Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
4.1.3 Ocean heat content
Key messages
• The warming of the oceans has accounted for approximately 93 % of the warming of the Earth since the 1950s. Warming
of the upper (0–700 m) ocean accounted for about 64 % of the total heat uptake.
• A trend for increasing heat content in the upper ocean has become evident since the 1950s. Recent observations also
show substantial warming of the deeper ocean (between depths of 700 and 2 000 m and below 3 000 m).
• Further warming of the oceans is expected with the projected climate change. The amount of warming is strongly
dependent on the emissions scenario.
Relevance
The ocean is the most dominant component of the
Earth's heat balance, and most of the total warming
caused by climate change is manifested in increased
ocean heat content (OHC) (Hansen et al., 2011).
Isotherms (i.e. contour lines of a given temperature)
in the ocean have moved at comparable or faster
rates than on land, causing species distribution shifts
(Burrows et al., 2011; Poloczanska et al., 2013). Good
estimates of past changes in OHC are essential for
understanding the role of the oceans in past climate
change, and for assessing future climate change. OHC
integrates temperature change, the density of seawater
and specific heat capacity from the surface down
to the deep ocean. OHC is an anomaly calculated in
comparison with a reference period. OHC is estimated
based on temperature measurements or on reanalyses
using a combination of models and observations
(Levitus et al., 2012). Changes in heat content cause the
ocean to expand or contract, thereby changing global
sea level. This thermosteric effect has contributed
about one-quarter to global sea level rise since 1993
(Church et al., 2011) (see Section 4.2.2).
Past trends
The warming of the oceans has accounted for
approximately 93 % of the warming of the Earth since
the 1950s (Hansen et al., 2011; Rhein et al., 2013;
Howes et al., 2015). OHC has increased since around
1970 (Figure 4.3). Differences in the values for yearly
and five-yearly averages are the result of the particular
method used for spatial gap filling. The linear warming
trends of the uppermost 700 m of the ocean and the
700–2 000 m layer over the time period 1955–2013
were 0.27 W/m 2 and 0.39 W/m 2 (per unit area of the
ocean), respectively. It is likely that the ocean warmed
between 700 and 2 000 m from 1957 to 2014 and
between 3 000 m and the bottom of the ocean from
1992 to 2005, while trends in ocean temperature
between depths of 2 000 and 3 000 m were not
statistically significant (Purkey and Johnson, 2010;
Levitus et al., 2012; Abraham et al., 2013; Rhein et al.,
2013).
Two-thirds of the observed increase in OHC has
occurred in the upper 700 m of the ocean, with
increases in the layers below a depth of 700 m
accounting for the remaining third. The strongest
warming is found near the sea surface, with the upper
75 m having warmed by more than 0.1 °C per decade
since 1971. It has been estimated that heat uptake
has doubled in recent decades (Gleckler et al., 2016).
At a depth of 700 m, the warming decreases to about
0.015 °C per decade (Rhein et al., 2013). Recently, it
has been determined that past increases in OHC have
been substantially underestimated because of poor
sampling of the Southern Hemisphere and limitations
of the analysis methods (Durack et al., 2014). These
concerns have not yet been considered in the datasets
presented here.
Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report
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