The above figure shows a picture of warmer than average land surface temperatures almost everywhere. This image is from the NOAA report
It is created, as described in the NOAA article, as a
NOAA map by Dan Pisut, based on Global Historical Climatology Network data from the National Climatic Data Center (NCDC). Caption by Susan Osborne, NCDC. Reviewed by Jessica Blunden, NCDC Climate Monitoring Branch.
However, while it certainly shows a very warm period at the surface, there are caveats in this analysis:
1. The data is not as dense or as uniform as presented in this figure; eg. see the figure below
source of image from climanova.wordpress.com
Large land areas are dependent on just a few or no surface observing sites.
2. While the lower tropospheric data shows a very warm May, it is not as anomalous as at the surface as diagnosed by the Global Historical Climatology Network. The spatial map of lower tropospheric temperatures for May 2012 is shown below
In this data, May 2012 has a global composite lower tropospheric temperature anomaly of +0.29 C (about 0.52 degrees Fahrenheit) above 30-year average for May. The NOAA plot above has a global composite of “more than 1°F above the 20th century average” according to the NOAA article.
3. This divergence between the surface temperature analysis and the lower tropospheric temperature analyses is further demonstration of the divergence between these two data sets as we reported on in
Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2009: An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841.
Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2010: Correction to: “An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841″, J. Geophys. Res., 115, D1, doi:10.1029/2009JD013655.
In a paper in press, which I will post on soon, we show that there is a warm bias in the minimum land temperatures which are used to create the land temperature anomalies that are presented in the NOAA GHCN figure.
4. The reason that the surface temperature anomaly is so much larger than higher in the troposphere (for the mid tropospheric anomalies, see NOAA CPC) appears to be related to the exceptionally dry soil conditions across much of the USA, as shown for May 2012 below from NOAA’s Climate Prediction Center.
As we discussed in our paper
Pielke, R.A. Sr., K. Wolter, O. Bliss, N. Doesken, and B. McNoldy, 2007: The July 2005 Denver heat wave: How unusual was it? Nat. Wea. Dig., 31, 24-35
when the effect of temperature and humidity are combined (moist enthalpy), this provides a markedly different perspective, than using temperature alone. In the July 2005 heat wave discussed in the above paper, the Denver heat wave was less extreme using this combined metric, due to very low humidity accompanying the event. This is also a major factor in the current heat wave.
The mid-tropospheric anomalies for the past 15 days from the University of Albany is presented below which further documents that the tropospheric anomalies over most of the land areas, including the USA, are much less than at the surface.
Thus, the conclusion regarding the NOAA GHCN analysis and the news report based on it is neglects to also report that the same magnitude of anomaly does not exist higher in the troposphere. Thus the reason for the warm surface temperatures needs further explanation. In this post I pose that the larger surface temperature anomaly is due to
- the concurrent occurrence of dry soils results in a larger fraction of solar radiation being converted to sensible heat (i.e. measured by the dry bulb temperature) rather than into latent heat fluxes (evaporation and transpiration). This results in a higher dry bulb temperature than it otherwise would be. It is correct, however, that, based on the lower tropospheric temperature analyses, that most land areas appear to be warmer than average for May, but the surface anomalies are significantly larger.
- the surface data also, however, contains an effect of local microclimate changes that results in a local elevation of the nighttime minimum temperatures from what this temperature would have been in the past. The grid-averaging and homogenization algorithms used by NCDC smear this warm effect (which may be applicable only to a very small location) over large areas. This will be discussed further when our new paper is reported on. This is an issue in addition to the siting quality question in the study led by Anthony Watts that we reported on it Fall et al 2011.
- the use of the GHCN as a diagnostic for the magnitude of global warming has a number of major complications including the neglect of concurrent surface anomalies in water vapor, siting quality issues, and local microclimate effects at GHCN sites which are inappropriately extrapolated over large regions.