Atmospheric circulation as a source of uncertainty in climate change projections

703 The accepted evidence of anthropogenic climate change1 is based on multiple global indicators of change, including surface temperature, upper-ocean heat content, sea level, Arctic sea-ice extent, glaciers, Northern Hemisphere snow cover, large-scale precipitation patterns (especially as reflected in ocean salinity) and temperature extremes (Fig. 1a,b). All these global indicators are physically linked in a direct way to the first on the list, surface temperature, and the changes are robust in observations, theory and models1. Owing to the consistency of the evidence and the physical understanding of the changes, both scientific and public attention is rapidly shifting from the detection and attribution of global climate change — by all measures a settled scientific question — to the quantification and prediction of its manifestations at the regional scale, together with an increasing demand for uncertainties. This attention is heightened whenever there are record-breaking weather events, recent examples being Australian summertime heat waves, wintertime cold-air outbreaks over the continental United States and wintertime flooding in the United Kingdom. Although the proximate explanation of such events is always the synoptic weather patterns prevailing at the time, the inevitable question that arises is whether such events are now more likely and are harbingers of things to come2. On the regional scale, climate is strongly affected by aspects of atmospheric circulation, such as monsoons, jet streams and storm tracks. For example, there is a well-documented relationship between the North Atlantic Oscillation, with its associated modulation of the position of the North Atlantic storm track, and wintertime weather conditions over Europe3. More generally, there is a relationship between the amplitude of mid-latitude planetary waves and particular regional weather extremes, which varies with region and implies that opposite-signed extremes in different regions may reflect the same underlying driver4. Planetary waves also provide non-local teleconnections, for example, between El Niño/ Southern Oscillation (ENSO) and the Indian summer monsoon5. Circulation furthermore impacts atmospheric chemistry; for example, the observed changes in tropospheric ozone at Mauna Loa over the past 40 years have been attributed to changes in circulation rather than to changes in precursor emissions6. In contrast to the Atmospheric circulation as a source of uncertainty in climate change projections