Hydroclimate hostility
Hydroclimate refers to the interaction between climate and the hydrological cycle, including evapotranspiration, condensation, and precipitation. Global warming speeds up the hydrological cycle, and the amount of water vapour the atmosphere can contain increases exponentially with rising temperature.
However, the atmosphere is only saturated with water vapour when it can draw enough from water bodies, land surfaces, and plants. The difference between the atmosphere’s maximum potential humidity and its actual humidity is the vapour pressure deficit.
Warming tends to increase the vapour pressure deficit, thereby strengthening atmospheric evaporative demand. Add lower rainfall, and the result is drought.
“Drying – the drop in winter rainfall – is one of the most consistently predicted changes globally in all Mediterranean systems,” says Midgley. We already see this in data for the Mediterranean climate of the Western Cape.
Less rainfall on average can nonetheless be associated with increased flood risk. “All the models say that the Western Cape will lose the reliability of winter rainfall, but they’re mixed on the summer impacts,” says Midgley.
“We could get more rain and more extreme events, such as cut-off lows, in summer,” he elaborates. “We don’t really understand what generates cut-off lows, but they dump a lot of rain.”
As the atmosphere warms and the weather yo-yos between heavy rainfall and intense drying, we experience what climate scientists call hydroclimate whiplash.
When it rains, it pours
Cut-off lows are slow-moving pools of cold air in the upper atmosphere that have been cut off from the primary airstream. Cast adrift, these rotating air pockets spin in place or wander erratically, producing heavy rainfall until they are re-absorbed into air currents. Unlike cold fronts, which happen in winter, cut-off lows can occur at any time of year.
Although they are a normal part of Western Cape weather, extreme rainfall induced by cut-off lows has increased, likely due to greater atmospheric water-holding capacity. At current levels of 1.5 °C warming, the atmospheric water-holding capacity has already risen by ± 10% – that’s 10% more water that can fall when it rains.
Besides cut-off lows, extreme rainfall can result from atmospheric rivers. “These are concentrated flows of water vapour that are generated, possibly over the Amazon, and then flux across the Atlantic and into the Cape,” says Midgley.
Atmospheric rivers are responsible for most of the extreme winter-rainfall events in the Cape, disgorging their water when they hit mountains or low-pressure systems. And it can be a lot of water – on average, the flow in an atmospheric river is twice as much as in the Amazon River.
“We’ve seen evidence that atmospheric rivers are becoming more common,” says Midgley. “They were discovered relatively recently, and we’re learning a lot about their links to cut-off lows.”
Several studies have highlighted more frequent extreme weather events in SA, especially storms and flooding in the Western Cape. For example, most rainfall stations in the Western Cape have shown increases in 20- and 50-year single-day rainfall extremes.
However, the entire country is projected to experience more extreme single-day rainfall events and more extreme prolonged (more than five consecutive days) rainfall events.
Extreme rainfall is the main cause of flooding, with smaller catchments particularly at risk. Paradoxically, climate change may be reducing the frequency of minor floods, as drier soils absorb more water. But we can expect more of what used to be once-in-a-century floods, and we must plan accordingly.
Adding fuel to the fire
Large parts of the Western Cape were yet again ravaged by fires this past season. Although wildfires are a natural component of many ecosystems, including fynbos, there is no doubt that they are becoming more frequent and severe under climate change.
Ignition aside, three factors are key to wildfire occurrence and intensity: biomass amount, biomass dryness, and fire weather.
In California, which has suffered multiple devastating wildfires in recent years, researchers have found that high-intensity fires in scrub- and grass-dominated ecosystems occur after transitions of unusually wet to unusually dry periods.
During wet periods, native Californian vegetation grows rapidly, resulting in high fuel loads. When dry periods follow within six to 24 months, the thirsty atmosphere sucks the moisture from plants, leading to catastrophic wildfires within about six months.
The growth of indigenous South African vegetation, such as fynbos, is nutrient-limited and less responsive to rain, but Midgley thinks aliens could increase fire risk.
“The invasive acacias come from Western Australia, which has more extreme summers, so they’re pre-adapted to high temperatures. And they’re more likely to benefit from higher atmospheric CO2 than our endemic vegetation,” he says.
Fire weather is hot, dry, and windy. We already know that the weather is hotter and drier, but what about wind? According to Midgley, data indicate that the Western Cape is becoming windier.
Other researchers have also pointed out that one-hour gust speeds and wind runs (average wind speed multiplied by a specific period) have already increased, although the effects are not uniform across the Western Cape.
Beyond averages
Estimating the severity and probability of extreme events is challenging, but we need to start thinking beyond averages, as CO2 levels continue to rise.
“We’ve changed the climate to such an extent that there is no climate event anywhere on this planet that’s not affected by rising CO2 and greater atmospheric energy,” says Midgley. “Every event is different than it would have been. The question is how much?”
In the case of floods, we can anticipate that big events will occur more often. Analysing long-term climate data, Californian researchers calculated that their risk of a state-wide mega-flood has doubled over the past century. We don’t have similar numbers for SA, but we do know that flood risk is creeping up.
The effects of hydroclimate whiplash are not limited to fires after floods. Floods after fires have their own problems, as recently burned areas are more prone to erosion and landslides. Denuded soils also exacerbate flooding due to worse run-off. In California, researchers have estimated that the risk of flash floods is increased for up to eight years following wildfires.
Hydroclimate whiplash illustrates why planning for climate change must include planning for the worst. While it’s true that the pome- and stone-fruit industry can expect drier conditions on average, we should also be ready to deal with wetter wet periods, drier dry periods, and the consequences of abrupt transitions between them.
