Record  High & Low Daily Temperatures further Evidence of Current Warming

             (Nov. 17)    For a few decades weather stations have been reporting more record-setting daily high temperatures than daily low temperatures in the United States.  A new study ¹ by Meehl (of the National Center for Atmospheric Research) and four other authors (from the Weather Channel, Climate Central, and the National Climatic Data Center) reveals that in ten years ending this Fall, twice as many record high temperatures as record low temperatures have been reported.  This is fairly strong evidence that the annual average temperature from 2000 to now has jumped above the averages of the 20^th century, for most locations in the US, and above the 30-year averages that make up the climate “normals” used to describe the climate of a particular location. (Climate normals now are based on averaged observations from 1971 to 2000.) 

             The authors looked into whether any logical or physical reason could explain this surprising ratio of 2 to 1. They found that this ration is temporary and coincidental.  It has been increasing for some time, and may attain 20:1 by the year 2050 and 50:1 by the end of this century, if temperature projections of the Intergovernmental Panel on Climate Change (IPCC) come to pass. This result, that record-breaking high temperatures become more frequent as record-breaking low temperatures become less frequent, is statistically expected when the observed average temperature increases as time goes on.

             The ratio in the western United States is now about 3:1. This agrees with the observation that climate has warmed more rapidly in the western than the eastern US.

             The ratio of high-to-low record temperatures has also increased in climate model simulations of US climate, but the ratio in the models (now 4:1) is already higher than in the observations, and is increasing faster. Since the models have been overestimating the rate of climate warming in recent years, these higher ratios are not surprising.

CITATION

1. “The relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S.” by G.A. Meehl, C. Tebaldi, G. Walton, D. Easterling, L. McDaniel (2009): In press, Geophysical Research Letters (accepted 20 October 2009), doi:10.1029/2009GL040736.

_______________________________________________________

Return to Main Page

More News Briefs

 Airborne Particles Multiply Climatic Impact of Greenhouse Gases

        (Nov. 23) Certain common gases interact with particles in the air to cause more warming or more cooling of the Earth than the gases or particles alone would cause, say Drew Shindell and five colleagues at NASA’s Goddard Institute for Space Studies. In a new published report¹, they write that airborne particles chemically interact with methane, ozone, and carbon monoxide in ways that magnify the warming that these gases cause. Similarly, nitrogen oxides are known to cool the earth, but when sulfur-containing particles are also present, the cooling is from two to five times greater than this gas alone can cause. Such particles cool the climate without the help of any gas (story on page one), but the effects of particles and gases are not simply adding up, but multiplying due to chemical reactions that generate new particles.

        The climatic impact of a gas is often measured by its “Global Warming Potential” (GWP), a measure that compares the ability of a gas to warm the atmosphere to  the warming that one kilogram (1 kg) of carbon dioxide causes. The GWP came to be used for the Kyoto treaty agreements and in carbon trading markets.    It is common to measure the global warming potential of a gas over a 100 year period. Obviously this underestimates the impact of transient gases and of all particles (which float in the air for mere days or weeks).  Nevertheless, Shindell reported the 100-year GWP for the three gases in keeping with standard practice.

        Among greenhouse gases, methane is the #2 contributor to warming (after CO₂ which is #1). Interactions with airborne particles magnify this warming by 10% to 15% when only the direct effect of particles on solar and thermal radiation is considered. As we have seen on page 1, particles also affect global temperature indirectly by aiding the formation of clouds. When this large indirect effect is considered, the warming is magnified 20% to 40% above the pure greenhouse warming of methane alone -- quite a bit larger than the calculation of the Intergovernmental Panel on Climate Change (IPCC), now used as a baseline for economic planning, carbon trading, and climate projections into the future.

        Other gases such as carbon monoxide also interact with particles and ozone so that their warming is magnified. On the other hand,   nitrogen oxides “cause substantial cooling on all time scales,” Shindell writes,  a cooling greatly magnified by airborne particles.  The global warming potential of methane is about as great as, but in the opposite direction from, the “global cooling potential” of nitrogen oxides. The direct effect of particles magnifies the cooling due to nitrogen oxides by two to five times, and the indirect (cloud) effects plus the direct effects magnify it by 10 times or more.

        It should be mentioned that the chemistry of nitrogen oxides in the air was so complex, and the effects so uncertain, that the IPCC did not estimate their global warming or cooling potential in 2007.        

        Though the authors do not mention this, greenhouse “cooling” by nitrogen oxides has not made the news largely because the gas is much less abundant than the well-known CO₂ and methane.  Nitrogen oxides, like ozone, carbon monoxide, and particles in the air, are local pollutants that trouble cities and regions with smog, smoke, and poor air quality. Shindell asserts that their global impacts are regarded as minor compared to their major local impacts on health, mortality, visibility, and quality of life. Local pollutants will be regulated more effectively by policies aimed at air pollution, he adds, rather than efforts to stabilize climate. But their global impacts have been underrated because each gas or pollutant is considered in isolation.

CITATION

1. “Improved attribution of climate forcing to emissions” by Drew Shindell and five other authors at NASA Goddard Institute for Space Studies, Science, vol. 326, 716-718,   30 Oct. 2009, doi:10.1126/science.1174760.

Return to Main Page       

More News Briefs