New York grapples with climate-driven deluge: Historic rainfall and flash floods are the new normal

On September 27, 2023, a fierce storm unleashed its fury upon New York City, resulting in unprecedented rainfall and flash floods across significant portions of the city. 

The torrential downpour resulted from an intense collision between cold air masses from Canada and warm, moisture-laden air surging in from the Gulf of Mexico.

This clash led to New York’s John F Kennedy International Airport receiving nearly 200 millimetres of rain within 24 hours — the highest recorded in the city since 1948 — according to the National Weather Service (NWS).

Following the event, New York’s LaGuardia airport, too, witnessed the wettest September since 1939.

Moreover, heavy rain and flash floods in New York and other regions of the Northeast prompted Governor Kathy Hochul to declare a state of emergency. 

“Temperatures were about average everywhere in September, but the big story during the month was the rain,” NWS, New York, wrote on X (formerly known as Twitter) on October 3, 2023.

Apart from these regions, Central Park saw its second-wettest September since 1869. Other parts, including Islip, Newark and Bridgeport, had also breached their rainfall records in September, according to NWS. 

 Source: National Weather Service, New York, October 3, 2023  

A new rapid-attribution study has attributed New York’s heavy rainfall to human-induced climate change.

According to a study led by scientists from ClimaMeter, the nature of this storm has undergone a notable transformation. It is now observed to be 10-20 per cent wetter than it would have been in the previous century, a change attributed to the influence of climate change.

ClimaMeter is a rapid experimental framework designed for the assessment of weather extremes within the context of climate.

The low-pressure systems that cause flooding in New York are 3-15 mm/day wetter than they were in the past, the research added. These systems frequently result in higher amounts of precipitation in other regions as well. Low-pressure systems associated with rains and storms are largely strengthened by human-induced climate change, it concluded.

Such extreme weather events have become more frequent in September, doubling their frequency of occurrence, said the analysis. 

The new normal 

The September floods unfolded less than three months after a devastating storm in July that unleashed catastrophic flooding across New York’s Hudson Valley.

Moreover, New York has witnessed several extreme weather events in recent years, the New York State Climate Action Council Scoping Plan pointed out in December 2022. 

This includes significant events, such as the historic deluge brought by Hurricane Ida, which spanned from August 26-September 4, 2021, and the floods caused by Tropical Storm Isaias in August 2020.

New York’s climate is changing faster than national and global averages, and it is necessary to plan for the new normal, which includes more frequent and intense flooding, stated the New York State Water Resources Institute in its projections.

“The reality we have to deal with is this is our new normal,” said Kathy Hochul, New York governor. The state will need to prepare for a “new normal” of regularly occurring extreme weather events, she stated in a press conference on September 30, 2023, according to a media report.

The Intergovernmental Panel on Climate Change, in its study titled Climate Change 2021: The Physical Science Basis, had warned about the increasing frequency and intensity of extreme weather events almost everywhere in the United States. 

New York may see more frequent and severe heatwaves due to an increase in global temperature brought on by human-induced climate change, said the IPCC study. 

IPCC also projected heavy precipitation, leading to urban flooding, elevated storm surge risks and chronic flooding from sea-level rise. Local terrain, regional climatic patterns and global warming influence the scale and impact of these extreme events. 

The study analysed how phenomena, such as the low-pressure systems causing the recent New York floods, have altered compared to what they might have looked like in the past (1979-2000).

According to scientists involved in the study, the heavy deluge is a sign of a warming globe since a hotter atmosphere can contain more moisture, which is subsequently released in storms.

The storm and torrential rains leading to floods in large parts of New York have been primarily driven by climate change caused by humans, with a minor contribution from natural variability, concluded the study.