On May 9, 2013, the daily concentration of carbon dioxide in the atmosphere exceeded 400 parts per million (ppm) at the Mauna Loa monitoring station for the first time ever at least at one monitoring location according to the Scripps Institute of Oceanography.  A separate monitoring location on Mauna Loa measured 399.9 ppm.  This concentration is not a science-based threshold.  However, according to scientists at the Scripps Institute of Oceanography, carbon dioxide concentration may reach 450 ppm by 2040 if the current rate of carbon dioxide concentration increase continues.  A prior Blog Post discusses the long term trends in carbon dioxide concentrations at Mauna Loa.  Obviously, a number of climate change mitigation efforts are underway, others are likely to be initiated, and increased use of natural gas vis a vis coal is lowering GHG emissions in the U.S., so the current rate of carbon dioxide increase may not be predictive of future carbon dioxide ambient levels.  A prior Blog Post concerning recent trends in US carbon dioxide emission levels can be found here.

It is interesting that the 2007 Intergovernmental Panel on Climate Change (IPCC) recommended stabilizing ambient carbon dioxide equivalent levels at 445 to 490 ppm by 2050 to prevent average temperatures from rising above 2.0 to 2.4 degrees Celsius (3.6 to 4.3 degrees Fahrenheit), a stabilization level deemed necessary to avert severe climate impacts.  Of course, the recommended stabilization level (or at least the date by which it should be achieved) may change when the latest edition of the IPCC report on climate change is released later this year.

As a practical matter, the psychological milestone of 400 ppm is unlikely to have any effect on the probability of any dramatic new U.S. actions (particularly those which require legislation).   In fact, the United States has proposed at this year's international climate meeting that each country "define their own mitigation contributions, taking into account national circumstances, capacity, and other factors that they consider relevant."  These mitigation contributions would then be included in some form of international agreement.  Given the political deadlock over any ambitious climate change requirements, not only in the U.S., but worldwide, and the continuing fragility of the economic recovery, this U.S. proposal seems appropriately modest.

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