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| | Surface temperature changes | | |  The global mean surface temperature was lower in 1999 than in the record warm year of 1998, mainly because temperatures in the tropical Pacific changed from the warm El Niño state to the colder La Niña state. Nevertheless, 1999 was the fifth warmest year since the beginning of the instrument temperature record (1860), and the 1990s was the warmest decade during this period (around 0.6°C above the temperature during the late 19th century). The temperature shown for 2000 only includes data up to August. Over the continents, the night-time temperatures generally warmed more than those during the day, reducing the daily temperature range. Proxy measurements (such as tree rings and coral) indicate that the temperatures observed over the past decade are higher than any which have occurred during the last 1000 years. The cooling between 1998 and 1999 illustrates that, even when there is a strong long-term warming trend, sizeable year-to-year changes (in either direction) can occur as a result of natural processes within the climate system: it does not negate long-term trends. (Figure and text by courtesy of the Hadley Centre,UK Met Office.)
See More... | | | | Atmospheric temperature change | | |  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)
See More... | | | | Oceanic temperature change | | |  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. (Figure: Temperatures between 1.5 and 2.5 km depth near Bermuda, by courtesy of T Joyce / WOCE.)
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