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giovedì 24 marzo 2016

Mass Extinctions and Climate Change

We now know that greenhouse gases are rising faster than at any time since the demise of dinosaurs, and possibly even earlier. According to research published in Nature Geoscience this week, carbon dioxide (CO₂) is being added to the atmosphere at least ten times faster than during a major warming event about 50 million years ago.

With increasing CO₂ levels, temperatures and ocean acidification also rise, and it is an open question how ecosystems are going to cope under such rapid change.
Coral reefs, our canary in the coal mine, suggest that the present rate of climate change is too fast for many species to adapt: the next widespread extinction event might have already started.
In the past, rapid increases in greenhouse gases have been associated with mass extinctions. It is therefore important to understand how unusual the current rate of atmospheric CO₂ increase is with respect to past climate variability.

Into the ice ages

There is no doubt that atmospheric CO₂ concentrations and global temperatures have changed in the past.
Ice sheets, for example, are reliable book-keepers of ancient climate and can give us an insight into climate conditions long before the thermometer was invented. By drilling holes into ice sheets we can retrieve ice cores and analyse the accumulation of ancient snow, layer upon layer.
These ice cores not only record atmospheric temperatures through time, they also contain frozen bubbles that provide us with small samples of ancient air. Our longest ice core extends more than 800,000 years into the past.
During this time, the Earth oscillated between cold ice ages and warm “interglacials”. To move from an ice age to an interglacial, you need to increase CO₂ by roughly 100 ppm. This increase repeatedly melted several kilometre-thick ice sheets that covered the locations of modern cities like Toronto, Boston, Chicago or Montreal.
With increasing CO₂ levels at the end of the last ice age, temperatures increased too. Some ecosystems could not keep up with the rate of change, resulting in several megafaunal extinctions, although human impacts were almost certainly part of the story.
Nevertheless, the rate of change in CO₂ over the past million years was tame when compared to today. The highest recorded rate of change before the Industrial Revolution is less than 0.15 ppm per year, just one-twentieth of what we are experiencing today.

Looking further back

To find an analogue for present-day climate change, we therefore have to look further back, to a time when ice sheets were small or did not exist at all. Several abrupt warming events occurred between 56 million and 52 million years ago. These events were characterised by a rapid increase in temperature and ocean acidification.
The most prominent of these events was the Palaeocene Eocene Thermal Maximum (PETM). This event resulted in one of the largest known extinctions of life forms in the deep ocean. Atmospheric temperatures increased by 5-8C within a few thousand years.
Reconstructions of the amount of carbon added to the atmosphere during this event vary between 2000-10,000 billion tonnes of carbon.
The new research, led by Professor Richard Zeebe of the University of Hawaii, analysed ocean sediments to quantify the lag between warming and changes in the carbon cycle during the PETM.
Although climate archives become less certain the further we look back, the authors found that the carbon release must have been below 1.1 billion tonnes of carbon per year. That is about one-tenth of the rate of today’s carbon emissions from human activities such as burning fossil fuels.

What happens when the brakes are off?

Although the PETM resulted in one of the largest known deep sea extinctions, it is a small event when compared to the five major extinctions in the past.
The Permian-Triassic Boundary extinction, nicknamed “The Great Dying”, wiped out 90% of marine species and 70% of land vertebrate families 250 million years ago. Like its four brothers, this extinction event happened a very long time ago. Climate archives going that far back lack the resolution needed to reliably reconstruct rates of change.
There is, however, evidence for extensive volcanic activity during the Great Dying, which would have led to a release of CO₂ as well as the potential release of methane along continental margins. Ocean acidification caused by high atmospheric CO₂ concentrations and acid rain have been put forward as potential killer mechanisms.
Other hypotheses include reduced oxygen in the ocean due to global warming or escape of hydrogen sulfide, which would have caused both direct poisoning and damage to the ozone layer.
These past warming events occurred without human influence. They point to the existence of positive feedbacks within the climate system that have the power to escalate warming dramatically. The thresholds to trigger these feedbacks are hard to predict and their impacts are hard to quantify.
Some examples of feedbacks include the melting of permafrost, the release of methane hydrates from ocean sediments, changes in the ocean carbon cycle, and changes in peatlands and wetlands. All of these processes have the potential to quickly add more greenhouse gases to the atmosphere.
Given that these feedbacks were strong enough in the past to wipe out a considerable proportion of life forms on Earth, there is no reason to believe that they won’t be strong enough in the near future, if triggered by sufficiently rapid warming.
Today’s rate of change in atmospheric CO₂ is unprecedented in climate archives. It outpaces the carbon release during the most extreme abrupt warming events in the past 66 million years by at least an order of magnitude.
We are therefore unable to rely upon past records to predict if and how our ecosystems will be able to adapt. We know, however, that mass extinctions have occurred in the past and that these extinctions, at least in the case of the PETM, were triggered by much smaller rates of change.

ICE MELT SEA LAVEL RISE AND SUPERSTORMS
Surreal Wyoming Superstorm Captured in Incredible Time-Lapse
Last summer, prior to countries’ United Nations negotiations in Paris, Hansen and 16
collaborators authored a draft paper that suggested we could see at least 10 feet of
sea-level rise in as few as 50 years.
If that sounds alarming to you, it is—10 feet of sea-level rise is more than enough to 
effectively kick us out of even the most well-endowed coastal cities. Stitching 
together archaeological evidence of past climate change, current observations, and 
future-telling climate models, the authors suggested that even a small amount of global 
warming can rack up enormous consequences—and quickly.
Now, the final version of the paper has been published in the journal Atmospheric
Chemistry and Physics. It’s been reviewed and lightly edited, but its conclusions are still 
shocking—and still contentious.
So what’s the deal? The authors highlight several of the threats they believe we’ll face this 
century, including many feet of sea-level rise, a halting of major ocean circulatory currents,
and an outbreak of super storms. These are the big threats we’ve been afraid of—and 
Hansen et al. say they could be here before we know it — well before the Intergovernmental
Panel on Climate Change’s sanctioned climate models predict.
Here we help you understand their new paper:

Sea-level Rise

The scientists estimate that existing climate models aren’t accounting well enough for
current ice loss off of the Greenland and Antarctic ice sheets. Right now, Antarctica and 
Greenland ice sheets both contribute under or near 1 millimeter to sea-level rise every year; 
they each contain enough stored ice to drive up ocean levels by 20 and 200 feet, respectively.
This study suggests that, since the rate of ice loss is increasing, we should think of it not
as a straight line but as an exponential curve, doubling every few years. But how much time it 
takes to double makes a big difference. Right now, measurements of ice loss aren’t clear 
enough to even make a strong estimate about how long that period might be. Is it 10 years 
or is it 40? It’s hard to say based on the limited data we have now, which would make
a big difference either way.
But then again, we don’t even know that ice loss is exponential. Ian Joughin—a 
University of Washington researcher unaffiliated with the paper and who has studied the
 tipping points of Antarctic glaciers—put it this way: Think about the stock market in the ’80s. 
If you observed a couple years of accelerating growth, and decided that rate would double
every 4 years—you’d have something like 56,000 points in the Dow Jones Industrial by now.
Or if stocks aren’t your thing, think about that other exponentially expanding force of 
nature: bacteria. Certain colonies of bacteria can double their population in a matter of hours.
Can they do this forever? No, or else we’d be nothing but bacteria right now (and while
we’re certainly a high percentage of bacteria, there’s still room for a couple other things).
Nature tends to put limits on exponential growth, Joughin points out — and the same probably
goes for ice loss: “There’s only so fast you can move ice out of an ice sheet,” Joughin 
explained. While some ice masses may be collapsing at an accelerating rate, others won’t
be as volatile.
This means, while some parts of ice sheet collapse may very well proceed exponentially, we
can’t expect such simple mathematics to model anything in the real world except the terror spike of the Kingda Ka.

Ocean Turnover

Mmm mm, ocean turnover: Is it another word for a sushi roll or a fundamental process that
keeps the climate relatively stable and moderate?
That’s right—we’re talking the Atlantic Meridonal Overturning Circulation, or AMOC, and
other currents like it.
As cold meltwater flows off of glaciers and ice sheets at enormous rates, it pools at the
ocean’s surface, trapping the denser but warmer saltwater beneath it. This can seriously 
mess with the moving parts of the ocean, the so-called “conveyor belts” that cycle deep
nutrient-rich water to the surface. These slow currents are driven by large-scale climate
processes, like wind, and drive others, like the carbon cycle. But they also rely on
gradients in temperature and density to run; if too much cold water from the glaciers pools
at the surface, the whole conveyor belt could stutter to a stop.
In the North Atlantic, this would mean waters get colder, while the tropics, denied their 
influx of colder water, would heat up precipitously. Hansen says we’re already 
seeing the beginnings of AMOC’s slowdown: There’s a spot of unusually cool water hanging
out off of Greenland, while the U.S. East Coast continues to see warmer and warmer
temperatures. Hansen said it plainly in a call with reporters: “I think this is the 
beginning of substantial slowdown of the AMOC.”

Superstorms

Pointing to giant hunks of rock that litter the shore of the Bahamas, among other 
evidence of ancient climates, the study’s authors suggest that past versions of Earth may
have featured superstorms capable of casually tossing boulders like bored Olympians.
And as the temperature gradient between the tropic and the polar oceans gets steeper, 
thanks to that slowing of ocean-mixing currents, we could see stronger storms, too.
This is surprisingly intuitive: Picture a temperature gradient like a hill, with the high 
temperatures up at the top and the low temperatures down at the bottom. As the highs get 
higher and the lows get lower, that hill gets a lot steeper—and the storms are the bowling 
balls you chuck down the hill. A bowling ball will pick up a lot more speed on a steep hill, and 
hurt a lot more when it finally runs into something. Likewise, by the time these supercharged 
storms are slamming into coasts in the middle latitudes, they will be carrying a whole lot of 
deadly force with them.

So What Does it all Mean?

Whether other scientists quibble over these results or not—and they probably will—the 
overall message is hardly new. It’s bad, you guys. It might be really, truly, deeply bad, or it 
might be slightly less bad. Either way, says Hansen, what we know for sure is that it’s time 
to do something about it. “Among the top experts, there’s a pretty strong agreement that
we’ve reached a point where this is truly urgent,” he said.
So Hansen is frustrated once more with the failure of humanity to respond adequately. The 
result he’d hoped for when he released an early version of the paper online last summer was 
to get world leaders to come together in Paris to agree on a global price on carbon. As 
he told Grist’s Ben Adler at the time, “It’s going to happen.” (It didn’t happen, but some
Still, true urgency would require more of us than just slowing the growth of emissions—it 
requires stopping them altogether. In a paper published in 2013, Hansen found that we have 
to cut 6 percent of our use of carbon-based fuels every year, if we want to avoid dangerous 
climate change.
Carbon prices and emissions cuts are more the purview of politicians and diplomats, 
but if anything, Hansen has shown he is unafraid to stray beyond the established 
protocol of academic science.
“I think scientists, who are trained to be objective, have something to offer by analyzing the 
problem all the way to the changes that are needed in order to address it,” he said on a 
press call. “That 6 percent reduction—that’s not advocacy, that’s science. And then I 
would advocate that we do that!”
And to pre-empt the haters, Hansen wants you to remember one thing. “Skepticism is the life
blood of science. You can be sure that some scientists will find some aspects in our long 
paper that they will think of differently,” he said. “And that’s normal.”
So while scientists continue their debate over whether the ice sheets are poised to collapse
in the next 50 years or the next 500, the prognosis is the same: The future is wetter,
stranger, stormier unless we make serious moves to alternative energy sources now. 
Will we? Maybe. We’ve started but we still have a long, long way to go. If it’s a race 
between us and the ice sheets, neither I nor James Hansen nor anyone else can tell you 
for sure who will win.

CLIMATE CHANGE IS MUCH WORSE THAN WE THOUGHT, ACCORDING TO THE SCIENTIST WHO FIRST WARNED AMELIA URRY 3/22/16


E' difficile confrontare l'estinzione di massa attualmente in corso sul pianeta per opera dell'uomo, che ha modificato gli ecosistemi della Terra e sconvolto il clima al punto da spazzar via intere specie animali dal pianeta, con le cinque che l'hanno preceduta, avvenute in epoca preistorica. Infatti l'unico modo che abbiamo per catalogare le specie che un tempo c'erano e che da un certo punto in poi non ci sono state più sono i loro resti fossili, tracce indelebili del loro passaggio lasciate sul pianeta, ritrovate dagli archeologi e studiate dai peleontologi. Una nuova ricerca condotta da tre studiosi di paleontologia, e pubblicata su Ecology Letters, dimostra che le specie che si stanno estinguendo nella nostra epoca potrebbero scomparire senza lasciare una tale traccia permanente. Questo fa sospettare che lo stesso possa essere successo a molte altre specie anche in passato, il che significa che le estinzioni precedenti possono essere state sottovalutate.
"Il confronto tra l'attuale crisi della biodiversità, spesso chiamata la "sesta estinzione", con quelle del passato geologico richiede dati equivalenti", spiega Roy Plotnick, professore di Scienze della Terra e dell'ambiente presso la University of Illinois a Chicago. Insieme a due colleghi, Plotnick ha confrontato la Lista rossa delle specie minacciate con diversi database ecologici delle specie viventi e con tre database paleontologici di fossili catalogati. Gli autori hanno poi svolto un'analisi statistica per evidenziare quali specie minacciate hanno più probabilità di scomparire senza lasciare alcun segno della loro esistenza e sono rimasti sorpresi nello scoprire che per oltre l'85 per cento delle specie di mammiferi ad alto rischio di estinzione manca una documentazione fossile. Quelle a più alto rischio hanno circa la metà delle probabilità di essere incorporati nella documentazione fossile rispetto a quelle a più basso rischio.
Ad aumentare le probabilità di essere ricordati dai posteri è la stazza: le cose più grandi tendono a lasciare una traccia fossile, così come gli animali che hanno una maggiore diffusione geografica. I mammiferi più piccoli, come roditori e pipistrelli, sono quelli che hanno minori probabilità di essere ritrovati in forma di fossili. E lo stesso vale per altri vertebrati terrestri: solo del 3 per cento delle specie diuccelli minacciate di oggi e appena dell'1,6 per cento delle specie direttili minacciate vi è una testimonianza fossile conosciuta.
"Ci sono specie che si stanno estinguendo oggi che non sono mai state descritte", ha detto Plotnick. "Altre sono note solo perché qualcuno ne ha scritto". Ma tutte queste specie sarebbero sconosciute in un lontano futuro, se la documentazione storica scritta andasse perduta, cosa che non si può escludere. La pietra, insomma, parla più a lungo della carta: la documentazione fossile, fa notare Plotnick, è molto più resistente di qualsiasi documentazione umana. "Man mano che l'umanità si è evoluta, i nostri metodi di registrazione delle informazioni sono diventati sempre più effimeri", ha spiegato l'autore. "Le tavolette di argilla durano più dei libri. E chi oggi è in grado di leggere un floppy da 8 pollici? Se mettiamo tutto su supporti elettronici, quella documentazione esisterà ancora tra un milione di anni?", si chiede Plotkick. "I fossili sì".

Animali: la sesta estinzione di massa potrebbe non lasciare tracce 23 marzo 2016


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