Atmospheric temperature observed by satellites

Since 1979 meteorological satellites flown by the National Oceanic and Atmospheric Administration (NOAA) of the United States have carried a microwave instrument, the Microwave Sounding Unit (MSU), for the remote observation of the average temperature of the lower part of the atmosphere up to about 7 km in altitude.

Figure 4.3b shows the record of global average temperature deduced from the MSU and compares it with data from sounding instruments carried on balloons for the same region of the atmosphere, showing very good agreement for the period of overlap. Figure 4.3c shows the record of surface air temperature for the same period. All three measurements show similar variability, the variations at the surface tracking well with those in the lower troposphere. The plots also illustrate the difficulty of deriving accurate trends from a short period of record where there is also substantial variability. Since 1979 the trend in the MSU observations of 0.12 to 0.19 °C per decade shows good agreement with the trend in surface observations of 0.16 to 0.18 °C per decade.

In the stratosphere the temperature trends are reversed (Figure 4.3a) ranging from a decrease of about 0.5 °C per decade in the lower stratosphere to 2.5 °C per decade in the upper stratosphere.

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' Satellite data : Balloon instruments

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Figure 4.3 Time series of analyses of observations of global average temperature (°C) (relative to average for 1979-97) (a) for the lower stratosphere (~13 to 20 km) from balloon instruments (blue and red) and since 1979 from satellite MSUs (purple and brown); (b) for the lower troposphere (up to ~7 km) from balloon instruments (blue and red) and since 1979 from satellite MSUs (purple and brown); (c) for the surface. All time series are monthly mean anomalies relative to 1979 to 1997 smoothed with a seven-month running mean filter. Times of major volcanic eruptions are indicated by vertical lines.

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Figure 4.3 Time series of analyses of observations of global average temperature (°C) (relative to average for 1979-97) (a) for the lower stratosphere (~13 to 20 km) from balloon instruments (blue and red) and since 1979 from satellite MSUs (purple and brown); (b) for the lower troposphere (up to ~7 km) from balloon instruments (blue and red) and since 1979 from satellite MSUs (purple and brown); (c) for the surface. All time series are monthly mean anomalies relative to 1979 to 1997 smoothed with a seven-month running mean filter. Times of major volcanic eruptions are indicated by vertical lines.

140-year period. Changes in sea surface temperature have been estimated by processing over 60 million observations from ships - mostly merchant ships -over the same period. All the observations, from land stations and from ships, are then located within a grid of squares, say 1° of latitude by 1° of longitude, covering the Earth's surface. Observations within each square are averaged; the global average is obtained by averaging (after weighting them by area) over the averages for each of the squares.

A number of research groups in different countries have made careful and independent analyses of these observations. In somewhat different ways they have made allowances for factors that could have introduced artificial changes in the records. For instance, the record at some land stations could have been affected by changes in their surroundings as these have become more urban. In the case of ships, the standard method of observation used to be to insert a thermometer into a bucket of water taken from the sea. Small changes of temperature have been shown to occur during this process; the size of the changes varies between day and night and is also dependent on several other factors including the material from which the bucket is made - over the years wooden, canvas and metal buckets have been variously employed. Nowadays, a large proportion of the observations are made by measuring the temperature of the water entering the engine cooling system. Careful analysis of the effects of these details on observations both on land and from ships has enabled appropriate corrections to be made to the record, and good agreement has been achieved between analyses carried out at different centres.

Confidence that the observed variations are real is increased by noticing that the trend and the shape of the changes are similar when different selections of the total observations are made. For instance, the separate records from the land and sea surface (Figures 4.1b and 4.2) and from the northern and southern hemispheres are closely in accord. Further indirect indicators such as changes in borehole temperatures and sub-surface ocean temperatures, decrease in snow cover and glacier shrinkage provide independent support for the observed warming (Table 4.1.)

During the last 30 years or so observations have been available from satellites orbiting around the Earth. Their great advantage is that they automatically provide data with global coverage, which are often lacking in other data sets. The length of the record from satellites, however, is less than 30 years, a comparatively short period in climate terms. At the time of the IPCC 2001 assessment there were suggestions that the satellite measurements of lower atmospheric temperature since 1979 showed a substantially smaller warming trend than the surface observations. However, more careful analyses since 2001 of the satellite observations now bring the two trends into agreement within their respective uncertainties (see box).

The most obvious feature of the climate record illustrated in Figure 4.1 is that of considerable variability, not just from year to year, but from decade to decade. Some of this variability will have arisen through causes external to the atmosphere and the oceans, for instance as a result of volcanic eruptions such as those of Krakatoa in 1883 or of Pinatubo in the Philippines in 1991 (the low global average temperature in 1992 and 1993, compared with neighbouring years, is almost certainly due to the Pinatubo volcano). But there is no need to invoke volcanoes or other external causes to explain all the variations in the record. Many of them result from internal variations within the total climate system, for instance between different parts of the ocean (see Chapter 5 for more details).

The warming during the twentieth century has not been uniform over the globe. For instance, the recent warming has been greatest over northern hemisphere continents at mid to high latitudes. There have also been areas of cooling, for instance over some parts of the North Atlantic ocean associated with changes in ocean circulation (see Chapter 5). Some of the regional patterns of temperature change are related to different phases of atmosphere-ocean oscillations, such as the El Niño Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). The positive phase of the NAO, with high pressure over the sub-tropical Atlantic and southern Europe and mild winters over northwest Europe, has tended to be dominant since the mid 1980s.

An interesting feature of the increasing temperature during the last few decades has been that, in the daily cycle of temperature, minimum temperatures over land have increased about twice as much as maximum temperatures. A likely explanation for this, in addition to the effects of enhanced greenhouse gases, is an increase in cloud cover which has been observed in many of the areas with reduced temperature range. An increase in cloud tends to obstruct daytime sunshine and tends also to reduce the escape of terrestrial radiation at night.

As might be expected the increases in temperature have led on average to increases in precipitation, although precipitation shows even more variability in both space and time than temperature. The increases have been particularly noticeable in the northern hemisphere in mid to high latitudes, often appearing particularly as increases in heavy rainfall events (see Table 4.1).

The broad features of these changes in temperature and precipitation are consistent with what is expected because of the influence of increased greenhouse gases (see Chapter 5), although there is much variability in the record that arises for reasons not associated with human activities. For instance, the particular increase from 1910 to 1940 (Figure 4.1a) is too rapid to have been due to the rather

Table 4.1 Twentieth-century changes in the Earth's atmosphere, climate and biophysical system

Indicator

Concentration indicators

Atmospheric concentration of CO2

Terrestrial biospheric CO2 exchange Atmospheric concentration of CH4 Atmospheric concentration of N2O Tropospheric concentration of O3

Stratospheric concentration of O3

Atmospheric concentrations of HFCs, PFCs and SF6

Weather indicators

Global mean surface temperature

Northern hemisphere surface temperature

Diurnal surface temperature range

Hot days/heat index Cold/frost days

Continental precipitation

Heavy precipitation events Drought

Observed changes

280 ppm for the period 1000-1750 to 368 ppm in year 2000 (31 ± 4% increase) - 380 ppm in 2006

Cumulative source of about 30 GtC between the years 1800 and 2000; but during the 1990s a net sink of about 10 ± 6 GtC 700 ppb for the period 1000-1750 to 1750 ppb in year 2000 (151 ± 25% increase) - 1775 ppb in 2005 270 ppb for the period 1000-1750 to 316 ppb in the year 2000 (17 ± 5% increase) - 319 ppb in 2005

Increased by 35 ± 15% from the years 1750 to 2000, varies with region

Decreased since 1970, varies with altitude and latitude

Increased globally over the last 50 years

Increased by 0.6 ± 0.2 °C over the twentieth century - 0.74 ± 0.18 over 100 years 1906-2005; land areas warmed more than the oceans (very likely)

Increase over the twentieth century greater than during any other century in the last 1000 years; 1990s warmest decade of the millennium (likely)

Decreased over the years 1950 to 2000 over land; night-time minimum temperatures increased at twice the rate of daytime maximum temperatures (likely)

Increased (likely)

Decreased for nearly all land areas during the twentieth century (very likely)

Increased by 5-10% over the twentieth century in the northern hemisphere (very likely), although decreased in some regions (e.g. north and west Africa and parts of the Mediterranean) Increased at mid and high northern latitudes (likely)

Increased summer drying and associated incidence of drought in a few areas (likely). Since 1970s, increase in total area affected in many regions of the world (likely)

Tropical cyclones Intense extratropical storms

Biological and physical indicators

Global mean sea level

Duration of ice cover of rivers and lakes

Arctic sea-ice extent and thickness

Non-polar glaciers Snow cover

Permafrost

El Niño events

Growing season

Plant and animal ranges

Breeding, flowering and migration

Coral reef bleaching Economic indicators

Weather-related economic losses

Since 1970s, trend towards longer lifetimes and greater storm intensity but no trend in frequency (likely) Since 1950s, net increase in frequency/intensity and poleward shift in track (likely)

Increased at an average annual rate of 1-2 mm during the twentieth century - rising to about 3 mm from 1993-2003

Decreased by about two weeks over the twentieth century in mid and high latitudes of the northern hemisphere (very likely)

Thinned by 40% in recent decades in late summer to early autumn (likely) and decreased in extent by 10-15% since the 1950s in spring and summer

Widespread retreat during the twentieth century Decreased in area by 10% since global observations became available from satellites in the 1960s (very likely) Thawed, warmed and degraded in parts of the polar, sub-polar and mountainous regions

Became more frequent, persistent and intense during the last 30 years compared to the previous 100 years

Lengthened by about one to four days per decade during the last 50 years in the northern hemisphere, especially at higher latitudes

Shifted poleward and up in elevation for plants, insects, birds and flsh

Earlier plant flowering, earlier bird arrival, earlier dates of breeding season and earlier emergence of insects in the northern hemisphere

Increased frequency, especially during El Niño events

Global inflation-adjusted losses rose by an order of magnitude over the last 50 years. Part of the observed upward trend is linked to socio-economic factors and part is linked to climatic factors

Note: This table provides examples of key observed changes and is not an exhaustive list. It includes both changes attributable to anthropogenic climate change and those that may be caused by natural variations or anthropogenic climate change. Confidence levels (for explanation see Note 1) are reported where they are explicitly assessed by the relevant Working Group of the IPCC.

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Figure 4.4 Global average sea level from tide gauge (blue) and satellite (red) data relative to the 1961-90 mean.

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Figure 4.4 Global average sea level from tide gauge (blue) and satellite (red) data relative to the 1961-90 mean.

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small increase in greenhouse gases during that period. The particular reasons for this will be discussed in the next chapter where comparisons of observed temperatures with simulations from climate models for the whole of the twentieth century will be presented, not just as they concern the global mean but also the regional patterns of change. We conclude therefore that, although the expected signal is still emerging from the noise of natural variability, most of the observed warming over the last 50 years is very likely to have been due to the increase in greenhouse gas concentrations.

Significant cooling of the lower stratosphere (the region at altitudes between about 10 and 30 km) has been observed over the last two decades (Figure 4.3a). This is to be expected both because of the decrease in the concentration of ozone (which absorbs solar radiation) and because of the increased carbon dioxide concentration which leads to increased cooling at these levels (see Chapter 2). Because warming of the troposphere and cooling of the stratosphere are occurring because of the increase in greenhouse gases, an increase on average in the height of the tropopause, the boundary between the troposphere and the stratosphere, is also expected. There is now observational evidence for this increase.

A further source of information regarding climate change comes from measurements of change in sea level (Figure 4.4). Over the twentieth century sea level rose by 17 ± 5 cm. The rate of rise increased to 3.1 ± 0.7 cm over the decade 1993 to 2003 of which 1.6 ± 0.5 cm is estimated to be from thermal expansion of the ocean as its average temperature increased and 1.2 ± 0.4 cm from the melting of glaciers that have generally been retreating over the last century. The net contribution from the Greenland and Antarctic ice caps is more uncertain but is believed to be small.

In Chapter 1, we mentioned the increasing vulnerability of human populations to climate extremes, which has brought about more awareness of recent extremes in the forms of floods, droughts, tropical cyclones and windstorms. It is therefore of great importance to know whether there is evidence of an increase in the frequency or severity of these and other extreme events. The available evidence regarding how these and other relevant parameters have

Nukuoro Atoll, Federated States of Micronesia is home to 900 people around the 6-km wide lagoon. It is part of an island chain that stretches northeast of Papua New Guinea in the western Pacific. It was reported in November 2005 that the islands have progressively become uninhabitable, with an estimate of their total submersion by 2015. Storm surges and high tides continue to wash away homes, destroy vegetable gardens and contaminate fresh water supplies. Photographed from the International Space Station.

Nukuoro Atoll, Federated States of Micronesia is home to 900 people around the 6-km wide lagoon. It is part of an island chain that stretches northeast of Papua New Guinea in the western Pacific. It was reported in November 2005 that the islands have progressively become uninhabitable, with an estimate of their total submersion by 2015. Storm surges and high tides continue to wash away homes, destroy vegetable gardens and contaminate fresh water supplies. Photographed from the International Space Station.

changed during the twentieth century is summarised in Table 4.1 in terms of different indicators: concentrations of greenhouse gases; temperature, hydro-logical and storm-related indicators; and biological and physical indicators. To what extent these changes are expected to continue or to intensify during the twenty-first century will be addressed in Chapter 6.

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