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As the mercury soars, many of us will find comfort picnicking by a river, swimming at the beach or resting from a hike by soaking our feet in an alpine lake. Yet all these environments are facing the threat of climate change in the coming decades. We’re likely to see some sudden and even irreversible impacts.

In future, beachgoers will find the oceans a little warmer when they take a dip. But in other ways, our beaches could become a less reliable refuge.

If a big storm’s just blown through, you’ll also have to stay on the sand, as the overwhelmed wastewater systems may have flushed raw sewage into the seas.

Alternatively, your favourite sandy beach may be sacrificed to build a rock wall to protect the homes of residents behind it from the rising tides.

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You may turn up at the coastline to find there’s a “red tide” (an algal bloom), meaning you and your pets will have to stay out of the water, said Niwa marine ecologist Matt Pinkerton​.

If there’s still some sand remaining, you might notice fewer shells and larger dumps of seaweed, ripped out by more intense storms. The seaweed dumps could rot, making life unpleasant, Pinkerton said.

Large swarms of jellyfish may be another unwelcome sight, he added.

And that’s if you can get to the sea at all – steps, handrails and jetties are likely to suffer more frequent damage, from storms and erosion.

Divers may notice a shift in the clarity of the seawater, though this is harder to predict, Pinkerton said. In general, the numbers of microscopic plankton in the water is falling around parts of the North Island and increasing around the South Island.

“Around a lot of the coast, it seems to be clearing, which is a good thing if you’re a diver maybe… but not so good if you’re a fish looking for food,” he added.

However, some coastal areas, such as the Hauraki Gulf and Raglan, have a growing amount of run-off from the land entering the ocean, which can make things murky courtesy of microscopic plankton. The big storms we’ll experience as the earth warms could worsen this issue, he said.

Our local lakes and rivers will experience literal ups and downs in future, as the rains that feed them are predicted to alter.

In general, if you live in the west of the country, your rivers will most likely see more rain, and could burst their banks more frequently.

As at the coast, the banks of your favourite freshwater swimming spot could be concreted to provide protection for the homes and farmlands behind it from these overflows.

If you’re in the east, water will be scarcer. With increased evaporation from warmer days, some rivers and lakes and wetlands could dry up completely.

Low rainfall in these areas will mean underground aquifers, which provide water for drinking and irrigation, will be slower to fill. Consequently, nearby farms and cities will look to rivers to get water, which could see levels fall even lower.

Yet, when the clouds do arrive, the downpours may be intense.

Swimmers and boaties will also experience the knock-on effects. Rivers will be increasingly contaminated with sewage after big deluges wash through wastewater pipes.

On top of that, we’ll increasingly see toxic algae, which enjoy warmer, settled conditions and thrive when nutrients leak into the water from the land. These blooms make it unsafe to swim and can kill fish and pets. Deeper lakes, which can separate into a cool bottom layer and warmer top layer, can be particularly vulnerable to this phenomenon.

University of Canterbury freshwater scientist Angus McIntosh​ said small-scale floods flush out rivers, keeping them healthy.

“If we don’t get floods, we can get much longer algal blooms,” he added. “New Zealand systems are used to getting floods but the problem is the floods are getting more and more extreme as well.”

Coastal estuaries, lagoons or wetlands will experience between 15 and 20 centimetres of sea level rise by mid-century. It may be too slow to be spotted by the locals, but the increasing seawater incursions into the freshwater will raise the salinity of these environments, affecting the life within.

In unpopulated areas, these habitats may be able to migrate inland, but those fringed by houses, buildings and roads could be eventually squeezed out.

Those in search of kaimoana will also see some upheaval, relative to today. One impact of climate change – ocean acidification – will affect marine species right up the food chain.

As well as heating the planet, carbon dioxide also enters our water, making it more acidic, said Niwa marine scientist Cliff Law​.

That will affect the amount of dissolved carbonate – one of the components of limestone – in the water, Law added.

“There’s a lot of animals and organisms that build their shells or exoskeletons out of carbonate. If there’s less of that in the water, it obviously makes it harder for them to grow and maintain their shells and skeletons.”

Life will be tougher for shellfish – such as kina, mussels, oysters and paua – plus corals, algae, plankton and crustaceans, such as crabs, crayfish and shrimps.

By the middle of the century, the increasing acidity of the ocean could be affecting the youngest, and therefore most vulnerable, marine animals, Law said.

That could cause a bottleneck: “The adults and juveniles will be fine but if you’ve got a weak spot, which is these very early life stages, then they might not get to be juveniles or adults.”

Meanwhile, fishermen – whether at sea or in freshwater – may start to notice fish species they’ve caught for years become rare.

Temperature will be one factor driving this shift, Law said. Marine heatwaves are already being recorded in the spring and summer months. “Things that prefer cooler water will shift further south.”

Even the classic whitebait sandwich is at risk – four of the six native migratory fish species are already declining.

Extreme floods could put additional pressure on the whitebait species that lay eggs on river margins when water levels are high, McIntosh said.

Normally, these eggs hatch into larvae and are washed downstream during the next cycle of high water, he explained. “Those eggs might just be washed away before they hatch… Or we could lose lots of habitat.”

As these pressures build, catch regulations may get tighter in response to public concerns about extinction.

At sea, the intensifying storms will also crash through marine habitats, such as kelp beds, affecting the species using them as nursery grounds.

Up in the mountains, lakes and rivers are drying out, McIntosh said. The species calling these alpine environments home are being pushed to higher elevations. “But if there isn’t more mountain to move up, then there’s nowhere to go.”

Even some species that can deal with the dry, such as the Canterbury mudfish, have a bleak outlook, McIntosh said. As the name suggests, the species can survive out of the water, in moist conditions for several weeks.

“They’re like the sloths of streams,” he said. “As things get more and more and more extreme, their capacity to withstand those extremes is being exceeded.”

Pinkerton warns that the changes may not be gradual. “The numbers of some species may change abruptly or suddenly.”

With marine and freshwater wildlife already under pressure from pollution, invasive species, human infrastructure and loss of habitat, climate change could push it over the edge. “That’s difficult to predict,” he said.

Not all wildlife will suffer, Pinkerton added. Some species will find the effects of climate change more to their liking. “You’re going to see winners and losers.”

Easing additional pressures on our oceans, lakes and rivers could be our country’s best chance at helping at-risk species adapt.

In farming communities, we could decrease the nutrients entering the water, whether by capping fertiliser or through systems limiting nutrient loss. This is no mean feat – the protection of one lake requires the work of every person living and working by each one of its tributaries, McIntosh said.

We’ll also need to rethink how water flows through towns, he said. Urban rivers get a lot higher after storms. “We’ve designed all these channels and pipes and roofs and concrete into our cities to get that water down into the rivers faster.”

To boost resilience, urban areas will need to store rain, he said. “We need to find a happy medium there, where we can provide for storm water runoff, but also provide the habitats and decontamination… Heavy metals are a real problem.”

Wherever we live, our local lake, river or wetland would benefit from restoration, McIntosh said. Provided they don’t suck up lots of water, riverside plants can boost the environment’s health. “Shade is good because it reduces temperatures and it might stop some evaporation of the water.”

Replanted habitats also have more food, he said.

At the same time, we can replace barriers that get in the way of migrating fish with friendlier passages. And we may need to embrace unkempt riverbanks, McIntosh added. “The reality is that messy, less manicured riparian margins are good for fish during spawning periods.”

On the coasts, our willingness to retreat as the tides rise could determine the fate of wildlife populations, warned a recent report released by Parliamentary Commissioner for the Environment Simon Upton. We could lose both homes and beloved natural habitats “if we do not retreat early enough to give coastal species the space and time they need to move with rising tide lines,” the report warned.

Most importantly, the country must turn off the carbon tap, Law said. Rapidly decarbonising our world will avoid the worst effects of climate change, though the weather will still become less stable.

“I’d like to think that, post-Covid, there’s this recognition that we need to be moving, not just in this country but globally, on reduction of emissions.”

Upton hopes we may see healthier beaches, lakes and rivers in the coming decades.

“In many cases, the experience today may not be so wonderful,” he said. “We know what the problems are. We know what the pressures are. We have to start investing in some of the – I’m afraid – costly solutions.”

McIntosh believes our native species are capable of adapting, if given a fighting chance. After all, they developed in a land regularly hit by floods and earthquakes.

“There’s a good chance those things will recover,” he added. “We just need to make sure we’re not exceeding their capacity to adapt.”

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