Exploring Climate Events and Human Development
Past 100 Years: Putting the 20th Century
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."
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
Institute for Public Health and the Environment (RIVM) in the Netherlands.)
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.
shows multiple reconstructions of temperature anomalies from Mann,
et. al. 2002.
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History 100 Years.