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3. Human Caused Forcings
Pages 9-14

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From page 9...
... are very potent and nearly inert greenhouse gases with atmospheric lifetimes much longer than 1000 years. The natural atmosphere contained many greenhouse gases whose atmospheric concentrations were determined by the sum of the ongoing geophysical, biological, and chemical reactions that produce and destroy them.
From page 10...
... A major removal process depends on the transfer of the carbon content of near-surface waters to the deep ocean, which has a century time scale, but final removal stretches out over hundreds of thousands of years. Reductions in the atmospheric concentrations of these gases following possible lowered emission rates in the future will stretch out over decades for methane, and centuries and longer for carbon dioxide and nitrous oxide.
From page 11...
... There is no definitive scientific basis for choosing among several possible explanations for these variations in the rates of change of global methane concentrations, making it very difficult to predict its future atmospheric concentrations. Both carbon dioxide and methane were trapped long ago in air bubbles preserved in Greenland and Antarctic ice sheets.
From page 12...
... However, the increased burning of fossil fuels and the increased burning of biomass for land clearing may result in increased black carbon concentration globally. Sulfate The precursor to sulfate is sulfur dioxide gas, which has two primary natural sources: emissions from marine biota and volcanic emissions.
From page 13...
... Black carbon (soot) aerosols absorb sunlight and, even though this can cause a local cooling of the surface in regions of heavy aerosol concentration, it warms the atmosphere and, for plausible atmospheric loadings, soot is expected to cause a global surface warming.
From page 14...
... In the future, if greenhouse gases continue to increase rapidly while aerosol forcing moderates, solar forcing may be relatively less important. Even in that case, however, the difference between an increasing and decreasing irradiance could be significant and Other potentially important climate forcings include vol- affect interpretation of climate change, so it is important that canic aerosols, anthropogenic land use, and solar variability.

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