Unprecedented congestion at the ports of Long Beach and Los Angeles amid the coronavirus pandemic drove up emissions from the agencies’ operations after being down for years, according to a report released Monday.

Supply chain disruptions have plagued U.S. businesses and consumers for years, beginning with the Trump administration’s trade war with China in 2018. The pandemic-induced disruptions, however, caused historic congestion that forced a backlog of ships to sit anchored near the coast, emitting pollutants all the while.

“No one could have foreseen this once-in-a-lifetime event, but we are not discouraged by this temporary impediment, and our goal to be a zero-emissions port remains,” Long Beach Harbor Commission President Sharon Weissman said in a statement.

The port’s annual emissions inventory report shows that greenhouse gas emissions last year were 22% above 2005 levels. In the previous inventory, greenhouse gas emissions were actually 10% below 2005 levels.

The port uses 2005 as its emissions baseline because it was the year before the original San Pedro Bay Clean Air Action Plan was adopted.

Diesel particulates were down 88% last year after being down 90% the year prior, according to the report. Nitrogen oxides and sulfur oxides, meanwhile, decreased 49% and 96%, respectively, compared to 2005 levels. In the previous report, those emissions had decreased 62% and 97%, according to the port.

The emissions inventory is reviewed by the U.S. Environmental Protection Agency, the California Air Resources Board and the South Coast Air Quality Management District.

Despite the increases and ongoing challenges, the port continues to meet its 2023 targets for diesel particulate matter and silver oxides.

The pandemic caused supply chain congestion for a number of reasons, starting with the shutdown of Chinese ports—some of the largest in the world—early on. Simultaneously, the pandemic forced people around the world to stay in their homes, which increased online shopping.

Once the ports reopened, the backlog of goods awaiting shipping saw a constant stream of vessels calling on the San Pedro Bay ports. Prior to the pandemic, a backlog of ships waiting outside the twin ports was unheard of, despite the fact that, as the nation’s largest port complex, it moves 40% of all containerized cargo entering the U.S. and about 30% of all containerized exports.

“Many of the negative conditions which created this perfect storm have improved,” Port of Long Beach Executive Director Mario Cordero said in a statement. “In recent months, as we’ve left behind the surge of COVID, great strides have been made in reducing the number of ships waiting at anchor. Looking ahead, a vessel queuing program put into place last year to relieve congestion is also expected to have a positive impact on the next inventory.”

At its peak in January of this year, 109 container ships were awaiting their turn at berth in the San Pedro Bay. As of Sept. 30, the backlog was down to seven ships, according to the Marine Exchange of Southern California.

In November of last year, the shipping industry created a new ship queuing system, which reduced the number of ships sitting directly off the coast. The emissions, however, were just dispersed farther out at sea as more ships “slow-steamed” their way to the West Coast.

When ships finally made it to berth, many stayed longer than usual as terminal operators limited the size of work groups as a safety protocol due to COVID-19. Additionally, more cargo-handling equipment was used to keep up with activity, and trucks often idled for hours waiting to pick up or drop off containers.

The revelations of increased emissions amid the pandemic are a small blow to the Port of Long Beach, which is widely regarded as an industry leader when it comes to reducing its environmental footprint. Throughout the pandemic, though, the port continued working toward its goal of all zero-emission cargo-handling equipment by 2030 and a zero-emissions drayage truck fleet by 2035.

Last month, 4 Gen Logistics announced it had purchased 61 electric drayage trucks, with plans to convert its entire fleet to zero-emissions by 2025.

Today, about 17% of cargo-handling equipment at the port is electric-powered, making it the largest electric port fleet in the country, according to port staff. Since 2021, the port has implemented several initiatives to continue reducing emissions, including launching its Clean Truck Fund Rate and committing $150 million to zero and near-zero emissions demonstration projects.