2020 has also been hot. During one of the Northern Hemisphere’s warmest winters on record, the Great Lakes never froze, Russian officials in Moscow had to import fake snow for the holidays, and the fire season in parched California began months ahead of schedule. Temperatures soared in the Siberian Arctic, melting permafrost and fueling devastating, carbon-spewing fires. In Baghdad, where the mercury hit an unprecedented 125 degrees Fahrenheit in July, vegetation withered and metal door handles burned to the touch. Heat waves have smashed records from Phoenix to Hong Kong. Earth overall is on track to have its second-hottest year on record.
But how do scientists know this year’s weather disasters are linked to climate?
Let’s begin with the easiest to explain: coastal flooding. By melting polar ice sheets, human-caused warming has raised the global average sea level between eight and nine inches since the start of the industrial era. In some places, a variety of other factors — regional ocean currents, erosion, settling of the ground — can make the change even more extreme. One study of a “hot spot” along the Outer Banks of North Carolina found that sea levels were rising as fast as an inch per year.
The higher the baseline sea level, the easier it is for a simple high tide to send water surging into communities. According to the National Oceanic and Atmospheric Administration, flooding during high tides has doubled in the United States in the past 20 years.
Rising waters also increase the risk of flooding during hurricanes. In the Bay of Bengal, where sea levels are rising twice as fast as the global average, storm surge from Cyclone Amphan exceeded 16 feet and reached almost 10 miles inland.
But increased storm surge is just the start of ways climate change has made hurricanes worse. Storms draw strength from energy in the ocean. As water warms and evaporates, it can interact with weather disturbances to create a swirling cell of rising humid air, falling rain and raging winds. The warmer the water, the more intense the resulting storm. With global sea surface temperature increasing 0.13 degrees Fahrenheit per decade, studies show the chance of a given tropical storm becoming a hurricane that is Category 3 or greater has grown 8 percent every 10 years.
Higher ocean temperatures also make hurricanes more likely to rapidly intensify, catching forecasters and communities off guard. The Gulf Coast saw the consequences of this pattern in August, when the winds of Hurricane Laura increased 65 miles per hour in the 24 hours just before the storm made landfall. The storm killed 42 people and caused $14 billion in damage, ranking it among the 20 costliest Atlantic hurricanes on record.
In Lake Charles, La., a city of nearly 80,000 that was struck by Hurricane Delta six weeks after surviving Laura, the devastation has been profound. “We need help,” Mayor Nic Hunter pleaded in a Facebook video. “This truly is an American tragedy.”
The warmer air, meanwhile, allows for wetter hurricanes. This is a consequence of a physical phenomenon known as the Clausius-Clapeyron equation, which shows that for every 1 degree Celsius (1.8 degrees Fahrenheit) of warming, the atmosphere can hold 7 percent more moisture. Because the relationship between temperature and moisture isn’t linear, even small amounts of warming can create exponentially more destructive storms.
This was especially evident during Hurricane Harvey, which in 2017 dropped a stunning 60 inches of rain on South Texas, the most precipitation ever recorded from a single storm in the United States. Multiple studies have shown that climate change increased precipitation during the storm by at least 15 percent, and one study found that events like it are now six times more likely than they were just a few decades ago.
The flip side of the Clausius-Clapeyron equation is that a warmer atmosphere is able to suck more moisture from vegetation and soils, drying out fuels and setting the stage for worse wildfires. Scientists quantify this using a metric called the vapor pressure deficit (VPD), which calculates the difference between how much water vapor the air can hold and how much it actually does hold. A high VPD means an exceptionally “thirsty” atmosphere, and scientists say VPD levels in California this year were at their highest in 40 years.
A landmark study in 2016 found that climate change was responsible for more than half of the increase in fuel dryness in western U.S. forests in the past 50 years. As the world continues to warm, the West’s dry season will lengthen and the number of days when conditions are ripe for extreme fire behavior will spike.
The events of this year are yet another sign of how incremental increases in temperature can lead to exponentially worse natural disasters, experts say. The global average temperature has increased a little more than 1 degree Celsius (1.8 degrees Fahrenheit) since the preindustrial era, a number that may seem small. But research shows that human-caused warming has already doubled the amount of western forest burned since 1984. This year is California’s worst fire season on record, with five of California’s six biggest recorded wildfires, including the state’s first million-acre “gigafire” since 1932.
“It’s really been a shocking escalation,” said Daniel Swain, a climate scientist at the University of California at Los Angeles.
But that nonlinear relationship between rising temperatures and escalating impacts does have a silver lining, Swain said. If natural disasters become exponentially worse with every degree of warming, the future will become exponentially safer with every degree of warming humanity manages to avert.
“It means there is no point in the future at which it becomes pointless or hopeless to take action,” Swain said. “From here forward, everything we do is important.”
Do you have questions about climate change? Ask us here and maybe we’ll feature your question next!