“If we are to understand the real state of the world, we need to focus on the fundamentals and we need to look at realities, not myths.” Bjorn Lomborg, 2001
According to geological history the earth has been warming for about 18,000 years and over this period sea levels have risen over 100 metres.
While the overall temperature trend has been one of warming, there have been ups and downs due to natural climatic variations. So, if we consider the last 2,000 years of global temperature anomalies there was the Medieval Warm Period followed by the Little Ice Age and then a period of relative rapid warming during the 20th century, Figure 1.
Figure 1. 2,000 Years of Reconstructed Global Temperature Anomalies. Based on Loehle, C. 2007. Energy & Environment 18(7-8): 1049-1058. Thermometer data was added for 1850 -2007 by Roy W Spencer. From more information see http://www.weatherquestions.com/Roy-Spencer-on-global-warming.htm
It was not until the development of the thermometer that temperatures could be measured with accuracy.
The Central England Temperature Series is considered the world’s longest series; the monthly mean begins in 1659, during the Little Ice Age.
The Climate Research Unit (CRU) at the University of East Anglia in conjunction with the Hadley Centre of the UK Meteorology Office provide global temperature data going back only as far as 1850. This information is updated on a monthly basis.
It is this data set that is used by the Australian Bureau of Meteorology for global temperature trends, Figure 2.
Figure 2. The Global Temperature Anomaly for the period 1850 to 2007.
It is important to realize that Figures 1 and 2 show temperatures anomalies, not actual temperature. An anomaly is something that deviates from what is considered standard, normal or expected. The anomaly in Figure 2 shows the deviation from the mean temperature for the period 1961 to 1990.
We have become familiar with this representation of global temperatures but it is contrived to emphasis difference and in particular the extent to which temperatures have increased from 1850 to the present. When the same data is plotted just showing the actual global mean temperature for the same time period, the trend is no longer evident, Figure 3.
Figure 3. Global Mean Temperature from 1850 – 2007
The data from the Climate Research Unit (CRU) at the University of East Anglia in conjunction with the Hadley Centre of the UK Meteorology Office is generally accepted by those who subscribe to the idea that carbon dioxide is driving dangerous man-made global warming, as well as by the so-called climate skeptics. There are other organisations that collect information on global temperatures including the National Climatic Data Center (NCDC) in the US which claims to have the world’s largest archive of climate data.
Well known advisor to Al Gore, James Hansen, from NASA’s Goddard Space Institute has developed what is known as a GISS surface temperature analysis. This data set has shown more warming over recent years than the CRU data from the UK Meteorology Office and some argue that this is because the Hansen system overemphasizes temperatures at the North Pole.
There are ongoing arguments over the accuracy of the various data sets and methods of analysis. Ross McKitrick from Canada’s University of Guelph argues that 50 percent of global warming measured by land-based thermometers in the US since 1980 is due to local influences of man-made structures, also known as The Urban Heat Island Effect. When James Hansen recalculated temperatures in 2006 using a corrected algorithm, 1934 rather than 1998 was found to be the hottest in the last 100 years in the US. There have also been issues with the additions and losses of weather stations; for example many weather stations were lost in places like Siberia with the disintegration of the Soviet Union. Furthermore, thermometer temperature data has only been collected in the polar-regions since the 1940s and calculating the mean temperature at the poles is still difficult because of the sparseness of ground-based weather stations and the freezing environment takes its toll on equipment.
Bill Kininmonth, former head of Australia’s Bureau of Meteorology’s National Climate Centre, has suggested that because of the difficulty of assessing surface temperatures over ice surfaces it is more realistic to consider sea surface temperatures in places like the south pole and exclude areas of seasonal sea ice.
Dr Hansen has explained the general difficulty of measuring surface temperatures, “Even at the same location, the temperature near the ground may be very different from the temperature 5 ft above the ground and different again from 10 ft or 50 ft above the ground. Particularly in the presence of vegetation (say in a rainforest), the temperature above the vegetation may be very different from the temperature below the top of the vegetation. A reasonable suggestion might be to use the average temperature of the first 50 ft of air either above ground or above the top of the vegetation. To measure SAT (surface air temperature) we have to agree on what it is and, as far as I know, no such standard has been suggested or generally adopted. Even if the 50 ft standard were adopted, I cannot imagine that a weather station would build a 50 ft stack of thermometers to be able to find the true SAT at its location.”
Given these difficulties an alternative is to use temperature data from satellites.
Since 1979, Microwave Sounding Units (MSUs) on orbiting satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen with the intensity proportional to the temperature of vertical layers of the atmosphere. The research focus has been on getting a broadly representative measure of lower atmosphere temperature.
The satellite record of temperature in Figure 4, is from Roy Spencer at the University of Alabama in Huntsville (UAH) and corrects for previous errors including orbital drift. Dr Spencer is the US Science Team Leader for the Advanced Microwave Scanning Radiometer on NASA’s Aqua Satellite.
Figure 4. Monthly globally averaged lower atmospheric temperature anomaly since 1979 as measured by NOAA and NASA satellites. For more information see http://www.weatherquestions.com/Roy-Spencer-on-global-warming.htm
Figure 4 is another graph showing the temperature anomaly; in this case the deviation for the period 1979 – June 2008 from the mean temperature for the period 1979-1998.
Figure 3 is based on ‘Certainty clouds the IPCC’ by Sinclair Davidson and Alex Robson, IPA Review, March 2007, from page 7. You can read the article here: http://www.ipa.org.au/publications/898/certainly-clouds-the-ipcc