Over the past 150 years, we have activated the process of ocean warming, as the sea has absorbed up to 91 % of heat that was trapped by greenhouse gases. Stopping this warming is like trying to stop a speeding train; its inertia is so great that it cannot be stopped immediately.

The reason for this inertia is the physical properties of seawater and the vastness of the oceans. Anyone who has ever boiled water knows that the process of heating and cooling takes time. When the burner is turned off, the air above the water cools quickly, while the liquid itself returns to its original temperature more slowly. Large quantities of water, such as a pot of soup, take longer to cool than smaller portions, such as a cup of tea. Now imagine the 3.5 quintillion liters of water that make up the ocean, covering 71 % of our planet. After absorbing heat, that heat must sink to the depths of the ocean, often several miles, before it returns to the surface and is released back into the atmosphere. Each layer of the ocean slowly transfers heat to the layer below, a process that can take thousands of years. Once the heat is deep enough, it stays there practically forever. "The deep ocean is so disconnected from the surface that it has no idea of what's happening there. Even though the surface is cooling, the middle and deeper layers continue to warm,"

As seawater heats up, it expands, contributing to sea level rise through a process known as thermal expansion. This phenomenon is also tied to thousands of years. A 2017 study showed that if greenhouse gas emissions were to stop in 2050, thermal expansion caused by carbon dioxide could double by 2150 and quadruple by 2550.

The extended heat retention period is due to the fact that carbon dioxide persists in the atmosphere for hundreds to thousands of years. In addition, the strong inertia of the oceans means that even short-lived greenhouse gases such as methane, with a lifetime of only ten years, can cause thermal expansion for centuries. The study suggested that 75 % of the thermal expansion caused by methane will persist for 100 years after emissions cease; 40 % of this remains after 500 years. Although methane is considered a short-lived gas, its effects on the oceans are permanent.

Ocean warming and sea level rise are irreversible, the analysis says. Even if we achieve zero emissions, or even extract significant amounts of greenhouse gases from the atmosphere, the oceans will continue to warm because they are responding to conditions from the past. “The oceans live in a time frame that is separate from the present. They live in a period of global warming. “The ocean never forgets.” (CO2AI)