Observed climate variability and change of relevance to the biosphere

In this paper we review the current instrumental evidence regarding climate variations and change during the 20th century emphasizing those changes th at are likely to have direct interactions with the biosphere. Three basic q uestions are addressed: (1) Is the climate getting warmer, (2) is the hydro logic cycle changing, and (3) is the climate becoming more extreme. Based o n global near-surface temperature measurements for the 20th century, it is clear that a warming of similar to 0.5 degrees C has occurred. More importa ntly for biospheric systems, however, are the observed asymmetric changes i n daily maximum and minimum temperature? with the minimum temperatures incr easing at a rate approximately twice that of the maximum temperature. Other temperature-sensitive measures, such as glacial and snow cover extent, rei nforce the observed temperature trends. Examination of the hydrologic cycle indicates that changes also appear to be occurring, although less confiden ce can be placed on these analyses than those for temperature. Recent studi es suggest that precipitation has increased in higher latitudes, particular ly in the Northern Hemisphere. Increases in cloudiness, atmospheric water v apor, and changes in stream flow also suggest that changes to a more vigoro us hydrologic cycle are taking place. The final question regarding climate extremes is much more difficult to assess due to a lack of high temporal re solution climate databases. Of the few studies that have been performed, ho wever, there is evidence that precipitation extremes, particularly heavy ra infall events, are increasing in the United States and Australia, also sugg esting an enhanced hydrologic cycle as the planet warms.