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Explainer: climate modes and drought

The climate system operates with a range of discrete modes. Patrick Hoesly

While most people now understand that the enhanced greenhouse effect means a much warmer planet, communicating regional shifts in weather remains a significant challenge.

As with most complex science, nuance is everything. But how do you communicate complexity and nuance in a world increasingly geared to a 140-character limit? This is part three of a series looking at the relationship between climate change and rainfall. Part two is here.

Greenhouse climate change is forced by the buildup of greenhouse gas concentrations in the atmosphere. The climate system responds to this forcing by inexorable increases in temperature and by changing rainfall and circulation patterns. That response is superimposed on the natural fluctuations of climate, and is expressed through the natural modes of the system.

By “mode” here we mean a preferred circulation pattern. A crude metaphor for a related set of modes might be a bicycle with several gears. Each gear is a preferred mode, whereas the spaces between gears are not preferred. The bicycle is mostly in one of the gears/modes, with occasional transitions between the gears. The climate system has a range of different “bicycles” describing different types of modes that operate on different time and space scales.

Greenhouse climate change does not present a brand new way for the climate system to operate. The system still has the same ways of expressing variability through atmosphere and ocean circulation modes. The system in which those modes occur is warming. That does not make the modes go away, but it may change their behaviour, perhaps by making some modes more active and others less so.

In the metaphor of our bicycle, if we ride out of the flats and into the hills, we still have all the same gears, but we would use the gears differently in the hills. We might prefer to spend more time in the lower gears for example.

Climate favours different gears, depending on the circumstances, but will still generally use each of the gears. Rob J Brooks

Stewarts Franks’ contribution to The Conversation on the recent floods implies that we are not undergoing greenhouse climate change, just interannual and decadal variability as manifest by La Niña and the Pacific Decadal Oscillation respectively.

Franks points to natural modes of rainfall variability, as if the existence of these modes ruled out any role for greenhouse climate change. La Niña and decadal variability are intrinsic modes and expressions of the climate system, but that fact does not constitute evidence against greenhouse climate change.

We continue to have La Niña and El Niño events in a changing climate. Indeed, greenhouse climate change would express itself through the natural modes of the system such as La Niña and El Niño in the tropics and blocking (the tendency for persistent high pressure systems to form) in mid-latitudes. The climate system might change the frequency, length, position, or intensity of these natural modes in a warmer climate, but the modes themselves will persist.

Similarly, the climate system generates multi-decadal variation in circulation patterns because high frequency weather can generate a low frequency response in the ocean-atmosphere system. To point to such variability is not evidence against greenhouse climate change. A warmer climate system still has multi-decadal variability.

Such variability simply makes it a bit harder to extract and identify the climate change signal. Wet runs of years and dry runs of years will both continue, though the baseline about which these occur may change. If the rainfall baseline drops to lower averages (as seems plausible for southern Australia, based on the research), the dry runs will have larger impacts on the environment.

Multi-decadal variability is a feature of all aspects of the climate system and is not just a property of rainfall. Regional and global temperatures also undergo multi-decadal fluctations, which is why we don’t expect each year, or even decade, to set a new record for warming.

Droughts raise local temperatures, but from a higher point thanks to climate change. Surain Rajadurai

Some climate change critics have tried to have natural variability both ways to suit themselves. They are arguing that rainfall changes must be assessed over multiple decades on the one hand, but claiming inconsistently (and incorrectly) that any brief interruption in the upward march of temperature is evidence against greenhouse climate change. Global surface temperature is always going to bounce around on top of the warming trend.

We do have high confidence that temperatures will continue to rise under greenhouse forcing. The impact of those rising temperatures on the water balance during dry runs is disputed by Franks and he implies that increasing temperatures have no impact on the severity of drought.

Franks makes his argument by noting that temperatures are higher during droughts than wet periods due to the drying of soils, which changes the way energy is partitioned between evaporation and heating. This is true, is not in dispute, and has no bearing on the greenhouse role.

The temperature changes we are concerned about are not the natural increases that always occur during drought periods. Rather, we are concerned about the inexorable greenhouse-induced rise in global and regional temperature that raises temperature everywhere. Droughts will continue to raise local temperatures, but that rise will occur on top of a warmer baseline. The relevant question is whether droughts are exacerbated when they occur in a warmer climate due to the greenhouse-induced rise in temperature baseline.

If temperature were the only thing to change, then the atmospheric demand for water (termed the potential or Priestley-Taylor evaporation) would increase during droughts in warmed climates. That result is a consequence of the dependence of potential evaporation on temperature. Increases in potential evaporation increase aridity and reduce the amount of available water during droughts. Changes to other variables may oppose or support these changes, but the role of temperature increases will be to increase potential evaporation.

Wet and dry runs sit on a warmer baseline. Len Matthews

Rainfall is set to undergo a variety of different changes across Australia, and this issue is reviewed by Karl Braganza in this series. While the net impact of greenhouse-induced rainfall changes on drought depends on the precise regional changes in rainfall and on a better quantification of water loss processes, it is very likely that the contribution from greenhouse temperature increases will be to make each drought more stressful than it would otherwise have been.

Greenhouse climate change will generally be expressed through changes in the statistics of the preferred climate modes, but not by the extinction of those modes per se. The occurrence of a given mode or event such as a run of wet years doesn’t constitute evidence against greenhouse climate change. It is just an expression of a mode that is always present.

On the other hand, the spatial and temporal fingerprint of the century long changes in temperature is evidence of greenhouse forced climate change, and that in turn is changing the system in which the modes operate. We can expect changes in the statistics of wet runs (floods) and dry runs (droughts), superimposed on a warming baseline that tends to exacerbate both extremes of the hydrological cycle.

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