Ocean Maintained Steady Temperature Prior to 1990 Followed by Rapid Warming

[bluewave]

https://phys.org/news/2021-08-ocean-steady-temperature-20th-century.html
 

In estimations of ocean heat content—important when assessing and predicting the effects of climate change—calculations have often presented the rate of warming as a gradual rise from the mid-20th century to today. However, new research from UC Santa Barbara scientists Timothy DeVries and Aaron Bagnell could overturn that assumption, suggesting the ocean maintained a relatively steady temperature throughout most of the 20th century, before embarking on a steep rise. The newly discovered dynamics may have significant implications for what we might expect in the future.

There wasn't an onset of an imbalance until about 1990, which is later than most estimates," said DeVries, an associate professor in the Department of Geography, and a co-author on a paper that appears in the journal Nature Communications. According to the study, the period from 1950 to 1990 saw temperature fluctuations in the water column but no net warming. After 1990, the study continues, the entire water column switched from cooling to warming.

These findings are the result of the addition of a largely underexplored factor in ocean heat content (OHC): Deep ocean temperatures.

Prior studies didn't consider the deep ocean," said Bagnell, a graduate scholar in DeVries's laboratory and the paper's lead author. Because of the challenges involved in getting temperature measurements in the deep ocean (below 2,000 meters) that region has gone largely unaccounted for, and data has been sparse. "There is some existing data, from research cruises and autonomous floats," he added.

The researchers used an autoregressive artificial neural network (ARANN) and machine learning methods to connect the dots between data points and "produce a single consistent estimate of the top-to-bottom OHC change for 1946 to 2019." The result was a trend that delays warming by decades over previous models.

There are two main possibilities for why the effects of global warming took so long to reach the ocean, De Vries said.

"One is that anthropogenic warming might have been weaker than previously thought during the 20th century, perhaps due to the cooling effects of aerosol pollution," he said. The other is that the deep ocean may still be exhibiting the effects of climate events long past.

"It can take centuries for climate signals to propagate from the surface to the interior," he said. Thus, the effects of a cooling event such as the Little Ice Age might be deep history to us on the surface, but the echoes of the event may have continued to resonate in the deep oceaninto the 20th century, providing a buffer to the warming Earth.

The delayed cooling effect ended in 1990, after which ocean temperatures, according to the study, have been accelerating upward.

"The lag is catching up and the ocean is warming more strongly now," Bagnell said. The Atlantic Ocean and Southern Ocean are currently where most of the warming is, with the Pacific Ocean and Indian Ocean not far behind.

Ocean warming is a concern on many levels, as it can cause changes in circulation, reduce its ability to absorb carbon and fuel more intense storms, in addition to causing sea level rise and creating inhospitable environments for undersea life. If the trend continues, the effects might last centuries, thanks to the same lag that kept the oceans cool until the last 30 years.

"The ocean remembers," DeVries said.

https://www.nature.com/articles/s41467-021-24472-3

Abstract

The historical evolution of Earth’s energy imbalance can be quantified by changes in the global ocean heat content. However, historical reconstructions of ocean heat content often neglect a large volume of the deep ocean, due to sparse observations of ocean temperatures below 2000 m. Here, we provide a global reconstruction of historical changes in full-depth ocean heat content based on interpolated subsurface temperature data using an autoregressive artificial neural network, providing estimates of total ocean warming for the period 1946-2019. We find that cooling of the deep ocean and a small heat gain in the upper ocean led to no robust trend in global ocean heat content from 1960-1990, implying a roughly balanced Earth energy budget within −0.16 to 0.06 W m⁻² over most of the latter half of the 20th century. However, the past three decades have seen a rapid acceleration in ocean warming, with the entire ocean warming from top to bottom at a rate of 0.63 ± 0.13 W m⁻². These results suggest a delayed onset of a positive Earth energy imbalance relative to previous estimates, although large uncertainties remain.

Discussion

The ARANN reconstruction of full-depth OHC provides an internally consistent framework for monitoring EEI over time, showing that the Earth energy budget was in quasi-equilibrium, with substantial decadal variability, for the four decades from 1950 to 1990. The warming rate from the ARANN does not differ from that derived by objective mapping methods with statistical significance, and previous studies already support a slower ocean warming rate for the 1950–1990 period relative to the 21st century (Fig. 5). However, due to the combination of a smaller estimated change in 0–2000 m OHC for 1950–1990 and the contribution of deep ocean cooling, the ARANN implies a stronger and later shift toward accelerated EEI than previously recognized, and raises the question as to what may have caused this climate shift.

Anthropogenic radiative forcing has remained positive and continued to grow in magnitude over the past century¹, so the lack of global ocean warming implied by the ARANN results over the period from 1950 to 1990 may seem counterintuitive at first. However, Earth’s climate system is not currently at equilibrium. Due to the timescales of overturning in the ocean, propagating the entire forced climate signal from the surface to the interior may require decades to centuries to manifest as signals in the deep OHC^8,10, implying that the EEI is modulated by changes in external forcing on multi-decadal time-scales. In the deep ocean, cooling of the Pacific and Indian over much of the 20th century could result from a past climate event such as the Little Ice Age¹⁰. A cooling trend that derives itself from long-term modes of climate variability⁴⁹ would not be reflected in any of the components of the external radiative forcing budget for the 20th century.

Nonetheless, deep ocean cooling does not entirely account for the near zero warming trend in OHC prior to 1990, especially when considering that the 0–2000 m interval shows minimal change in the ARANN OHC estimate as well, averaging just 0.03 ± 0.09 W m⁻² from 1960 to 1990. The difference between the ARANN and the IAP reconstruction¹¹ of OHC in the upper 2000 m, is similar in magnitude to the ARANN estimate of deep ocean cooling (Supplementary Fig. 15), and in general the spread across OHC estimates in the top 2000 m is larger than the deep ocean cooling trend estimated by the ARANN (Fig. 1b–c). This spread indicates large uncertainties related to methodological differences in estimating OHC over the latter half of the 20th century. However, if the ARANN estimate of minimal upper ocean warming prior to 1990 is correct, it could indicate that anthropogenic or volcanic aerosol effects are larger than currently estimated for this time period⁵⁰ or that the transient climate response to anthropogenic forcing is affected by regional feedbacks arising from the pattern of ocean heat uptake^51,52. Changes in the ocean overturning can also affect the EEI by modifying the rate of ocean heat uptake⁵³, which could also lead to discrepancies between radiative forcing and upper ocean warming.

The recent accelerated warming since 1990 implied by the ARANN is consistent with the dominant effects of anthropogenic greenhouse gas forcing and negligible volcanic aerosol forcing^1,54, as well as estimates of increased radiative forcing⁵⁵ during the past three decades. Due to improved ocean temperature sampling over the past several decades, there is high confidence that the top 2000 m of the ocean have been gaining heat at an accelerating rate, as indicated by the convergence of OHC estimates across methodologies during this time period (Fig. 1b–c). In addition, the ARANN results suggest that the deep ocean below 2000 m has added 48 ± 19 ZJ since 1990, or about 10–28% of the ocean warming above 2000 m during this period, significantly contributing to the accelerating EEI in recent decades. This contribution is larger than that from non-ocean components of the Earth energy budget, including the land surface, cryosphere, and atmosphere, which together account for ~27 ± 8 ZJ of warming since 1990⁵⁶.

In all, the results presented here show that deep ocean cooling during the latter half of the 20th century has given way to deep ocean warming over the past three decades, contributing to a delayed response of the EEI to contemporary radiative forcing effects. If this recent shift toward warming of the deep ocean continues, it will have implications for Earth’s climate for decades to centuries to come due to the long overturning timescales of the deep ocean. Continued monitoring of the global OHC, and improved resolution of deep ocean temperature changes, will be key for developing accurate forecasts of Earth’s energy budget and future climate change.

 

[LibertyBell]

1990  is an interesting year to pick because I notice that's when we started having record heat overcome record cold outcomes around here.