Global Warming

Among various signs of climate change, global warming is the most significant.  In this Website we have followed Wikipedia  in defining global warming this way:

‘Global warming’ is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-twentieth century and its projected continuation.'

The first meteorological network in the world was formed in northern Italy in 1653.  Before long, records of atmospheric temperature measurements were being collected at various locations around the world (e.g. the “Central England Temperature” (CET) databank is a continuous record of atmospheric temperature data stretching right back all the way to 1659). By the middle of the 19th century, scientists were beginning to consider how to validly synthesise the enormous accumulations of such records collected under a wide variety of measurement systems and to draw up rules to standardise future measurements and collation procedures so that whole-of-Earth trends in climate might be better discerned. When the First Assessment Report of the IPCC was released in 1990, a disturbing trend was apparent in such data. According to that First Report

“Our judgement is that: global mean surface air temperature has increased by 0.3 to 0.6 oC over the last 100 years...; the size of this warming is broadly consistent with prediction of climate models, but it is also of the same magnitude as natural climate variability."

Since then, with each successive report of the IPCC, the evidence for global warming has become both clearer and more disturbing in its implications. In 2007, the IPCC  met for the fourth time.  The IPCC’s Fourth Assessment Report , in November of that year, concluded as follows:

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level. Eleven of the last twelve years (1995-2006) rank among the twelve warmest years in the instrumental record of global surface temperature (since 1850) … The temperature increase is widespread over the globe and is greater at higher northern latitudes. Land regions have warmed faster than the oceans."

This is a very important statement of what the IPCC scientists believe. What it says is that there can no longer be doubt that the oceans and atmosphere of the world have been heating up over the past 50 years or more. Figure 1 shows the Global Mean Temperature values for every year (the black dots), together with different ways of presenting the average trend in temperatures for different time periods between 1850 and 2005. By Global Mean Temperature, we mean an estimate of the average surface air temperature of the Earth based on the area-weighted global average of the sea surface temperature and land surface air temperature calculated from thousands of world-wide observation sites on land and sea.

Figure 1:  The rise in the Earth’s Global Mean Temperature since 1850 (Source:  IPCC 2007 b  p.253) .

The evidence is that since the late 1800s, the average temperature of the earth's surface has risen by 0.74 degrees Celsius.  It is important to realise, that there is considerable year to year variability around the average trends, and the gradient (i.e. steepness) of the trend depends very much on the particular period of time chosen for consideration; while there is some suggestion in the data that the warming trend has slowed down in the last decade, time-series analysis suggests on overall upward trend (for detailed discussion of this see ‘CSIRO Hot Topic’ , ‘Climate Sensitivity Reconsidered’    and  the draft of ‘Garnaut Climate Change Review’ Chapter 5 page 113).

In Australia, records kept by the Bureau of Meteorology for the past century show that:

‘Since the middle of the 20th century, Australian temperatures have, on average, risen by about 1°C with an increase in the frequency of heatwaves and a decrease in the numbers of frosts and cold days.’  (Source: Australian Bureau of Meteorology   [Accessed October 2008])

This can be seen in Figure 2, which shows the average temperature for Australia in each year between 1910 and 2007 expressed as an anomaly (i.e. a deviation; negative or positive) from the mean temperature for the thirty year period from 1961 to 1990. In 2005, Australia recorded its warmest year on record since reliable temperature observations began in 1910, with an average temperature of 22.87°C i.e. 1.06°C above the standard 1961-90 averaging period.

Records reveal that more warming has occurred over inland Australia and less in the southeast. Night time temperatures have increased more than daytime temperatures.

Figure 2:  Annual mean temperature anomalies for Australia between 1910 and 2007. 11-year running averages are shown by the black curve.  ( Source: Australian Bureau of Meteorology  [Accessed October 2008] )