Professor Neville Nicholls, Monash University
Contrary to the impression you might have gained from the media, the global climate is NOT cooling. In fact, the last twelve months, June 2009 – May 2010, has been the hottest June-May period on record, in both the 31-year satellite record of lower atmosphere global temperature and the 131-year surface global temperature record. In both data series the last 12 months have been more than 0.4C hotter than the average temperature of the last two decades of the 20th century.
The figure below plots the time series of twelve-month (June-May) global mean temperature anomalies. The data in the figure are the Spencer-Christy lower atmospheric temperatures from satellites (labelled “UAH” in the figure) and the surface temperatures from the NASA Goddard Institute for Space Studies (labelled “GISS”). Both datasets are freely available (UAH from http://vortex.nsstc.uah.edu/data/msu/t2lt/uahncdc.lt; GISS from http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts+dSST.txt). Both datasets have been plotted as anomalies relative to 1979/80-1998/99, ie the first twenty years of the satellite observations.
Simply eyeballing the graphs of the surface and satellite temperature record should convince anyone that global warming never “stopped”. Fitting linear trends to the data since the start of the satellite observations produces virtually identical trends in the two data sets. Even the variations from year-to-year in the two temperature series are close matches.
The close match between the surface and satellite variations and trends confirms that the warming trend at the surface is NOT due to the urban heat island effect. Nor is it due to changes in the numbers of stations used in the surface analysis, or any problems with the locations of the surface instruments. None of these potential problems affects the satellite data, and the satellite data are completely independent of the surface data.
Nor is the warming due to the Sun getting stronger. Satellite measurements show that total solar irradiance has decreased since the start of the 21st century, and this would probably have caused some weak cooling rather than any warming (http://acrim.com/TSI%20Monitoring.htm).
Neville Nicholls is an Australian Research Council Professorial Fellow in the School of Geography and Environmental Science at Monash University, President of the Australian Meteorological and Oceanographic Society (www.amos.org.au), and an Executive Editor of Wiley Interdisciplinary Reviews: Climate Change (wires.wiley.com).