Greenhouse gases

Greenhouse gases

• Studies of show that carbon dioxide levels rise and fall with or after (as much as 1000 years) temperature variations This has been used to argue that the current rise in CO₂ is a result of warming and not a cause. While it is generally agreed that variations before the industrial age are mostly timed by astronomical forcing, the current variations, of whatever size, are claimed to be timed by anthropogenic releases of CO₂ (thus returning the argument to the importance of human CO₂ emissions). Analysis of carbon isotopes in atmospheric CO₂ shows that the recent observed CO₂ increase cannot have come from the oceans, volcanoes, or the biosphere, and thus is not a response to rising temperatures as would be required if the same processes creating past lags were active now
• Carbon dioxide accounts for about 383 parts per million by volume (ppm) of the Earth's atmosphere, increasing from 278 ppm in the 1880s to over 380 ppm in 2005.Carbon dioxide causes between 9 and 26% of the natural greenhouse effect.
• In the of the about 450 million years ago), the Earth had an atmospheric CO₂ concentration estimated at 4400ppm (or 0.44% of the atmosphere), while also having evidence of some glaciation. Modeling work has shown that it is possible for local areas at elevations greater than 300–500 meters to contain year-round snow cover even with high atmospheric CO₂ concentrationsA 2006 study suggests that the elevated CO₂ levels and the glaciation are not synchronous, but rather th associated with the uplift and erosion of the greatly reduced atmospheric greenhouse gas concentrations and permitted the observed glaciation.

As noted above, are only able to simulate the temperature record of the past century when GHG forcing is included, being consistent with the findings of the IPCC which has stated that: "Greenhouse gas forcing, largely the result of human activities, has very likely caused most of the observed global warming over the last 50 years"

The "standard" set of scenarios for future atmospheric greenhouse gases are the IPCC scenarios. The purpose of the range of scenarios is not to predict what exact course the future of emissions will take, but what it may take under a range of possible population, economic and societal trends. Climate models can be run using any of the scenarios as inputs to illustrate the different outcomes for climate change. No one scenario is officially preferred, but in practice the "A1b" scenario roughly corresponding to 1%/year growth in atmospheric CO₂ is often used for modelling studies.

There is debate about the various scenarios for fossil fuel consumption. Global warming skeptic stated that "some good experts believe" that atmospheric CO₂ concentration will not double since economies are becoming less reliant on carbon.