Query: “The global reef crisis does not necessarily mean extinction for coral species.

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Water scarcity <=SEP=> Water scarcity is the lack of sufficient available water resources to meet water needs within a region . It affects every continent and around 2.8 billion people around the world at least one month out of every year . More than 1.2 billion people lack access to clean drinking water .   Water scarcity involves water shortage , water stress or deficits , and water crisis . The relatively new concept of water stress is difficulty in obtaining sources of fresh water for use during a period of time ; it may result in further depletion and deterioration of available water resources . Water shortages may be caused by climate change , such as altered weather-patterns ( including droughts or floods ) , increased pollution , and increased human demand and overuse of water . The term water crisis labels a situation where the available potable , unpolluted water within a region is less than that region 's demand . Two converging phenomena drive water scarcity : growing freshwater use and depletion of usable freshwater resources .   Water scarcity can result from two mechanisms :   physical ( absolute ) water scarcity  economic water scarcity   Physical water scarcity results from inadequate natural water resources to supply a region 's demand , and economic water scarcity results from poor management of the sufficient available water resources . According to the United Nations Development Programme , the latter is found more often to be the cause of countries or regions experiencing water scarcity , as most countries or regions have enough water to meet household , industrial , agricultural , and environmental needs , but lack the means to provide it in an accessible manner .   Many countries and governments aim to reduce water scarcity . The UN recognizes the importance of reducing the number of people without sustainable access to clean water and sanitation . The Millennium Development Goals within the United Nations Millennium Declaration aimed by 2015 to `` halve the proportion of people who are unable to reach or to afford safe drinking water . ''

Conservation biology <=SEP=> Conservation biology is the management of nature and of Earth 's biodiversity with the aim of protecting species , their habitats , and ecosystems from excessive rates of extinction and the erosion of biotic interactions . It is an interdisciplinary subject drawing on natural and social sciences , and the practice of natural resource management .   The conservation ethic is based on the findings of conservation biology .

Extinction risk from global warming <=SEP=> The extinction risk of global warming is the risk of species becoming extinct due to the effects of global warming .

Cretaceous–Paleogene extinction event <=SEP=> The Cretaceous -- Paleogene ( K -- Pg ) extinction event , also known as the Cretaceous -- Tertiary ( K -- T ) extinction , was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 65 million years ago . With the exception of some ectothermic species like the leatherback sea turtle and crocodiles , no tetrapods weighing more than 25 kg survived . It marked the end of the Cretaceous period and with it , the entire Mesozoic Era , opening the Cenozoic Era that continues today .   In the geologic record , the K -- Pg event is marked by a thin layer of sediment called the K -- Pg boundary , which can be found throughout the world in marine and terrestrial rocks . The boundary clay shows high levels of the metal iridium , which is rare in the Earth 's crust but abundant in asteroids .   As originally proposed in 1980 by a team of scientists led by Luis Alvarez , it is now generally thought that the K -- Pg extinction was caused by a massive comet or asteroid impact , estimated to be 10 km wide , 66 million years ago and its catastrophic effects on the global environment , including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis . The impact hypothesis , also known as the Alvarez hypothesis , was bolstered by the discovery of the 180 km Chicxulub crater in the Gulf of Mexico in the early 1990s , which provided conclusive evidence that the K -- Pg boundary clay represented debris from an asteroid impact . The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K -- Pg extinction was caused by the asteroid . It was possibly accelerated by the creation of the Deccan Traps . However , some scientists maintain the extinction was caused or exacerbated by other factors , such as volcanic eruptions , climate change , or sea level change , separately or together .   A wide range of species perished in the K -- Pg extinction . The best-known victims are the non-avian dinosaurs . However , the extinction also destroyed a plethora of other terrestrial organisms , including certain mammals , pterosaurs , birds , lizards , insects , and plants . In the oceans , the K -- Pg extinction killed off plesiosaurs and the giant marine lizards ( Mosasauridae ) and devastated fish , sharks , mollusks ( especially ammonites , which became extinct ) and many species of plankton . It is estimated that 75 % or more of all species on Earth vanished . Yet the devastation caused by the extinction also provided evolutionary opportunities . In the wake of the extinction , many groups underwent remarkable adaptive radiations -- a sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches resulting from the event . Mammals in particular diversified in the Paleogene , producing new forms such as horses , whales , bats , and primates . Birds , fish and perhaps lizards also radiated .