"nightline," "global warning," will be right back.

this is a special edition of "nightline." "global warning." >> good evening. thanks for joining us. the fallout from president trump's historic announcement from the paris climate accord met with celebration from his base, but condemnation from many ceos and global leaders. the french president saying the u.s. has turned its back on the

world. but the decision could reverberate far beyond the political realm. tonight abc's matt gutman takes us on a journey around the globe to confront what scientists see as disturbing signs of global warning. >> reporter: across the globe reports of bizarre, seemingly unrelated phenomena. cicadas emerging four years early in washington, d.c. poppy in the middle east spontaneously growing much more potent, producing heroin of astonishing strength. airline pilots reporting turbulence more violent than ever experienced before. record numbers of great white sharks -- >> got to be frightening if you're thinking about going into the water here. >> reporter: swarming feet off of popular california beaches. scientists insist those strange phenomena are actually related, triggered by global warming. which critics say president trump disavowed today. >> the united states will

withdraw from the paris climate accord. >> reporter: the president pledged to "keep america first" by rejecting that 2015 nonbinding agreement signed by 174 countries to cut back on carbon emissions. >> i was elected to represent the citizens of pittsburgh, not paris. i promised i would exit or renegotiate any deal which fails to serve america's interests. >> reporter: but all of the world's pollution has imperiled the planet as we learned deeping up with scientists across the pacific, in australia, hawaii, and california on a forensic investigation of what they say is a matter of terrifying urgency. >> if we do not stay the warming on high pressure planet, corals will be the last of our worries. it's our species that will be vulnerable. >> reporter: there have been sayi sightings. >> approximately 15 great white

sharks. >> reporter: beach closures, last month an attack. every week professor chris lowe, head of cal state long beach's shark lab, deploys a small armada of boats, jet skis, drones, and helicopters to take a rough census of the swarming great whites by spearing them with a tag like this. on a foggy morning last month, we joined them. >> hey, chris, good to see you. >> reporter: a trip out shockingly short because the predators are shockingly close, cruising just feet off the beach, flanked by the multi-million dollar homes. so white sharks are born at about 4 1/2 to 5 feet long, a baby white shark. about 60 pounds. >> okay. looks like you're about to go spear fishing. >> sort of. trying to find sharks. >> you jab this and what releases? >> the dart this will come off. this will stay under the skin. it will just get pulled away as the shark swim swims away this will be dangling off the shark.

>> reporter: to tag them you can't stay in the boat. >> you've got to go on the tug board? >> yeah, yeah. >> reporter: the water is murky and the fog is thick. the sharks announce themselves, fins cutting right through the water. >> here, fishy fishy fishy. oh! okay, so he missed. >> reporter: student finds his groove from this drone, watch him stealthily sneaking up on this great white. the tags relay the shark's information to receivers placed up and down the california coast. >> were you surprise the at the sheer number of shark wet saw today? >> well, yeah. i had to pinch myself. this is crazy. this is long beach. i've been working here 30 years. studying sharks. i never thought i would actually see the day where i would see that many in one morning. >> reporter: but why are they here? >> the water's warm. it's shallow. so there's not a lot of predators. then there's lots of food.

>> reporter: warmer water has been a boon to great whites. saving many pacific sharks their annual migration south to mexico for the winter. but it is cooking a species anchored to the seabed. coral are tiny, mysterious creatures living in dazzling, colorful colonies. but they're builders. over generations their exoskeletons create tree-like shapes which 25% of the world's fish depend on. the granddaddy of coral is the great barrier reef, the world's largest living organism. it sprawls across 1,400 miles of the australian coast, basically the distance between miami and maine. but warmer than normal sea temps cause coral bleaching which occurs which the corals expel the algae that live in their tissue. what you're saying that is 50% of what is arguably the largest living organism on earth is dead? or will have died in 18 months

of bleaching? >> 50% of the corals died in the last 18 months. >> and this is not -- they're not going to recover? >> once you're dead, you're dead. right? >> reporter: last year the northern third of the barrier reef experienced bleaching, killing 30% of the reef overall. this year an estimated 19% died in the middle section of the reef and the worst may not be over. that's why we've flown across the globe towards the epicenter of this environmental disaster, townsville, australia. >> glad to meet you, welcome to australia, welcome to the great barrier reef. >> it's beautiful. >> reporter: scott herren is part of a team from noaa's reef watch which predicted the past two bleachings at the great barrier reef in 2016 and 2017. they're terrified about the prospects of a potentially cass alcoholismic third consecutive bleaching. herren is the kind who rides his bike to work. but we are taking him on a much longer ride.

up the coast to mission beach. the evening before our big dive, we map out the route with scott and our guide, karen bell. >> all right, we're going to depart from here. we'll go all the way out here along this sort of -- through the potter roofs. a lot of this has limited damage to it. >> reporter: suddenly i realize the mortality is happening now. much of it not even documented. >> this is 90% dead all along this wall. and it was dead pretty much two or three weeks ago. so anything that was left, that mortality level will be even higher. >> what's the like to see the stuff that you've grown to love and basically care for die like that? >> it just rips your heart out. >> reporter: the next morning, karen brings the boat and we load in. you may ask yourself, why do they have so much gear? why the pelican cases? the answer is that we have a couple of really cool gizmos. we've got your 360 cameras, your underwater cameras, some of the

stuff has never been tried before. so fingers crossed. the water's flat and we skim across for an hour to the first pristine reef. the idea is that before we see the devastation, they want me to see a vibrant, healthy reef. there's just one problem. >> [ bleep ]. [ bleep ]. >> reporter: much is already dead. >> at least 50% mortality. >> reporter: two other spots and a lot of cursing later, we decide it's as good as we'll get. scott and i splash in. >> these are astounding colors. astounding colors. but that's not how coral are supposed to look. >> reporter: what we see slaps the words out of everyone's mouth. >> i want you to come and look in here. the structure and the variety of shapes in these pieces that have bleached.

>> reporter: we swim back to the surface. where do we head to now? >> i dived on this reef over ten years ago with some other folks that i work with and worked with at the time. at noaa. and at that time it was this amazing, beautiful, vibrant reef. there were sharks swimming around. cuttlefish that we saw. moray eels coming out. and the corals looked magnificent. >> reporter: karen shows us what eddy reef looked like months ago. those coral radiating with color. branches long and unbroken. the water clear as vodka. but a startling difference awaits when we go back under. >> i can remember this reef and the dive i had ten years ago. that diversity of corals. this is the same reef. but it's not the same reef.

>> reporter: there's so much stuff floating around in the water, it's hard to see because it's basically coral carcass. >> on this coral, you see the full transition from right to left. floresing coral recently dead coral with algae and dead coral for quite some time. >> reporter: still, there are fish. there are colors. we see this giant clam, to a novice like me it's still beautiful. and that is part of the problem. >> what percent would you say is alive and thriving? >> around 80% to 90% of this reef is impacted. >> reporter: we hop back on board. bleached coral can recover but much doesn't. once the coral dies the skeleton gets colonized by a shaggy carpet of algae. >> it's all bleached. it's all brown. that's all -- the brown is all morbidity.

>> nothing dead here 12 weeks ago? nothing dead? wow. >> reporter: the food chain that feeds the larger fish that in turn feed us, potentially cass alcoholismicly affected. people who work on the reef, like karen, say it's like having a death in the family. many australian lawmakers dispute terry hughes' numbers, even mocking conservationists. so do many in the country's $5 billion a year coral tourism industry. we meet cole mckenzie, chief executive of marine park tourism operators in cannes, the capital of reef tourism. you can take your pick of transportation. boats or helicopters. people like terry hughes and noaa say that basically over the past 18 months, nearly 50% of the reef has died. an additional portion has been bleached and may survive. but that's a significant mortality. >> it is a significant mortality. and we would be absolutely in

bits if it was true. fortunately it's simply in make believe land. >> make believe land? these are scientists, the government entrusts them, my government entrusts them. what's going on here? are you saying they're lying? they're making it up? >> i would think noaa is taking their advice from some of the scientists here and trying to make a point. >> reporter: is there an exit ramp from the highway so many say we're on to ecological doom? the possible answer may lie 4,600 miles away on this tiny sliver of a hawaiian island. ...i kept looking for ways to manage my symptoms. i thought i was doing okay... then it hit me... ...managing was all i was doing. when i told my doctor,... ...i learned humira is for people who still have symptoms of moderate to severe crohn's disease... ...even after trying other medications. in clinical studies,... the majority of people on humira... saw significant symptom relief... ...and many achieved remission. humira can lower your ability...

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we witnessed the devastation of the great barrier reef of australia, the largest living organism on earth now in the throes of death. as scientists point to warming waters due to climate change. on the island of oahu, one researcher has discovered a glimmer of hope. >> reporter: the once resplendent reefs of australia revealed the ugly truth. huge mortality to the cradle of much of what lives in the sea. dive after dive revealed dead or dying reef. but 4,600 miles away, these infant corals, babies coddled in cargo nets in the gentle currents of hawaii, could hold a key to the species' survival. we've met up with the shepherd of this little flock of sea creatu creatures in the bay off oahu. quite a rendezvous at sea.

hi, ruth. >> nice to meet you. we can do things here that can't be done anywhere else in the world. >> this is the great location to experiment, because this is the future. >> that's the holy grail. to plant a reef that can actually persist in the conditions that we know are probably going to kill them if we don't attempt to adapt them for the future. >> on a scale that's large enough to make a difference? >> absolutely. >> reporter: we suit up for another coral dive. something miraculous appears to be happening. >> you can see we have almost 100% cover of coral here. >> astonishing. >> really not a lot of species diversity, but many, many corals packed into the space. >> it's so different than what we saw in the great barrier reef.

you think this is 100%. we saw maybe 40% that was alive. >> unfortunately, if you got the great barrier reef prior to the bleaching event, it would have look the like this fields of coral. >> reporter: we swim to the underwater lab. the cargo nets are holding super coral, growing at a super-fast rate to be able to withstand higher temperatures and ocean acidification. >> how would you define the process that these coral are going through? >> call it training. we're essentially putting coral through their paces. in different environments that we find throughout this bay. we want to find the best conditions that enable coral to grow well and thrive. >> reporter: the thing about this coral, for decades most of oahu's sewage flowed into this bay. six species managed to survive after the sewage was repiped

offshore. those species now make up one of the healthiest reefs in the world. >> here we are. >> cool! >> reporter: we dry off and meet in her lab in coconut island. the lab's basically a boot camp for coral. >> the apparatus you see around us on this side is all selective breeding. >> essentially choosing the offspring of the fittest of the fittest to combine them with the other fittest. >> that's right, like taking two super athletes, putting them together, and looking at what their offspring can do. >> reporter: gates acknowledges the limitations. there are coral scientists who say, listen, it's great, but it's not feasible over something that is as large as the great barrier reef. >> yep. >> 1,300, 1,400 miles long. >> in many ways, they may be right. that it's not feasible over that space. but it is feasible on reefs around island nations who depend on those reefs. do we sit down, watch, and give

up? or do we try and be prepared to fail? >> reporter: as we walk through this maze of tanks harboring these super corals, dr. gates, like most of the scientists we spoke to, offers this warning. if things continue the way they are in terms of warming oceans, acidification, how much time does the world's coral have left? >> i think we need to be actively intervening in the next ten years to stabilize more than the 10% we predict could survive to 2050. >> reporter: her work isn't going to repopulate the world's coral. it's geared to create a recipe that can be used by anyone to quickly grow the strongest of the strong. from the caribbean to the great barrier reef. and just maybe give us all a sliver of hope. for "nightline," i'm matt gutman reporting across the pacific. >> our thanks to matt for that report. we'll be right back. ♪

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paris accords and the flood of reactions from around the globe, head to abcnews.com and our "nightline" facebook page. thanks for watching. good night, america.