Global climate indicators: ocean heat content, acidification, deoxygenation and blue carbon

WMO has published annual State of the Global Climate reports since 1993. In 2020, it published a five-year climate report for 2015 to 2019 incorporating data and analyses from the State of the Global Climate across this period. The initial purpose of the annual report was to inform Members on climate trends, extreme events and impacts. In 2016, the purpose was expanded to include summaries on key climate indicators to inform delegates in Conference of Parties (COP) of the United Nations Framework Convention on Climate Change (UNFCCC). The summaries cover the atmosphere, land, ocean and cryosphere, synthesizing the past year’s most recent data analysis. There are four ocean related climate indicators: ocean heat content, sea level, sea ice and ocean acidification.

This article highlights the heat content summary from the State of the Global Climate 2020, ocean acidification, deoxygenation and blue carbon, covered in the WMO State of the Global Climate 2018, 2019 and 2020.

Ocean Heat Content

Ocean heat content measurements back in the 1940s relied mostly on shipboard techniques, which constrained the availability of subsurface temperature observations at global scale and at depth (Abraham et al., 2013). Global-scale estimates of ocean heat content are thus often limited to the period from 1960 onwards, and to a vertical integration from the surface down to a depth of 700 metres (m). With the deployment of the Argo network of autonomous profiling floats, which reached target coverage in 2006, it is now possible to routinely measure ocean heat content changes down to a depth of 2000 m (Roemmich et al., 2019) (Figure 1).

The summary on ocean heat content, provided by Mercator Ocean, France, states that the increasing emission of greenhouse gases is causing a positive radiative imbalance at the top of the atmosphere – called the Earth Energy Imbalance (EEI) – which is driving global warming through an accumulation of heat energy in the Earth system (Hansen et al., 2011; Rhein et al., 2013; von Schuckmann et al., 2016). The EEI is the portion of the forcing that the Earth’s climate system has not yet responded to (James Hansen et al., 2005), and is an indicator of the global warming that will occur without further change in forcing (Hansen et al., 2017). Ocean heat content is a measure for this heat accumulation in the Earth system from a positive EEI, the majority (~90%) is stored in the global ocean, it is thus a critical indicator for the changing climate.

Consequently, ocean warming is having wide-reaching impacts on the Earth climate system. For example, ocean heat content increase contributes to more than 30% of observed global mean sea-level rise through the thermal expansion of sea water (WCRP, 2018). Ocean warming is altering ocean currents (Yang et al., 2016; Voosen, 2020; Yang et al., 2020, Hoegh-Guldberg et al., 2018) and indirectly altering storm tracks (Hoegh-Guldberg et al., 2018; Trenberth et al., 2018; Yang et al., 2016). The implications of ocean warming are widespread across Earth’s cryosphere too, as floating ice shelves become thinner and ice sheets retreat (e.g. Serreze and Barry, 2011, Shi et al. 2018, Polyakov et al., 2017; Straneo et al., 2019; Shepherd et al., 2018). Ocean warming increases ocean stratification (Li et al., 2020) and, together with ocean acidification and deoxygenation, can lead to dramatic changes in ecosystem assemblages and biodiversity, to population extinction and to coral bleaching (e.g. Gattuso et al., 2015, Molinos et al., 2016, Ramirez et al., 2017).

Figure 1: 1960–2019 ensemble mean time series

Figure 1: 1960–2019 ensemble mean time series and ensemble standard deviation (2-sigma, shaded) of global ocean heat content anomalies relative to the 2005–2017 climatology for the 0 to 300 m (grey), 0 to 700 m (blue), 0 to 2000 m (yellow) and 700 to 2000 m depth layer (green). The ensemble mean is an outcome of a concerted international effort, and all products used are referenced in the legend of Fig. 2. The trends derived from the time series are given in Table 1. Note that values are given for the ocean surface area between 60°S–60°N, and limited to the 300 m bathymetry of each product, respectively. Source: Updated from von Schuckmann et al. (2020). The ensemble mean OHC (0-2000 m) anomaly (relative to the 1993-2020 climatology) has been added as a red point, together with its ensemble spread, and is based on CMEMS (CORA), Cheng et al., 2017 and Ishii et al., 2017 products.

Ocean Acidification

The IOC-UNESCO, supported by the Global Ocean Acidification Observing Network (GOA-ON), has provided a summary on ocean acidification for the annual State of the Global Climate since 2017.

Over the past decade, the oceans absorbed around 23% of annual anthropogenic CO2 emissions (Friedlingstein et al. 2020). Absorbed CO2 reacts with seawater and changes the pH of the ocean. This process is known as ocean acidification. Changes in pH are linked to shifts in ocean carbonate chemistry that can affect the ability of marine organisms, such as molluscs and reef-building corals, to build and maintain shells and skeletal material. This makes it particularly important to fully characterize changes in ocean carbonate chemistry. Observations in the open ocean over the last 30 years have shown a clear trend of decreasing pH (Figure 2). There has been a decrease in the surface ocean pH of 0.1 units since the start of the industrial revolution (1750) with a decline of 0.017-0.027 pH units per decade since late 1980s (IPCC 4AR and SROCC). Trends in coastal locations, however, are less clear due to the highly dynamic coastal environment, where a great many influences such as temperature changes, freshwater run-off, nutrient influx, biological activity and large ocean oscillations affect CO2 levels. In order to characterize the variability of ocean acidification, and to identify the drivers and impacts, a high temporal and spatial resolution of observations is crucial.

In line with previous reports and projections, the State of the Global Climate 2020 report states that ocean acidification is ongoing and that global pH levels continue to decrease. More recently established sites for observations in New Zealand show similar patterns, while filling important data gaps in ocean acidification monitoring in the southern hemisphere. Availability of operational data is currently limited, but it is expected that the newly introduced Methodology for the Sustainable Development Goal (SDG) Indicator 14.3.1 (“Average marine acidity (pH) measured at agreed suite of representative sampling stations”) will lead to an expansion in the observation of ocean acidification on a global scale.

Figure 2: pCO2 and pH records from three long-term ocean observation stations

Figure 2: pCO2 and pH records from three long-term ocean observation stations. Top: Hawaii Ocean Time- Series (HOTS) in the Pacific Ocean; Middle: Bermuda Atlantic Time Series (BATS); Bottom: European Station for Time-Series in the Ocean Canary Islands (ESTOC) in the Atlantic Ocean. Credit: Richard Feely (NOAA- PMEL) and Marine Lebrec (IAEA OA-ICC), IOC-UNESCO, GOA-ON.

Coastal blue carbon

The IOC-UNESCO together with the Blue Carbon Initiative (co-organized by Conservation International, IOC-UNESCO and IUCN) supports scientists, coastal managers and governments in measuring carbon stocks in coastal and marine ecosystems. Together they contribute on the blue carbon indicator to the annual report. In climate mitigation, coastal blue carbon (also known as “coastal wetland blue carbon”; Howard et al. 2017) is defined as the carbon stored in mangroves, tidal salt marshes and seagrass meadows within the soil, the living biomass above ground (leaves, branches, stems), the living biomass below ground (roots and rhizomes) and the non-living biomass (litter and dead wood). When protected or restored, coastal blue carbon ecosystems act as carbon sinks (Figure 4a). They are found on every continent except Antarctica and cover approximately 49 million hectares (Mha).

Currently, for a blue carbon ecosystem to be recognized for its climate mitigation value within international and national policy frameworks, it is required to meet the following criteria:

  1. Quantity of carbon removed and stored or prevention of emissions of carbon by the ecosystem is of sufficient scale to influence climate
  2. Major stocks and flows of greenhouse gases can be quantified
  3. Evidence exists of anthropogenic drivers impacting carbon storage or emissions
  4. Management of the ecosystem that results in increased or maintained sequestration or emissions reductions is possible and practicable
  5. Management of the ecosystem is possible without causing social or environmental harm.
Figure 4 (a): In intact coastal wetlands

Figure 4 (a): In intact coastal wetlands (from left to right: mangroves, tidal marshes, and seagrasses), carbon is taken up via photosynthesis (purple arrows) where it gets sequestered long-term into woody biomass and soil (red dashed arrows) or exhaled (black arrows).

Figure 4 (b): When soil is drained from degraded coastal wetlands

Figure 4 (b): When soil is drained from degraded coastal wetlands, the carbon stored in the soils is consumed by microorganisms that release CO2 as a metabolic waste product when they exhale. This happens at an increased rate when soils are drained and more oxygen is available, which leads to greater CO2 emissions. The degradation, drainage and conversion of coastal blue carbon ecosystems from human activity (i.e. deforestation and drainage, impounded wetlands for agriculture, dredging) results in a reduction in CO2 uptake due to the loss of vegetation (purple arrows) and the release of globally important greenhouse gas emissions (orange arrows).

However, the ecosystem services provided by mangroves, tidal marshes and seagrasses are not limited to carbon storage and sequestration. They also support improved coastal water quality, provide habitats for economically important and iconic species, and protect coasts against floods and storms. Recent estimates revealed that mangroves are worth at least US$1.6 billion each year in ecosystem services.

Despite their importance for ocean health and human wellbeing, mangroves, tidal marshes and seagrasses are being lost at a rate of up to 3% per year. When degraded or destroyed, these ecosystems emit the carbon they have stored for centuries into the ocean and atmosphere and become sources of greenhouse gases (Figure 4b).

The Intergovernmental Panel on Climate Change (IPCC) estimates that as much as a billion tons of CO2 being released annually from degraded coastal blue carbon ecosystems – mangroves, tidal marshes and seagrasses – which is equivalent to 19% of emissions from tropical deforestation globally (IPCC 2006).

For complete bulletin see The ocean, our climate and weather.

World Meteorological Organization, 31 March 2021. Full article.

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