| Pre-human GlobalWarming? |
| |
The Earth has experienced natural global warming and cooling many times in the past, and can offer useful insights into present processes. It is thought by some geologists that a rapid buildup of greenhouse gases caused the Earth to experience global warming in the early Jurassic period, with average temperatures rising by 5 °C (9.0 °F). Research by the Open University published in Geology (32: 157–160, 2004 [33]) indicates that this caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, carbon dioxide levels dropped back to normal over roughly the next 150,000 years.
Sudden releases of methane from clathrate compounds (the Clathrate Gun Hypothesis) have been hypothesized as a cause for other past global warming events, including the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last glacial period is thought not to be due to methane release [34]. Instead, natural variations in the Earth's orbit (Milankovitch cycles) are believed to have triggered the retreat of ice sheets by changing the amount of solar radiation received at high latitude and led to deglaciation.
The greenhouse effect is also invoked to explain how the Earth made it out of the Snowball Earth period 600 million years ago. During this period all silicate rocks were covered by ice, thereby preventing them from combining with atmospheric carbon dioxide. The atmospheric carbon dioxide level gradually increased until it reached a level that could have been as much as 350 times the current level. At this point temperatures were raised enough to melt the ice, even though the reflective ice surfaces had been reflecting most sunlight back into space. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere, and thick layers of abiotic carbonate sediment have been found on top of the glacial rocks from this period.
Using paleoclimate data for the last 500 million years Veizer et al. (2000, Nature 408, pp. 698–701) concluded that long-term temperature variations are only weakly related to carbon dioxide variations. Most paleoclimatologists believe this is because other factors, such as continental drift and mountain building have larger effects in determining very long term climate. However, Shaviv and Veizer (2003, [35]) proposed that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy, and the ways in which this influences the atmosphere by altering the flux of cosmic rays received by the Earth. Afterwards, they argued that over geologic times a change in carbon dioxide concentrations comparable to doubling pre-industrial levels, only results in about 0.75 °C (1.3 °F) warming rather than the usual 1.5–4.5 °C (2.7–8.1 °F) reported by climate models [36]. They acknowledge (Shaviv and Veizer 2004) however that this conclusion may only be valid on multi-million year time scales when glacial and geological feedback have had a chance to establish themselves. Rahmstorf et al. 2004 [37] argue that Shaviv and Veizer arbitrarily tuned their data, and that their conclusions are unreliable.
18 Nov
2006 |
| |
| Top Sites for "GlobalWarming Awareness2007" Yahoo |
| |
-
www.global-pink-hat-warming.org.uk
-
del.icio.us/tag/globalwarming
-
earth-globalwarming-awareness2007.com
-
www.championcheap.com
-
api.moreover.com/ping
-
www.seoworldchampionship.com
-
www.kevinjb.com
-
www.bookner.com
-
www.globalawareness.biz
-
chanstarwars.free.fr/globalwarming-awareness2007.html
|
| |
| Top Sites for "GlobalWarming
Awareness2007" MSN |
| |
-
itsglobalwarmingawareness2007.blogspot.com
-
globawarmingawareness.blogspot.com
-
www.2007globalwarming.com
-
www.globalwarmingawarenessblog.com
-
www.globalwarming--awareness2007.info
-
www.jerkasmarknad.com/contest/archives/3
-
globalwarmingxawareness2007.com
-
www.2007globalwarmingawareness.co.uk
-
globalwarmingawareness07.spaces.live.com/
friends
|
| |
| Top Sites
for "GlobalWarming Awareness2007" : Google |
| |
-
www.global-warming-awareness2007.org/
-
globalwarmingawareness2007.org.uk/
-
www.championcheap.com/
-
www.globalwarmingawareness2007contest.com
-
www.e-globalwarming-awareness2007.com
-
www.seoworldchampionship.com/
-
www.pink-globalwarming-awareness2007.com
-
www.globalwarmingawareness2007.be/
-
www.globalwarming-awareness2007.com/
|
