By George Blake, Kingfisher Writer

Published May 26th, 2022

Central Chile is currently experiencing a mega-drought on a scale rarely, if ever, seen in the last millennia. River flows have dropped low enough that water restrictions are now in place in Chile’s capital Santiago, a city of some six million people. Researchers, policymakers, and local stakeholders are now urgently trying to establish if this is a sign of things to come?

Chileans are no strangers to droughts, but the most recent dry spell gripping central Chile is unprecedented. The long-term decline in rainfall observed over central Chile has been exacerbated by an uninterrupted sequence of dry years since 2010. Dubbed the Central Chile Mega-Drought, it is by far the longest observed in Chilean records (dating back to 1914), with researchers suggesting it may have few (if any) analogues in the last millennia.

Over the last 13 years, annual rainfall deficits have ranged between 25% and 80%, and as the drought persists, the picture is becoming increasingly grim. Ranchers have seen tens of thousands of cattle die, lakes are drying up, the Andean snowpack has reduced, annual mean river discharges are down by up to 90%, and there have been wide-scale reductions in groundwater and reservoir levels across central Chile and westernmost Argentina.

The lack of rainfall has also had severe impacts on vegetation; landscapes that would normally be a lush green have withered. The Normalised Vegetation Index (NDVI), which provides a measure of the health and greenness of vegetation based on the amount of infrared light that is reflected, demonstrates this decline.

The map below contrasts the 2019 NDVI against the 2000 to 2010 average, with brown areas indicating that vegetation health has declined (healthy vegetation with lots of chlorophyll reflects more infrared light and thus appears more green). Barren landscapes and the lack of flowers are also causing beehives to disappear, with agricultural officials declaring a state of emergency in more than 100 farming communities. The only areas where vegetation is thriving are in valleys fed by mountain glaciers, likely a reflection of enhanced melt rates.

This now record-breaking drought has reduced flows in the Maipo and Mapocho rivers in central Chile—which account for much of the capital Santiago’s water supply—to such an extent that in April 2022, the government announced plans to implement water rationing measures.

‘Over the last 13 years, annual rainfall deficits have ranged between 25% and 80%.’

The plan consists of four tiers, beginning with public service announcements, before moving on to restricting water pressure and ending with rotating water cuts across four, six, or twelve days. These cuts will not see the water supply permanently switched off for twelve days, rather water supply will be temporarily switched off and on, with the maximum continuous cut being 24 hours. How long the rotating cuts last (four, six, or twelve-day periods) and their duration (five, nine, twenty hours) is dependent on water deficit rates, measured in litres per second, in the Maipo and Mapocho rivers.

Areas fed by boreholes, or other sources beyond these two rivers, will be exempt from restrictions, but the measures could still affect up to 1.7 million people.

The pressing question now is when will this drought end? The answer depends on the driving forces behind a given drought.

Chile is characterised by a typical Mediterranean-like climate, annual rainfall totals are highly variable and most rainfall would normally fall during a relatively small number of heavy winter storms. Intense short-lived droughts are commonplace in these climates, including in Chile, where rainfall can vary between 100 and 2000 millimetres per year. These large interannual fluctuations are driven by natural climate variability phenomena such as El Niño, which is the leading driver of drought in many regions worldwide.

If one concludes that a drought event primarily reflects natural forcing (such as El Niño/La Niña cycles). In that case, it is reasonable to expect that wetter conditions should return in the foreseeable future, although the exact timing will remain unknown.

Whereas, if it is established that drought is primarily due to climate change, one should assume that there will be a sustained trend towards drier and drier conditions. Given the drought over central Chile has now lasted for 13 years, it is evident the driving mechanism is different to that driving short one to two-year droughts.

However, identifying whether the drought is fundamentally a response to natural or anthropogenic forces is challenging, and researchers believe it is a combination of both.

‘The drought over central Chile has now lasted for 13 years.’

Modelling shows the dry signal is predominantly a function of sea surface temperature (SST) anomalies in the subtropical south-west Pacific (SSWP), in a region known as the Southern Blob. This ‘blob’ of water in the SSWP is larger than the continental United States and has exhibited a clear warming trend over the last 20 years. Through a series of complex atmospheric processes, warming in this giant area of ocean can lead to persistent high-pressure conditions over western South America. This effect is particularly strong in the southern hemisphere winter and pushes storms poleward, south of central Chile.

The origin of these elevated SSTs in the SSWP is related to anthropogenic warming and natural variability. Pre-industrial climate models, which do not account for anthropogenic carbon emissions, are able to replicate this warming and subsequent long-term dry periods over central Chile. Hence, researchers believe natural factors are the dominant cause of the recent drying, although they acknowledge that current warming rates exceed the range of natural variability. This means that human activity is exacerbating the drought.

So while natural variability may partially reverse the rainfall signal at some point in the future, anthropogenic emissions will continue to relentlessly push Chile and westernmost Argentina towards ever drier conditions. The intensity of this push will depend on the emission scenario we choose to follow in the coming decades.

Although uncertainties remain, scientists worry this mega-drought could be heralding the new dry conditions we should expect in the 21st century. Regional climate projections suggest that mean rainfall could reduce by up to 40% (relative to current values) in the second half of the century under high emission scenarios.

Hence, the priority for stakeholders—such as national water authorities and agricultural leaders—is to establish ways to adapt to this new reality. The Chilean government is hoping to mitigate future drought impacts by building 26 new reservoirs, and 20 more desalination plants, and has also added a clause in agency budgets that accounts for the ‘costs’ of climate change. As the impacts of climate change worsen, this scramble to update water infrastructure is likely to be repeated across semi-arid climates.

Featured Image: International Monetary Fund | Flickr

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