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In recent years, a variety of independent studies, employing different methodologies, have found evidence for the widening of the tropical region, as defined by climate scientists (reviewed by Seidel et al. 2008). For example, Hudson et al (2006) used long term satellite measurements of ozone concentration and estimated that the area of the northern hemisphere occupied by the tropics has expanded by approximately 1 degree per decade in the period 1979-2003 – a total widening of 2.5 degrees. Fu et al. (2006), who used satellite temperature observations from 1979-2005, estimated a smaller net widening of 2 degrees latitude across both hemispheres. However, Seidel & Randal (2006), using data from weather balloons and climate models, have
estimated a larger increase in the tropical region of more than 5 degrees in the same time period. In addition to these observations of the poleward expansion of the tropical belt, a number of studies have also demonstrated that the height of the tropics, as measured by the height of the tropical tropopause, has also increased by some tens of metres over the past few decades (Seidel & Randal 2006; Zhou et al. 2001; Santer et al. 2003). Taking all estimates into account, Seidel et al. (2008) proposes that the overall three-dimensional growth of the tropical zone over the past 25 years is around 5% (Seidel et al. 2008). Shifts in other climatic phenomenon have also been reported for the same period. For example, a number of studies show a longitudinal, westward extension of around 10 degrees in the western Pacific subtropical high (WPSH) over the past 30 years (Ho et al. 2004; Wu et al. 2005). This is significant, as the WPSH is the predominant driver of climate and precipitation patterns across
Asia. Hu & Fu (2006) suggest that other changes in more regional circulation patterns, such as the WPSH, may also be contributing to the general expansion of the tropical zone. However, while evidence is accumulating for the widening of the tropical belt and shifts in other climatic events, there is still much uncertainty regarding the degree of the expansion and the mechanisms which are driving it. For example, across the studies reviewed above the estimates of the increase in the tropics vary from 2.0 to more than 5 degrees approximately every 25 years.