Box 7.2: Changes in natural modes of the climate system.

Observed changes in climate over the Northern Hemisphere in winter reveal large warming over the main continental areas and cooling over the North Pacific and North Atlantic. This “cold ocean – warm land” pattern has been shown to be linked to changes in the atmospheric circulation, and, in particular, to the tendency in the past few decades for the North Atlantic Oscillation (NAO) to be in its positive phase. Similarly, the Pacific-North American (PNA) teleconnection pattern has been in a positive phase in association with a negative Southern Oscillation index or, equivalently, the tendency for El Niño-Southern Oscillation (ENSO) to prefer the warm El Niño phase following the 1976 climate shift (Chapter 2). Because of the differing heat capacities of land and ocean, the “cold ocean-warm land” pattern has amplified the Northern Hemisphere warming. A fingerprint of global warming from climate models run with increasing greenhouse gases indicates greater temperature increases over land than over the oceans, mainly from thermodynamic (heat capacity and moisture) effects. This anthropogenic signal is therefore very similar to that observed, although an in-depth analysis of the processes involved shows that the dynamical effects from atmospheric circulation changes are also important. In other words, the detection of the anthropogenic signal is potentially masked or modified by the nature of the observed circulation changes, at least in the northern winter season. The detection question can be better resolved if other seasons are also analysed (Chapter 12). Attribution of the cause of the observed changes requires improved understanding of the origin of the changes in atmospheric circulation. In particular, are the observed changes in ENSO and the NAO (and other modes) perhaps a consequence of global warming itself?

There is no simple answer to this question at present. Because the natural response of the atmosphere to warming (or indeed to any forcing) is to change large-scale waves, some regions will warm while others cool more than the hemispheric average, and counterintuitive changes can be experienced locally. Indeed, there are preferred modes of behaviour of the atmospheric circulation, sometimes manifested as preferred teleconnection patterns (see this chapter) that arise from the planetary waves in the atmosphere and the distribution of land, high topography, and ocean. Often these modes are demonstrably natural modes of either the atmosphere alone or the coupled atmosphere-ocean system. As such, it is also natural for modest changes in atmospheric forcing to project onto changes in these modes, through changes in their frequency and preferred sign, and the evidence suggests that changes can occur fairly abruptly. This is consistent with known behaviour of non-linear systems, where a slow change in forcing or internal mechanisms may not evoke much change in behaviour until some threshold is crossed at which time an abrupt switch occurs. The best known example is the evidence for a series of abrupt climate changes in the palaeoclimate record apparently partly in response to slow changes in sea level and the orbit of the Earth around the Sun (Milankovitch changes, see Chapter 2). There is increasing evidence that the observed changes in the NAO may well be, at least in part, a response of the system to observed changes in sea surface temperatures, and there are some indications that the warming of tropical oceans is a key part of this (see this chapter for more detail). ENSO is not simulated well enough in global climate models to have confidence in projected changes with global warming (Chapter 8). It is likely that changes in ENSO will occur, but their nature, how large and rapid they will be, and their implications for regional climate change around the world are quite uncertain and vary from model to model (see this chapter and Chapter 9). On time-scales of centuries, the continuing increase of greenhouse gases in the atmosphere may cause the climate system to cross a threshold associated with the Atlantic thermohaline circulation: beyond this threshold a permanent shut-down of the thermohaline circulation results (see this chapter and Chapter 9).

Therefore, climate change may manifest itself both as shifting means as well as changing preference of specific regimes, as evidenced by the observed trend toward positive values for the last 30 years in the NAO index and the climate “shift” in the tropical Pacific about 1976. While coupled models simulate features of observed natural climate variability such as the NAO and ENSO, suggesting that many of the relevant processes are included in the models, further progress is needed to depict these natural modes accurately. Moreover, because ENSO and NAO are key determinants of regional climate change, and they can possibly result in abrupt changes, there has been an increase in uncertainty in those aspects of climate change that critically depend on regional changes.