Report on the 2023 North Atlantic Marine Heat Extreme: Drivers, Impacts, and Implications for Sustainable Development Goals

Abstract

A comprehensive analysis of the record-breaking 2023 North Atlantic sea surface temperature (SST) anomalies reveals that the event was a result of internal climate variability superimposed on a long-term warming trend driven by human activity. This aligns with the urgent call for action under SDG 13 (Climate Action). The unprecedented warming, particularly in the subtropics and eastern basin, was initiated by extreme air-sea surface fluxes. The intensity of this event was significantly amplified by a long-term increase in ocean stratification, a consequence of anthropogenically-driven ocean warming. While the event would have been impossible without the background state of human-induced climate change, its regional characteristics highlight the complex interplay between forced trends and natural variability. This complexity is critical for assessing risks to marine ecosystems (SDG 14: Life Below Water) and dependent economies (SDG 8: Decent Work and Economic Growth). The event is classified as a decadal-level occurrence for the entire North Atlantic but a centennial-level event for the most affected regions, underscoring the necessity of regional analysis for effective climate risk management and sustainable development planning.

1. Introduction: A Warming Ocean in the Context of Global Goals

The year 2023 registered unprecedented global surface temperatures, driven by a combination of factors atop the dominant long-term warming trend forced by human activities. This trend directly challenges the global community’s ability to meet the objectives of SDG 13 (Climate Action). The oceans have absorbed approximately 91% of the excess anthropogenic heat, leading to a significant increase in ocean heat content and a global average sea surface temperature (SST) rise of 0.97°C between the pre-industrial era (1850-1900) and the recent decade (2014-2023).

These warming trends are punctuated by extreme events known as marine heatwaves (MHWs), which have increased in frequency and intensity. MHWs pose a direct threat to SDG 14 (Life Below Water) by causing severe, often irreversible, damage to marine ecosystems and biodiversity. They also generate significant socio-economic consequences, impacting fisheries, trade, and coastal communities, thereby affecting progress towards SDG 8 (Decent Work and Economic Growth) and SDG 11 (Sustainable Cities and Communities). This report focuses on the exceptional 2023 North Atlantic MHW to understand its drivers and assess its probability in the current climate, providing critical insights for future risk assessment and sustainable adaptation strategies.

2. Analysis of the 2023 North Atlantic Event

2.1. Extreme Ocean-Atmosphere Conditions

The 2023 North Atlantic SST anomaly began with a sharp increase in May-June, reaching +0.76°C above the 1991-2020 average. The warming was not uniform, exhibiting a distinct horseshoe-shaped pattern (HS) of extreme warmth in the tropical and eastern North Atlantic. This pattern is associated with a known mode of internal climate variability.

The primary drivers of this surface warming were anomalous atmospheric conditions:

• Weakened Azores High: A significant weakening and contraction of the subtropical high-pressure system.
• Reduced Trade Winds: An outstanding reduction in trade winds, leading to decreased evaporative cooling (latent heat loss) from the ocean surface.
• Reduced Cloud Cover: A reduction in cloudiness over the HS region, which increased the amount of solar radiation reaching the ocean surface.

These atmospheric drivers created an extreme air-sea heat flux event, forcing the ocean to absorb an exceptional amount of heat and triggering the MHW. This demonstrates a direct physical mechanism through which atmospheric variability can exacerbate ocean warming, a critical interaction for understanding and modeling climate risks under SDG 13.

2.2. Ocean Subsurface Preconditioning and Amplification

The impact of the 2023 atmospheric forcing was amplified by pre-existing conditions in the ocean subsurface, a direct consequence of long-term, human-caused warming.

1. Increased Ocean Stratification: Decades of warming have heated the surface layer more than the subsurface, increasing the density difference between them (stratification). In 2023, the upper-ocean stratification in the HS region reached a record high.
2. Positive Feedback Loop: Higher stratification acts as a barrier, trapping heat from the atmosphere in the surface layer and making it more difficult for the ocean to mix this heat downwards. This amplifies the surface warming response to atmospheric heat fluxes.
3. Quantified Impact: Analysis shows that the long-term trend in MLD (Mixed Layer Depth), driven by increased stratification, boosted the 2023 extreme temperature surge by approximately 20%.

This finding reveals a critical feedback mechanism where the consequences of past emissions (warming and stratification), a core issue of SDG 13, are making the ocean more susceptible to extreme events, further endangering marine life as outlined in SDG 14.

2.3. Attribution and Probability in a Warming Climate

An attribution study using large ensembles of climate models was conducted to contextualize the 2023 event.

• Human Influence is Unequivocal: The 2023 event would have been impossible to occur without the background warming caused by human activities. In a counterfactual world without this influence, the observed temperatures are statistically unattainable. This is a stark confirmation of the failure to curb emissions as required by SDG 13.
• Return Period Varies by Region:
  • For the entire North Atlantic basin, the 2023 event is assessed as a rare but not exceptional event, with a return period of approximately 10 years in the current climate.
  • For the horseshoe (HS) region, where the warming was most intense, the event was exceptional, with a return period of about 100 years.
• Internal Variability vs. Forced Trend: The spatial pattern of the 2023 warming (the HS shape) is spatially orthogonal (anticorrelated) to the pattern of long-term warming expected from anthropogenic forcing alone. This indicates that the 2023 event was an extreme expression of internal climate variability superimposed on the warming trend, not a simple preview of the future average state.

3. Implications for Sustainable Development Goals (SDGs)

3.1. SDG 13: Climate Action

The 2023 event is a clear manifestation of the consequences of insufficient climate action. It demonstrates that human-induced warming has not only raised baseline temperatures but has also “doped” the climate system, enabling internal variability to produce extremes of greater magnitude. The positive feedback from ocean stratification implies that the climate system’s sensitivity to atmospheric fluctuations is increasing, making future climate action even more urgent.

3.2. SDG 14: Life Below Water

The extreme temperatures and increased stratification directly threaten marine biodiversity and ecosystem functions. MHWs of this magnitude can cause:

• Mass mortality of marine species, including corals, fish, and marine mammals.
• Disruption of food webs and fisheries.
• Irreversible loss of critical habitats.

The regional nature of the event highlights that specific, highly productive areas of the ocean are at risk of centennial-level extremes even at current global warming levels, jeopardizing the conservation and sustainable use of marine resources.

3.3. SDG 8 & 11: Economic Growth and Sustainable Communities

The health of the North Atlantic is tied to the economic well-being of numerous nations through fisheries, aquaculture, shipping, and tourism. Extreme events like the 2023 MHW can disrupt these sectors, leading to economic losses and threatening livelihoods in coastal communities. Accurate risk assessment that accounts for regional extremes is vital for building resilient infrastructure and economies.

3.4. SDG 17: Partnerships for the Goals

This study relies on international collaborative efforts, such as the Coupled Model Intercomparison Project (CMIP6) and data from multiple space agencies and research institutions. It highlights the need to strengthen these partnerships to:

• Improve climate models to better represent internal variability and its interaction with forced warming.
• Maintain and expand global ocean observing systems.
• Share data and findings to inform global policy and local adaptation strategies effectively.

4. Conclusions and Recommendations for Sustainable Development

The 2023 North Atlantic marine heat extreme was a complex event driven by an extreme phase of internal atmospheric variability acting on an ocean preconditioned and made more sensitive by human-caused climate change. The event would not have been possible without the anthropogenically warmed background state, a direct challenge to the ambitions of SDG 13. Its regional intensity poses a centennial-level threat to marine ecosystems, undermining SDG 14.

To advance sustainable development in the face of such climate realities, the following actions are recommended:

1. Advance Beyond Basin-Wide Averages: Risk assessments for climate impacts must consider regional patterns of variability, as basin-wide averages can mask the true level of extreme risk faced by specific marine ecosystems and the communities that depend on them.
2. Utilize Large Ensemble Simulations: To properly account for internal variability and its potential to amplify or mask forced climate trends, impact assessments and adaptation planning should use large ensembles of climate simulations. This is crucial for robustly assessing near-future risks.
3. Account for Non-Stationarity: The relationship between climate modes and their impacts is changing in a warming world. Research and policy must account for these evolving relationships, such as the amplifying effect of ocean stratification, to ensure that adaptation measures remain effective.

Addressing the systemic model biases in representing North Atlantic climate variability is critical. Improving these models through international collaboration (SDG 17) is essential for providing the reliable projections needed to safeguard our oceans (SDG 14) and build a sustainable future for all (All SDGs).

Analysis of Sustainable Development Goals in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 13: Climate Action

   • The article’s central theme is the analysis of an extreme climate event—the 2023 North Atlantic marine heatwave. It directly attributes the possibility and intensity of this event to “human-induced climate change” and “anthropogenically-driven ocean warming,” which are core concerns of SDG 13. The entire study is an effort to “take urgent action to combat climate change and its impacts.”

2. SDG 14: Life Below Water

   • The article explicitly discusses the consequences of ocean warming on marine life. It states that marine heatwaves (MHWs) have “severe and persistent impacts on marine ecosystems,” cause “irreversible losses,” and “threaten global biodiversity.” This directly aligns with the goal to “conserve and sustainably use the oceans, seas and marine resources for sustainable development.”

3. SDG 17: Partnerships for the Goals

   • The research presented in the article is a product of global scientific collaboration. It relies on data from multiple international bodies and projects, including the “Coupled Model Intercomparison Experiment (CMIP6),” the “World Climate Research Programme,” “European Space Agency (ESA),” and the “IPCC.” This demonstrates the importance of partnerships to address complex global challenges like climate change.

4. SDG 8: Decent Work and Economic Growth

   • The article connects the environmental impacts of ocean warming to economic activities. It notes that MHWs have “significant socio-economic consequences” and affect “worldwide trades and related economic stability,” specifically mentioning impacts on “fisheries management.” This links the climate issue to economic stability and industries that depend on marine resources.

5. SDG 7: Affordable and Clean Energy

   • The article identifies the root cause of the anthropogenic heat excess as “primarily associated with fossil fuel burning.” While not the main focus, this points to the energy sector’s role in driving climate change and implicitly supports the transition to cleaner energy sources to mitigate such impacts.

2. What specific targets under those SDGs can be identified based on the article’s content?

1. Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters.

   • The article analyzes an “extreme event” and a “natural disaster” in the form of a marine heatwave. By investigating its drivers and probability, the study contributes to the knowledge base needed for “risk assessment,” which is a critical first step in strengthening resilience and adaptive capacity.

2. Target 14.2: By 2020, sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts.

   • The article highlights that the warming event has “severe and persistent impacts on marine ecosystems” and can cause “irreversible losses.” This underscores the urgency of this target and provides scientific evidence of the “significant adverse impacts” that need to be managed and mitigated.

3. Target 14.a: Increase scientific knowledge, develop research capacity and transfer marine technology.

   • The study is a direct example of this target in action. It uses advanced “observational data through a process-based approach and outputs of large ensembles of simulations from the sixth phase of the Coupled Model Intercomparison Experiment (CMIP6)” to increase scientific understanding of ocean dynamics and climate change impacts.

4. Target 17.16: Enhance the Global Partnership for Sustainable Development.

   • The research methodology relies heavily on global partnerships. The authors “acknowledge the World Climate Research Programme, which, through its Working Group on Coupled Modelling, coordinated and promoted CMIP6” and use data from various international sources (ESA, WMO). This exemplifies a multi-stakeholder partnership for science and sustainable development.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

1. Sea Surface Temperature (SST) Anomaly

   • This is a primary indicator used throughout the article to quantify the extreme heat event. Specific values are given, such as the “2023 annual mean anomaly is +0.71 °C” and a peak “anomaly of +2.02 °C.” This directly measures the intensity of a climate-related hazard (relevant to Target 13.1).

2. Ocean Heat Content (OHC)

   • The article states that “about 91% of the anthropogenic heat excess… has accumulated in the oceans” and measures the “upper ocean (0–200 m) ocean heat content anomaly” as “3.1 × 10⁸ J m⁻².” OHC is a key indicator of the total energy stress on marine ecosystems (relevant to Target 14.2).

3. Upper Ocean Stratification

   • The article identifies a “long-term ocean stratification increase” as a critical factor amplifying the heatwave. It quantifies the “record in the upper-ocean stratification anomaly of 1.4 × 10⁻⁵ s⁻².” This is an important indicator of changing physical conditions that impact marine life and ocean dynamics (relevant to Target 14.2).

4. Frequency and Intensity of Marine Heatwaves (MHWs)

   • The article notes that MHWs “have been increasing globally in both frequency and intensity” and that the “number of MHW days has increased from about 12 to 30 days per year in less than a century.” This serves as a direct indicator of the increasing prevalence of climate-related hazards (relevant to Target 13.1).

5. Global Warming Level (GWL)

   • The analysis is framed within the context of “global warming levels (GWL) with respect to the 1850-1900 reference period,” with the current climate at “1.3 °C GWL.” This is a fundamental indicator used by the IPCC and the global community to track overall progress on climate action (relevant to SDG 13).

4. Table of SDGs, Targets, and Indicators

Source: nature.com