Figure 6.11.1 Future return periods (years) of current climate 1-in-20 year PED events, for four scenarios: CSIRO 2080s 25% scaling (top) and Hadley 2080s 75% scaling (bottom), and for standard scenario (left) and with ad hoc 5% PET reduction to represent increased stomatal resistance under CO₂ enrichment (right)

Text description of figure

The figure consists of four maps which, like Figures 3.4 and 6.9.5, show future return periods for what is currently a 1-in-20 year drought. The maps apply to the CSIRO model with 25% scaling (top 2 panels) and to the Hadley model with 75% scaling (bottom 2 panels). The two left-hand panels are reproduced from Figure 3.4, and the right-hand panels show the expected result if the CO₂ fertilization effect reduces overall PET by 5%, as described in the text.

For both scenarios, the CO₂ effect limits the increase in drought risk seen in the left-hand panels. For the CSIRO 2080s 25% case (top right), there is now only a small increase in drought risk relative to the current climate. What is currently a 1-in-20 year drought could become a 1-in-15 to 1-in-20 year drought for the Bay of Plenty, East Cape, Hawke’s Bay, Wairarapa, Marlborough down to North Otago and inland Otago. There are some scattered regions within these districts that could experience a 1-in-10 to 1-in-15 year drought.

For the Hadley 2080s 75% case (bottom right), what is currently a 1-in-20 year drought could become a 1-in-5 to 1-in-10 in Northland (except in the Far North), Coromandel, Bay of Plenty, East Cape, Wairarapa, Nelson, and from Marlborough down to North Otago. For the east coast of Northland, parts of the Coromandel, the regions around Tauranga and Gisborne, Hawke’s Bay, northern and coastal Wairarapa and the coastal regions of Kaikoura, North and South Canterbury, the current 1-in-20 year drought could occur every 3 to 5 years.