There is no single thermometer measuring the global temperature. Instead, individual thermometer measurements taken every day at several thousand stations over the land areas of the world are combined with thousands more measurements of sea surface temperature taken from ships moving over the oceans to produce an estimate of global average temperature every month.
From these records, the ten warmest years in the instrumental record of global temperature (since around 1880) all occur within the 12 year period 1997-2008. Although 2008 data below show it was the coolest since 2000 due to the moderate to strong La Niña that developed in the latter half of 2007. However, the total global temperature increase from the 1850s throught to 2005 is 0.76°C (1.36°F) and the rate of warming averaged over the last 50 years is nearly twice that for the last 100 years. So don't be lulled by one year where it is relatively cooler, what we need to look at is decadal or long term trends.
An increasing rate of warming has taken place over the last 25 years. Above the surface, global observations since the late 1950s show that the troposphere (up to about 10 km) has warmed at a slightly greater rate than the surface, while the stratosphere (about 10–30 km) has cooled markedly since 1979. This is in accord with physical expectations and most model results.
Confirmation of a global temperature rise comes from the observed temperature increases in the oceans, observations of sea level rise, glacial melt, sea ice retreat in the Arctic and diminished snow cover in the Northern Hemisphere. Global average temperature is forecast to rise 4°C (7.2°F) toward the end of the 21st century, and this is a mere 90 years away! Even if began today, and stopped most of our greenhouse gas emissions overnight, we would still see a temperature rise of around 2°C (3.6°F) by 2090-2100. David Spratt and Philip Sutton explain in their paper Code Red what will happen with three degrees of warming, and how dangerous this is.
This rapid rise in temperature is unmatched in the last million years, and even then, the data indicate that the global warming at the end of an ice age was a gradual process taking about 5,000 years. Our human ( anthropogenic ) actions have ramped up the rate of change not evidenced in any record, and we are leaving ourselves very little time to adapt.
Figure Above: Annual average global temperature (black dots) along with simple fits to the data. The left hand axis shows global temperature anomalies relative to the 1961 to 1990 average and the right hand axis shows the estimated actual temperature (°C). Linear trend fits to the last 25 (yellow), 50 (orange), 100 (purple) and 150 years (red) are shown, and correspond to 1981 to 2005, 1956 to 2005, 1906 to 2005, and 1856 to 2005, respectively. Note that for shorter recent periods, the slope is greater, indicating accelerated warming.
The 4th IPCC Report also states that, "analysis of long-term changes in daily temperature extremes has recently become possible for many regions of the world (parts of North America and southern South America, Europe, northern and eastern Asia, southern Africa and Australasia). Especially since the 1950s, these records show a decrease in the number of very cold days and nights and an increase in the number of extremely hot days and warm nights. The length of the frost-free season has increased in most mid- and high-latitude regions of both hemispheres. In the Northern Hemisphere, this is mostly manifest as an earlier start to spring."
The 4th IPCC Report also states that, "analysis of long-term changes in daily temperature extremes has recently become possible for many regions of the world (parts of North America and southern South America, Europe, northern and eastern Asia, southern Africa and Australasia). Especially since the 1950s, these records show a decrease in the number of very cold days and nights and an increase in the number of extremely hot days and warm nights. The length of the frost-free season has increased in most mid- and high-latitude regions of both hemispheres. In the Northern Hemisphere, this is mostly manifest as an earlier start to spring."
Regional Temperature Anomalies
2008 was a year with above-average temperatures all over Europe. A large geographical domain, including north-western Siberia and part of the Scandinavian region, recorded a remarkably mild winter. January and February were very mild over nearly all of Europe. Monthly mean temperature anomalies for these months exceeded +7°C in some places in Scandinavia. In most parts of Finland, Norway and Sweden, winter 2007-08 was the warmest recorded since the beginning of measurements. In contrast, the boreal winter was remarkably cold for a large part of Eurasia extending eastward from Turkey to China. Some places in Turkey had their coldest January nights in nearly 50 years. This extreme cold weather caused hundreds of casualties in Afghanistan and China.
In January 2009 Melbourne experienced three successive days of temperatures above 43°C for the first time in recorded history. In March 2008, southern Australia experienced a record heatwave that brought scorching temperatures across the region. Adelaide experienced its longest running heatwave on record, with 15 consecutive days of maximum temperatures above 35°C. Persistent extreme heat affected much of eastern Australia from late December 2005 until early March with many records being set (e.g. second hottest day on record in Sydney with 44.2°C/111.6°F on 1 January). Spring 2006 (September-November) was Australia’s warmest since seasonal records were first compiled in 1950. Heat waves were also registered in Brazil from January until March 2006 (e.g. 44.6°C/112.3°F in Bom Jesus on 31 January – one of the highest temperatures ever recorded in Brazil).
Several parts of Europe and the USA experienced heat waves with record temperatures in July and August of 2006. Air temperatures in many parts of the USA reached 40°C/104°F or more. The July European-average land-surface air temperature was the warmest on record at 2.7°C above the climatological normal.
Autumn 2006 (September-November) was exceptional in large parts of Europe at more than 3°C warmer than the climatological normal from the north side of the Alps to southern Norway. In many countries it was the warmest autumn since official measurements began: records in central England go back to 1659 (1706 in The Netherlands and 1768 in Denmark).
Arctic sea ice extent during the 2008 melt season dropped to its second-lowest level since satellite measurements began in 1979, reaching the lowest point in its annual cycle of melt and growth on 14 September 2008. Average sea ice extent over the month of September, a standard measure in the scientific study of Arctic sea ice, was 4.67 million km2. The record monthly low, set in 2007, was 4.3 million km2.
Summary
Based on science from the IPCC 4th Report, and consistent with observed increases in global temperature, there have been:
- decreases in the length of river and lake ice seasons.
- worldwide reduction in glacial mass and extent in the 20th century.
- melting of the Greenland Ice Sheet has recently become apparent.
- snow cover has decreased in many Northern Hemisphere regions.
- sea ice thickness and extent have decreased in the Arctic in all seasons.
- the oceans are warming
- sea level is rising due to thermal expansion of the oceans and melting of land ice.
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