Recent climate change

Eleven of the last twelve years (1995 -2006) rank among the 12 warmest years in the instrumental record of global surface temperature9 (since 1850). The updated 100-year linear trend (1906-2005) of 0.74 [0.56 to 0.92]�C is therefore larger than the corresponding trend for 1901-2000 given in the TAR of 0.6 [0.4 to 0.8]�C. The linear warming trend over the last 50 years (0.13 [0.10 to 0.16]�C per decade) is nearly twice that for the last 100 years. The total temperature increase from 1850 - 1899 to 2001 - 2005 is 0.76 [0.57 to 0.95]�C. Urban heat island effects are real but local, and have a negligible influence (less than 0.006�C per decade over land and zero over the oceans) on these values.

Temperature changes in the atmosphere continue to be a topic of debate. Measurements made using balloons and satellites both suggest that the lower layers of the atmosphere (between approximately 1 km and 8 km) have warmed, but that the warming trend is lower than at the surface. This difference could be due to either instrument uncertainties or a real physical mechanism; this is an area of active research. In the stratosphere, between approximately 12 km and 50 km, the measurements show a cooling trend (left); this is expected and is partly due to increasing greenhouse gas concentrations, increases in water vapour, and also the depletion of stratospheric ozone. The general warming trend has been accompanied by an increase in the number of heatwaves and a reduction in the frequency of frosts in many parts of the world. There are also indications that globally we are experiencing more days with heavy rainfall. (Figure and text by courtesy of the Hadley Centre, UK Met Office)

 

While ocean observations from satellites reveal surface features very well, they seldom give much information about what is happening below the surface. In fact, there are very few sub-surface measurements, even from ships and moorings, that extend over periods of 10 years or longer. Where such data exist, we see significant long-term changes. These changes are almost as large as those at the surface, have significant vertical extent and are seen across whole ocean basins. One of the challenges facing coupled ocean-atmosphere models is to see whether they can reproduce these changes.