A5. Since about 1950 many marine species across various groups have undergone shifts in geographical range and
seasonal activities in response to ocean warming, sea ice change and biogeochemical changes, such as oxygen loss, to
their habitats (high confidence). This has resulted in shifts in species composition, abundance and biomass production of
ecosystems, from the equator to the poles. Altered interactions between species have caused cascading impacts on
ecosystem structure and functioning (medium confidence). In some marine ecosystems species are impacted by both the
effects of fishing and climate changes (medium confidence). {3.2.3, 3.2.4, Box 3.4, 5.2.3, 5.3, 5.4.1, Figure SPM.2}
A5.1 Rates of poleward shifts in distributions across different marine species since the 1950s are 52 ±
33 km per decade and 29 ± 16 km per decade (very likely ranges) for organisms in the epipelagic (upper 200 m from sea
surface) and seafloor ecosystems, respectively. The rate and direction of observed shifts in distributions are shaped by
local temperature, oxygen, and ocean currents across depth, latitudinal and longitudinal gradients (high confidence).
Warming-induced species range expansions have led to altered ecosystem structure and functioning such as in the North
Atlantic, Northeast Pacific and Arctic (medium confidence). {5.2.3, 5.3.2, 5.3.6, Box 3.4, Figure SPM.2}
A5.2 In recent decades, Arctic net primary production has increased in ice-free waters (high confidence)
and spring phytoplankton blooms are occurring earlier in the year in response to sea ice change and nutrient availability

with spatially variable positive and negative consequences for marine ecosystems (medium confidence). In the Antarctic,
such changes are spatially heterogeneous and have been associated with rapid local environmental change, including
retreating glaciers and sea ice change (medium confidence). Changes in the seasonal activities, production and distribution
of some Arctic zooplankton and a southward shift in the distribution of the Antarctic krill population in the South Atlantic
are associated with climate-linked environmental changes (medium confidence). In polar regions, ice associated marine
mammals and seabirds have experienced habitat contraction linked to sea ice changes (high confidence) and impacts on
foraging success due to climate impacts on prey distributions (medium confidence). Cascading effects of multiple climaterelated drivers on polar zooplankton have affected food web structure and function, biodiversity as well as fisheries (high
confidence). {3.2.3, 3.2.4, Box 3.4, 5.2.3, Figure SPM.2}
A5.3 Eastern Boundary Upwelling Systems (EBUS) are amongst the most productive ocean ecosystems.
Increasing ocean acidification and oxygen loss are negatively impacting two of the four major upwelling systems: the
California Current and Humboldt Current (high confidence). Ocean acidification and decrease in oxygen level in the
California Current upwelling system have altered ecosystem structure, with direct negative impacts on biomass production
and species composition (medium confidence). {Box 5.3, Figure SPM.2}
A5.4 Ocean warming in the 20th century and beyond has contributed to an overall decrease in maximum
catch potential (medium confidence), compounding the impacts from overfishing for some fish stocks (high confidence).
In many regions, declines in the abundance of fish and shellfish stocks due to direct and indirect effects of global warming
and biogeochemical changes have already contributed to reduced fisheries catches (high confidence). In some areas,
changing ocean conditions have contributed to the expansion of suitable habitat and/or increases in the abundance of
some species (high confidence). These changes have been accompanied by changes in species composition of fisheries
catches since the 1970s in many ecosystems (medium confidence). {3.2.3, 5.4.1, Figure SPM.2}
A6. Coastal ecosystems are affected by ocean warming, including intensified marine heatwaves, acidification, loss of
oxygen, salinity intrusion and sea level rise, in combination with adverse effects from human activities on ocean and land
(high confidence). Impacts are already observed on habitat area and biodiversity, as well as ecosystem functioning and
services (high confidence). {4.3.2, 4.3.3, 5.3, 5.4.1, 6.4.2, Figure SPM.2}
A6.1 Vegetated coastal ecosystems protect the coastline from storms and erosion and help buffer the
impacts of sea level rise. Nearly 50% of coastal wetlands have been lost over the last 100 years, as a result of the
combined effects of localised human pressures, sea level rise, warming and extreme climate events (high confidence).
Vegetated coastal ecosystems are important carbon stores; their loss is responsible for the current release of 0.04–1.46
GtC yr–1 (medium confidence). In response to warming, distribution ranges of seagrass meadows and kelp forests are
expanding at high latitudes and contracting at low latitudes since the late 1970s (high confidence), and in some areas

episodic losses occur following heatwaves (medium confidence). Large-scale mangrove mortality that is related to
warming since the 1960s has been partially offset by their encroachment into subtropical saltmarshes as a result of
increase in temperature, causing the loss of open areas with herbaceous plants that provide food and habitat for
dependent fauna (high confidence). {4.3.3, 5.3.2, 5.3.6, 5.4.1, 5.5.1, Figure SPM.2}.
A6.2 Increased sea water intrusion in estuaries due to sea level rise has driven upstream redistribution
of marine species (medium confidence) and caused a reduction of suitable habitats for estuarine communities (medium
confidence). Increased nutrient and organic matter loads in estuaries since the 1970s from intensive human development
and riverine loads have exacerbated the stimulating effects of ocean warming on bacterial respiration, leading to
expansion of low oxygen areas (high confidence). {5.3.1}.
A6.3 The impacts of sea level rise on coastal ecosystems include habitat contraction, geographical shift
of associated species, and loss of biodiversity and ecosystem functionality. Impacts are exacerbated by direct human
disturbances, and where anthropogenic barriers prevent landward shift of marshes and mangroves (termed coastal
squeeze) (high confidence). Depending on local geomorphology and sediment supply, marshes and mangroves can grow
vertically at rates equal to or greater than current mean sea level rise (high confidence). {4.3.2, 4.3.3, 5.3.2, 5.3.7, 5.4.1}
A6.4 Warm-water coral reefs and rocky shores dominated by immobile, calcifying (e.g., shell and
skeleton producing) organisms such as corals, barnacles and mussels, are currently impacted by extreme temperatures
and ocean acidification (high confidence). Marine heatwaves have already resulted in large-scale coral bleaching events
at increasing frequency (very high confidence) causing worldwide reef degradation since 1997, and recovery is slow (more
than 15 years) if it occurs (high confidence). Prolonged periods of high environmental temperature and dehydration of the
organisms pose high risk to rocky shore ecosystems (high confidence). {SR1.5; 5.3.4, 5.3.5, 6.4.2.1, Figure SPM.2}