In a stable climate, the ratio of new record highs to new record lows is approximately even. However in our warming climate, record highs have begun to outpace record lows, with the imbalance growing for the past three decades. This trend is one of the clearest signals of climate change that we experience directly.
Trends in California driven by climate change—including higher temperatures and drier conditions—are elevating the risk of dangerous and destructive wildfires across California and the western United States.
On October 8, a group of fires exploded across a wide swath of Northern California. Taken as a group, the fires are among the worst on record in the state in terms of lives and property lost. After just one week, the fires have killed at least 41 people, burnt more than 200,000 acres, destroyed or damaged more than 5,500 homes, and displaced 100,000 people. Governor Jerry Brown declared a state of emergency in Butte, Lake, Mendocino, Napa, Nevada, Orange, Sonoma and Yuba counties.
The combination of a wet winter followed by extreme heat and dryness has fueled record wildfires in many Western states. Climate change compounds the risks of wildfires by extending the length of the fire season and adding to the intensity of droughts and heat waves. Park Williams, a climate and drought expert, noted, “... the combination of dry fuel, extreme heat and climate change is a recipe for what we are seeing."
A vast area of high pressure brought unusual heat and humidity to the Great Lakes and Ohio Valley after the end of astronomical summer. This late-season heat wave occurred the same year and in much of the same region that experienced record hot and unseasonably early temperatures in February. Formal attribution work has identified the fingerprint of global warming in February's record hot temperatures, as climate change increased the likelihood of such heat by threefold.
Chicago had seven days in a row of record daily high temperatures, with the mercury reaching or passing the 92 degree-mark each day from September 20th to 26th. This is the latest stretch of seven consecutive 90-degree days on record in Chicago. Further east, Cleveland had a five-day streak of daily record highs, and Lansing, Michigan had a five-day streak of highs that tied or broke the previous records set in 1891 and 1895. A handful of other cities saw their highest temperatures on record for this late in the year, and Burlington, Vermont warmed above 90°F for the first time since 1939.
The fingerprint of global warming has been firmly identified in the increasing intensity, duration and frequency of extreme heat events globally. Heat waves exacerbate the risks associated with heat exposure, and urban residents are particularly vulnerable to threats of heat waves due to the urban heat island effect.
The record-breaking rainfall and flooding driven by Hurricane Maria—as well as Hurricanes Harvey and Irma just weeks before—is consistent with the long-term trend driven by climate change. Hurricane Maria made landfall in Dominica as a Category 5 hurricane on September 18, then hit southeast Puerto Rico on September 20 with 155 mph winds and a central pressure of 917 millibars. It was the third strongest storm to make landfall in the United States. "1,000-year" rains inundated much of eastern and northwestern Puerto Rico. The storm knocked out power to the entire island of Puerto Rico, home to 3.5 million people, leading to a prolonged humanitarian crisis.
Extreme rainfall is increasing worldwide due to climate change. In Puerto Rico, rain falling in very heavy events increased at least 33 percent from 1958-2012. Seas are now higher due to global warming, so storm surge drives much further inland. There has also been a global increase in the observed intensity of the strongest tropical cyclones, correlated with observed trends in sea surface temperatures in recent decades.
NFIP claims rise alongside upward trends in extreme storm frequency . . .
The increasing frequency of US National Flood Insurance Program (NFIP) claims parallels the increasing frequency of extreme rainfall and flooding in the United States.
Higher temperatures and drier conditions—both linked to climate change—are increasing wildfire risk in California.
The La Tuna Fire erupted north of downtown Los Angeles and, at more than 7,000 acres, the fire is the largest to burn within Los Angeles' city limits.
While small compared to the biggest fires in California's history, the La Tuna Fire is notable for its proximity to a major city and its rapid growth amid unseasonably warm temperatures. The fire also erupted at the edge of the Angeles National Forest, a region where bark beetle activity has increased in recent years of extreme heat and drought.
"The greatest statewide heat wave ever recorded in California" began in late August and lasted through early September, as a major high pressure system stalled over the western United States. Many locations broke daily, monthly, and all time temperature records. Most notably, San Francisco broke its all time heat record, reaching 106°F on September 1.
Record breaking heat waves are a classic signal of climate change. The trend in global warming has contributed to the severity and probability of 82 percent of record-hot days globally over the 1961-2010 period.
The heat wave bears several signatures of heat waves on a warming planet: record-breaking heat, hot nights, high humidity, long duration, and increasing frequency.
This heat wave comes on the heels of June’s heat wave, which also broke temperatures records across the state.
Climate change is amplifying the damage done by hurricanes, by elevating sea levels and thus extending the reach of storm surge, and by loading storms with additional rainfall and thereby increasing flood risk.
In addition there is significant evidence linking climate change to the observed shift in the track of hurricanes such as Irma toward the US coast.
Hurricane Irma maintained maximum wind speeds of at least 180 mph for 37 hours, longer than any storm on Earth on record, passing Super Typhoon Haiyan, the previous record holder (24 hours). Irma’s maximum accumulated energy over 24 hours was the highest for any Atlantic hurricane on record. The storm intensified into a Category 5 with 185 mph winds on September 5, making it the most powerful Atlantic hurricane ever recorded outside of the Caribbean and Gulf of Mexico where warmer waters make those areas more prone to stronger cyclones.
Hurricanes are fueled by available heat. As global warming heats sea surfaces, the energy available to power hurricanes increases, raising the limit for potential hurricane wind speed.
Irma intensified in the Atlantic from September 4 to 5 as it entered a region of sea surface temperatures ranging from 0.9°F to 2.25°F (0.5°C to 1.25°C) above average, relative to a 1961-1990 baseline.
Mass destruction of property, unprecedented flooding, and widespread power outages occurred in Southern Texas as a result of Hurricane Harvey. At least 60 deaths are related or suspected to be related to the storm. After a loss of power at a chemical plant, failing refrigeration units led to fires and explosions in Crosby, Texas.
In just 57 hours, Harvey intensified from a regenerated tropical depression into a Category 4 hurricane, making landfall northeast of Corpus Christi on August 25 with wind speeds of 130 mph. As wind speeds lowered on August 26th, Harvey was downgraded to a tropical storm but remained destructive. The storm's center moved back out over the Gulf of Mexico and made a second landfall just west of Cameron, Louisiana on August 30th. By the morning of the 31st, Harvey had dumped an estimated total of 24.5 trillion gallons of water on Southeast Texas and southern Louisiana.
Climate change raises or amplifies the three primary hazards associated with hurricanes: storm surge, rainfall, and the power ceiling, aka potential speed limit, for hurricane winds.
Sea level rise has elevated and dramatically extended the storm surge driven by hurricanes - the main driver of damage for coastal regions.
Climate change has been found to have significantly increased the rainfall in hurricanes. A warmer atmosphere holds more water vapor, feeding more precipitation into all storms including hurricanes, significantly amplifying extreme rainfall and increasing the risk of flooding.
Hurricanes are fueled by ocean heat. As climate change warms sea surfaces, the heat available to power hurricanes has increased, raising the limit for potential hurricane wind speed and with that an exponential increase in potential wind damage. Harvey intensified rapidly amid sea surface temperatures in the Gulf of Mexico up to 2.7 - 7.2°F (1.5 - 4°C) above average, relative to a 1961-1990 baseline.
A very small, slow-moving storm pinpointed parts of New Orleans, Louisiana on August 5, dropping heavy rainfall that overwhelmed city pumps and helped to drive heavy flooding. Some neighborhoods witnessed as much as 8 inches of rainfall in the span of three hours, an extreme micro-burst expected to occur roughly every hundred years on average according to NOAA rainfall frequency tables and New Orleans city officials.
Just two weeks prior the same areas endured extreme rainfall that mounted to as much as 4.5 inches in less than hour, a burst expected to occur roughly every fifty years. New Orleans has seen a 62 percent increase in the number of heavy downpours since the 1950s.
Increasing extreme rainfall is one of the clearest observed signals of climate change, and the fingerprint of global warming has been firmly identified in the global trend of increasing extreme precipitation. Back-to-back years of extreme August rainfall events are consistent with the global trend of increasingly frequent extreme and record-breaking rainfall events.