The arid and semi-arid regions of the northern mid-latitudes - land masses in Asia, Europe and North America - have experienced the fastest global warming since 1990, as reported in a paper published online this week in Nature Climate Change.

Land surface warming began in the subtropical and subpolar regions of the Northern Hemisphere, where, between 1950 and 1985, warming bands expanded and subsequently merged to cover the entire region. Warming does not occur in a uniform way through time and space, and understanding how it evolves will allow better evaluation of potential impacts as the warming trend continues.

Zhaohua Wu and colleagues use a nonlinear trend-detection method to determine the spatial pattern of global land surface warming. This method estimates the numerous components of natural climate variability and subtracts them from the data. It allows the data, a temperature time series, to determine the trend. From this, they find that the northern mid-latitudes have warmed at a rate close to 0.3 degrees Celsius per decade, whereas the Southern Hemisphere subtropical region has only warmed at about half that rate.

Many studies have assumed linearity in the evolution of warming but this study highlights the nonlinearity in its acceleration during the twentieth century.