Future megadroughts could be worse than we thought – New Zealand’s past shows why

The Committee on Climate Change’s latest national risk assessment notes that these events are becoming more severe over time, particularly in the country’s drier north and east.

Recent events have given us a glimpse of what it all looks like: brown paddocks, shrinking reservoirs, dry riverbeds and farmers struggling for feed. They also illustrate the severe economic damage that drought can cause.

An ongoing drought in 2007-08 hit the Waikato particularly hard, causing feed shortages and quadrupling silage prices, costing the national economy billions of dollars.

Another drought four years later covered the entire North Island and was later estimated to have reduced national economic output by up to 0.7% of GDP.

The Reserve Bank has since considered the 2012-13 event as a possible worst-case drought scenario in some agricultural lending risk assessments.

But looking further back in history, we find evidence of more severe meteorological droughts (long periods of abnormally low rainfall) that occurred in the early 20th century.

Our new research shows that some of New Zealand’s most extreme drought history has actually been ignored in modern policymaking.

Reconstructing New Zealand’s drought history

When scientists and planners assess the severity of droughts, they often rely on data sets called virtual climate station networks. It uses rainfall, temperature and other weather observations to provide a detailed look at climate conditions across New Zealand.

While this is an invaluable tool, reliable data on many variables only date back to the 1970s. This means that many modern drought assessment methods are less useful for understanding the severe events that occurred in the early 20th century.

To get a clearer picture of these early droughts, we focused on a reliable indicator: the rainfall deficit. Thousands of rain gauges across New Zealand were recording this data as early as the 1860s.

Next, we selected weather stations with long-term reliable records, including at least 70 years of data and observations before 1914. This creates a nationwide network of 97 high-quality weather stations.

Finally, we compare historical rainfall data to long-term averages to understand how drought conditions develop over weeks, months, and longer periods of time.

Contrary to the belief that New Zealand’s most severe events have occurred in recent decades, the results show that the three most extreme meteorological droughts in the instrumental record all occurred before 1950.

Forgotten Daqian

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The most widespread and severe drought began in the winter of 1914 and lasted until at least February 1915. Most weather stations recorded the worst rainfall deficits on record between July and October, with severe shortfalls continuing throughout much of the North Island and eastern South Island throughout the summer.

Nationally, no other eight-month period can compare. More than half of the sites recorded one of the five driest periods on record, with the following July-February period ranking second, suggesting an unusual multi-year drought sequence.

New Zealand has experienced similar late-winter droughts recently, particularly in 1993, when Auckland’s reservoir capacity dropped to about a third during the city’s water crisis. But a drought on the scale of 1914-15 could have a more serious impact.

Looking specifically at summer droughts, the period from July to February 1908 saw the most widespread extreme rainfall deficit in recorded history.

Many weather stations in the South Island are experiencing the driest conditions on record, with more than double the number of stations recording extreme drought compared to the second worst summer drought in 1945/46.

[1945-46年的干旱在北岛北部和东部也很突出，许多气象站都出现了有记录以来最严重的降雨短缺。这些地区的降雨量短缺比人们记忆最深刻的2012-13年干旱期间增加了约22%。

被低估的风险

最近的干旱经历与这些早期历史极端事件之间的差距是惊人的，这凸显了超越过去几十年的重要性。

新西兰气候历史上已经存在过比近期记忆中经历过的严重得多的干旱。

然而，许多现代风险评估仍然严重依赖最近发生的事件，可能低估了该国可能经历的干旱变化的真实规模。

气候变化增加了另一层风险，气温上升预计将进一步导致土壤干燥，并在长期干旱期间增加蒸发，从而加剧影响。

所有这些都意味着最近的干旱可能不再为未来的情况提供可靠的指导。

气候变暖可能会使新西兰陷入人们记忆中从未经历过的干旱状况——未来的规划和风险评估需要紧急考虑这一点。 GRS

GRS

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