The Earth is home to a range of climates, from the scorching dunes of the Sahara to the freezing ridges of Antarctica. Given this diversity, why are climate scientists so alarmed about a worldwide temperature increase of just 2.7 degrees Fahrenheit (1.5 degrees Celsius)?
Changing the average temperature of an entire planet, even if it's just by a few degrees, is a big deal, said Peter deMenocal, a paleoclimate scientist at Lamont-Doherty Earth Observatory at Columbia University in New York.
"A person living in any one location can experience huge changes in weather and even in climate, but those are often compensated by changes on opposite sides of the world," deMenocal told Live Science. [Is Global Warming Melting Antarctica's Ice?]
Right now, the world is about 2.1 degrees F (1.2 degrees C) warmer than it was during preindustrial times, deMenocal said. The 144 countries participating in the 2016 Paris Agreement announced that the world should limit the global increase in this century to 2.7 degrees F (1.5 degrees C), a stricter limit than the former goal of a 3.6 degrees F (2 degrees C) increase.
To put 2.7 degrees F into perspective, just about 9 degrees F (5 degrees C) separates the modern world from the last ice age, which ended about 15,000 years ago, deMenocal said. During that time, sea levels were about 350 feet (106 meters) lower than they are today, because an extensive amount of water was stored as ice at the poles, he said. During that ice age, about 32 percent of Earth was covered in ice, compared to just about 10 percent today, according to the National Snow and Ice Data Center.
Earth's climate changes over time — the last ice age is evidence of that — but it's the rapid rate of change and the amount of the greenhouse gas carbon dioxide filling up the atmosphere that have scientists concerned, deMenocal said. Moreover, global warming doesn't just increase temperatures; it also threatens the food, water, shelter, energy grid and health of humans, he said.
Climate change affects the ecosystems that provide food, "and therefore our security of food is linked to the security of those ecosystems," deMenocal said.
The oceans, for instance, provide people with about 20 percent of their dietary protein, deMenocal said. However, ocean acidification caused by climate change makes it difficult, if not impossible, for thousands of species, including oysters, crabs and corals, to form their protective shells, which in turn disrupts the food web, Live Science previously reported.
On land, an increase of 3.6 degrees F (2 degrees C) would almost double the water deficit and would lead to a drop in wheat and maize harvests, according to NASA.
Northern latitudes may see a temporary boost in soy and wheat farming, partly because of the warmer temperatures farther north and partly because increased carbon dioxide helps plants grow, NASA said. But at an increase of 3.6 degrees F (2 degrees C), this advantage almost disappears for soy, and entirely vanishes for wheat, NASA reported.
If temperatures get too hot when these plants are flowering, their growth can become stunted, leading to decreased or no edible food crop, such as corn or grain, NASA said. [How Often Do Ice Ages Happen?]
As temperatures warm and glaciers melt, the corresponding sea-level rise can destroy homes and cities. About 40 percent of the world's population lives within 62 miles (100 kilometers) of the coast, deMenocal said. In 2010, more than 123 million people, or 39 percent of the United States' population, lived in counties touching the shoreline, according to the National Ocean Service.
"Collectively, that is the single biggest investment at risk due to climate change as sea level rises," deMenocal said.
From 1901 to 1990, the average global sea levels rose about 0.04 inches (1.2 millimeters) per year, but from 1993 to 2010, the levels rose about 0.11 inches (3 mm) a year, meaning the rate of rise more than doubled, according to a 2015 report in the journal Nature.
About 7 percent of the United States' electricity generation in 2013 came from hydropower, which accounted for 52 percent of the nation's generated renewable energy that year, according to the Department of Energy.
However, reduced snowpack and shifting rainfall patterns may reduce hydropower in the long run, deMenocal said.
"This is now threatening the American West and some European areas as well," he said.
Increases in temperature and changing rain patterns are associated with the spread of vector-borne diseases (which another organism transmits between humans or from animals to humans), such as Lyme disease and malaria, deMenocal said.
"Even if it [a vector-borne disease] is eradicated locally in a particular region, the weather changes associated with climate change can lead to migrations of these vector-borne diseases to new regions," he said.
Furthermore, because of health concerns, some regions of the globe, such as parts of the Middle East and the American West, may become inhabitable to humans because of extreme temperatures, deMenocal said.
That's because humidity often increases with the heat index. When both are high, the human body is unable to evaporate sweat to cool itself. "If you're unable to evaporate [sweat], you can actually die from exposure," deMenocal said.
Extreme temperatures can also lower productivity among workers. According to a 2014 Bloomberg report on the economic risks of climate change, extreme heat, especially in the American Southeast, may lead to a 3 percent drop in outdoor worker productivity, including among people who work in construction, utility maintenance, landscaping and agriculture. This drop is twice that of the "productivity slowdown" that happened in the 1970s, which likely occured because of high inflation and economic instability, the report said.
All of these threats are just around the corner, deMenocal said. The Earth is anticipated to exceed the 2.7 degrees F (1.5 degrees C) milestone in about 15 years — between 2032 and 2039, deMenocal said. The planet is expected to surpass the 3.6 degrees F (2 degrees C) benchmark between 2050 and 2100, he said.
"If we're on our current emissions scenario, it's even sooner than that," he said. "Even over the last 8,000 years, we haven't seen a temperature extreme this rapid and this fast and large."
Original article on Live Science.
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Laura is the archaeology and Life's Little Mysteries editor at Live Science. She also reports on general science, including paleontology. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.