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 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

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.

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 3: Oxygen Minimum Zones (blue) and areas with coastal hypoxia (red; dissolved oxygen concentrations <2 mg/L) in the ocean. Coastal hypoxic sites mapped here are those in which anthropogenic nutrients are a major cause of oxygen decline (data from Diaz and Rosenberg, 2008 and Diaz, unpublished. Figure adapted after Isensee et al., 2015, Breitburg et al. 2018, GO2NE 2018).

Deoxygenation of open ocean and coastal waters

The IOC-UNESCO Global Ocean Oxygen Network (GO2NE) coordinates the annual report’s summary on deoxygenation, with a focus on understanding its multiple aspects and impacts.

Both observations and numerical models indicate that oxygen is declining in the modern open and coastal oceans, including estuaries and semi-enclosed seas. Since the middle of the last century, there has been an estimated 1% to 2 % decrease (i.e. 2.4-4.8 Pmol or 77-145 billion tons) in the global ocean oxygen inventory (Bopp et al., 2013; Schmidtko et al., 2017). In the coastal zone, many hundreds of sites are known to have experienced oxygen concentrations that impair biological processes or are lethal for many organisms. Regions with historically low oxygen concentrations are expanding, and new regions are now exhibiting low oxygen conditions. While the relative importance of the various mechanisms responsible for the loss of the global ocean oxygen content is not precisely known, global warming is expected to contribute to this decrease directly because the solubility of oxygen decreases in warmer waters, and indirectly through changes in ocean dynamics that reduce ocean ventilation, which is the introduction of oxygen to the ocean interior. Model simulations for the end of this century project a decrease of oxygen in the open ocean under both high and low emission scenarios (Figure 3).

In coastal areas, increased river export of nitrogen and phosphorus since the 1950s has resulted in eutrophication of water bodies worldwide. Eutrophication, leading to higher primary production and decomposition of this material increases oxygen consumption and, when combined with low ventilation, leads to the occurrence of oxygen deficiencies in subsurface waters. Climate change is expected to further amplify deoxygenation in coastal areas influenced by anthropogenic nutrient discharges, decreasing oxygen solubility, reducing ventilation by strengthening and extending periods of seasonal stratification of the water column, and in some cases where precipitation is projected to increase, by increasing nutrient delivery.

The volume of anoxic regions of the ocean’s oxygen minimum zones has expanded since 1960 (Schmidtko et al., 2017), altering biogeochemical pathways by allowing processes that consume fixed nitrogen and releasing phosphate, iron, hydrogen sulfide (H2S) and, possibly, nitrous oxide (N2O). The relatively limited inventory of essential elements, like nitrogen and phosphorus, means such alterations are capable of perturbing the equilibrium of the chemical composition of the ocean. We do not know to how positive feedback loops (e.g. remobilization of phosphorus and iron from sediment particles) may speed up the run away from equilibrium.

Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters. For example, deoxygenation impacts biodiversity and food webs, and can reduce growth, reproduction and survival of marine organisms. Low-oxygen-related changes in spatial distributions of harvested species may force people to change their fishing locations and practices and can reduce the profitability of fisheries. Deoxygenation can also increase the difficulty of providing sound advice on fishery management.

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 (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, 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).

Authors

Kirsten Isensee, Intergovernmental Oceanographic Commission of UNESCO Secretariat

Katherina Schoo, Intergovernmental Oceanographic Commission of UNESCO Secretariat

John Kennedy, Met Office Hadley Centre, UK 

Karina von Schuckmann, Mercator Ocean international, France 

Omar Baddour, WMO Secretariat 

Maxx Dilley, WMO Secretariat