Lecture Climate ChangeLesson 8

The oceans: Thermohaline circulation

Rene Orthrene.orth@bgc-jena.mpg.de

Course webpagehttps://www.bgc-jena.mpg.de/bgi/index.php/Lectures/HydroBioClimClimateChange

Lesson 8The oceans: Thermohaline circulation

1) Thermohaline circulation2) Ocean acidification

“Global conveyor belt”

Driven by water density differences (temperature, salinity), and rotation of the Earth (Coriolis force)

Thermohaline circulationwikipedia.org

1

Spatial differences in ocean salinity driven bydifferences in evaporation minus precipitation

Spatial differences in ocean temperature driven by differences in (the incidence angle of) radiation -

highest at equator, lowest at poles

Thermohaline circulation

IPCC (2013)

2

Gulf stream

Gulf stream is part of the global thermohaline

circulation

Gulf stream visible in sea surface

temperature pattern

Stream called florida stream initially,

Benjamin Franklin introduced the name

gulf stream, corresponding to the

Gulf of Mexico

wikipedia.org

3

European vs. American temperatureswikipedia.org

4

Europe clearly warmer than America across similar latitudes

Caused by transport of heat towards Europe through(1) Gulf stream, and more importantly (2) south-westerly winds

European vs. American temperatures5

Summary

● Gulf stream is north Atlantic branch of global thermohaline circulation driven by water density differences due to salinity and temperature anomalies

No detectable trend in Gulf Stream activity

Thermohaline circulation in recent years6

IPCC SROCC (2019)

No detectable trend in activity of North-Atlantic thermohaline circulation

Ocean’s salinity and temperature changes impact strength of thermohaline circulation

Future of thermohaline circulation

IPCC (2013)

7

Weakening of North-Atlantic thermohaline circulation projected by climate models, but collapse very unlikely

Future of thermohaline circulation8

IPCC SROCC (2019)

Collapse of North-Atlantic thermohaline circulation could occur after massive global warming, after gradual weakening

Future of thermohaline circulationIPCC SROCC (2019)

9

Diverse impacts in Europe, and elsewhereHighlighting relevance of North-Atlantic thermohaline circulation

Impacts of thermohaline circulation weakening10

IPCC SROCC (2019)

Distribution of total-column anthropogenic CO2 in the ocean

Most CO2 taken up in deep water formation areas

Imprints of thermohaline circulation

IPCC (2013)

11

Additional heat from greenhouse gas-induced radiative forcing mostly taken up by shallow

ocean

Transport into deep ocean in deep water formation areas

Imprints of thermohaline

circulation

IPCC (2013)

12

Warming of very deep ocean restricted to southern ocean

Imprints of thermohaline circulation

IPCC SROCC (2019)

13

Oceans have taken up by far most of the additional heat

Imprints of thermohaline

circulation

IPCC (2013)

14

Clear differences between scenarios

Ocean warming affects marine organisms15

IPCC SROCC (2019)

Surface warming → stronger stratification → less upwelling →

less nutrients → less plankton → less fish

Ocean warming affects marine organisms16

IPCC SROCC (2019)

Decreasing fish catch potential depends linearly on global warming

Ocean warming affects marine organisms17

IPCC SROCC (2019)

Strongest impacts in the tropics where fish is very popular

Ocean warming affects marine organisms18

IPCC SROCC (2019)

Summary

● Gulf stream is north Atlantic branch of global thermohaline circulation driven by water density differences due to salinity and temperature anomalies

● Gulf stream and thermohaline circulation projected to weaken, collapse unlikely

● Thermohaline circulation controls oceanic uptake of CO2 and related additional heat, most uptake in deep water formation areas

● Considerable loss of fish and biodiversity due to surface ocean warming

Lesson 8The oceans: Thermohaline circulation

1) Thermohaline circulation2) Ocean acidification

Atmospheric CO2 propagates into ocean water, forms carbonic acid which in turn increases H+ concentration (pH value)

Ocean acidification

IPCC (2013)

19

Decreasing pH in concert with increasing atmospheric and oceanic CO2

Ocean acidificationIPCC (2013)

20

Strong differences across scenarios

Ocean acidification21

IPCC SROCC (2019)

Formation of shells is more difficult with ocean acidification

Loosing corals and their reefs22

oneillseaodyssey.org

Corals are important for fish and biodiversity

Loosing corals and their reefs23

change.org

AFTERBEFORE

Summary

● Gulf stream is north Atlantic branch of global thermohaline circulation driven by water density differences due to salinity and temperature anomalies

● Gulf stream and thermohaline circulation projected to weaken, collapse unlikely

● Thermohaline circulation controls oceanic uptake of CO2 and related additional heat, most uptake in deep water formation areas

● Considerable loss of fish and biodiversity due to surface ocean warming

● Increased atmospheric CO2 enters ocean, forms carbonic acid and hence acidifies the ocean; nevertheless overall alkaline conditions (pH > 7) likely to persist into the future

● Loss of coral reefs and corresponding biodiversity