Stockholm+50: Oceans on simmer

Oceans regulate the climate by absorbing atmospheric carbon dioxide (CO2) and by altering the energy budget, carbon cycle and nutrient cycle.

They have helped reduce the worst impacts of climate change by absorbing over 90 per cent of excess global temperature rise and about 25 per cent of CO2 emission. However, global warming is causing (i) warming, (ii) acidification and (iii) deoxygenation of oceans.

The oceans have warmed unabated since 2005, continuing the clear multi-decadal trends documented by the Intergovernmental Panel on Climate Change (IPCC).

At the ocean surface, temperature has on average increased by 0.88 degrees Celsius from 1850-1900 to 2011-20, with 0.60°C of this warming having occurred since 1980.

The ocean surface temperature is projected to increase by about 1.5°C by the 2050s with respect to the 1850-1990 threshold. The rise has been observed not only at the surface but also in deep ocean waters.

The major impact of ocean warming is seen over the Arctic Ocean which will likely become practically sea ice-free during the seasonal sea ice minimum for the first time before 2050.

It is virtually certain that global mean sea level will rise through the 2050s because all contributors to global the mean sea level will likely continue.

Relative to 1995-2014, the global mean sea level will rise 0.18-0.23 m by 2050 and by 0.38-0.77 m by 2100. The rise is mainly due to thermal expansion and mass loss from glaciers and ice sheets.

Recent studies have suggested occurrence of marine heatwaves (MHW), which are periods of extreme high sea surface temperatures. Unlike atmospheric heatwaves, MHWs can extend millions of square kilometres and persist for weeks to months.

MHW can lead to severe and persistent impacts on marine ecosystems, coral bleaching, changes in phytoplankton blooms, shifts in species composition and toxic algal blooms. The IPCC climate change projections suggest that MHWs will become four times more frequent by 2080 compared to the 1995-2011 threshold.

The rate of ocean uptake of atmospheric CO2 has continued to strengthen in the recent two decades in response to the rising atmospheric CO2. The oceans are continuing to acidify in response to the carbon uptake.

The open ocean surface water pH is observed to be declining by 0.017-0.027 pH units per decade since the late 1980s. There is a growing consensus that the open ocean is losing oxygen overall, with a very likely loss of 0.5-3.3 per cent between 1970-2010 from the ocean surface to 1,000 m. The oxygen minimum zones are expanding by a very likely range of 2-8 per cent, most notable in the tropical oceans.

Marine ecosystem hit 

The warming of oceans has serious adverse impacts. It affects marine organisms at multiple levels, impac-ting fisheries and food production. Global warming can fundamentally alter ocean biodiversity.

“The Global Assessment Report on Biodiversity and Ecosystem Services” finds that 66 per cent of the global ocean is impacted by human pressures, with severe impacts in declining richness and abundance of ocean biodiversity.

There is a high confidence in inferring that fisheries, catches and their composition in many regions are already impacted. The erosion of ocean biodiversity and ecosystems particularly threatens the livelihoods of local communities. For example, 80 per cent of all tourism is based near the sea, but the destruction of coral reefs is affecting that.

Coastal stress

Costal ecosystems are observed to be under stress from ocean warming and sea level rise that are exacerbated by non-climatic pressures from human activities on ocean and land.

The global wetland area has declined by nearly 50 per cent relative to pre-industrial level. Warming related mangrove encroachment into subtropical salt marshes has been seen in the past 50 years.

Since the early 1980s, the occurrence of harmful algal blooms and patho-genic organisms has increased in coastal areas due to warming, deoxygenation and eutrophication with negative impacts of food provisioning, tourism, economy and human health.

Extreme events 

The ocean warming also will have impacts on weather systems around the world. A tropical cyclone (TC) is the most important severe weather system, which ocean warming can influence.

An increasing trend in sea surface temperatures and ocean heat content could help TCs intensify quickly and travel long distances without losing appreciable energy. Studies have suggested that there is a 6 per cent per decade increase in major TC exceedance probability of 50.

There is also evidence that TC intensification rates and the frequency of rapid intensification events have increased within the satellite era. The climate change projections suggest that average peak TC wind speeds and the proportion of very severe TCs will very likely to increase with global warming.

It is also very likely that average TC rain-rates will increase with warming. The average locations where TCs reach their peak wind-intensity will migrate poleward as the tropics expand with warming.

The Indian Ocean is also warming and the trend is likely to continue. A warm Indian Ocean can lead to more intense precipitation events over India during the monsoon, as it will help to more moisture advection and convergence.

Mitigation ways 

In view of the projected adverse impacts of warming oceans, there is a concern about the efficacy of ocean governance, highlighting the need for timely mitigation and adaptation responses.

There are three major risks to oceans that arise from governance-related issues: (i) the impacts of the overexploitation of marine resou-rces; (ii) inequitable distribution of access to and benefits from marine ecosystem services; and (iii) inadequate or inappropriate adaptation to changing ocean conditions.

The actions that can reduce these risks relate to the process of co-creation and implem-entation of improved, compre-hensive and integrated ocean management and enhancement of decision-making processes.

The 2030 Agenda for Sustainable Development, adopted by all UN member states, provides a blueprint for peace and prospe-rity for the planet. There are 17 Sustainable Development Goals (SDGs), which are an urgent call for action.

SDG 14 is to conserve and sustainably use the oceans, seas and marine resources for sustain-able development. The UN has proclaimed a Decade of Ocean Science for Sustainable Develop-ment (2021-2030) to support efforts and gather ocean stakeholders worldwide behind a common framework that will ensure ocean science can help countries to create improved conditions for sustainable development of the ocean.

India is actively participa-ting in both the initiatives.

M Rajeevan is a Distinguished Scientist, Ministry of Earth Science, National Centre for Earth Science Studies, Kerala

Climate Change Oceans and Seas World