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Extreme heat waves have blanketed the , , , and this summer, with and exceeding 50 C.

As temperatures near outdoor survival thresholds, individuals who do not have easy access to air conditioning or cooling stations, or are , may .

These . But as tragic as they are to human health, they are only part of a larger climate catastrophe story — the wide-scale damage to the ecosystems that people depend upon, including agriculture, fisheries and freshwater.

Most wildlife cannot seek refuge from extreme heat. An estimated during the heatwave this past June in the Pacific Northwest alone.

Fisheries in hot water

Many people may perceive lakes and rivers to be refuges from unprecedented heat, but freshwater systems are no less sensitive. as temperatures exceeded the lethal limit for coldwater fishes.

This year’s hot and dry summer could . In British Columbia and Yukon, salmon numbers have declined by as much as 90 per cent and have led the federal government to shut down .

Coldwater fish, such as trout and salmon, are being squeezed out of their cool, well-oxygenated, deep-water habitat. As water contains less oxygen at higher water temperatures, this forces the fish to move into nearshore regions. While these shallower waters may be better oxygenated, they are even warmer and may exceed thermal tolerances of coldwater species.

By the same token, invasive fishes such as smallmouth bass are thriving in warmer temperatures and displacing native Canadian fishes like walleye and lake trout.

Water is on the move — too little and too much

The combination of a warming climate, drought and human activities, including irrigation for agriculture, can have drastic consequences for both the quality and quantity of our freshwater supply — ultimately leading to shortages of potable water.

By the end of the century, . Lakes closer to the equator, which are already experiencing the highest evaporation rates, are expected to experience the ���˴�Ƭ increase.

In regions with seasonal ice cover, evaporation rates can increase with warmer air temperatures and when ice cover is shorter or lost completely. This essentially “lifts the lid” on a lake during winter and could potentially lead to year-round evaporation, accelerating the rate at which water is lost. Salts and nutrients are concentrated in the remaining water, leading to further decline in water quality.

Potable water in countries with limited freshwater are , including the . . Even in water-rich areas like the Arctic, , including some , are already drying out.

On the other hand, ice-dammed glacial lakes in both polar and alpine regions are sensitive to outburst floods as dams melt, potentially flooding downstream ecosystems and the communities that depend on them, including population-rich areas such as and . Climate change is a crisis multiplier and threatens to make water scarcity or flooding an impending reality for increasingly more people.

Algal blooms on the rise

Warmer summers, coupled with that deliver large quantities of nutrients and pollutants in bursts, are creating the perfect conditions for . Harmful toxin-producing cyanobacteria (blue-green algae that frequently form floating surface blooms) can lead to mass mortality of fish and birds, as well as pose a serious health threat for cattle, pets, wildlife and humans.

In 2014, over half a million people could not use their water supply in Toledo, Ohio, because of a . Lake Taihu, China, which often has blooms so large that they can be and leave millions of people in a drinking water supply crisis.

In Ontario, there are now in formerly pristine northern lakes occurring as late as November. Study after study now links warmer conditions and the associated lake changes as important contributing factors to toxic blooms.

Rapid change requires rapid responses

Climatic extremes are now occurring more frequently and with greater intensity than were . We are already crossing ecosystem thresholds and tipping points that were not even projected to occur until the end of this century.

Climatic extremes will not appear gradually, but impacts will be felt quickly and often without warning, leaving little time for adaptation. We need to immediately , so that we are prepared for the inevitable emergencies already underway, including massive wildfires, coastal and , disruption of food supplies and freshwater shortages.

The apocalyptic future, once portrayed only in books and movies, is becoming our reality and the time for assessing our options is running out. Numerous studies have shown the benefits of reducing greenhouse gas emissions. Human innovation and originality, coupled with a sense of urgency, are required to lessen future impacts.

Without mitigation efforts, we must prepare for the fallout of the developing climate catastrophe and protect our citizens and ecosystems.

, Associate Professor and York University Research Chair in Global Change Biology, ; , Research Fellow, Climate Office, , and , Distinguished University Professor and Canada Research Chair in Environmental Change,

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