Climate Change 2001: Working Group I: The Scientific Basis

	Other reports in this collection

Changes in temperature during the pre-instrumental period

The past millennium

• New analyses indicate that the magnitude of Northern Hemisphere warming over the 20th century is likely to have been the largest of any century in the last 1,000 years.
• The 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere and 1998 is likely to have been the warmest year. Because less data are available, less is known about annual averages prior to 1,000 years before the present and for conditions prevailing in most of the Southern Hemisphere prior to 1861.
  
• Evidence does not support the existence of globally synchronous periods of cooling or warming associated with the ‘Little Ice Age' and ‘Medieval Warm Period'. However, reconstructed Northern Hemisphere temperatures do show a cooling during the 15th to 19th centuries and a relatively warm period during the 11th to 14th centuries, though the latter period is still cooler than the late 20th century.
• Analyses of borehole temperatures indicate a non-linear increase in global average ground surface temperature over land of 1.0 ± 0.3°C over the last 500 years, with most of the increase occurring since the late 19th century. There may be additional uncertainties due to the assumptions used in this technique, and decreasing resolution back in time limits confidence in the exact timing of the warming.

Changes across the last 500,000 years

• It is very likely that large and rapid decadal temperature changes occurred during the last glacial and its deglaciation (between about 100,000 and 10,000 years ago), particularly in higher latitudes of the Northern Hemisphere. During the last deglaciation, local increases in temperature are likely to have been as large as 5 to 10°C over a few decades. Over the same period there is evidence of less pronounced but nearly synchronous changes worldwide, except in high southern latitudes.
• Antarctic ice cores have provided new evidence of almost in-phase changes of temperature, carbon dioxide and methane through the ice age cycles over the past 420,000 years.
  
• There is emerging evidence for significant, rapid (time-scales of several decades or more), regional temperature changes during the last 10,000 years. However, the evidence does not indicate that any such events were global in scale.

Changes in Precipitation and Related Variables

Precipitation

• Instrumental records of land-surface precipitation continue to show an increase of 0.5 to 1%/decade in much of the Northern Hemisphere mid- and high latitudes. A notable exception includes parts of eastern Russia. In contrast, over much of the sub-tropical land areas rainfall has decreased during the 20th century (by -0.3%/decade), but this trend has weakened in recent decades. Other precipitation indicators suggest that large parts of the tropical oceans have had more precipitation in recent decades, and that precipitation has significantly increased over tropical land areas during the 20th century (2.4%/century). The increase in precipitation over the tropics is not evident during the past few decades.
• In the Southern Hemisphere, the pattern of island rainfall in parts of the South Pacific has changed since the mid-1970s, associated with the more frequent occurrence of the warm phase of the El Niño-Southern Oscillation (ENSO).
• Where data are available, changes in annual streamflow usually relate well to changes in total precipitation.

Water vapour

• Changes in water vapour mixing ratio have been analysed for selected regions using in situ surface observations as well as lower-tropospheric measurements based on satellites and weather balloons. A pattern of overall surface and lower-tropospheric water vapour mixing ratio increases over the past few decades is emerging, although there are likely to be some time-dependent biases in these data and regional variations in trends. The more reliable data sets show that it is likely that total atmospheric water vapour has increased several per cent per decade over many regions of the Northern Hemisphere since the early 1970s. Changes over the Southern Hemisphere cannot yet be assessed.
  
• Satellite observations of upper-tropospheric humidity from 1980 to 1997 show statistically significant positive trends of 0.1%/year for the zone 10°N to 10°S. Other trends are not statistically significant, but include a 0.04%/year positive trend for the zone 60°N to 60°S but a negative trend of -0.1%/year over the region 30°S to 60°S.
• Balloon observations of stratospheric water vapour above 18 km show an increase of about 1%/year for the period from 1981 to 2000. Shorter satellite records show a similar positive trend, suggesting that the change is global in character, but they also indicate a slowing of the positive trend after 1996.
  

Clouds

• It is likely that there has been an increase in total cloud cover of about 2% over many mid- to high latitude land areas since the beginning of the 20th century. The increases in total cloud amount are positively correlated with decreases in the diurnal temperature range. Changes in total cloud amount are uncertain both over sub-tropical and tropical land areas as well as over the oceans.

Changes in Atmospheric/Oceanic Circulation

El Niño-Southern Oscillation (ENSO)

• The frequency and intensity of ENSO has been unusual since the mid-1970s compared with the previous 100 years. Warm phase ENSO episodes have been relatively more frequent, persistent, or intense than the opposite cold phase during this period.
• This recent behaviour of ENSO is related to variations in precipitation and temperature over much of the global tropics and sub-tropics and some mid-latitude areas. The overall effect is likely to have made a small contribution to the increase in global surface temperature during the last few decades.

Other Oscillations

• The Inter-decadal Pacific Oscillation is likely to be a Pacific-wide manifestation of the Pacific Decadal Oscillation. Both are associated with decadal climate variability over the Pacific basin. It is likely that these related phenomena modulate ENSO-related climate variability.
• The winter North Atlantic Oscillation (NAO) and the associated Arctic Oscillation (AO), which appear to be largely the same phenomenon, show decadal to multi-decadal variability. Since the 1970s these oscillations have been in a phase that gives stronger westerly winds over much of extra-tropical Eurasia in the winter half year. This is associated with cold season warming over extra-tropical Eurasia, but cooling in some regions further south.
• The High Latitude Mode (HLM) or Antarctic Oscillation (AAO) in the Southern Hemisphere has been in an enhanced positive phase in the last fifteen years, with stronger westerly winds over the Southern Ocean.
• It is likely that rapid (time-scales of several decades or more) changes of atmospheric and ocean circulation occurred during inter-glacial periods, affecting regional climate, without human interference.

Changes in Extreme (within the upper or lower ten percentiles) Weather and Climate Events

Precipitation

• New analyses show that in regions where total precipitation has increased it is very likely that there have been even more pronounced increases in heavy and extreme precipitation events. The converse is also true.
  
• In some regions, heavy and extreme precipitation events have increased despite the fact that total precipitation has decreased or remained constant. This is attributed to a decrease in the frequency of precipitation events. Changes in the frequency of heavy precipitation events can arise from several causes, e.g., changes in atmospheric moisture or circulation.
• Over the latter half of the 20th century it is likely that there has been a 2 to 4% increase in the frequency of heavy precipitation events reported by the available observing stations in the mid- and high latitudes of the Northern Hemisphere.
• Trends for severe drought and wet area statistics for 1900 to 1995 are relatively small over global land areas. However, during the last two or three decades there have been some increases in the globally combined severe dry and wet areas.

Temperature

• In many regions inter-daily temperature variability has decreased. Increases in the daily minimum temperatures are lengthening the freeze-free season in most mid- and high latitude regions.
• A significant reduction in the frequency of extreme low monthly and seasonal average temperatures across much of the globe has occurred since the late 19th century. However, a relatively smaller increase in the frequency of extreme high monthly and seasonal average temperatures has been observed.

Storms

• Changes in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal to multi-decadal variations, with no significant trends over the 20th century evident. 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.