climatologists have been able to reconstruct paleoclimatic periods over the Earth's geologic past. The following list summarizes the major climate change episodes.
Ancient Climates—2 MYA to 4.6 BYA
Faint Young sun Paradox snowball earth Hothouse Earth
The Pleistocene Climates—10,000 years to 2 MYA
Penultimate Interglacial Period Dansgaard-oeschger events Heinrich events
Holocene/Recent Climate—10,000 years ago to today
Younger dryas cooling Mid-Holocene Thermal Maximum Late Holocene Neoglacial fluctuations The little ice Age
As scientists better understand the climatic events from the past, they are better able to apply their knowledge to the complexities of the climate system today and in the future in an effort to deal with the issues of global warming.
experts disagree, citing instead the fact that humans are causing an accelerated greenhouse effect through the burning of fossil fuels and other carbon-emitting activities.
The earliest-kept records of temperature measured officially by thermometers began in western Europe in the late 1600s. As more weather reporting stations came into existence and began recording official temperatures, by the early 1900s temperature networks had been established nearly worldwide. The exceptions were the polar regions. Collections began there in the 1940s and 1950s. Today, NOAA compiles the records from a climatology network of more than 7,000 stations worldwide so that studies can be made.
As this chapter has illustrated, the climate over the past several hundred thousand years has had a number of series of cold and warm periods. Today, the Earth is in a relatively calm period following the last ice age, but the temperature is slowly rising and at present-day levels passing earlier record high temperatures—this time due to human influence.
Although scientific understanding has progressed and dating techniques have improved, allowing scientists to increase their understanding about the Earth's past climates and how the climate system not only varies naturally but also responds to changes in climate forcing, key uncertainties still exist about the Earth's ancient climate. Even though scientists have a good understanding of the glacial-interglacial variations in climate and greenhouse gases, they recognize that there are still unanswered questions as to the exact factors that control these changes. According to the IPCC, climatologists need a better understanding of what causes abrupt climate change as well as a deeper knowledge of the main thresholds that can trigger, when they are crossed, rapid sea level rise and regional climate change.
Another area that needs improvement is climate modeling, particularly the simulation of abrupt changes in ocean circulation, flood frequency, drought cycles, monsoon behavior, the frequency of El Niño events, and the processes that control the advance and retreat of ice sheets.
Specific issues that present problems in recreating the past are that there are not enough paleoclimate records from the Earth's tropical areas, Southern Hemisphere, and oceans. Along with this, it would be beneficial to have data from areas evenly distributed over the Earth's surface, enabling climatologists to gain a more global view rather than a sporadic regional view, whereby large geographic areas must be interpolated. Experts believe if they could collect a larger global paleoda-tabase, they could compare the changes contained within it with the changes being seen today on a global scale and gain insights on how today's inputs are affecting the climate system in an unnatural way and how humans are upsetting the natural balance.
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