The last hundred years

The 1980s and 1990s and the early years of the twenty-first century have brought unusually warm years for the globe as a whole as is illustrated in Figure 4.1, which shows the global average temperature since 1850, the period for which the instrumental record is available with good accuracy and coverage. An increase over this period has taken place of 0.76 ± 0.19 °C (Figure 4.1a). The two warmest years in the record are 1998 and 2005, 1998 ranking highest on one estimate and 2005 highest on two other estimates. Also 12 of the 13 years 1995 to 2007 rank amongst the 13 warmest years in the whole record. A further striking statistic is that each of the first eight months of 1998 was very likely1 the warmest of those months in the record up to that date. Although there is a distinct trend in the record, the increase is by no means a uniform one. In fact, some periods of cooling as well as warming have occurred and an obvious feature of the record is the degree of variability from year to year and from decade to decade.

Note also that there has been little if any average increase in warming during the years 2001-2006. Some have tried to argue that this shows the warming is over. However, as the figure illustrates, seven years of record is too short a period to establish a trend. Although the year 2007 was slightly cooler that 2006, the first seven years of the twenty-first century were on average nearly 0.2 °C warmer than the last seven years of the twentieth century, even though 1998 was the warmest year so far. Further, studies of interannual variability in the record demonstrate the strong influence of variations in El Niño and suggest that interannual variability may continue to offset anthropogenic warming until around 2009.2

Shown also in Figure 4.1b are the patterns of recent warming at the surface and averaged over the troposphere. Warming within the atmosphere is more spatially uniform than the surface record which shows more warming over land than over the ocean (see also Figure 4.2).

A sceptic may wonder how diagrams like those in Figure 4.1 can be prepared and whether any reliance can be placed upon them. After all, temperature varies from place to place, from season to season and from day to day by many tens of degrees. But here we are not considering changes in local temperature but in the average over the whole globe. A change of a few tenths of a degree in that average is a large change.

First of all, just how is a change in global average temperature estimated from a combination of records of changes in the near-surface temperature over land and changes in the temperature of the sea surface? To estimate the changes over land, weather stations are chosen where consistent observations have been taken from the same location over a substantial proportion of the whole

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Annual mean Smoothed series 5-95% decadel error bars

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°C per decade

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0.177+0.052

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0.128+0.026

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0.074+0.018

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2000

u to TS

tfl "D

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Surface

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Figure 4.1 (a) Variations of the globally averaged Earth's surface temperature (combined land surface air temperature and sea surface temperature) for 1850-2006 relative to the 1961-90 mean. The black dots show annual means and the right-hand axis shows the estimated actual average temperature. Linear trend fits are shown to the last 25, 50, 100 and 150 years indicating accelerated warming. (b) Patterns of linear global temperature trends from 1979 to 2005 estimated at the surface (left), and for the troposphere (right) from the surface to about 10 km altitude from satellite records. Grey areas indicate incomplete data.

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CRUTEM3

HadMAT

HadSSt2

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HadMAT

Figure 4.2 Decadally smoothed annual anomalies of global average sea surface temperature (blue), night marine air temperature (green) and land surface air temperature (red) relative to their 1961-90 means.

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Figure 4.2 Decadally smoothed annual anomalies of global average sea surface temperature (blue), night marine air temperature (green) and land surface air temperature (red) relative to their 1961-90 means.

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