1. Has the climate changed during the 20th century?

1.1. Has the world warmed?
       1.1.1. Has the earth surface temperature increased?
       1.1.2. Has the atmosphere temperature increased?
       1.1.3. Have the snow cover and ice extent decreased?
       1.1.4. Have sea level and ocean heat content increased?

1.2. What other climate changes have been observed?

1.3. What aspects of our climate have NOT changed?

"An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.

Since the release of the Second Assessment Report (SAR4), additional data from new studies of current and palaeoclimates, improved analysis of data sets, more rigorous evaluation of their quality, and comparisons among data from different sources have led to greater understanding of climate change."

Source & © : IPCC TAR SPM of WG1 page 2

1.1. Has the world warmed?

       1.1.1. Has the earth surface temperature increased?
       1.1.2. Has the atmosphere temperature increased?
       1.1.3. Have the snow cover and ice extent decreased?
       1.1.4. Have sea level and ocean heat content increased?

1.1.1. Has the earth surface temperature increased?

"The global average surface temperature has increased over the 20th century by about 0.6°C.

Figure 1a

Globally, it is very likely7 that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record, since 1861 (see Figure 1a).

Source & © : IPCC TAR SPM of WG1 pages 2 & 3

1.1.2. Has the atmosphere temperature increased?

"Temperatures have risen during the past four decades in the lowest 8 kilometres of the atmosphere.

Since the late 1950s (the period of adequate observations from weather balloons), the overall global temperature increases in the lowest 8 kilometres of the atmosphere and in surface temperature have been similar at 0.1°C per decade.

Since the start of the satellite record in 1979, both satellite and weather balloon measurements show that the global average temperature of the lowest 8 kilometres of the atmosphere has changed by +0.05 ± 0.10°C per decade, but the global average surface temperature has increased significantly by +0.15 ± 0.05°C per decade. The difference in the warming rates is statistically significant. This difference occurs primarily over the tropical and sub-tropical regions.

The lowest 8 kilometres of the atmosphere and the surface are influenced differently by factors such as stratospheric ozone depletion, atmospheric aerosols, and the El Niño phenomenon. Hence, it is physically plausible to expect that over a short time period (e.g., 20 years) there may be differences in temperature trends. In addition, spatial sampling techniques can also explain some of the differences in trends, but these differences are not fully resolved."

Source & © : IPCC TAR SPM of WG1 page 4

1.1.3. Have the snow cover and ice extent decreased?

"Snow cover and ice extent have decreased.

Satellite data show that there are very likely7 to have been decreases of about 10% in the extent of snow cover since the late 1960s, and ground-based observations show that there is very likely7 to have been a reduction of about two weeks in the annual duration of lake and river ice cover in the mid- and high latitudes of the Northern Hemisphere, over the 20th century.

There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century.

Northern Hemisphere spring and summer sea-ice extent has decreased by about 10 to 15% since the 1950s. It is likely7 that there has been about a 40% decline in Arctic sea-ice thickness during late summer to early autumn in recent decades and a considerably slower decline in winter sea-ice thickness."

Source & © : IPCC TAR SPM of WG1 page 4

1.1.4. Have sea level and ocean heat content increased?

"Global average sea level has risen and ocean heat content has increased.

Tide gauge data show that global average sea level rose between 0.1 and 0.2 metres (10 to 20 cm, 4 to 8") during the 20th century.

Global ocean heat content has increased since the late 1950s, the period for which adequate observations of sub-surface ocean temperatures have been available."

Source & © : IPCC TAR SPM of WG1 page 4

more info on sea level's rise in IPCC Technical Summary Box 2

1.2. What other climate changes have been observed?

"Changes have also occurred in other important aspects of climate:

It is very likely7 that precipitation has increased by 0.5 to 1% per decade in the 20th century over most mid- and high latitudes of the Northern Hemisphere continents, and it is likely7 that rainfall has increased by 0.2 to 0.3% per decade over the tropical (10°N to 10°S) land areas. Increases in the tropics are not evident over the past few decades. It is also likely7 that rainfall has decreased over much of the Northern Hemisphere sub-tropical (10°N to 30°N) land areas during the 20th century by about 0.3% per decade. In contrast to the Northern Hemisphere, no comparable systematic changes have been detected in broad latitudinal averages over the Southern Hemisphere. There are insufficient data to establish trends in precipitation over the oceans.

In the mid- and high latitudes of the Northern Hemisphere over the latter half of the 20th century, it is likely7 that there has been a 2 to 4% increase in the frequency of heavy precipitation events. Increases in heavy precipitation events can arise from a number of causes, e.g., changes in atmospheric moisture, thunderstorm activity and large-scale storm activity.

It is likely7 that there has been a 2% increase in cloud cover over mid- to high latitude land areas during the 20th century. In most areas the trends relate well to the observed decrease in daily temperature range.

Since 1950 it is very likely7 that there has been a reduction in the frequency of extreme low temperatures, with a smaller increase in the frequency of extreme high temperatures.

Warm episodes of the El Niño-Southern Oscillation (ENSO) phenomenon (which consistently affects regional variations of precipitation and temperature over much of the tropics, sub-tropics and some mid-latitude areas) have been more frequent, persistent and intense since the mid-1970s, compared with the previous 100 years.

Over the 20th century (1900 to 1995), there were relatively small increases in global land areas experiencing severe drought or severe wetness. In many regions, these changes are dominated by inter-decadal and multi-decadal climate variability, such as the shift in ENSO towards more warm events.

In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades."

Source & © : IPCC TAR SPM of WG1 pages 4 & 5

1.3. What aspects of our climate have NOT changed?

"Some important aspects of climate appear not to have changed.

A few areas of the globe have not warmed in recent decades, mainly over some parts of the Southern Hemisphere oceans and parts of Antarctica.

No significant trends of Antarctic sea-ice extent are apparent since 1978, the period of reliable satellite measurements.

Changes globally in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal to multi-decadal variations, with no significant trends evident over the 20th century. Conflicting analyses make it difficult to draw definitive conclusions about changes in storm activity, especially in the extra-tropics.

No systematic changes in the frequency of tornadoes, thunder days, or hail events are evident in the limited areas analysed."

Source & © : IPCC TAR SPM of WG1 page 5