Is the Nightime Warming in Arizona Due to Global Warming?

A news release on March 27, 2006 by Tony Davis of the Arizona Daily Star discusses the observed increase in winter nightime temperaturres in Arizona over the past 70 years. The article is generally well written, but it does not present the warm bias in minimum temperatures that we have discussed several times on this weblog (e.g see).

The article, which is headlined “Ariz. temps on the rise in winter for last 70 years” includes the text,

“Winter nights have warmed significantly across Arizona over 70 years, raising questions about whether human-caused global warming is part of the cause, said a University of Arizona researcher.

From 1931 to 2001, average wintertime low temperatures rose by as little as 0.03 a of a degree per decade in Safford to as much as 1.11 degrees in Mesa, according to the UA-analyzed data. ”

As shown on Climate Science, however, any climate change that results in less cooling to space at night (such as a long term trend in increased cloud cover at night, greater air pollution, and/or greater water vapor in the air overhead) necessarily results in an amplified temperature increase at the surface. The radiative forcing of the added well-mixed greenhouse gases certainly can reduce cooling to space, but these other climate forcings also need to be considered.

As just one example, the role of aerosols in altering minumum temperatures was discussed in the Climate Science weblog of March 19, 2006 (see). An extract from the peer reviewed paper on this subject stated in part,

““A regional coupled climate-chemistry-aerosol model is developed to examine the impacts of anthropogenic aerosols on surface temperature and precipitation over East Asia. Besides their direct and indirect reduction of short-wave solar
radiation, the increased cloudiness and cloud liquid water generate a substantial downward positive long-wave surface forcing; consequently, nighttime temperature in winter increases by +0.7°C, and the diurnal temperature range decreases by -0.7°C averaged over the industrialized parts of China.”

With the urban growth in Arizona and the large amounts of aerosols transported into the state from industrial activities in adjacent areas of Mexico, the aerosol influence is also likely to be important in this region.

Thus, to use the observed surface temperature increase at night as support that the radiative forcing of CO2 and the other well-mixed greenhouse gases dominates climate variabiltiy and change in Arizona needs further scrutiny.

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