Why Climate Matters

Even though our planet is called Earth, 71% of its surface is covered by water<a href='https://www.usgs.gov/special-topics/water-science-school/science/how-much-water-there-earth#:~:text=About%2071%20percent%20of%20the,in%20you%20and%20your%20dog' target='_blank'><img src='/img/courses/ref.svg'/></a>. For most of our history, the ocean has been regarded as a nearly inexhaustible resource, so large that we could never affect it. This has led us to mistreat the ocean, filling it with waste and sewage<a href='https://www.epa.gov/ocean-dumping/learn-about-ocean-dumping' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.jstor.org/stable/43265424?seq=1' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://doi.org/10.1016/j.rser.2016.06.031' target='_blank'><img src='/img/courses/ref.svg'/></a>. Combined with the effects of climate change, this represents a serious threat to life in the ocean<a href='https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/09_SROCC_Ch05_FINAL-1.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.

You probably don’t live underwater, and maybe you can imagine a diet without fish. So why should you care? Well, in short, because the ocean absorbs and stores vast amounts of heat energy and carbon dioxide.

It takes much more heat energy to warm 1kg of water by 1°C than it takes to warm 1kg of air by 1°C<a href='https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/2%3A_The_Chemical_Foundation_of_Life/2.2%3A_Water/2.2C%3A_Water%E2%80%99s_High_Heat_Capacity' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://thermtest.com/thermal-resources/materials-database' target='_blank'><img src='/img/courses/ref.svg'/></a>. In scientific terms, this means water has a very high specific heat capacity<a href='https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/2%3A_The_Chemical_Foundation_of_Life/2.2%3A_Water/2.2C%3A_Water%E2%80%99s_High_Heat_Capacity' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://thermtest.com/thermal-resources/materials-database' target='_blank'><img src='/img/courses/ref.svg'/></a>.

The ocean has absorbed over 90% of the heat energy gained by the planet over the last 50 years<a href='https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content' target='_blank'><img src='/img/courses/ref.svg'/></a>. If this energy had been absorbed by the air in the lowest 10km of our atmosphere, we would have already seen a temperature increase of around 36°C<a href='https://www.imperial.ac.uk/media/imperial-college/grantham-institute/public/publications/briefing-papers/Ocean-heat-uptake---Grantham-BP-15.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>!

Instead, all that energy has warmed the ocean<a href='https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/09_SROCC_Ch05_FINAL-1.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>. Most warming has occurred in the top 75m of the ocean surface, which gained 0.4-0.5°C between 1971-2010<a href='https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter03_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>. This is much less than 36°C because of the water's heat capacity.

And that's not all - the ocean also stores carbon dioxide…

The ocean also stores almost 25% of the carbon dioxide we emit into the atmosphere<a href='https://doi.org/10.1038/s41467-020-18203-3' target='_blank'><img src='/img/courses/ref.svg'/></a>. This storage is driven by life within the ocean<a href='https://zenodo.org/record/5517457' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='http://cmore.soest.hawaii.edu/summercourse/2015/documents/Church_05-27/Riser_Johnson_2008.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://zenodo.org/record/5656910' target='_blank'><img src='/img/courses/ref.svg'/></a>, and by CO₂ dissolving in seawater<a href='http://chemistry.elmhurst.edu/vchembook/174temppres.html' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-Chap30_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Storing both energy and CO₂ allows the ocean to keep Earth’s climate stable, despite human activities<a href='https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/joc.926' target='_blank'><img src='/img/courses/ref.svg'/></a>. This is known as buffering climate change<a href='https://doi.org/10.1038/s41598-019-55039-4' target='_blank'><img src='/img/courses/ref.svg'/></a>.

It doesn’t matter how much we change the climate, the ocean will always buffer as much as it has in the past

The oceans aren’t able to endlessly buffer our actions. If we continue to increase CO₂ emissions, the ocean will absorb less carbon<a href='https://zenodo.org/record/5656910' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://pure.mpg.de/rest/items/item_1841117/component/file_1841119/content' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Let’s look at an example of the effects these have.

Coral reefs are home to hundreds of thousands, if not millions, of species<a href='https://www.icriforum.org/about-coral-reefs/what-are-corals' target='_blank'><img src='/img/courses/ref.svg'/></a>. This makes them some of the most biodiverse places on Earth<a href='https://www.icriforum.org/about-coral-reefs/what-are-corals' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.frontiersin.org/articles/10.3389/fmars.2020.567428/full' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://gcrmn.net/2020-report-v1-2023/' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Just like trees in rainforests, corals give reefs their structure<a href='https://coral.org/coral-reefs-101/coral-reef-ecology/how-reefs-are-made/' target='_blank'><img src='/img/courses/ref.svg'/></a>. Corals are invertebrate animals that allow algae to live inside them<a href='https://oceanservice.noaa.gov/education/tutorial_corals/coral02_zooxanthellae.html#:~:text=Most%20reef%2Dbuilding%20corals%20contain,compounds%20they%20need%20for%20photosynthesis' target='_blank'><img src='/img/courses/ref.svg'/></a>. In return, algae pay their rent by providing corals with food<a href='https://www.journals.uchicago.edu/doi/abs/10.2307/1540123' target='_blank'><img src='/img/courses/ref.svg'/></a>. This is called symbiosis<a href='https://oceanservice.noaa.gov/education/tutorial_corals/coral02_zooxanthellae.html#:~:text=Most%20reef%2Dbuilding%20corals%20contain,compounds%20they%20need%20for%20photosynthesis' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.pnas.org/content/92/7/2850.short' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Coral reefs directly support around 500 million people across the world in several ways<a href='https://www.iucn.org/resources/issues-brief/coral-reefs-and-climate-change' target='_blank'><img src='/img/courses/ref.svg'/></a>:

While buffering by the ocean may be good for us on land, corals are taking the hit underwater<a href='https://www.frontiersin.org/articles/10.3389/fmars.2020.567428/full' target='_blank'><img src='/img/courses/ref.svg'/></a>. When stressed by temperature increases, algae produce molecules that can damage their coral hosts<a href='https://link.springer.com/chapter/10.1007/978-94-007-0114-4_23' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://link.springer.com/chapter/10.1007/978-3-319-75393-5_8' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.marinebiology.org/coralbleaching.htm' target='_blank'><img src='/img/courses/ref.svg'/></a>. This causes the corals to get rid of the algae<a href='https://link.springer.com/chapter/10.1007/978-94-007-0114-4_23' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://link.springer.com/chapter/10.1007/978-3-319-75393-5_8' target='_blank'><img src='/img/courses/ref.svg'/></a>.

As well as providing food, algae are responsible for the corals’ colour<a href='https://oceanservice.noaa.gov/education/tutorial_corals/coral02_zooxanthellae.html#:~:text=Most%20reef%2Dbuilding%20corals%20contain,compounds%20they%20need%20for%20photosynthesis' target='_blank'><img src='/img/courses/ref.svg'/></a>. Corals without algae, therefore, turn white and starve<a href='https://pdf.wri.org/reefs_at_risk_revisited.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-AnnexII_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0152693' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.tandfonline.com/doi/abs/10.4155/cmt.11.21?journalCode=tcmt20' target='_blank'><img src='/img/courses/ref.svg'/></a>. This is called coral bleaching.

Since the 1870s, we have lost around half of the live coral in the ocean<a href='https://zenodo.org/record/5517457' target='_blank'><img src='/img/courses/ref.svg'/></a>! Severe bleaching events, where entire areas of coral are bleached, used to occur once every 25 years. Now they happen every 6 years<a href='https://www.science.org/doi/full/10.1126/science.aan8048' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Coral reefs are some of the most biodiverse places on Earth

Coral reefs provide homes for a nearly quarter of all fish species

Coral reefs account for 25% of global fishery catches annually

Coral reefs can protect people from the impacts of floods and storms

We have already reached 1.2°C warming<a href='https://essd.copernicus.org/articles/15/2295/2023/essd-15-2295-2023.html' target='_blank'><img src='/img/courses/ref.svg'/></a>. Coral reefs are so sensitive that if temperatures reach 1.5°C above pre-industrial levels, it is predicted that 70 to 90% of them will be lost<a href='https://report.ipcc.ch/ar6syr/pdf/IPCC_AR6_SYR_LongerReport.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.ipcc.ch/2018/10/08/summary-for-policymakers-of-ipcc-special-report-on-global-warming-of-1-5c-approved-by-governments/' target='_blank'><img src='/img/courses/ref.svg'/></a>. If we reach 2°C, nearly all coral reefs will be lost<a href='https://www.ipcc.ch/2018/10/08/summary-for-policymakers-of-ipcc-special-report-on-global-warming-of-1-5c-approved-by-governments/' target='_blank'><img src='/img/courses/ref.svg'/></a>.

This is all made worse by an issue known as ocean acidification: when CO₂ reacts chemically with water (H₂O), it results in an increase in the number of hydrogen ions (H⁺) that are found in the water<a href='https://www.researchgate.net/publication/319405916_Future_of_the_sea_ocean_acidification/figures?lo=1' target='_blank'><img src='/img/courses/ref.svg'/></a>. This makes the ocean more acidic, like a lemon, although not quite as strong as that<a href='https://www.bbc.co.uk/bitesize/guides/zsmgpbk/revision/3' target='_blank'><img src='/img/courses/ref.svg'/></a>! Higher acidity slows the chemical reactions that corals need to build their hard skeletons, making them even more vulnerable<a href='https://reefresilience.org/pdf/Reefs_at_Risk_Revisited.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Normally, marine species are more widely distributed than land species and can travel further distances more easily<a href='https://www.sciencedirect.com/science/article/pii/S0960982217305055' target='_blank'><img src='/img/courses/ref.svg'/></a>. This means that complete extinctions in the ocean are rarer and harder to keep track of<a href='https://royalsocietypublishing.org/doi/full/10.1098/rspb.2018.1698' target='_blank'><img src='/img/courses/ref.svg'/></a>. However, there are several ways in which population reductions or movement in species are felt throughout an entire ecosystem<a href='https://link.springer.com/article/10.1007/s10393-004-0093-7' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.nationalgeographic.org/encyclopedia/keystone-species/' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.science.org/doi/10.1126/science.1255641' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Let’s look at an example of how the abundance of a species can matter to an ecosystem:

Southern sea otters are very important in kelp forests because they eat sea urchins, thus keeping their population in check<a href='https://link.springer.com/article/10.1007/s10393-004-0093-7' target='_blank'><img src='/img/courses/ref.svg'/></a>. However, sea otters are endangered<a href='https://www.iucnredlist.org/species/7750/219377647' target='_blank'><img src='/img/courses/ref.svg'/></a>. When sea otter populations are low, urchins are left to overgraze the kelp, and the kelp forest as a whole eventually disappears<a href='https://link.springer.com/article/10.1007/s10393-004-0093-7' target='_blank'><img src='/img/courses/ref.svg'/></a>.

The consequences of warming on interactions between species

Deeper water, and water close to the poles, is colder, so many species are moving towards these areas<a href='https://www.ipcc.ch/srocc/chapter/summary-for-policymakers/' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://besjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2664.2008.01488.x' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.nature.com/articles/nclimate1958' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/04_SROCC_TS_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>. Since the 1950s, we have seen the area in which species live move by up to 50 kilometres every ten years<a href='https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/04_SROCC_TS_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>!

But some species are less able to move, and the size of the area they can live in has become smaller<a href='https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/04_SROCC_TS_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2012.2829' target='_blank'><img src='/img/courses/ref.svg'/></a>. This is because regions become too warm or experience stronger heatwaves and, as we saw in the previous chapter, higher temperatures at earlier times of the year can alter species' life cycles and food availability<a href='https://www.nature.com/articles/nature02808' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2486.2010.02165.x' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2012.2829' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/04_SROCC_TS_FINAL.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Changes to where and when species are found in the ocean don’t necessarily happen at the same time or speed for every species<a href='https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2486.2010.02165.x' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.nature.com/articles/nature02808' target='_blank'><img src='/img/courses/ref.svg'/></a>. As a result, some species that would normally interact fail to find each other because they look in the wrong place or at the wrong time<a href='https://www.nature.com/articles/nature02808' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.int-res.com/articles/cr2007/33/c033p271.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>. This is called mismatching<a href='https://www.nature.com/articles/nature02808' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.int-res.com/articles/cr2007/33/c033p271.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Disruptions to species interactions change the arrangement of marine ecosystems<a href='https://www.nature.com/articles/nature02808' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.ncbi.nlm.nih.gov/pubmed/14668864' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.577.6967&rep=rep1&type=pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>. Predator populations shrink when they mismatch with their prey, or when their prey declines and they have less food to eat<a href='https://www.ncbi.nlm.nih.gov/pubmed/14668864' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://royalsocietypublishing.org/doi/abs/10.1098/rstb.2006.1953' target='_blank'><img src='/img/courses/ref.svg'/></a>. This can result in a change to the dominant species within the ecosystem<a href='https://www.nature.com/articles/nature02996' target='_blank'><img src='/img/courses/ref.svg'/></a>.

The image below shows one example of how temperatures affecting food availability for one species can have effects on many other species along a food chain<a href='https://www.nationalgeographic.com/animals/invertebrates/group/krill/' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.nature.com/articles/nature02996' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2656.2006.01148.x' target='_blank'><img src='/img/courses/ref.svg'/></a>:

The ocean is threatened by many other human activities which combine to make the situation worse<a href='https://www.sciencedirect.com/science/article/pii/S0960982207008822' target='_blank'><img src='/img/courses/ref.svg'/></a>.

We continue to use the ocean to dump waste, including sewage, industrial chemicals, agricultural fertilisers, and up to 14 million tonnes of plastic every year<a href='https://www.pnas.org/content/pnas/111/28/10239.full.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.int-res.com/articles/meps/37/m037p295.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.iucn.org/resources/issues-briefs/marine-plastics' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.jstor.org/stable/4315461?seq=1' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.sciencedirect.com/science/article/pii/S1568988308001078' target='_blank'><img src='/img/courses/ref.svg'/></a>.

Still, the biggest threat facing life in the ocean today is exploitation through fisheries<a href='https://ipbes.net/sites/default/files/downloads/spm_unedited_advance_for_posting_htn.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://www.cambridge.org/core/services/aop-cambridge-core/content/view/29944BD8AA1EED41210B777389F34952/S0959270912000020a.pdf/div-class-title-seabird-conservation-status-threats-and-priority-actions-a-global-assessment-div.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>. To learn more about this, you can check out our chapter on Fish Farming in the Agriculture course.

Essential Biodiversity Variables are important factors for describing the state of ecosystems, such as whether populations are stable, and aspects like the length and complexity of food chains<a href='https://zenodo.org/record/5517457' target='_blank'><img src='/img/courses/ref.svg'/></a>. Fisheries cause 29% of changes to these variables in marine systems, while climate change and pollution account for 16% and 15%, respectively<a href='https://ipbes.net/sites/default/files/ipbes_global_assessment_chapter_2_2_nature_unedited_31may.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.

But, the impact of climate change is worsening, and as important ecosystems disappear, so do the habitats for many species<a href='https://www.icriforum.org/about-coral-reefs/what-are-corals' target='_blank'><img src='/img/courses/ref.svg'/></a><a href='https://ipbes.net/sites/default/files/ipbes_global_assessment_chapter_2_2_nature_unedited_31may.pdf' target='_blank'><img src='/img/courses/ref.svg'/></a>.