title: On avoiding dangerous anthropogenic interference with the climate system: formidable challenges ahead.
abstract: The observed increase in the concentration of greenhouse gases (GHGs) since the preindustrial era has most likely committed the world to a warming of 2.4 degrees C (1.4 degrees C to 4.3 degrees C) above the preindustrial surface temperatures. The committed warming is inferred from the most recent Intergovernmental Panel on Climate Change (IPCC) estimates of the greenhouse forcing and climate sensitivity. The estimated warming of 2.4 degrees C is the equilibrium warming above preindustrial temperatures that the world will observe even if GHG concentrations are held fixed at their 2005 concentration levels but without any other anthropogenic forcing such as the cooling effect of aerosols. The range of 1.4 degrees C to 4.3 degrees C in the committed warming overlaps and surpasses the currently perceived threshold range of 1 degrees C to 3 degrees C for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer arctic sea ice, Himalayan-Tibetan glaciers, and the Greenland Ice Sheet. IPCC models suggest that approximately 25% (0.6 degrees C) of the committed warming has been realized as of now. About 90% or more of the rest of the committed warming of 1.6 degrees C will unfold during the 21st century, determined by the rate of the unmasking of the aerosol cooling effect by air pollution abatement laws and by the rate of release of the GHGs-forcing stored in the oceans. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO(2) mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4 degrees C.
authors.affiliation: Scripps Institution of Oceanography, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0221, USA. vramanathan@ucsd.edu
authors.forename: V
authors.initials: V
authors.lastname: Ramanathan
authors.forename: Y
authors.initials: Y
authors.lastname: Feng
mesh_terms: D000336:Aerosols / Q000009:adverse effects; D002245:Carbon Dioxide / Q000378:metabolism; D003247:Conservation of Natural Resources / Q000379:methods*; D004787:Environmental Pollution / Q000517:prevention & control; D005544:Forecasting; D005579:Fossil Fuels; D017752:Greenhouse Effect; D006802:Human Activities; D013696:Temperature
chemical_list: D000336:Aerosols; D005579:Fossil Fuels; D002245:Carbon Dioxide
references.citation: Science. 2005 Mar 18;307(5716):1766-9
references.pmid: 15774756
references.citation: Proc Natl Acad Sci U S A. 2000 Nov 21;97(24):13286-93
references.pmid: 11087870
references.citation: Science. 2007 Oct 26;318(5850):629-32
references.pmid: 17962560
references.citation: Science. 2005 Mar 18;307(5716):1769-72
references.pmid: 15774757
references.citation: Proc Natl Acad Sci U S A. 2008 Feb 12;105(6):1786-93
references.pmid: 18258748
references.citation: Proc Natl Acad Sci U S A. 2005 Nov 1;102(44):15728-35
references.pmid: 16150711
references.citation: Proc Natl Acad Sci U S A. 2005 Apr 12;102(15):5326-33
references.pmid: 15749818