How Oceans Are Transformed by Climate Change

In many respects, our oceans are the engine of Earth’s climate system. Covering more than two-thirds of the planet’s surface, they are our primary reservoir of heat and moisture, and our largest single store of carbon. However, they are also impacted profoundly by climate change.

Last year, multiple records were broken: global sea levels were the highest ever observed; sea-surface temperatures reached new highs; and marine heatwaves were exceptionally widespread.

But what exactly is happening beneath the ocean’s surface, and why? How will ecosystems and societies be affected? And how bad could the damage be?

How Climate Change Affects Oceans

Tom Strachan

There are two main ways that oceans are affected by anthropogenic climate change: firstly, oceans absorb around 90% of added heat; and secondly, they sequester around 30% of all carbon dioxide emissions.

These two dynamics — one thermal, and one chemical — are transforming our oceans from a stabilizing force in our planet’s climate system, into a source of profound systemic risk.

Beneath the surface, three interlinked processes are reshaping the ocean: sea level rise, ocean acidification, and ocean stratification. Each is driven by oceans absorbing excess heat and carbon dioxide; and the impacts are already being felt in our ecosystems and economies.

Sea Level Rise

The global average rate of sea level rise in the last 10 years is more than double that of the previous decade. Rising average temperatures drive this acceleration in two key ways: firstly, as water warms, it expands; and secondly, as land ice melts, it flows into the ocean (note that the term sea level rise is interchangeable with ocean level rise).  

• Accelerated coastal erosion and land loss.
• More frequent and severe coastal flooding.
• Contamination of freshwater sources by saltwater.
• Habitat shift and loss of intertidal areas.

Coastal properties and infrastructure are most at risk of damage or disruption, including ports, where reduced trade can lead to economic losses throughout an entire region. Communities also contend with health risks, such as drinking water contaminated by saltwater, and waterborne disease linked to flooding.

Coastal ecosystems are also greatly affected, with benthic (seafloor dwelling), intertidal (shoreline), and brackish (mixed freshwater and seawater) habitats especially vulnerable to sea level rise, placing communities that rely on these ecosystems at further risk.

Ocean Acidification

Today’s oceans are on average 30% more acidic than they were in the 1850s, and they continue to acidify at an increasing rate. As atmospheric concentrations of CO₂ rise, so does the amount of CO₂ that gets absorbed by the oceans, which reacts with seawater to produce carbonic acid (H₂CO₃).

Ocean acidification threatens marine ecosystems on a global scale:

• Impairing shell and skeleton formation in calcifying species (such as corals, molluscs, crustaceans, plankton).
• Interfering with the development and behavior of a large proportion of marine species.
• Disrupting the balance of marine ecosystems due to the unequal impact on different species.

The impacts of ocean acidification on calcifying species cascades through marine food webs, harming fisheries and aquaculture — especially those focused on shellfish and their predators — leading to economic losses and food insecurity.

Marine habitats themselves are also affected, with coral reefs — which support almost a quarter of all marine life — growing slower and smaller, harming marine biodiversity.

Moreover, ocean acidification reduces the ability of oceans to sequester carbon, further accelerating climate change. For example, studies have shown that certain species of plankton grow smaller in acidified water, reducing their ability to absorb carbon from the atmosphere and deposit it on the seafloor upon their death — an important natural process known as the biological pump.

Ocean Stratification

Ocean stratification describes how ocean waters tend to separate into horizontal layers according to density: colder, saltier water sinks, and warmer, fresher water rises. By forming these horizontal layers — warmer and fresher water on top, and colder and saltier water below — ocean stratification naturally reduces the vertical mixing of water, restricting the exchange of heat, oxygen, nutrients and carbon within oceans.

Climate change is increasing ocean stratification, further reducing the vertical mixing of water below normal levels. Global ocean stratification increased by as much as 5.8% between 1960 and 2018. This is driven by rising temperatures in two main ways: firstly, surface waters absorb more of the added heat than deeper waters; and secondly, melting ice sheets add freshwater to surface waters, decreasing their salinity.

Greater stratification has several wide-ranging impacts:

• Amplification of marine heatwaves due to the greater concentration of heat in surface waters.
• Deoxygenation of deeper waters, threatening marine species that depend on well-oxygenated environments.
• Reduced upward flow of nutrients from deeper waters, limiting phytoplankton’s ability to photosynthesize and diminishing the base of marine food webs.
• Reduced carbon sequestration as less carbon is transported to deep ocean sinks.

The ecological and socioeconomic consequences of these changes are substantial. Declining oxygen levels combined with reduced phytoplankton productivity could disrupt fisheries and aquaculture on a global scale, while shifting species distributions may impact food security and livelihoods for millions of people worldwide.

Additionally, the frequency of marine heatwaves has roughly doubled since the 1980s, which has been especially disastrous for coral reefs. Between 2023 and 2025, around 84% of the world’s reef area experienced bleaching-level heat stress, with devastating consequences for local biodiversity and fisheries. The loss of these habitats — many of which are considered important natural heritage sites — also impacts marine tourism sectors in countries like Australia, the Maldives, and the Caribbean.

Systemic Impacts

Sea level rise, ocean acidification, and ocean stratification are not isolated problems —   they are interlinked stressors that each compound the effects of the other. Together, they can destabilize some of the largest elements of the global climate system, amplifying both climate change itself and the risks to societies.

One of the most significant systemic risks lies in the disruption of large-scale ocean circulation, including the Atlantic Meridional Overturning Circulation (AMOC). This circulation is driven by the sinking of dense, cold, and salty waters at high latitudes. As stratification intensifies, that sinking becomes less efficient, slowing down the conveyor belt that redistributes heat across the globe. A weakened AMOC could profoundly alter weather systems, including monsoons that sustain billions of people, storm tracks in the North Atlantic, and rainfall patterns across the Americas, Europe, and Africa.

Polar regions are also especially vulnerable to these systemic effects. Warmer and more stratified waters can trap heat below the surface and direct it toward the undersides of ice shelves, accelerating their melt from below. This process not only contributes further to sea level rise but also risks crossing thresholds that could destabilize the Greenland and Antarctic ice sheets. Such tipping points would lock in many meters of future sea level rise over centuries to millennia, reshaping coastlines worldwide.

As mentioned previously, the combined impacts of stratification and acidification disrupt the biological pump — the process by which marine organisms transport carbon to the deep ocean. Reduced nutrient upwelling limits phytoplankton growth, while acidification weakens calcifying organisms that play a central role in carbon burial. As a result, less carbon is stored in the deep sea, leaving more in the atmosphere to accelerate global warming.

Parting Thoughts

The ocean is shifting from being a stabilizing force in the climate system to a source of compounding risks due to climate change. As sea levels rise, acidification and stratification intensify, and reinforce one another in ways that amplify climate change and increase the likelihood of crossing tipping points. These systemic impacts highlight the fact that protecting the ocean is more than an environmental imperative — it is a matter of survival, security and prosperity for our societies.

To learn more about climate science, and how to identify, assess and manage climate and nature-related risks facing your business, consider signing up for our Sustainability and Climate Risk (SCR®) Certificate.

Tom Strachan is Assistant Vice President at the GARP Risk Institute, specializing in sustainability and climate and nature risk management. He holds a BA in Geography from the University of Exeter and a MSc in Environmental Technology from Imperial College London.