The Lungs of the Earth

Try­ing harder and harder to breathe

Asian Geographic - - Nature - Cov­er­ing over 70 per­cent of the Earth’s sur­face, oceans are es­sen­tial to life. Un­doubt­edly, our oceans are chang­ing in re­sponse to cli­mate change. What does this mean for our liq­uid planet?

The

oceans are cru­cial to reg­u­lat­ing cli­mate and act as “the lungs of the Earth”, with al­gae and cyanobac­te­ria in sea­wa­ter pro­vid­ing up to 80 per­cent of the at­mo­spheric oxy­gen which we rely on to breathe. The oceans also house over 230,000 marine species, with es­ti­mates that there are be­tween one and 10 mil­lion species still undis­cov­ered. Along­side their own in­trin­sic value, many of these marine species pro­vide im­por­tant goods and ser­vices. Col­lec­tively, ocean-re­lated ser­vices and busi­ness are es­ti­mated to con­trib­ute over USD500 bil­lion to the world’s econ­omy.

Our sur­vival is un­de­ni­ably de­pen­dent on a healthy ocean. How­ever, cli­mate change, in tan­dem with other hu­man im­pacts, such as pol­lu­tion and over­fish­ing, threaten the very re­source that life on Earth de­pends on. These threats will con­tinue to in­ten­sify as the global pop­u­la­tion grows, plac­ing an ever-in­creas­ing strain on the world’s marine ecosys­tems.

The oceans have a two-way re­la­tion­ship with the Earth, with the oceans in­flu­enc­ing cli­mate, weather and coastal land­scapes, while the Earth’s cli­mate di­rectly al­ters the oceans’ phys­i­cal and chem­i­cal con­di­tions. Con­se­quently, the in­creas­ing tem­per­a­tures on Earth, owing to global warm­ing over the past 50 years, have also re­sulted in warmer sur­face wa­ters and greater heat stor­age in the world’s oceans. Fur­ther­more, the oceans cur­rently ab­sorb ap­prox­i­mately a quar­ter of all ex­cess car­bon diox­ide gen­er­ated by hu­man

ac­tiv­i­ties – sig­nif­i­cantly re­duc­ing the lev­els of car­bon diox­ide in the at­mos­phere, which helps to reg­u­late the Earth’s tem­per­a­ture. The ab­sorp­tion of this ex­cess car­bon diox­ide, how­ever, causes a fun­da­men­tal shift in sea­wa­ter chem­istry, which ul­ti­mately re­sults in the oceans be­com­ing more acidic. Warmer and more acidic oceans are the re­sult of cli­mate change and are linked to nu­mer­ous im­pacts that sci­en­tists glob­ally are still un­cov­er­ing.

Warmer Oceans

The oceans have a high la­tent heat ca­pac­ity, which means they are very good at stor­ing en­ergy – so ef­fi­cient, in fact, that they have ab­sorbed an es­ti­mated 93 per­cent of the ad­di­tional en­ergy cre­ated from the green­house ef­fect. This, com­bined with the slow mix­ing time of the world’s oceans, means that it can take up to a decade for changes in cli­mate to al­ter ocean tem­per­a­tures. That be­ing said, since the start of the 20th cen­tury, there has been warm­ing of the deep oceans, and an in­crease in the global mean sea sur­face tem­per­a­tures.

Ris­ing sea level is a di­rect im­pact of ocean warm­ing. Warmer tem­per­a­tures cause sea­wa­ter to phys­i­cally ex­pand (known as ther­mosteric sea level rise), while melt­ing glaciers, snow and ice add vol­ume to the oceans (known as eu­static sea level rise). Since the mid-19th cen­tury, sea level has risen at a greater rate than the mean val­ues from the last two mil­len­nia. The con­se­quence of this is that habi­tats are be­ing lost – glacial habi­tats that house an­i­mals such as the iconic po­lar bear are di­min­ish­ing, with the si­mul­ta­ne­ous ef­fect of coastal habi­tats be­ing flooded. Sci­en­tific es­ti­mates sug­gest that sea level is ris­ing at a rate of 3.5 mil­lime­tres per year, a trend that threatens

coastal com­mu­ni­ties glob­ally and could mean low-ly­ing is­lands, such as the Mal­dives, are lost to the sea.

Melt­ing ice and glaciers also trans­fer fresh water into the oceans, which changes the salinity (how much salt is in the water) of sea­wa­ter. Over the last 50-odd years, changes in ocean salinity linked to cli­mate change have cor­re­sponded with shifts in rain­fall pat­terns and an ac­cel­er­a­tion in the evap­o­ra­tion and rain­fall cy­cle. Changes in where and how of­ten it rains has pro­found reper­cus­sions for crop pro­duc­tion and food se­cu­rity.

Vari­a­tions in rain­fall pat­terns and fresh­wa­ter in­put, along with el­e­vated tem­per­a­tures, also threaten to dis­rupt ocean cur­rents. The oceans are in con­stant move­ment, re­sult­ing from sur­face wind-driven cur­rents and deep-water ther­mo­ha­line cur­rents ( thermo mean­ing tem­per­a­ture; ha­line mean­ing salinity). Colder and more saline sea­wa­ter sinks and is re­placed by warmer sur­face wa­ters – cre­at­ing the Great Ocean Con­veyor Belt. Dis­rup­tion to the oceans’ cur­rents has the po­ten­tial to al­ter global weather pat­terns, as well as the mi­gra­tion and dis­per­sal of marine or­gan­isms. Al­ready, changes in ther­mal strat­i­fi­ca­tion (heat lay­er­ing in the ocean) have been de­tected, re­sult­ing in re­duced mix­ing of sea­wa­ter in the deep ocean. Changes in sea­wa­ter mix­ing can de­crease nu­tri­ent avail­abil­ity, lim­it­ing the fun­da­men­tal build­ing blocks needed by marine or­gan­isms to grow and sus­tain life.

As un­pre­dictable as the weather

Changes in ocean cur­rents and pre­cip­i­ta­tion pat­terns also con­trib­ute to the fre­quency and in­ten­sity of ex­treme weather events, which are pre­dicted to be­come more com­mon. Large storms such as hur­ri­canes and cy­clones can cause sig­nif­i­cant habi­tat loss, with in­creased storm surges re­sult­ing in dra­matic coastal ero­sion. The im­pact of large storms can be dev­as­tat­ing on both the en­vi­ron­ment and lo­cal com­mu­ni­ties.

El Niño events are also be­com­ing more com­mon, warm­ing the eastern and cen­tral Pa­cific above their nor­mal sea­sonal av­er­ages. El Niño events al­ter global weather pat­terns, which af­fect ex­treme weather sys­tems world­wide. The po­ten­tial in­crease in se­vere El Niño events threatens ecosys­tems and could have large so­cio-eco­nomic con­se­quences. For ex­am­ple, 2016 saw the third mass global co­ral bleach­ing event, re­sult­ing from warmer-than-nor­mal sea­wa­ter, which was at­trib­uted to it be­ing an El Niño year. Bleach­ing is a stress re­sponse of corals, re­sult­ing in the loss of their mi­cro­scopic al­gae that they de­pend on for en­ergy pro­duc­tion. Sci­en­tists be­lieve that up to a third of the north­ern Great Bar­rier Reef was lost from the El Niño in 2016. With pre­dic­tions that El Niño events will be­come more fre­quent, the abil­ity of the reef to re­cover is wor­ry­ing, es­pe­cially when other cli­mate im­pacts, such as ocean acid­i­fi­ca­tion, are in­ten­si­fy­ing.

The Os­teo­poro­sis of the Sea

The oceans ab­sorb at­mo­spheric car­bon diox­ide, which ini­ti­ates chem­i­cal re­ac­tions that re­duce sea­wa­ter ph and car­bon­ate ions in sea­wa­ter. ph is the scale used to mea­sure how acidic some­thing is. The scale ranges from 0 to 14, with the lower val­ues mea­sur­ing that which is more acidic. Sea­wa­ter is slightly ba­sic (higher than seven on the ph scale) mean­ing that the process of ocean acid­i­fi­ca­tion is a shift to­wards ph neu­tral (ph equals seven) rather than acidic con­di­tions. The shift in chem­istry also re­duces the car­bon­ate ions in sea­wa­ter, which are the fun­da­men­tal build­ing blocks needed for marine or­gan­isms that have a cal­cium car­bon­ate shell or skele­ton. The greater acid­ity also in­creases the risk of dis­so­lu­tion, mak­ing ocean acid­i­fi­ca­tion “the os­teo­poro­sis of the sea” – com­pro­mis­ing the struc­tural in­tegrity of or­gan­isms made of cal­cium car­bon­ate.

The im­pact of cli­mate change on the world’s oceans is ex­ten­sive. Col­lec­tively, the ef­fects threaten to dis­rupt the bal­ance of the oceans’ ecosys­tems. Al­ready, there are re­ports of the pole­ward mi­gra­tion of marine or­gan­isms to cooler wa­ters where con­di­tions are more op­ti­mal.

Un­for­tu­nately, not all species are ca­pa­ble of re­lo­cat­ing; many species face an in­creas­ing risk of ex­tinc­tion.

As sci­en­tists con­tinue to un­der­stand and un­cover the planet’s re­spon­sive pro­cesses, we are see­ing how in­tri­cately bal­anced life on Earth is. Op­ti­misti­cally, sci­en­tists do not be­lieve the tip­ping point of ir­re­versible change has yet oc­curred. There is, how­ever, an ur­gent need to im­ple­ment poli­cies and prac­tices to re­duce car­bon diox­ide emis­sions, to en­sure the ef­fects of cli­mate change do not neg­a­tively change our oceans . ag

Op­ti­misti­cally, sci­en­tists do not be­lieve the tip­ping point of ir­re­versible change has yet oc­curred

Warmer water tem­per­a­tures can re­sult in co­ral bleach­ing. When water is too warm, corals will ex­pel the al­gae ( Zoox­an­thel­lae) liv­ing in their tis­sues, caus­ing the co­ral to turn com­pletely white

Above A wa­ter­spout formed dur­ing a large storm off the coast of the Cay­man Is­lands above right Melt­ing water gushes from ice in North East Land, Sval­bard in Nor­way. In both the Arc­tic and Antarc­tic re­gions, ice is re­treat­ing

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