Record ocean heat content

Last year was the warmest on record for the heat content of the world’s oceans. Ocean heat content (OHC) has increased by around 417 zettajoules — a billion trillion joules — since the 1940s. The heat increase in 2021 alone, compared to 2020 — about 14 zettajoules — is around 23 times more than the total energy used by everyone on Earth in 2019 (the latest year in which global primary energy statistics are available).

Human-emitted greenhouse gases trap extra heat in the atmosphere. While some of this warms the Earth’s surface, the vast majority — around of 93 per cent — goes into the oceans. About two-thirds of this accumulates in the top 700 metres, but some also end up in the deep oceans. Annual OHC estimates between 1950 and present for both the upper 700m and 700m-2000m depths of the ocean are shown in the figure below.

In many ways, OHC represents a much better measure of climate change than global average surface temperatures. It is where most of the extra heat ends up and is much less variable on a year-to-year basis than surface temperatures.

Changes in the amount or rate of warming are much easier to detect in the OHC record than on the surface. For example, OHC shows little evidence of the modest slowdown in warming at the surface in the mid-2000s that got so much attention at the time. It also shows a distinct acceleration after 1991, matching the increased rate of greenhouse gas emissions over the past few decades.

Just about every year since 1991 has set a new OHC record, showing that heat has continued to accumulate in the Earth system as concentrations of atmospheric greenhouse gases have increased.

Fifth-to-seventh warmest year on the surface

Global surface temperatures in 2021 were among the warmest measured since records began in the mid-1800s. Data from NASA, NOAA and Berkeley Earth showed that it was the sixth warmest year on record, while data from Copernicus ERA5 has it as the fifth warmest. The release of the Hadley Centre/UEA HadCRUT5 record has been delayed due to data reporting problems, but it will likely show a similar ranking for 2021.

Read the full article about global warming in 2021 by Zeke Hausfather at Eco-Business.