3.9 Synthesis: Consistency Across Observations

This section briefly compares variability and trends within and across different climate variables to see if a physically consistent picture enhances confidence in the realism of apparent recent observed changes. Therefore, this section looks ahead to the subsequent observational chapters on the cryosphere (Chapter 4) and oceans (Chapter 5), which focus on changes in those domains. The emphasis here is on inter-relationships. For example, increases in temperature should enhance the moisture-holding capacity of the atmosphere as a whole and changes in temperature and/or precipitation should be consistent with those evident in circulation indices. Variables treated in this chapter are summarised in the executive summary, with some discussion below. The example of increases in temperature that should also reduce snow seasons and sea ice and cause widespread glacier retreat involves cross-chapter variables. The main sections where more detailed information can be found are given in parentheses following each bullet.

• The observed temperature increases are consistent with the observed nearly worldwide reduction in glacier and small ice cap (not including Antarctica and Greenland) mass and extent in the 20th century. Glaciers and ice caps respond not only to temperatures but also to changes in precipitation, and both winter accumulation and summer melting have increased over the last half century in association with temperature increases. In some regions, moderately increased accumulation observed in recent decades is consistent with changes in atmospheric circulation and associated increases in winter precipitation (e.g., southwestern Norway, parts of coastal Alaska, Patagonia, Karakoram, and Fjordland of the South Island of New Zealand), even though enhanced ablation has led to marked declines in mass balances in Alaska and Patagonia. Tropical glacier changes are synchronous with higher-latitude ones and all have shown declines in recent decades; local temperature records all show a slight warming, but not of the magnitude required to explain the rapid reduction in mass of such glaciers (e.g., on Kilimanjaro). Other factors in recent ablation include changes in cloudiness, water vapour, albedo due to snowfall frequency and the associated radiation balance (Sections 3.2.2, 3.3.3, 3.4.3 and 4.5).
• Snow cover has decreased in many NH regions, particularly in spring, consistent with greater increases in spring as opposed to autumn temperatures in mid-latitude regions, and more precipitation falling as rain instead of snow. These changes are consistent with changes in permafrost: temperatures of the permafrost in the Arctic and subarctic have increased by up to 3°C since the 1980s with permafrost warming also observed on the Tibetan Plateau and in the European mountain permafrost regions. Active layer thickness has increased and seasonally frozen ground depth has decreased over the Eurasian continent (Sections 3.2.2, 3.3.2, 4.2 and 4.8).
• Sea ice extents have decreased in the Arctic, particularly in spring and summer, and patterns of the changes are consistent with regions showing a temperature increase, although changes in winds are also a major factor. Sea ice extents were at record low values in 2005, which was also the warmest year since records began in 1850 for the Arctic north of 65°N. There have also been decreases in sea ice thickness. In contrast to the Arctic, antarctic sea ice does not exhibit any significant trend since the end of the 1970s, which is consistent with the lack of trend in surface temperature south of 65°S over that period. However, along the Antarctic Peninsula, where significant warming has occurred, progressive breakup of ice shelves has occurred beginning in the late 1980s, culminating in the breakup of the Larsen-B ice shelf in 2002. Decreases are found in the length of the freeze season of river and lake ice (Sections 3.2.2, 3.6.5, 4.3 and 4.4).
• Radiation changes at the top of the atmosphere from the 1980s to 1990s, possibly ENSO-related in part, appear to be associated with reductions in tropical cloud cover, and are linked to changes in the energy budget at the surface and in observed ocean heat content in a consistent way (Sections 3.4.3, 3.4.4, 3.6.2 and 5.2.2).
• Reported decreases in solar radiation from 1970 to 1990 at the surface have an urban bias. Although records are sparse, pan evaporation is estimated to have decreased in many places due to decreases in surface radiation associated with increases in clouds, changes in cloud properties and/or increases in air pollution (aerosol), especially from 1970 to 1990. There is evidence to suggest that the solar radiation decrease has reversed in recent years (Sections 2.4.5, 2.4.6, 3.3.3, 3.4.4, 7.2 and 7.5).
• Droughts have increased in spatial extent in various parts of the world. The regions where they have occurred seem to be determined largely by changes in SSTs, especially in the tropics, through changes in the atmospheric circulation and precipitation. Inferred enhanced evapotranspiration and drying associated with warming are additional factors in drought increases, but decreased precipitation is the dominant factor. In the western USA, diminishing snowpack and subsequent summer soil moisture reductions have also been a factor. In Australia and Europe, direct links to warming have been inferred through the extreme nature of high temperatures and heat waves accompanying drought (Sections 3.3.4 and 4.2, FAQ 3.2 and Box 3.6).
• Changes in the freshwater balance of the Atlantic Ocean over the past four decades have been pronounced, as freshening has occurred in the North Atlantic and south of 25°S, while salinity has increased in the tropics and subtropics, especially in the upper 500 m. The implication is that there have been increases in moisture transport by the atmosphere from the subtropics to higher latitudes, in association with changes in atmospheric circulation, including the NAO, thereby producing the observed increases in precipitation over the northern oceans and adjacent land areas (Sections 3.3.2, 3.6.4, 5.3.2 and 5.5.3).
• Sea level likely rose 1.7 ± 0.5 mm yr^–1 during the 20th century, but the rate increased to 3.1 ± 0.7 mm yr^–1 from 1993 through 2003, when confidence increases from global altimetry measurements. Increases in ocean heat content and associated ocean expansion are estimated to have contributed 0.4 ± 0.1 mm yr^–1 from 1961 to 2003, increasing to an estimated value of 1.6 ± 0.5 mm yr^–1 for 1993 to 2003. In the same interval, glacier and land ice melt has increased ocean mass by approximately 1.2 ± 0.4 mm yr^–1. Changes in land water storage are uncertain but may have reduced water in the ocean. The near balance for 1993 to 2003 gives increased confidence that the observed sea level rise is a strong indicator of warming, and an integrator of the cumulative energy imbalance at the top of atmosphere (Sections 4.5, 4.6, 4.8, 5.2 and 5.5).

In summary, global mean temperatures have increased since the 19th century, especially since the mid-1970s. Temperatures have increased nearly everywhere over land, and SSTs and marine air temperatures have also increased, reinforcing the evidence from land. However, temperatures have increased neither monotonically nor in a spatially uniform manner, especially over shorter time intervals. The atmospheric circulation has also changed: in particular, increasing zonal flow is observed in most seasons in both hemispheres, and the mid- to high-latitude annular modes strengthened until the mid-1990s in the NH and up until the present in the SH. In the NH, this brought milder maritime air into Europe and much of high-latitude Asia from the North Atlantic in winter, enhancing warming there. In the SH, where the ozone hole has played a role, it has resulted in cooling over 1971 to 2000 for parts of the interior of Antarctica but large warming in the Antarctic Peninsula region and Patagonia. Temperatures generally have risen more than average where flow has become more poleward, and less than average or even cooled where flow has become more equatorward, reflecting the PDO and other patterns of variability.

Over land, a strong negative correlation is observed between precipitation and surface temperature in summer and at low latitudes throughout the year, and areas that have become wetter, such as the eastern USA and Argentina, have not warmed as much as other land areas (see especially FAQs 3.2 and 3.3). Increased precipitation is associated with increases in cloud and surface wetness, and thus increased evapotranspiration. The inferred increased evapotranspiration and reduced temperature increase is physically consistent with enhanced latent vs. sensible heat fluxes from the surface in wetter conditions.

Consistent with the expectations noted above for a warmer climate, surface specific humidity has generally increased after 1976 in close association with higher temperatures over both land and ocean. Total column water vapour has increased over the global oceans by 1.2 ± 0.3% per decade from 1988 to 2004, consistent in patterns and amount with changes in SST and a fairly constant relative humidity. Upper-tropospheric water vapour has also increased in ways such that relative humidity remains about constant, providing a major positive feedback to radiative forcing. In turn, widespread observed increases in the fraction of heavy precipitation events are consistent with the increased water vapour amounts.

The three main ocean basins are unique and contain very different wind systems, SST patterns and currents, leading to vastly different variability associated, for instance, with ENSO in the Pacific, and the THC in the Atlantic. Consequently, the oceans have not warmed uniformly, especially at depth. SSTs in the tropics have warmed at different rates and help drive, through coupling with tropical convection and winds, teleconnections around the world. This has changed the atmospheric circulation through ENSO, the PDO, the AMO, monsoons and the Hadley and Walker Circulations. Changes in precipitation and storm tracks are not as well documented but clearly respond to these changes on interannual and decadal time scales. When precipitation increases over the ocean, as it has in recent years in the tropics, it decreases over land, although it has increased over land at higher latitudes. Droughts have increased over many tropical and mid-latitude land areas, in part because of decreased precipitation over land since the 1970s but also from increased evapotranspiration arising from increased atmospheric demand associated with warming.

Changes in the cryosphere (Chapter 4), ocean and land strongly support the view that the world is warming, through observed decreases in snow cover and sea ice, thinner sea ice, shorter freezing seasons of lake and river ice, glacier melt, decreases in permafrost extent, increases in soil temperatures and borehole temperature profiles (see Section 6.6), and sea level rise (Section 5.5).

Acknowledgments

The authors gratefully acknowledge the valuable assistance of Sara Veasey (NCDC, Asheville) and Lisa Butler (NCAR, Boulder) in the development of diagrams and text formatting.