Why rising rainfall in the Arctic is bad news for the whole world

Loughborough, UK, December 2 (Talk) Before the end of this century, most of the Arctic will receive more rain than snow for the first time in a whole year. This is one of the key findings of a new study of Arctic rainfall, which has major implications – not just for the polar region, but for the whole world.

While a reduction in the surface of frozen oceans is one of the most recognized effects of Arctic warming, it has long been anticipated that a warmer Arctic will be wetter, with more intense water cycles between land, atmosphere and ocean.

The transition from a frozen region to a warmer, wetter Arctic is determined by the ability of a warmer atmosphere to retain more moisture, increased rates of evaporation from ice-free oceans, and relaxation of jet stream.

The Arctic water cycle is expected to change from a snow-dominated to a rain-dominated one during the 21st century, although the timing is uncertain. Now, a team of scientists has published a study in the journal Nature Communications, which suggests that this change will take place sooner than previously thought.

The effect will be particularly strong in the fall, with most of the Arctic Ocean, Siberia and the Canadian Archipelago becoming dominated by rain in the 2070s instead of the 2090s.

Warmer and wetter is not necessarily better. Such a profound change in the Arctic water cycle will inevitably affect land and ocean ecosystems. Intuitively, you might expect that a warmer, wetter Arctic would be very conducive to ecosystems – rainforests have far more species than tundra, after all. But Arctic plants and animals have evolved in cold conditions over millions of years, and their relatively simple food web is vulnerable to disturbance.

For example, warmer temperatures can cause larval insects to appear earlier, before the fish species that feed on them hatch. More rainfall means more nutrients washed into the rivers, which should benefit the microscopic plants at the base of the food chain.

However, this also makes rivers and coastal waters more turbulent, blocking the light needed for photosynthesis and can clog filter-feeding animals, including some whales or sharks. Brackish water typically supports fewer species than freshwater or seawater, so increasing freshwater flows from the open sea can reduce the range of animals and plants along the Arctic coast.

Further into the Arctic Ocean, there are several reasons to doubt the potential benefits of warmer temperatures and increased freshwater circulation. Dissolved constituents of precipitation, river water and melting snow and ice reduce the alkalinity of Arctic surface waters, making it difficult for marine organisms to build shells and skeletons and limiting the chemical neutralization of the acidifying effects of CO₂ absorbed in seawater.

At the same time, rivers flowing through degrading permafrost will largely wash away organic materials that bacteria can turn into CO₂, making the ocean more acidic. Fresh water essentially floats on denser sea water.

This causes the ocean to become stratified, preventing the exchange of nutrients and organisms between the depths of the sea and the surface and limiting biological activity. Therefore, the likely impact of a warmer and wetter Arctic on food webs, biodiversity and food security is uncertain, but is unlikely to be uniformly positive.

Arctic change is decades ahead of the global average Arctic temperature rises have exceeded the global average. This will only be strengthened as the snow is reduced and rainfall increases, as the snow reflects the sun’s energy back into space. As the earth becomes less snowy and less reflective, the bare earth will absorb more solar energy and thus heat up.

The Arctic will continue to warm faster than elsewhere, further reducing the temperature difference between the warmest and coldest parts of the planet, with complex implications for the oceans and atmosphere.

The recent COP26 climate summit in Glasgow focused on efforts to “keep 1.5 ° C alive”. It is worth remembering that the figure of 1.5 ° C is a global average and that the Arctic will heat up at least twice as much as this, even for modest projections.

The new study highlights the importance of the global 1.5 ° C target for the Arctic. For example, at that level of warming, Greenland will move to a rain-dominated climate for most of the year. While at 3 ° C warming, which is close to the current path based on existing policies rather than commitments, most Arctic regions will move to a rain-dominated climate before the end of the 21st century.

It is a research that adds more weight to the demands for improved monitoring of Arctic hydrological systems and a growing awareness of the considerable impact of even small increases in global warming. (Conversation) SCY SCY

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