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2024-11-21 21:11:12 [百科] 来源：有聲有色網

Climate 101 is a Mashable series that answers provoking and salient questions about Earth’s warming climate.

Some things are not what they appear.

At a giant climate summit, global nations are meeting for the 26th time (COP26) to find ways to limit Earth's overall warming this century to no more than 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-Industrial Revolution temperatures. This ambitious warming target is certainly much lower than where civilization is currently headed — to an extreme 3 C (5.4 F) or so.

Yet 2 C, an average global temperature, would not actually be a low or small level of warming. Crucially, the amounts of warming where people live will be much greater, and the impacts in the ocean will be significant and long-lasting. These momentous effects are often overshadowed by a single, slight number.

"We really need to avoid the trap of emphasizing the global average," said Zeke Hausfather, a climate scientist and the director of climate and energy at the Breakthrough Institute, an environmental research center.

Earth scientists continually emphasize that humanity isn't inescapably doomed by the coming, inevitable disruptions to the climate. It's just the opposite. Society still has an extraordinary amount of influence in the matter: The more warming, the worse the impacts. But Earth's inhabitants should be aware that a 2 C world has extreme effects. It's all the more reason to avoid any warming above 2 C.

a graphic showing the average global temperatures on earth rising

Average global temperatures on Earth.Credit: NASA

Heating the land

Since the late 19th century, humanity has warmed Earth's total surface by 1.2 C, or a little over 2 F. This number is deceiving, because the phenomenally absorbent oceans soak up bounties of heat, which lowers the global average. But on land, significantly more warming has occurred, around 1.9 C, or some 3.5 F, Hausfather explained.

In other words, humanity will blow through the 2 C warming target in places people live and experience environmental extremes like severe heat, wildfires, drought, and beyond.

Climate change boosts the odds for record-breaking or more frequent extreme events. "It pushes the envelope of what is possible higher," emphasized Hausfather. "What really affects people is the extremes."

"What really affects people is the extremes."

To illustrate, higher overall temperatures:

amp the odds for record-breaking heat waves.
make severe wildfires more frequent, because a warmer atmosphere dries out the vegetation and trees, which more easily burn. ("It takes just a little bit of warming to lead to a lot more burning.")
melt some of Earth's largest ice sheets, like on Greenland. This raises sea levels, and will contribute to sea level rise for centuries.
make droughts more severe, like the prolonged Southwestern megadrought. Historically, droughts have significantly decreased crop yields.
increase the chances for more pummeling, extreme downpours and floods, because a warmer atmosphere can hold more water vapor. (This happens over oceans and land.)

A recent, potent example of boosted odds for extremes was the record-shattering heat wave in the Pacific Northwest and parts of Canada in June 2021. Temperatures hit 116 F in Portland, breaking the previous record by a whopping nine degrees.

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"In a world without climate change, that would have been essentially impossible," said Hausfather, citing research that found the heat wave would have been a 1-in-150,000-year event.

But in a 2 C world, the researchers found such an event would occur "roughly every 5 to 10 years."

That's intense.

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The relentlessly warming oceans

If it weren't for theimpressively absorbent oceans, Earth today would be significantly hotter. That's because much of the planet's unnatural, human-made heat is going into the water — not the air.

"Over 90 percent of heat from global warming is warming the oceans," said NASA oceanographer Josh Willis.

The ocean, then, is doing a great service for us, but at its own expense and the life there. Even if humanity stabilizes the climate at a relatively modest 2 C of warming, the oceans will still keep absorbing heat. This happens naturally, and will continue for around a millennium.

"The amount of warming will change the oceans for the next 50 generations or so," explained Willis.

Deep circulating currents, sometimes dubbed conveyor belts, cycle water from the ocean's surface to the cold depths. Generally, this takes 1,000 years, or more. The ocean's uptake of humanity's excess heat will only stop when the entire ocean reaches an equilibrium, or match, with the warmer surface.

The top 2,300 feet of the seas, where most marine life dwells, has already warmed some 1.5 F since the early 1900s. The consequences of a continually warming ocean are many:

When the ocean soaks up heat, it expands. This "thermal expansion" raises sea levels, and since 2004 has contributed to one-third of sea level rise. (By the end of the century, climate scientists estimate sea levels will rise by another 1.5 to 2.5 feet, and still continue rising.) "Even if we stop warming at 1.5 [C] degrees, the oceans are still going to keep rising for 1,000 years, because they're still absorbing that heat," said Willis.
Warmer oceans, particularly around the balmy equator, are already forcing animals to leave and find homes in cooler waters.
Higher water temperatures are eating away at the ends of massive glaciers in Greenland, destabilizing them and further adding to sea level rise.
A warming ocean is losing oxygen (warmer liquids hold less gas), with serious consequences for sea creatures. This includes less oxygen for animals like fish to breathe, and even causes blindness in some marine dwellers.
Similar to heat waves on land, warmer oceans make marine heat waves — which cause major coral die-offs — more frequent and extreme. Marine biologists expect marine heat waves this century to have "significant" and "widespread" ecological impacts, like on the fish that depend on coral ecosystems.
Warm ocean temperatures are jet fuel for hurricanes. Though hurricane development hinges on a number of complex weather factors, and the science of hurricane intensification is still unfolding, the U.S. National Oceanic and Atmospheric Administration expects cyclone intensities to increase in a 2 C world.

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That's not all. The oceans also naturally absorb carbon dioxide from the atmosphere, which acidifies the water. Crucially, today's CO2 levels are the highest they've been in some 3 million years, meaning the oceans are now acidifying at a relatively fast pace. Biologists are researching what this portends for ocean life, but already know that more acidic waters can hinder shell-building in vital marine creatures like corals, oysters, and clams, exposing them to predators and harm.

"It's devastating to some ecosystems," said Willis.

When out at sea, Wills has marveled at the expansive ocean, particularly the Pacific, which makes up about one-third of Earth's surface.

"It's so big. It seems so immovable and permanent," said Willis. "But it's changing because of us."

UPDATE: Nov. 8, 2021, 9:38 a.m. EST: This article has been corrected to say the top 2,300 feet of the ocean has warmed 1.5 F since the early 1900s. The article originally said 1.5 C.