Auckland floods are a sign of things to come

*This story is new conversation Reprinted with permission*

Until New Zealand’s storm drainage systems adapt to the rising climate, they won’t be able to handle the levels of flooding seen in Auckland on Friday night, writes James Renwick.

The unusual flooding event Auckland experienced on the night of January 27, the eve of the city’s anniversary weekend, was caused by rainfall that was literally off the charts.

249 mm of rain fell in 24 hours, well above the previous record of 161.8 mm. An emergency declaration was issued at midnight.

Three people have been confirmed dead and one is missing, causing extensive damage to Aucklanders. Millions of dollars in damage to homes, cars, roads and infrastructure.

Looking at the images posted on social media on Friday night, I thought I’d seen this kind of photo before. But mostly they come from North America or Asia or Europe. But it was New Zealand’s largest city. No place is safe from extreme weather these days.

How did this happen

Torrential rains are due to North Tasman Sea storms associated with water sources from the tropics. This is what meteorologists call “rivers of the atmosphere.”

The storm moved very slowly. This is because it was carried south by a huge anticyclone (anticyclone) and stopped moving rapidly across the country.

A severe thunderstorm embedded in the main band of rain developed in unstable air over the Auckland area. These brought the heaviest rainfall, and MetService figures show Auckland Airport received his average monthly rainfall in January in less than an hour.

However, the type of storm that caused the mayhem was not particularly noteworthy. A similar storm passed through Auckland. But as the climate continues to warm, the amount of water vapor in the air will increase.

We believe climate change has played a large part in the incredible amount of rain in Auckland this time around.

warmer air means more water

Through careful analysis of historical records and numerous simulations using climate models, Return period The impact of this flood (according to the past climate, at least hundreds of years certainly).

The contribution of climate change to total precipitation will be part of these calculations. However, it is clear that this event is exactly what we expect as a result of climate change.

One degree of warming in the air averages about 7% more water vapor in that air. The Earth and New Zealand have experienced more than moderate warming in the last 100 years, and measured increases in water vapor content.

However, storms can increase precipitation by 7% or more. Air “converges” (is drawn into) the storm system near the Earth’s surface. So all the moist air is collected and “squeezed out” to make it rain.

Severe thunderstorms are the same thing on a smaller scale. Air is drawn in above ground, rises and cools rapidly, losing much of its moisture in the process.

The atmosphere currently holds 7% more water vapor, but this convergence of air masses means rainfall could be 10% or 20% heavier.

Beyond the capacity of stormwater systems

National Institute of Water and Atmosphere (Niwa) Estimate In Auckland, 1 degree of warming equates to about a 20% increase in hourly precipitation for a 1-in-50-year event.

The longer the climate warms, the more severe the storms will be.

Given what we’ve seen so far, how do we adapt? Flooding occurs when rainwater can’t drain fast enough. So what we need is bigger drains, bigger storm pipes and stormwater systems that can handle such extreme conditions.

The country’s storm drainage system was designed for the climate more than 50 years ago. What we need is a stormwater system designed for the climate now and 50 years from now.

Another part of the response is the “softening” of urban environments. Tar seals and concrete surfaces keep water on the surface, pooling and flowing.

If part of the stream diverted into the culvert can be reexposed and some wetlands can be rebuilt within the development area, a more natural spongy surface environment can be created that can handle heavy rainfall. These are the responses we must think and act on now.

We also need to stop burning fossil fuels and reduce global emissions of carbon dioxide and other greenhouse gases as soon as possible. New Zealand has an emissions reduction plan. We need to see the effect from this year. And all countries should follow suit.

As I said at the beginning, no community is immune from these extremes and we all have to work together.