Earth Has Become Less Cold in Recent Decades
A massive analysis encompassing 50 years of daily weather data collected from around the world concludes that more significantly than the planet becoming warmer, it has become less cold. The most dramatic trend among various measures of temperature extremes is in the number of cold or warm nights. This is when a high or low temperature reaches the outer fringe of what is normal for a location, registering in the 10th percentile.
For three-quarters of the earth's land surface, cold nights declined in frequency between 1951 and 2003, while the number of warm nights increased. On a global scale, this amounts to a difference each year of 20 fewer cold nights and 25 more warm nights. In comparison, the frequency of warm days increased for 41% of the land area.
The trend towards a less cold planet is also quite apparent in minimum daily temperatures, which rose by nearly 5 °C (9 °F) globally during the same period. As well, areas of the planet commonly subject to freezing are now receiving 16 fewer days of frost per year on average than they did five decades ago. The largest rises in extreme minimum temperatures and declines in frosty days have taken place in Europe and Asia.
Minimum temperatures have warmed much faster than have maximum temperatures. Consequently, the range of temperature during the course of 24 hours has shrunk for 39% of the land.
The shift to less cold conditions was not a steady transition. Instead, the climatic trend began markedly in the mid-1970s. While warmer temperature extremes are occurring year-round, the extent of change in temperatures is most pronounced for both hemispheres in the months of March, April and May, and least noticeable in September, October and November.
The one major exception to this global warming trend is in central and eastern United States. Much of the country is more frequently experiencing cold spells, where temperatures fall into the 10th percentile of what's normal for six or more consecutive days. Portions of the central US have seen the annual number of cold days climb by two days per decade. Meanwhile some parts of the eastern US have had a decrease in the number of days a year that temperatures top 25 °C (77 °F).
Precipitation has also changed, although not as dramatically nor consistently around the globe as have temperatures. The overall trend since the 1950s however, is to a wetter climate. Many places, including the southern US and northern Canada, are receiving more storms each year that dump extremely heavy amounts of rain or snow.
L.V. Alexander, X. Zhang, T.C. Peterson, J. Caesar, B. Gleason, A.M.G. Klein Tank, M. Haylock, D. Collins, B. Trewin, F. Rahimzadeh, A. Tagipour, K. Rupa Kumar, J. Revadekar, G. Griffiths, L. Vincent, D.B. Stephenson, J. Burn, E. Aguilar, M. Brunet, M. Taylor, M. New, P. Zhai, M. Rusticucci and J.L. Vazquez-Aguirre. 2006. Global observed changes in daily climate extremes of temperature and precipitation. Journal of Geophysical Research. 111: D05109.