Why are low temperatures lethal?

Why are low temperatures lethal?

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Why can't we (human, or even unicellular organisms) withstand low (body) temperatures (5 - 25°C). I have a limited background in chemistry and biology, so this might be obvious, but not to me.

I know that low temperature reduce the speed of chemical reactions, so any creature cold enough should just perform slower? Or are there any other cold-induced effects that prevent certain reactions? (a reaction requiring more energy to happen, would still happen, but at a lesser rate, right?)

And, supposing a bacteria/virus/small&simple organism was subjected to low temperature (above freezing, but low enough to stop working), what would prevent this organism from booting up again when subjecting to "normal" temperature?

Low temperatures change the speed of different chemical reactions, throwing delicately balanced systems out of balance. How resilient a creature is to temperature changes is largely a function of how complex it is, and how buffered it is against this kind of change.

Tardigrades as kmm mentioned essentially can't be killed by cold, surviving down to almost absolute zero (absolute zero ≈ minus 273.15°C). Most unicellular organisms can be frozen (especially if frozen quickly or cryoprotected with something like DMSO) and revived later without any change. Most strain libraries of bacteria are kept frozen for cost reasons. Even human cell cultures can be frozen and thawed, even though some of the cells will die. (If even 10% of your cells died you as a person would probably die, so freezing is off the table unless you are very clever indeed.)

Humans and most mammals are pretty sensitive to temperature changes, even though reptiles are not. Most fish are cold-blooded and are adapted to cope with the relatively cold temperatures, but the human heart is not well adapted to even relatively warm temperatures. The atria and the ventricles of the heart stop communicating below 27°C (80.6°F)or so, and below 20°C (68°F) the heart can stop beating entirely. Nerves are complex systems, and absent regulation systems in the nerves themselves to compensate for the changes in temperature, they can stop firing completely. A simple heart can run with less sophisticated orchestration, and is, therefore, more resistant to temperature change. A complicated heart involving the coordination of the atria and the ventricles to produce effective pumping action is affected much more strongly by the drop in temperature.

So, in general, more complicated implies more vulnerable to temperature changes. There are exceptions, however. Bears can hibernate and drop their core body temperatures without any ill-effects, so it's possible to adapt to. Fish deal very well with cold temperatures if exposed gradually, and some fish could live quite happily in a slushie, salinity notwithstanding.

Low Temperature Storage: Chiling and Freezing

Chilled foods are those foods stored at temperatures near, but above their freezing point, typically 0-5 °C. This commodity area has shown a massive increase in recent years as traditional chilled products such as fresh meat and fish and dairy products have been joined by a huge variety of new products including complete meals, prepared and delicatessen salads, dairy desserts and many others.

Three main factors have contributed to this development:

(1) The food manufacturers’ objective of increasing added value to their products

(2) Consumer demand for fresh foods while at the same time requiring the convenience of only occasional shopping and ease of preparation and

(3) The availability of an efficient cold chain – the organization and infrastructure which allows low temperatures to be maintained throughout the food chain from manufacture/harvest to consumption.

Chill storage can change both the nature of spoilage and the rate at which it occurs. There may be qualitative changes in spoilage characteristics as low temperatures exert a selective effect preventing the growth of mesophiles and leading to a microflora dominated by psychrotrophs.

This can be seen in the case of raw milk which in the days of milk churns and roadside collection had a spoilage microflora comprised largely of mesophiliclactococci which would sour the milk.

Nowadays in the UK, milk is chilled almost immediately it leaves the cow so that psychrotrophic Gram-negative rods predominate and produce an entirely different type of spoilage. Low temperatures can also cause physiological changes in micro­organisms that modify or exacerbate spoilage characteristics.

Two such examples are the increased production of phenazine and carotenoid pigments in some organisms at low temperatures and the stimulation of extracellular polysaccharide production in Leuconostoc spp. and some other lactic acid bacteria.

In most cases, such changes probably represent a disturbance of metabolism due to the differing thermal coefficients and activation energies of the numerous chemical reactions that comprise microbial metabolism.

Though psychrotrophs can grow in chilled foods they do so only relatively slowly so that the onset of spoilage is delayed. In this respect temperature changes within the chill temperature range can have pronounced effects.

For example, the generation time for one pseudomonad isolated from fish was 6.7 hours at 5°C compared with 󈥺.6 hours at 0°C. Where this organism is an important contributor to spoilage, small changes of temperature will have major implications for shelf-life.

The keeping time of haddock and cod fillets has been found to double if the storage temperature is decreased from 2.8 °C to — 0.3°C. Mathematical modelling techniques of the sort can be useful in predicting the effect of temperature fluctuations on shelf-life, but, as a general rule, storage temperature should be as low, and as tightly controlled, as possible.

The ability of organisms to grow at low temperatures appears to be particularly associated with the composition and architecture of the plasma membrane. As the temperature is lowered, the plasma membrane undergoes a phase transition from a liquid crystalline state- to a rigid gel in which solute transport is severely limited.

The temperature of this transition is lower in psychrotrophs and psychrophiles largely as a result of higher levels of unsaturated and short chain fatty acids in their membrane lipids. If some organisms are allowed to adapt to growth at lower temperatures they increase the proportion of these components in their membranes.

There seems to be no taxonomic restriction on psychrotrophic organisms which can be found in the yeasts, moulds, Gram-negative and Gram-positive bacteria. One feature they share is that in addition to their ability to grow at low- temperatures, they are inactivated at moderate temperatures.

A number of reasons for this marked heat sensitivity have been put forward including the possibility of excessive membrane fluidity at higher temperatures. Low thermal stability of key enzymes and other functional proteins appears to be an important factor, although thermo-stable extracellular lipases and proteases produced by psychrotrophic pseudomonads can be a problem in the dairy industry.

Though mesophiles cannot grow at chill temperatures, they are not necessarily killed. Chilling will produce a phenomenon known as cold shock which causes death and injury in a proportion of the population but its effects are not predictable in the same way as heat processing.

The extent of cold shock depends on a number of factors such as the organism (Gram-negatives appear more susceptible than Gram- positives), its phase of growth (exponential-phase cells are more susceptible than stationary phase cells), the temperature differential and the rate of cooling (in both cases the larger it is, the greater the damage), and the growth medium cells grown in complex media are more resistant).

The principal mechanism of cold shock appears to be damage to membranes caused by phase changes in the membrane lipids which create hydroppores through which cytoplasmic contents can leak out. An increase in single-strand breaks in DNA has also been noted as well as the synthesis of specific cold-shock proteins.

Since chilling is not a bactericidal process, the use of good microbiological quality raw materials and hygienic handling are key requirements for the production of safe chill foods. Mesophiles that survive cooling, albeit in an injured state, can persist in the food for extended periods and may recover and resume growth should conditions later become favourable.

Thus chilling will prevent an increase in the risk from mesophilic pathogens, but will not assure its elimination. There are however pathogens that will continue to grow at some chill temperatures and the key role of chilling in the modern food industry has focused particular attention on these.

Risks posed by these organisms, may increase with duration of storage but this process is likely to be slow and dependent on the precise storage temperature and composition of the food.

Some foods are not amenable to chill storage as they suffer from cold injury where the low temperature results in tissue breakdown which leads to visual defects and accelerated microbiological deterioration. Tropical fruits are particularly susceptible to this form of damage.

Freezing Technique:

Freezing is the most successful technique for long term preservation of food since nutrient content is largely retained and the product resembles the fresh material more closely than in appertized foods.

Foods begin to freeze somewhere in the range — 0.5 to — 3 °C, the freezing point being lower than that of pure water due to the solutes present. As water is converted to ice during freezing, the concentration of solutes in the unfrozen water increases, decreasing its freezing point still further so that even at very low temperatures, e.g. — 60 °C, some water will remain unfrozen.

The temperatures used in frozen storage are generally less than — 18 °C. At these temperatures no microbial growth is possible, although residual microbial or endogenous enzyme activity such as lipases can persist and eventually spoil a product.

This is reduced in the case of fruits and vegetables by blanching before freezing to inactivate endogenous polyphenol oxidases which would otherwise cause the product to dis-colour during storage.

Freezer burn is another non-microbiological quality defect that may arise in frozen foods, where surface discolouration occurs due to sublimation of water from the product and its transfer to colder surfaces in the freezer. This can be prevented by wrapping products in a water-impermeable material or by glazing with n layer of ice.

Low temperature is not the only inhibitory factor operating in frozen foods they also have a low water activity produced by removal of water in the form of ice. Table 4.11 describes the effect of temperature on water activity. As far as microbiological quality is concerned, this effect is only significant when frozen foods are stored at temperatures where microbial growth is possible (above — 10 C).

In this situation, the organisms that grow on a product are not those normally associated with its spoilage at chill temperatures but yeasts and moulds that are both psychrotrophic and tolerant of reduced water activity.

Thus meat and poultry stored at — 5 to — 10 °C may slowly develop surface defects such as black spots due to the growth of the mould Cladosporium herbarum, white spots caused by Sporotrichum carnis or the feathery growth of Thamnidium elegans.

Micro-organisms are affected by each phase of the freezing process. In cooling down to the temperature at which freezing begins, a proportion of the population will be subject to cold shock.

At the freezing temperature, further death and injury occur as the cooling curve levels out as latent heat is removed and the product begins to freeze. Initially ice forms mainly extracellularly, intracellular ice formation being favoured by more rapid cooling.

This may mechanically damage cells and the high extracellular osmotic pressures generated will dehydrate them. Changes in the ionic strength and pH of the water phase as a result of freezing will also disrupt the structure and function of numerous cell components and macromolecules which depend on these factors for their stability.

Cooling down to the storage temperature will prevent any further microbial growth once the temperature has dropped below — 10 °C. Finally, during storage there will be an initial decrease in viable numbers followed by slow decline over time. The lower the storage temperature, the slower the death rate.

As with chilling, freezing will not render an unsafe product safe – its microbial lethality is limited and preformed toxins will persist. Frozen chickens are, after all, an important source of Salmonella.

Survival rates after freezing will depend on the precise conditions of freezing, the nature of the food material and the composition of its microflora, but have been variously recorded as between 5 and 70%. Bacterial spores are virtually unaffected by freezing, most vegetative Gram-positive bacteria are relatively resistant and Gram-negatives show the greatest sensitivity.

While frozen storage does reliably inactivate higher organisms such as pathogenic protozoa and parasitic worms, food materials often act as cryoprotectants for bacteria so that bacterial pathogens may survive for long periods in the frozen state. In one extreme example Salmonella has been successfully isolated from ice cream stored at — 23 °C for 7 years.

The extent of microbial death is also determined by the rate of cooling.

Maximum lethality is seen with slow cooling where, although there is little or no cold shock experienced by the organisms, exposure to high solute concentrations is prolonged. Survival is greater with rapid freezing where exposure to these conditions is minimized. Food freezing processes are not designed however to maximize microbial lethality but to minimize loss of product quality.

Formation of large ice crystals and prolonged exposure to high osmotic pressure solutions during slow cooling also damage cells of the food material itself causing greater drip loss and textural deterioration on thawing, so fast freezing in which the product is at storage temperature within half an hour is the method of choice commercially.

The rate of freezing in domestic freezers is much slower so, although microbial lethality may be greater, so too is product quality loss.

Thawing of frozen foods is a slower process than freezing. Even with moderate size material the outside of the product will be at the thawing temperature some time before the interior. So with high thawing temperature, mesophiles may be growing on the surface of a product while the interior is still frozen. Slow thawing at lower temperature is generally preferred.

It does have some lethal effect as microbial cells experience adverse conditions in the 0 to — 10 °C range for longer, but it will also allow psychrotrophs to grow. Provided the product is not subject to contamination after thawing, the microflora that develops will differ from that on the fresh material due to the selective lethal effect of freezing.

Lactic acid bacteria are often responsible for the spoilage of defrosted vegetables whereas they generally comprise only about 1% of the microflora on fresh chilled produce which is predominantly Gram-negative.

Freezing and defrosting may make some foods more susceptible to microbiolo­gical attack due to destruction of antimicrobial barriers in the product and condensation, but defrosted foods do not spoil more rapidly than those that have not been frozen. Injunctions against refreezing defrosted products are motivated by the loss of textural and other qualities rather than any microbiological risk that is posed.


Emerging infectious diseases (EIDs) are increasing in incidence and are responsible for plant and animal population declines in managed and wild systems [20, 29, 68]. To understand the drivers of EIDs, the rapidly developing field of disease ecology integrates traditional approaches of parasite biology into ecological and evolutionary frameworks [16]. One recent focus has been to understand the effects of environmental temperature fluctuations on disease [11, 69]. Differences in magnitude, range, and variability of daily temperature fluctuations have been shown to affect transmission intensity of malaria [40, 45], transmission rates of dengue virus [9, 30], biocontrol of the chagas disease vector [18], and susceptibility of black abalone to withering syndrome [4]. Temperature can profoundly influence disease outcomes due to the thermal sensitivity of host and pathogen traits, including pathogen growth and reproduction [1, 10, 58]. Because pathogen growth and reproduction are tied to virulence [38], understanding the responses of these life history traits to thermal heterogeneity may reveal important patterns in infectious disease.

Temperature can affect the disease ecology of chytridiomycosis [57, 76], a lethal emerging infectious disease of amphibians that is responsible for global amphibian declines [63, 64]. Chytridiomycosis is caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), which has a complex and temperature-sensitive life history [5, 35]. Motile Bd zoospores encyst in keratinized amphibian tissues and develop into zoosporangia [5, 48]. Zoosporangia produce the next generation of zoospores and release them into the environment or back onto the amphibian host [6, 35]. This life cycle requires temperatures between approximately 2–27 °C in vitro, with an optimal temperature range between 15 and 25 °C and a drop in reproduction and viability above 27 °C [51, 66, 73, 78]. Because increases in Bd loads correlate with the severity of chytridiomycosis [72, 74], exposure to temperatures above the Bd thermal maximum that negatively affect Bd growth and reproduction may decrease infection intensities and slow disease progression [23, 26, 62].

To date, temperature studies have predominantly focused on Bd responses at constant temperatures (e.g. [51, 78]). However, amphibian hosts live in microhabitats with remarkable thermal heterogeneity across daily, seasonal, and annual cycles (e.g. [43, 75]). Constant temperature studies have provided critical insights into Bd biology but have not discerned how realistic fluctuating thermal environments may influence Bd growth and reproduction. Recent work by Raffel et al. [55] and Greenspan et al. [23] suggest that fluctuating thermal conditions can have profound effects on Bd growth in vitro and on chytridiomycosis development in vivo. These studies add to evidence in other disease systems that constant temperature experiments may not be generalizable to disease dynamics in wild populations because thermal fluctuations can have disproportionate biological consequences on pathogen traits [9, 30, 40, 45]. In addition, thermal heterogeneity may influence the persistence of free-living Bd in water bodies used by amphibian hosts. While the mechanisms or duration of Bd persistence in natural environments remain unclear [7, 41], models suggest that extended environmental persistence of Bd outside amphibian hosts is likely to increase local extinction risk [39]. Understanding how dynamic thermal regimes affect Bd outside of hosts may be an important conservation tool to target where Bd is (and is not) on a landscape [21].

In this study, we examined responses of Bd in culture to biologically realistic temperature fluctuations that simulate the thermal conditions of water bodies used by the Yosemite toad (Anaxyrus [Bufo] canorus). The Yosemite toad is a federally threatened California endemic that is highly susceptible to lethal chytridiomycosis in controlled exposure experiments [32]. While Bd infection has been detected in all life stages of wild Yosemite toads [19] C. Dodge unpublished data), the role of Bd in the decline of this species is not well understood. Yosemite toads breed and develop in shallow pools in high elevation meadows in the Sierra Nevada Mountains of California, USA that undergo large daily temperature fluctuations compared to the Bd thermal range (Fig. 1a [13, 28, 43]). However, it is unclear how these temperature fluctuations affect Bd growth and reproduction and in turn, the disease ecology of this system.

Observed and experimental diurnal temperature fluctuations. a Water temperature over a 24-h period of 10 different breeding pools containing Yosemite toad tadpoles (grey lines yellow line represents pool fluctuating within 27.5 and 7.5 °C). b Incubator temperature profiles over a 24-h period. Fluctuating temperature = black constant temperature at daily thermal maximum (27.5 °C) = red constant temperature at daily thermal minimum (7.5 °C) = blue constant temperature at daily thermal mean (17.5 °C) = green. Bd thermal optimum (green shaded band) and Bd thermal tolerance (grey shaded band) shown for reference

To better understand Bd responses to fluctuating thermal regimes, we collected temperature data from Yosemite toad breeding pools and cultured Bd under fluctuating thermal conditions that simulated pool temperatures (Fig. 1). To assess Bd responses to thermal fluctuation, we compared multiple reproductive life history traits of a single Bd isolate grown at fluctuating or constant temperatures. Our constant temperature treatments span the Bd thermal range and represent the daily minimum, daily mean, and daily maximum of the fluctuating temperature profile (Fig. 1b). We quantified Bd growth over time using measurements of culture optical density, motile zoospore counts, culture fecundity (ratio of motile zoospores to optical density), and zoosporangia viability assays. We hypothesized that fluctuating temperatures would reduce Bd growth as compared to Bd grown at the constant daily mean temperature of 17.5 °C. We predicted that exposure to daily temperature fluctuations would reduce Bd growth rate, fecundity, zoosporangia viability, zoospore production, and time to peak zoospore release as compared to Bd grown at 17.5 °C.

This Is Why Global Warming Is Responsible For Freezing Temperatures Across The U.S.

In January of 2014, a displaced polar vortex brought extremely cold temperatures to many parts of . [+] the United States, causing Lake Michigan near Chicago to freeze over, as shown here. The current cold snap is extremely similar in nature, and is wreaking havoc across much of the continential United States right now, in 2019.

Edward Stojakovik / flickr

The country is freezing in an unprecedented fashion, and global warming is to blame. Sound crazy? The cold snap that North America is experiencing east of the rocky mountains, with temperatures at Arctic-like levels, is real, but it's only part of the story. Simultaneously, there are record warm temperatures happening in other parts of the world, from Australia to the actual Arctic.

While a small but vocal minority of people might use the faulty logic of, "it's cold where I am, therefore global warming isn't real," even schoolchildren know that weather isn't climate. But these extreme cold snaps have gotten more severe in recent years, due to a combination of global warming and a phenomenon you've likely heard of: the polar vortex. Here's the science of how it works, and why global warming is paradoxically playing a major role in today's record-low temperatures.

The difference between a strong, stable polar vortex (L) and a weak, unstable one that can cause . [+] cold snaps and extremely cold weather across the mid-latitudes (R), such as the event we're experiencing now.

When you think about the Earth, including its weather, climate, and temperature, what picture do you get in your head?

The best way to picture Earth is as a sphere rotating on its axis, but with two additional effects: the atmosphere and the oceans. As the Earth rotates on its axis, we experience warming during the day (in direct sunlight) and cooling at night (in the dark), as the Earth radiates its stored heat away into the depths of space. When our hemisphere is tilted towards the Sun, we experience summer months when our hemisphere is tilted away from the Sun, we experience winter months.

The ocean stores tremendous amounts of heat, with ocean currents transporting that heat from one location to another. But in terms of these particular weather events we're experiencing right now, the atmosphere is the biggest factor.

This graphic shows the global circulation of Earth's atmosphere. Included in this display are Hadley . [+] cells, Ferrell cells and polar cells, along with the six different zones in the northern and southern hemispheres displaying the prevailing winds.

Wikimedia Commons user Kaidor

On any planet that rotates, you'll have an effect called the prevailing winds. As the atmosphere circulates around the world, planet Earth typically experiences three different types of winds, normally confined to three different latitude zones:

  • 0° to 30°: where we get the trade winds, which blow from east to west and converge at the equator.
  • 30° to 60°: which give us the westerlies, which blow from west to east, and rise up towards the Arctic (or down towards the Antarctic) circle.
  • 60° to 90°: the polar cells, which are normally confined to the highest-latitude regions on Earth.

Although the latitude bands differ, this phenomenon is common to most rapidly rotating planets with atmospheres, including Venus, Mars, Jupiter and Saturn. Earth, though, is a little bit special.

The ocean temperatures are warm enough in the equatorial regions, during the right seasons, to form . [+] tropical cyclones, and are cool enough, in the winter seasons, to form extreme polar vortices.

Berkeley Earth Surface Temperature (BEST) team

Because of the thinness of Earth's atmosphere, our substantial axial tilt, the behavior of cloud cover and reflectivity at the poles, and a number of other factors, our planet has an extremely large temperature difference between the equator and the poles. This temperature difference is smallest in the summer, when the polar areas experience nearly 24 hours of continuous sunlight, and largest in the winter, where it's almost always night.

As a result of these severe temperature differences, there is a persistent, large-scale, low-pressure zone that rotates in a cyclone-like fashion at each pole: from west to east. (Counterclockwise at the north pole, clockwise at the south pole.) These two zones are known as polar vortices, and they each start a few miles up in the atmosphere and extend well into the stratosphere.

The interplay between the atmosphere, clouds, moisture, land processes and the ocean all governs the . [+] evolution of Earth's equilibrium temperature. The stratosphere, in particular, is of tremendous importance for phenomena like the Arctic's polar vortex.

NASA / Smithsonian Air & Space Museum

Beneath them, you'll typically find a large mass of cold, dense air surrounding each of the poles. Normally, these vortices are stable enough, as temperature and pressure differences are severe enough, to keep them in place throughout the year.

When the vortices are at their strongest, you get a single cell, and the air is extremely well-confined. When the vortices weaken, they can break up into two or more cells, and begin to migrate away from the poles. When they're extremely weak, they can fragment, and some of that low pressure, low temperature air can begin to interact with the higher pressure, higher temperature air from outside the polar regions.

Earth in 2013 (at left) with a well-defined, single-cell, strong polar vortex, along with Earth in . [+] 2014 (at right) where the polar vortex became extremely weak, and migrated over the populous land masses of the mid-latitudes.

Although the term has been around since the 1850s, few people heard of the polar vortex until earlier this decade, when it became so weak that it migrated over the North American and Eurasian continents, causing some of the coldest winter weather we've seen in recent history.

When the vortex at the north pole becomes extremely weak, the high pressure zones found in the middle latitudes of Earth (where the westerlies are) can push towards the poles, displacing the cold air. This causes the polar vortex to move farther south. In addition, the jet stream buckles, and deviates towards more populous, southern latitudes. As the cold, dry air from the poles comes in contact with the warm, moist air of the mid-latitudes, you get a dramatic weather change that we conventionally refer to as a cold snap.

When the polar vortex around the North Pole weakens, it causes much of the cold air at high . [+] latitudes to mix with the warm air in the mid-latitudes. This pushes the jet stream south, brings cold air to highly populous areas, and creates the conditions for a cold snap.

The weather we're experiencing across much of the northern hemisphere is due to exactly this phenomenon, occurring right now.

But how is global warming to blame?

The answer is simple: because the phenomenon that causes the polar vortex to break down is known as sudden stratospheric warming, where the upper layers of the atmosphere increase in temperature by approximately 30–50 °C (54–90 °F) over the span of only a few days. The fact that there are land masses located where they are in the northern hemisphere means that as those land temperatures increase, they transport their heat to even more northern latitudes.

The polar vortex, typically, is a single-cell or double-cell region concentrated at polar latitudes. . [+] However, warming events along the land and in the sea near the poles have changed the temperature and pressure gradients in recent years, and are causing the polar vortex to destabilize. This results in the extreme weather events we're experiencing more recently.

The exact details of how this works are complex, but the explanation is simple: warmer land temperatures, particularly in northern North America and northern Eurasia, allow more heat to be transported into the Arctic stratosphere. A warmer Earth makes sudden stratospheric warming events more likely and more frequent. And those events destabilize the polar vortex, bring cold air down into the mid-latitudes, and cause the extreme weather we're experiencing right now.

The temperature map of Earth on the day of Sunday, January 27th. Note how the southern, Antarctic . [+] region has its cold air relatively confined, while the northern, Arctic region has colder and warmer areas in uneven, perhaps unexpected locations.

As the Earth continues to warm, there will be reduced snow cover and less sea ice in these critical regions, which alters the pressure and temperature gradients of the regions at the boundary of the polar vortex. In extreme cases, the polar vortex weakens or collapses as a result. The migration of the jet stream is one of the first signs, and it has become an all-too-frequent phenomenon in recent years.

The enormous cold snap we experienced in 2014 wasn't a one-off event. Although for many, that storm was so memorable it feels like it was only yesterday, we can absolutely expect these types of extreme weather events to become commonplace in the coming years. The climate is changing, and it's affecting our weather in a variety of ways all across the globe.

In January of 2014, the term polar vortex came into the popular lexicon with a catastrophic cold . [+] snap that affected large portions of North America, causing enormous portions of Niagara Falls to freeze over, among other things. We can expect these events to be far more frequent going forward.

Perhaps paradoxically, it's a strong, extremely cold polar vortex that results in stable, warm temperatures across the more populous mid-latitudes in winter. This is one effect of climate change that's already here, and will take centuries, in the best-case scenario, to reverse. There are freezing temperatures and an extraordinary cold snap affecting huge portions of the land mass in the northern hemisphere's mid-latitudes right now, but this won't feel extraordinary for long.

As the Earth continues to warm, extreme weather events like this will become commonplace, with many climatologists predicting an unstable polar vortex bringing storms like this to us multiple times per decade. Welcome to the new normal, courtesy of global warming, where the Arctic can't even remain cold in the dead of winter.

A Degree of Concern: Why Global Temperatures Matter

If you could ask a sea turtle why small increases in global average temperature matter, you&rsquod be likely to get a mouthful. Of sea grass, that is.

Of course, sea turtles can&rsquot talk, except in certain animated movies. And while on-screen they&rsquore portrayed as happy-go-lucky creatures, in reality it&rsquos pretty tough to be a sea turtle, dude (consider the facts), and in a warming world, it&rsquos getting tougher.

Since the temperature of the beach sand that female sea turtles nest in influences the gender of their offspring during incubation, our warming climate may be driving sea turtles into extinction by creating a shortage of males, according to several studies. 1

A few degrees make a huge difference. At sand temperatures of 31.1 degrees Celsius (88 degrees Fahrenheit), only female green sea turtles hatch, while at 27.8 degrees Celsius (82 degrees Fahrenheit) and below, only males hatch.

While the plight of sea turtles is illustrative, it&rsquos a fact that all natural and human systems are sensitive to climate warming in varying degrees. To assess the likely impacts of global warming on our planet at various temperature thresholds above pre-industrial levels (considered to be the time period between 1850 and 1900), the Intergovernmental Panel on Climate Change (IPCC) in October released a Special Report on Global Warming of 1.5 Degrees Celsius (2.7 Degrees Fahrenheit). The IPCC is the United Nations body tasked with assessing the science related to climate change.

The report examined the impacts of limiting the increase in global average temperature to well below 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels, and projected the impacts Earth is expected to see at both 1.5 degrees and 2 degrees Celsius warming above those levels. The 1.5-degree Celsius threshold represents the target goal established by the Paris Agreement, adopted by 195 nations in December 2015 to address the threat of climate change.

The following interactive presents selected highlights from the report:

The report, prepared by 91 authors and review editors from 40 countries along with 133 contributing authors, cites more than 6,000 scientific references and includes contributions from thousands of expert reviewers around the world, including from NASA. NASA data were critical to enabling understanding of how each half degree of warming will impact our planet. NASA models contributed to the report&rsquos projections, while NASA satellite and airborne observations provided critical inputs.

&ldquoUnfortunately, warming has progressed so much that we now have observations of what happens when you have an extra half a degree,&rdquo said Drew Shindell, professor of Climate Sciences at the Nicholas School of the Environment at Duke University in Durham, North Carolina. Shindell is a coordinating lead author of one chapter of the Special Report and an author of its Summary for Policy Makers. &ldquoHaving an extra five to 10 years since the last IPCC Assessment, along with modern monitoring systems, many of which are from NASA, really lets us see what happens to the planet with an extra half a degree of warming much more clearly than in the past.&rdquo

The report says that since the pre-industrial period, human activities are estimated to have increased Earth&rsquos global average temperature by about 1 degree Celsius (1.8 degrees Fahrenheit), a number that is currently increasing by 0.2 degrees Celsius (0.36 degrees Fahrenheit) every decade. At that rate, global warming is likely to reach 1.5 degrees Celsius above pre-industrial levels sometime between 2030 and 2052, with a best estimate of around 2040.

Warming that&rsquos already been introduced into the Earth system by human-produced emissions since the start of the pre-industrial period isn&rsquot expected to dissipate for hundreds to thousands of years. That already &ldquobaked in&rdquo warming will continue to cause further long-term changes in our climate, such as sea level rise and its associated impacts. However, the report says that these past emissions alone are not considered likely, by themselves, to cause Earth to warm by 1.5 degrees Celsius. In other words, what we as a society do now matters. The urgency with which the world addresses greenhouse gas emission reductions now will help determine the degree of future warming in essence, whether we&rsquoll be hit by a climate change hardball or a wiffle ball.

You might be thinking, &ldquoWhy should I care if temperatures go up another half a degree or one degree? Temperatures go up and down all the time. What difference does it make?&rdquo

The answer is, a lot. Higher temperature thresholds will adversely impact increasingly larger percentages of life on Earth, with significant variations by region, ecosystem and species. For some species, it literally means life or death.

&ldquoWhat we see isn&rsquot good &ndash impacts of climate change are in many cases larger in response to a half a degree (of warming) than we&rsquod expected,&rdquo said Shindell, who was formerly a research scientist at NASA&rsquos Goddard Institute for Space Studies in New York City. &ldquoWe see faster acceleration of ice melting, greater increases in tropical storm damages, stronger effects on droughts and flooding, etc. As we calibrate our models to capture the observed responses or even simply extrapolate another half a degree, we see that it&rsquos more important than we&rsquod previously thought to avoid the extra warming between 1.5 and 2 degrees Celsius.&rdquo

Shindell said the report was able to use scientists&rsquo understanding from observations to assess how many more people would be at risk from the impacts of climate change with an additional half a degree of warming. &ldquoIt&rsquos hundreds of millions,&rdquo he said, &ldquowhich makes clear the importance of keeping warming as low as possible.&rdquo

NASA&rsquos global climate change website, and its vital signs section, document what a 1-degree Celsius temperature increase has already done to our planet. The impacts of global warming are being felt everywhere, from rising sea levels to more extreme weather, more frequent wildfires, and heatwaves and increased drought, to name a few. Because our society has been built around the climate Earth has had for the past approximately 10,000 years, when it changes noticeably, as it has done in recent decades, people begin to take notice. Today, most people realize Earth&rsquos climate is changing. A December 2018 report by Yale and George Mason Universities found that seven in 10 Americans think global warming is happening, with about six in 10 saying it is mostly caused by humans.

We live in a world bound by the laws of physics. For example, at temperatures above 0 degrees Celsius (32 degrees Fahrenheit), ice, including Earth&rsquos polar ice sheets and other land ice, begins to melt and changes from a solid to a liquid. When that water flows downward into the ocean, it raises global sea level.

Similarly, temperature plays a critical role in biology. We all know the average temperature of a healthy adult human is about 37 degrees Celsius (98.6 degrees Fahrenheit). You don&rsquot have to ask anyone running a fever of 38.3 degrees Celsius (101 degrees Fahrenheit) if a couple of degrees matters. Our bodies are optimized to run at a certain temperature. According to most studies, humans feel most comfortable, are most productive and function best when the ambient temperature around us is roughly 22 degrees Celsius (71.6 degrees Fahrenheit). Vary that temperature by more than a few degrees in either direction and we seek to warm or cool ourselves if we can. Our bodies also make adjustments, such as sweating.

When ambient temperatures become too extreme, the impacts on human health can be profound, even deadly.

Plants and other animals have it tougher. While they also adjust to their external temperature environment through various mechanisms, they can&rsquot just turn on an air conditioner or furnace like we can, and they may not be able to migrate. They survive within specific, defined habitats.

For all living organisms, the faster climate changes, the more difficult it is to adapt to it. When climate change is too rapid, it can lead to species extinction. As greenhouse gas concentrations continue to increase, the cumulative impact will be to accelerate temperature change. Limiting warming to 1.5 degrees Celsius decreases the risks of long-lasting or irreversible changes, such as the loss of certain ecosystems, and allows people and ecosystems to better adapt.

So just how may another half- or full-degree Celsius of warming affect our planet? In part two of our feature, we look at some of the IPCC special report&rsquos specific projections.

What does cold temperature do to enzymes?

At very cold temperatures, the opposite effect dominates &ndash molecules move more slowly, reducing the frequency of enzyme-substrate collisions and therefore decreasing enzyme activity. As a result, enzyme-substrate collisions are extremely rare once freezing occurs and enzyme activity is nearly zero below freezing.

Likewise, do enzymes denature at cold temperatures? The shape of an enzyme also depends on its temperature. When enzymes get too warm, they get too loose. And when they get too cold, then they get too tight. If the temperature is increased too much, the rate of reaction will diminish due to denaturing or change in shape of the enzyme.

In this regard, why do cold temperatures slow down enzymes?

Low temperatures result in lower kinetic energy of particles, so this translates to less/slower activity by both the enzyme AND the substrate. Therefore, fewer substrates will come in contact with the enzyme. That said, different enzymes have different optimal temperature ranges.

What temperature do enzymes denature?

This optimal temperature is usually around human body temperature (37.5 o C) for the enzymes in human cells. Above this temperature the enzyme structure begins to break down (denature) since at higher temperatures intra- and intermolecular bonds are broken as the enzyme molecules gain even more kinetic energy.

Feeding behaviour and nutrition requirements

Tilapia ingest a wide variety of natural food organisms, including plankton, some aquatic macrophytes, planktonic and benthic aquatic invertebrates, larval fish, detritus, and decomposing organic matter. With heavy supplemental feeding, natural food organisms typically account for 30 to 50 percent of tilapia growth. (In supplementally fed channel catfish only 5 to 10 percent of growth can be traced to ingestion of natural food organisms.) Tilipia are often considered filter feeders because they can efficiently harvest plankton from the water.

However, tilapia do not physically filter the water through gill rakers as efficiently as true filter feeders such as gizzard shad and silver carp. The gills of tilapia secrete a mucous that traps plankton. The plankton-rich mucous, or bolus, is then swallowed. Digestion and assimilation of plant material occurs along the length of the intestine (usually at least six times the total length of the fish).

The Mozambique tilapia is less efficient than the Nile or Blue tilapia at harvesting planktonic algae. Two mechanisms help tilapia digest filamentous and planktonic algae and succulent higher plants:

  1. Physical grinding of plant tissues between two pharyngeal plates of fine teeth
  2. A stomach pH below 2, which ruptures the cell walls of algae and bacteria.

The commonly cultured tilapias digest 30 to 60 percent of the protein in algae blue-green algae is digested more efficiently than green algae.

When feeding, tilapias do not disturb the pond bottom as aggressively as common carp. However, they effectively browse on live benthic invertebrates and bacteria-laden detritus. Tilapias also feed on midwater invertebrates. They are not generally considered piscivorous, but juveniles do consume larval fish.

In general, tilapias use natural food so efficiently that crops of more than 2,700 pounds of fish per acre (3,000 kg/ha) can be sustained in well-fertilised ponds without supplemental feed. The nutritional value of the natural food supply in ponds is important, even for commercial operations that feed fish intensively.

In heavily fed ponds with little or no water exchange, natural food organisms may provide one-third or more of total nutrients for growth. In general, tilapia digest animal protein in feeds with an efficiency similar to that of channel catfish, but are more efficient in the digestion of plant protein, especially more fibrous materials.

Tilapia require the same ten essential amino acids as other warm water fish, and, as far as has been investigated, the requirements for each amino acid are similar to those of other fish. Protein requirements for maximum growth are a function of protein quality and fish size and have been reported as high as 50 percent of the diet for small fingerlings. However, in commercial food fish ponds the crude protein content of feeds is usually 26 to 30 percent, one tenth or less of which is of animal origin. The protein content and proportion of animal protein may be slightly higher in recirculating and flow-through systems.

The digestible energy requirements for economically optimum growth are similar to those for catfish and have been estimated at 8.2 to 9.4 kcal DE (digestible energy) per gram of dietary protein. Tilapia may have a dietary requirement for fatty acids of the linoleic (n-6) family. Tilapia appear to have similar vitamin requirements as other warm water fish species. Vitamin and mineral premixes similar to those added to catfish diets are usually incorporated in commercial tilapia feeds. The feeding behaviour of tilapia allows them to use a mash (unpelleted feeds) more efficiently than do catfish or trout, but most commercial tilapia feeds are pelletised to reduce nutrient loss. In the absence of feeds specifically prepared for tilapia, a commercial catfish feed with a crude protein content of 28 to 32 percent is appropriate in the United States.


How can you prevent hypothermia?

When it is cold, you should wear a hat that covers the ears and warm, dry clothing. Older people and children should take extra care to prevent hypothermia by:

  • Dressing in layers and keeping warm clothes nearby
  • Keeping homes at a temperature above 68° F
  • Moving around when you feel cold so you can increase your body temperature
  • Eating and drinking warm foods and beverages
  • Wearing appropriate clothing outdoors, including hats, mittens, coats and footwear
  • Taking regular breaks and coming inside to warm up whenever spending time outside

Median Lethal Dose Limitations

While the LD50 is a useful indicator of toxicity, there are also some inherent limitations associated with this method. Such drawbacks include:

  1. Variability between testing facilities, which can produce unreliable results.
  2. Genetic variability in the tested subjects. Thus, depending on the sample population, the LD50 may vary.
  3. The route of delivery (e.g., intravenous, intermuscular, subcutaneous, etc.).
  4. Animal species used for testing. For example, a substance that is innocuous in one species could be lethal in another (e.g., chocolate is lethal to dogs but safe for humans).

1. A substance with a low LD50 is considered to be: (Multiple Choice)
A. Lethal at a lower dose
B. Lethal at a higher dose
C. Highly toxic
D. Minimally toxic

What isn’t known

There are still many mysteries about this virus and coronaviruses in general – the nuances of how they cause disease, the way they interact with proteins inside the cell, the structure of the proteins that form new viruses and how some of the basic virus-copying machinery works.

Another unknown is how COVID-19 will respond to changes in the seasons. The flu tends to follow cold weather, both in the northern and southern hemispheres. Some other human coronaviruses spread at a low level year-round, but then seem to peak in the spring. But nobody really knows for sure why these viruses vary with the seasons.

What is amazing so far in this outbreak is all the good science that has come out so quickly. The research community learned about structures of the virus spike protein and the ACE2 protein with part of the spike protein attached just a little over a month after the genetic sequence became available. I spent my first 20 or so years working on coronaviruses without the benefit of either. This bodes well for better understanding, preventing and treating COVID-19.

By Benjamin Neuman, Professor of Biology, Texas A&M University-Texarkana. This article is republished from The Conversation under a Creative Commons license. Read the original article.