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Thursday, 5 February 2009
Page: 670

Dr Jensen asked the Minister representing the Minister for Climate Change and Water, in writing, on 24 November 2008:

(1)   What are the four sources of global temperature records.

(2)   What temperature trend and absolute difference do they provide for the period:

(a)   December 1998 to the present; and

(b)   December 2001 to the present.

(3)   Which of the four:

(a)   uses (i) satellites, and (ii) land thermometers; and

(b)   are affected by the urban heat island effect.

(4)   What are the drawbacks of using land thermometers.

(5)   What are the advantages of using satellites.

(6)   What were temperature projections for the period:

(a)   December 1998 to the present; and

(b)   December 2001 to the present, in the Intergovernmental Panel of Climate Change’s third and fourth assessment reports (TAR and AR4, respectively).

Mr Swan (Treasurer) —The Minister for Climate Change and Water has provided the following answer to the honourable member’s question:

(1)   A number of organisations produce records of global average surface temperatures. The most prominent data sets are

  • HadCRUT3, produced by the Climatic Research Unit of the University of East Anglia and the Hadley Centre in the UK Meteorological Office;
  • NASA-GISS, produced by NASA’s Goddard Institute for Space Studies; and
  • NOAA-NCDC, produced by the National Oceanic and Atmospheric Administration’s National Climatic Data Centre (USA).

   Temperature estimates of layers of the atmosphere are also produced by several organisations. These include the Microwave Sounding Unit (MSU) temperature estimates produced by Remote Sensing Systems with support from NASA and the University of Alabama, USA. Weather balloon-based temperature analyses are produced by the Climatic Research Unit of the University of East Anglia and the Hadley Centre in the UK Meteorological Office as well as by NOAA-NCDC.

   (2)   These primary records show sustained warming trends from the surface to the lower layer of the atmosphere (troposphere) with cooling above.

(a)   In two of the data sets of global average surface temperatures, HadCRUT3 and NASA-GISS, the linear trend over the interval 1998-2008 is one of warming. Absolute differences between the temperatures in the specific years 1998 and 2008 for the HadCRUT3 and NASA-GISS data sets are -0.202 and -0.150 oC, respectively. The NOAA-NCDC data set has not yet been updated to include 2008, but the interval 1998-2007 displays a warming trend. The absolute difference between temperatures in the specific years 1998 and 2007 is -0.027 oC.

(b)   For the NASA-GISS data set, the linear trend for the period 2001-2008 is one of warming, whereas HadCRUT3 shows cooling over this period. Absolute differences between 2001 and 2008 for the HadCRUT3 and NASA-GISS data sets are -0.087 and 0.000 oC, respectively. The NOAA-NCDC data set displays a warming trend for the period 2001-2007. The absolute difference between temperatures in 2001 and 2007 is +0.056 oC. The variation in trends is primarily due to the differences in spatial averaging techniques used to calculate the global average temperature. Trends seen in global surface temperature over short periods, such as from 1998 to the present, are not good indicators of longer term climate change. At short time scales, trends are strongly affected by year-to-year variability and by the start and end of times chosen. In 1998 and 2002, the Earth’s temperature was exceptionally hot because of El Niño events, and a La Niña event caused colder temperatures in 2000 and 2007. It is more robust to analyse data over many decades and climate scientists often use an 11-year moving average to remove short term variability from the record so that long term trends can be observed.



   (3)   (a)   (i)        HadCRUT3, NOAA-NCDC and NASA-GISS all use land thermometer measurements. Sea surface temperature measurements are made on ships and buoys.

   (ii)           There are versions of these data sets that also incorporate satellite measurements, but the the Intergovernmental Panel on Climate Change’s Fourth Assessment Report avoided using these data sets, since there is not a long history of satellite measurements, ie no measurements prior to 1979.

   (b)   Urban heat island effects are real but localised, and have not biased large scale trends in the global surface temperature. The effect of urbanisation on the land-based temperature record is neglible because the local effects are avoided or accounted for in the data sets used. Over the oceans there is clearly no urban heat island effect. Hence, the potential for urban heat islands to affect global surface temperature estimates is very limited.

   (4)   Advantages of using land thermometers:

  • thermometers measure temperature directly; and
  • there is a long history (around 150 years) of reliable thermometer measurements, which can be compared to detect long term trends.

   Disadvantages of using land thermometers:

  • some measurements in large cities must be adjusted to correct for urban heat island effects; and
  • there is limited data available from thermometers in the polar regions.

(5)   Advantages of using satellites:

  • satellites can provide temperature information for the ocean surface, which may supplement data from thermometers on ships and buoys;
  • satellites also provide information about changes in the temperature of the Earth’s atmosphere; and
  • satellites provide almost global information rapidly.

   Disadvantages of using satellites:

  • Satellites record data on radiation, which is an indirect measure of temperature;
  • Disparities remain among different tropospheric (lower atmosphere) temperature trends estimated from satellites since 1979, and are still likely to contain residual errors, although estimates have been substantially improved through adjustments for issues such as changing satellites and orbits; and
  • The vertical resolution of historical satellite data is poor, meaning that it is not possible to fully isolate cooling of the stratosphere from warming in the troposphere.

(6)   The temperature projections in the third and fourth assessment reports of the IPCC are, by their nature, multi-decadal projections in which the effects of internal climate variability on the decadal timescale are averaged out. It is therefore not meaningful to infer ‘projections’ over short time periods, such as December 1998 to the present or December 2001 to the present, from the published IPCC material.