Subscribe
The rate at which oceans are globally warming has doubled from the 1960s to the 2010s, a recent study has found. The top 2,000 metres of the global ocean has gained 351 zettajoules during 1958-2019. For context, a zettajoule is 10 to the power of 21 or 1,000,000,000,000,000,000,000 joules.
As humans emit greenhouse gases into the atmosphere, it causes the Earth to warm. The vast majority of the heat ends up in the ocean (more than 90 per cent). So, to understand how fast the Earth’s climate is changing, we must look to the ocean and track ocean heat content change.
Historic ocean warming is irreversible this century, with net warming dependent on the emission scenario, said the study published in Nature Reviews Earth & Environment. The rate of warming increased from 5 to 10 ZJ per year from the 1960s to the 2010s, it said.
Some ocean regions are warming faster than others, the study found. The Atlantic Ocean is warming the fastest, reporting the largest area-averaged warming at 1.42 joule per square metre. The southern ocean is next, at 1.40 joule per square metre for the upper 2,000 m from 1958–2019.
These warming patterns are, however, dominated by heat redistribution. The Pacific is projected to be the largest heat reservoir owing to its size, but the Atlantic and southern oceans report faster warming.
By 2100, the top 2,000 metres of the global ocean may skyrocket — it may be warmer by two-six times than the temperatures observed so far. It ranges from 1,030 ZJ for a low-emission scenario to 1,874 ZJ for a high-emission scenario, according to the projections by the study.
Study lead author Lijing Cheng of the Chinese Academy of Sciences said:
Relative to 2005–2019, future warming is projected to reach 1,030 ZJ for SSP1-2.6 and 1,874 ZJ for SSP5-8.5 at the end of this century, 2–4 (SSP1-2.6) to 4–6 times (SSP5-8.5) the observed 1958–2019 change.
Shared socio-economic pathways (SSP) are a set of plausible trajectories of societal development and radiative forcing. SSP1-2.6 is a relatively low-emission scenario and SSP5-8.5 is a higher emissions scenario.
The relentless increase in the ocean heat content has direct implications for the frequency, intensity and extent of marine heat waves (MHW) and other hotspots within the ocean.
Human-induced global warming and higher ocean heat content lead to more abundant, extensive and longer-lasting MHW. This has a huge impact on ocean ecosystems and marine life.
The prolonged MHW in the northeast Pacific and Gulf of Alaska from 2014 and 2016, known as ‘the blob’ saw seabird die-offs, declines in forage fish populations, and the appearance of subtropical marine taxa (ocean sunfish, skipjack tuna) in the northern Gulf of Alaska.
About 62,000 dead or dying common murres, a dominant fish-eating seabird of the North Pacific, washed ashore between the summer 2015 and spring 2016 on beaches from California to Alaska. Scientists estimated that the total mortality was around 1 million birds.
The MHW that persisted through 2014-2016 from California to Alaska created an enormous volume of ocean water (the blob) with temperatures exceeding “average by 2–3 standard deviations.”
Ocean warming also has an effect on tropical cyclones and associated changing ocean surface currents can indirectly affect pathways of storms. In August 2017, the Gulf of Mexico became the warmest on record to that point in the summertime.
The Gulf of Mexico ocean heat loss during Harvey matched the latent heat released by Harvey rainfall and thereby fueled the storm, said a May 9, 2018 study published in Earth’s Future. Ocean heat content was the highest on record just before the northern summer of 2017, supercharging Atlantic hurricanes Harvey, Irma and Maria.