Exploring Climate Events and Human Development
The Past 100 Years: Putting the 20th Century in Perspective

How does the 20th century compare with previous centuries? We know carbon dioxide in the atmosphere has increased significantly during the 20th century and that population has grown doubled and then doubled again. But is the .6 degrees Celsius of warming of the Earth's surface that has occurred during the 20th Century necessarily caused by human activity? Couldn't it be within the range of natural variability? Increasingly the answer appears to be "no." As the IPCC Synthesis Report states, "There is new and stronger evidence that most of the observed warming observed over the last 50 years is attributable to human activities." (IPCC, 2001.)

In addition to the increase in carbon dioxide and other greenhouse gases from human activities, there have been considerable changes in land cover and use over the past several centuries, but particularly during the 20th Century. (See landuse animation (275k) from the National Institute for Public Health and the Environment (RIVM) in the Netherlands.)

The figure below, using data from the National Climatic Data Center, shows the observed temperature changes as well as what would be expected in terms of warming and cooling from climate system forcing from natural sources.

In order to put the 20th century in perspective and understand what lag time there may be between higher levels of carbon dioxide and resulting higher temperatures, records longer than 100 years are required.

Figure A shows observed temperature changes based on NCDC data. Figure B projects estimated range of natural climate variability during 20th century. Image from Ruddiman, 2001 used by permission of W. H. Freeman & Co.

Figure below shows multiple reconstructions of temperature anomalies from Mann, et. al. 2002.

Return to Climate History 100 Years.