Climate change: encroaching seas to affect coastal homes by 2040s
Flooding will become an increasing reality for low-lying coastal properties in just two decades, well within the lifetime of an average mortgage, a new Niwa study warns. Without emissions cuts, our oceans could rise one metre or more by 2100. The increase is currently slow, though it is accelerating . Although the pace of change may feel gradual, new research from Niwa suggests low-lying properties could experience a "huge" increase in flooding events from the 2040s. This surge means coastal planning needs urgent attention, research author and Niwa coastal hazards scientist Dr Scott Stephens said. In the 2040s, the sea is expected to be 10 to 20 centimetres higher than it is today. This rise is primarily based on greenhouse gas released in the past and therefore is locked in. Any emissions cuts made today or in future will only reduce the scale of long-term sea level rise and delay its arrival. READ MORE: * Storm surges to get bigger in New Zealand's south, smaller in north * Homes of 240,000 Kiwis in way of worst case 2100 sea level rise, study indicates * South Canterbury needs to prepare for coastal erosion, climate change Stephens and a team of researchers looked back on a natural phenomenon that causes New Zealand's sea to rise (and then fall) by approximately 10cm each year known as the annual sea-level cycle. This gives us insight into the effect climate change will have on our coasts in the coming years. Stephens said summer warmth gently heats the ocean waters, causing them to expand. Because "there's a bit of a lag", the expansion in New Zealand peaks about May before the seas begin to contract again, he said. The new research, published recently in the journal Natural Hazards and Earth Science Systems , examined 120 years of oceanic records to determine how factors including the annual sea-level cycle were aligned to extremely high ocean levels. The study found floods are more likely to occur here between January and August because of the annual cycle. "It made a surprising difference to the number of extreme sea levels we saw," Stephens said. "What we learned from that is small sea level change makes a big difference." For low-lying communities and homeowners, the research results should be a wake-up call to the likely effects of climate change-induced sea level rise, Stephens said. "This shows we're going to see the impacts of sea level rise, on increasing flooding, within the lifetime of an average mortgage. It's not 100 years from now. It's starting to happen already," he said. "We really do need to get our planning in order." By 2040, the 10 to 20cm of climate change-caused sea level rise, on top of the 10cm of the annual sea-level cycle, combined with a spring or king tide could make sunny-day flooding inundation without a storm of coastal homes and roads increasingly frequent. "Sunny-day floods... are now starting to encroach into places they didn't before. That is going to become an increasing problem," Stephens said. The Niwa research also found certain weather patterns were linked to coastal inundation. These were "blocking" high-pressure systems that intensified the low-pressure systems behind them which can cause issues for the upper North Island's east coast and areas of low pressure in the Southern Ocean, which can cause flooding in the South Island and on the west coast of the North Island. Even so, the tide (which causes our seas to rise and fall up to 3.5 metres ) was the key cause of coastal flooding, Stephens said. "We don't get flooding unless we have a higher-than-normal tide, generally," he said. "The very biggest events need a big tide and also need a storm at the same time. It's the things coinciding."