. The Thames froze solidly during some winters in Shakespeare's time, an occasion for "Frost Fairs." The last Frost Fair occurred in 1814, the last time the Thames froze solidly.[1] The MWP and the LIA were global phenomena, although not all areas were equally affected. The modern warming trend started in 1850 and it too is a global phenomenon.

It is possible to estimate the actual temperature increases and decreases that occurred in the past one thousand years. The MWP was 1C or 2C warmer than the average temperature of the millennium. The LIA was 1C or 2C colder. The current warm era is about 0.75C warmer and thus not warmer than the MWP. Grape growing and wine making are still not possible in areas that supported it during the MWP. These temperature excursions, both positive and negative, are not very different from excursions occurring during the past 10,000 years.

Atmospheric carbon dioxide

Since carbon dioxide is the most significant greenhouse, it is natural to examine carbon dioxide levels during the past 1000 years. This graph shows what happened.

Carbon dioxide record

Prior to 1850, carbon dioxide concentration is in the range typical of a warm, interglacial period. Carbon dioxide concentration is perhaps a little higher during the MWP and a little lower during the LIA. The correlation is not very convincing. However, one thing jumps out. Carbon dioxide concentration started increasing decisively around 1850. By 1900, it is clear that a 400,000 year old pattern ended.

Data from Mauna Loa Observatory, Hawaii [3]

Here is a month by month record of the carbon dioxide in the atmosphere going back to 1958 and ending in 2002. 

Recent carbon dioxide record

The record looks like saw teeth because the amount of carbon dioxide in the atmosphere changes over the course of a year. Green plants absorb carbon dioxide during the summer when they are actively growing. There is more land area in the Northern Hemisphere and so carbon dioxide concentration decreases during the summer of the Northern Hemisphere. The carbon dioxide concentration peaks in March, just before the growing season starts in the Northern Hemisphere.

If you look closely, you will notice that there is a slight flattening starting in 1991 and lasting a year or so. Mount Pinatubo in the Philippines erupted in 1991 and released a large volume of sulfur dioxide. This reduced ocean surface temperature which increases the the solubility of carbon dioxide enough to be visible in this curve.

In March of 2004 Mauna Loa Observatory announced that carbon dioxide concentration had reached 379 parts per million, a fact noted in news sources all over the world.[4]  (This information has been added in green to the original graph.) The amount of carbon dioxide in the atmosphere has steadily increased since 1958. The rate of increase was 0.60 parts per million per year during the 60s, and currently it is 1.87 parts per million per year. (This information has been added in red to the original graph.) It is clear that the rate of increase is increasing. It is also clear that a 400,000 year old pattern has ended. These are alarming facts.

Atmospheric methane

Methane is the second most important greenhouse gas. The greenhouse effect of methane is 20 times greater than carbon dioxide, but there is less methane in the atmosphere. The net result is that methane contributes to about 25% of global warming. Like carbon dioxide, methane concentration started increasing decisively around 1850. Now methane concentration is at unprecedented levels. This is additional evidence that the 400,000 year old pattern has ended. This is an alarming fact.

Methane Record

Cows and termites produce methane. Agriculture, especially rice growing, produces methane. Anaerobic bacteria living in swamps and garbage dumps produce methane. Coal mining and oil wells produce methane. The amount of methane in the atmosphere has increased only slowly since 2000 although methane continues to enter the atmosphere at the same rate. No one has put forth a convincing explanation for this. 

The fact remains; the amount of methane in the atmosphere is twice as great than at any time during the last 400,000 years.

[1] http://en.wikipedia.org/wiki/Thames  Learn about the Frost Fairs on the Thames.

[3] http://www.mlo.noaa.gov/ The Mauna Loa Observatory of Hawaii has collected atmospheric data since 1958.

[3] http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm Carbon Dioxide Research Group, Scripps Institution of Oceanography, University of California, La Jolla, California.

[4] http://www.usatoday.com/weather/news/2004-03-21-co2-buildup_x.htm One  example among many.