Tag Archives: evolution

What does 7 Billion People Mean?

Making Sense of 7 Billion People | Wired Science | Wired.com.

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On the last day of October 2011, the global population of an upstart branch of the primate order will reach 7 billion.review smartphone android

What does it mean?

In itself, not much: Seven billion is just a one-digit flicker from 6,999,999,999. But the number carries a deep existential weight, symbolizing themes central to humanity’s relationship with the rest of life on Earth.

For context, let’s consider a few other numbers. The first: 10,000. That’s approximately how many Homo sapiens existed 200,000 years ago, the date at which scientists mark the divergence of our species from the rest of Homo genus, of which we are the sole survivors.

From those humble origins, humans — thanks to our smarts, long-distance running skills, verbal ability and skill with plants — proliferated at an almost inconceivable rate.


Some may note that, in a big-picture biological sense, humanity has rivals: In total biomass, ants weigh as much as we do, oceanic krill weigh more than both of us combined, and bacteria dwarf us all. Those are interesting factoids, but they belie a larger point.

We are the .00018 percent, and we use 20 percent.

Ants and krill and bacteria occupy an entirely different ecological level. A more appropriate comparison can be made between humans and other apex predators, which is precisely the ecological role humans evolved to play, and which — beneath our civilized veneer — we still are.

According to a back-of-the-envelope calculation, there are about 1.7 million other top-level, land-dwelling, mammalian predators on Earth. Put another way: For every non-human mammal sharing our niche, there are more than 4,000 of us.

In short, humans are Earth’s great omnivore, and our omnivorous nature can only be understood at global scales. Scientists estimate that 83 percent of the terrestrial biosphere is under direct human influence. Crops cover some 12 percent of Earth’s land surface, and account for more than one-third of terrestrial biomass. One-third of all available fresh water is diverted to human use.

Altogether, roughly 20 percent of Earth’s net terrestrial primary production, the sheer volume of life produced on land on this planet every year, is harvested for human purposes — and, to return to the comparative factoids, it’s all for a species that accounts for .00018 percent of Earth’s non-marine biomass.

We are the .00018 percent, and we use 20 percent. The purpose of that number isn’t to induce guilt, or blame humanity. The point of that number is perspective. At this snapshot in life’s history, at — per the insights of James C. Rettie, who imagined life on Earth as a yearlong movie — a few minutes after 11:45 p.m. on December 31, we are big. Very big.

However, it must be noted that, as we’ve become big, much of life had to get out of the way. When modern Homo sapiens started scrambling out of East Africa, the average extinction rate of other mammals was, in scientific terms, one per million species years. It’s 100 times that now, a number that threatens to make non-human life on Earth collapse.

In regard to that number, environmentalists usually say that humanity’s fate depends on the life around us. That’s debatable. Humans are adaptable and perfectly capable of living in squalor, without clean air or clean water or birds in the trees. If not, there wouldn’t be 7 billion of us. Conservation is a moral question, and probably not a utilitarian imperative.

But the fact remains that, for all of humanity to experience a material standard of living now enjoyed by a tiny fraction, we’d need four more Earths. It’s just not possible. And that, in the end, is the significance of 7 billion. It’s a challenge.

In just a few minutes of evolutionary time, humanity has become a force to be measured in terms of the entirety of life itself. How do we, the God species, want to live? For the answer, check back at 8 billion.

Permian Extinction Harder to recover from than thought

The Sun’ll Come Out Tomorrow? Maybe Not – ScienceNOW.



Survivors. The dicynodont Lystrosaurus browses on a stand of the lycopsid plant Pleuromeia. These species are two of the “disaster taxa” that proliferated in the wake of the end-Permian mass extinction.

The worst mass extinction of all time did far more than nearly denude the planet of life. This vast catastrophe—probably triggered about 252 million years ago by massive eruptions of the Siberian Traps volcanoes—destabilized life on Earth so drastically, according to a new study, that ecological aftershocks continued to hinder the recovery of life on land for millions of years.

Much of what paleontologists understand about the event, known as the end-Permian extinction, and its aftermath they learned from the fossil record of marine organisms. Up to 95% of known species, including the last of the trilobites, disappeared during the extinction. And the plentiful record of fossil fish and invertebrates indicates that ecosystems in the seas required about 5 million to 8 million years during the following Triassic period to regain their previous diversity and complexity. The story on land, however, has been unclear.

Now paleontologist Randall Irmis of the University of Utah and the Utah Museum of Natural History in Salt Lake City and geologist Jessica Whiteside of Brown University propose online today in the Proceedings of the Royal Society B that things were just as tough on dry land as in the seas. Irmis and Whiteside examined fossil vertebrate data sets from southern Africa’s Karoo Basin and the Ural region of Russia. They found that the number of different land-dwelling vertebrate genera dropped during the time interval when the extinction struck. In the post-extinction world, a small cadre of “disaster species” made the most of a bad situation. These survivors, including the “shovel lizard” Lystrosaurus (see picture), were hardy species that could make a living under distressed conditions. Such creatures quickly colonized and dominated the environments that had been shaken up by the mass extinction.

But life was not easy. According to previous geological studies, the global cycle through which carbon is recycled through land, air, and water was disrupted again and again during this time. Previous researchers blamed these cycles on continued volcanic activity. But Irmis and Whiteside propose instead that the perturbations point to a “boom-bust” cycle in which relatively minor changes in temperature, for example, caused some of the surviving species to go extinct. These smaller-scale losses then caused ecosystems to collapse. The events that triggered the end-Permian extinction drastically changed the climate, atmosphere, and other aspects of global ecology, hindering the recovery of life on land during the Early Triassic. Even as survivors of the mass extinction began to recover, the ecosystems they lived in were so fragile that the lingering influences of the end-Permian extinction—such as global warming—could cause those habitats to fail.

This ongoing cycle acted as a reset button, temporarily preventing land vertebrates from evolving the diversity their pre-extinction ancestors had enjoyed. “It seems that the rate of recovery in both marine organisms and terrestrial vertebrates was pretty similar,” Irmis says. “They didn’t really bounce back until the Middle Triassic, some 5 to 6 million years after the extinction.”

Paleontologist Peter Ward of the University of Washington, Seattle, agrees that the new study adds to the big picture of how life recovered from the end-Permian extinction. But our understanding of that time period is still changing, he notes. For one thing, what seems like a long recovery might look shorter as geologists revise figures of how long the Early Triassic actually lasted. If new studies find that the Early Triassic rocks were laid down during a shorter time frame than presently known, then the recovery of Early Triassic life would have happened more rapidly than presently thought.

However long it lasted, the Early Triassic world was likely a harsh one. Citing the work of geologist Lee Kump of Pennsylvania State University, University Park, Ward says that in the millennia after the mass extinction there would have been hot snaps, lowered oxygen levels in lakes and oceans, and possibly the persistence of the poisonous “swamp gas” hydrogen sulfide in the atmosphere. The powerful volcanic eruptions that likely triggered the extinction left a devastating imprint on the planet, which continued to shake up life on Earth even as ecological wounds began to heal.

The Last Great Global Warming

The Last Great Global Warming: Scientific American.

Surprising new evidence suggests the pace of Earth’s most abrupt prehistoric warm-up paled in comparison with what we face today. The episode has lessons for our future

Image: Illustration by Ron Miller

In Brief

  • Global temperature rose five degrees Celsius 56 million years ago in response to a massive injection of greenhouse gases into the atmosphere.
  • That intense gas release was only 10 percent of the rate at which heat-trapping greenhouse gases are building up in the atmosphere today.
  • The speed of today’s rise is more troubling than the absolute magnitude, because adjusting to rapid climate change is very difficult.

Polar bears draw most visitors to Spitsbergen, the largest island in Norway’s Svalbard archipelago. For me, rocks were the allure. My colleagues and I, all geologists and climate scientists, flew to this remote Arctic island in the summer of 2007 to find definitive evidence of what was then considered the most abrupt global warming episode of all time. Getting to the rocky outcrops that might entomb these clues meant a rugged, two-hour hike from our old bunkhouse in the former coal-mining village of Longyearbyen, so we set out early after a night’s rest. As we trudged over slippery pockets of snow and stunted plants, I imagined a time when palm trees, ferns and alligators probably inhabited this area.

Back then, around 56 million years ago, I would have been drenched with sweat rather than fighting off a chill. Research had indicated that in the course of a few thousand years—a mere instant in geologic time—global temperatures rose five degrees Celsius, marking a planetary fever known to scientists as the Paleocene-Eocene Thermal Maximum, or PETM. Climate zones shifted toward the poles, on land and at sea, forcing plants and animals to migrate, adapt or die. Some of the deepest realms of the ocean became acidified and oxygen-starved, killing off many of the organisms living there. It took nearly 200,000 years for the earth’s natural buffers to bring the fever down.



Are We in the Middle of a Sixth Mass Extinction?

Are We in the Middle of a Sixth Mass Extinction? – ScienceNOW.


Wipe out. This giant Irish elk roamed across Europe and Asia until it went extinct about 7700 years ago.
Credit: Brendon Dempsey, Trinity College, Dublin

Earth’s creatures are on the brink of a sixth mass extinction, comparable to the one that wiped out the dinosaurs. That’s the conclusion of a new study, which calculates that three-quarters of today’s animal species could vanish within 300 years. “This is really gloom-and-doom stuff,” says the study’s lead author, paleobiologist Anthony Barnosky of the University of California, Berkeley. “But the good news is we haven’t come so far down the road that it’s inevitable.”

Species naturally come and go over long periods of time. But what sets a mass extinction apart is that three-quarters of all species vanish quickly. Earth has already endured five mass extinctions, including the asteroid that wiped out dinosaurs and other creatures 65 million years ago. Conservationists have warned for years that we are in the midst of a sixth, human-caused extinction, with species from frogs to birds to tigers threatened by climate change, disease, loss of habitat, and competition for resources with nonnative species. But how does this new mass extinction compare with the other five?

Barnosky and colleagues took on this challenge by looking to the past. First, they calculated the rate at which mammals, which are well represented in the fossil record, died off in the past 65 million years, finding an average extinction rate of less than two species per million years. But in the past 500 years, a minimum of 80 of 5570 species of mammals have gone extinct, according to biologists’ conservative estimates—an extinction rate that is actually above documented rates for past mass extinctions, says Barnosky. All of this means that we’re at the beginning of a mass extinction that will play out over hundreds or thousands of years, his team concludes online today in Nature.

The picture gets even grimmer when all mammals currently endangered or threatened are added to the count. If those all disappear within a century, then by 334 years from now, 75% of all mammal species will be gone, says Barnosky. “Look outside of your window. Imagine taking away three-quarters of the living things you see and ask yourself if you want to live in that world.”

The team extended the same methods of analysis to amphibians, reptiles, birds, plants, mollusks, and other forms of life. They found fairly consistent patterns: From amphibians to birds to mammals, about 1% to 2% of species already are extinct today, and 20% to 50% are threatened—numbers that approach those of the great mass extinctions of the past. “Our best guess is that the current extinction rate is between three to 80 times too high” even without counting all threatened species, says Barnosky. “Assuming threatened species would actually go extinct—which is not inevitable—puts the extinction rate off the charts.”

“There’s been a lot of general talk on this issue, but attempts to draw more rigorously on the lessons of the fossil record have been rare,” says paleobiologist David Jablonski of the University of Chicago in Illinois, who was not involved in the study. “It’s really valuable to look at how current losses stack up against the past extinction events.”

The silver lining in this dark cloud is that if humans work quickly to protect endangered and threatened species and their habitats now, the mass extinction can be prevented or at least delayed by thousands of years, says Barnosky. Adds Jablonski, “This approach provides a way to gauge progress in walking the world back from that brink [of a mass extinction].”

Volcanoes wiped out Neanderthals

Volcanoes wiped out Neanderthals, new study suggests.

ScienceDaily (Oct. 7, 2010) — New research suggests that climate change following massive volcanic eruptions drove Neanderthals to extinction and cleared the way for modern humans to thrive in Europe and Asia.

The research, led by Liubov Vitaliena Golovanova and Vladimir Borisovich Doronichev of the ANO Laboratory of Prehistory in St. Petersburg, Russia, is reported in the October issue of Current Anthropology.

“[W]e offer the hypothesis that the Neanderthal demise occurred abruptly (on a geological time-scale) … after the most powerful volcanic activity in western Eurasia during the period of Neanderthal evolutionary history,” the researchers write. “[T]his catastrophe not only drastically destroyed the ecological niches of Neanderthal populations but also caused their mass physical depopulation.”

Evidence for the catastrophe comes from Mezmaiskaya cave in the Caucasus Mountains of southern Russia, a site rich in Neanderthal bones and artifacts. Recent excavations of the cave revealed two distinct layers of volcanic ash that coincide with large-scale volcanic events that occurred around 40,000 years ago, the researchers say.

Geological layers containing the ashes also hold evidence of an abrupt and potentially devastating climate change. Sediment samples from the two layers reveal greatly reduced pollen concentrations compared to surrounding layers. That’s an indication of a dramatic shift to a cooler and dryer climate, the researchers say. Further, the second of the two eruptions seems to mark the end of Neanderthal presence at Mezmaiskaya. Numerous Neanderthal bones, stone tools, and the bones of prey animals have been found in the geological layers below the second ash deposit, but none are found above it.

The ash layers correspond chronologically to what is known as the Campanian Ignimbrite super-eruption which occurred around 40,000 years ago in modern day Italy, and a smaller eruption thought to have occurred around the same time in the Caucasus Mountains. The researchers argue that these eruptions caused a “volcanic winter” as ash clouds obscured the sun’s rays, possibly for years. The climatic shift devastated the region’s ecosystems, “possibly resulting in the mass death of hominins and prey animals and the severe alteration of foraging zones.”

Enter Modern Humans

Anthropologists have long puzzled over the disappearance of the Neanderthals and the apparently concurrent rise of modern humans. Was there some sort of advantage that helped early modern humans out-compete their doomed cousins? This research suggests that advantage may have been simple geographic location.

“Early moderns initially occupied the more southern parts of western Eurasia and Africa and thus avoided much of the direct impact of the … eruptions,” the researchers write. And while advances in hunting techniques and social structure clearly aided the survival of modern humans as they moved north, they “may have further benefited from the Neanderthal population vacuum in Europe, allowing wider colonization and the establishment of strong source populations in northern Eurasia.”

While the researchers stress that more data from other areas in Eurasia are needed to fully test the volcanic hypothesis, they believe the Mezmaiskaya cave offers “important supporting evidence” for the idea of a volcanic extinction.

Mass Extinctions Change the Rules of Evolution

Mass Extinctions Change the Rules of Evolution | Wired Science | Wired.com.

ANOTHER giant ‘DUH’ for science: mass extinctions make things happen differently thereafter.

A reinterpretation of the fossil record suggests a new answer to one of evolution’s existential questions: whether global mass extinctions are just short-term diversions in life’s preordained course, or send life careening down wholly new paths.

Some scientists have suggested the former. Rates of species diversification — the speed at which groups adapt and fill open ecological niches — seemed to predict what’s flourished in the aftermath of past planetary cataclysms. But according to the calculations of Macquarie University paleobiologist John Alroy, that’s just not the case.

“Mass extinction fundamentally changes the dynamics. It changes the composition of the biosphere forever. You can’t simply predict the winners and losers from what groups have done before,” he said.

Alroy was once a student of paleontologist Jack Sepkoski, who in the 1980s formalized the notion that Earth has experienced five mass extinctions in the 550 million years since life became durable enough to leave a fossil record. Graphs of taxonomic abundance depict lines rising steadily as life diversifies, plunging precipitously during each extinction, and rising again as life proliferates anew.

As the fossil record is patchy and long-term evolutionary principles still debated, paleobiologists have historically disagreed about what these extinctions mean. Some held that, in the absence of extinctions, species would diversify endlessly. The Tree of Life could sprout new branches forever. Others argued that each taxonomic group had limits; once it reached a certain size, each branch would stop growing.

Sepkoski’s calculations put him on the limits side of this argument. He also proposed that, by looking at the rate at which each group produced new species, one could predict the winners and losers of each mass extinction’s aftermath. Groups that diversified rapidly would flourish. Their destiny was already established.

“It’s a clockmaker vision of evolution. Each group has fixed dynamics, and if there’s an extinction, it just messes it up a bit,” said Alroy. “That’s what I’m challenging in this paper. There are limits, and that’s why we don’t have a trillion species. But those limits can change.”

Alroy crunched marine fossil data in the Paleobiology Database, which gathers specimen records from nearly 100,000 fossil collections around the world. He used a statistical adjustment method designed to reduce the skewing influences of paleontological circumstance — the greater chances of finding young fossils rather than old, the ease of studying some types of rock rather than others.

Historical species diversity among marine animals of Cambrian, Paleozoic and Modern origin.

The analysis, published September 2 in Science, produced what Alroy considers to be the most accurate reflection of extinction dynamics to date. And while his data supported the notion that each group’s diversity eventually hits a limit, he didn’t find Sepkoski’s correlation between pre-mass-extinction diversity rates and post-extinction success. Each mass extinction event seemed to change the rules. Past didn’t indicate future.

In an accompanying commentary, paleontologist Charles Marshall of the University of California, Berkeley noted that Alroy’s statistical methods still need review by the paleobiology community. The Paleobiological Database, for all its thoroughness, might also be incomplete in as-yet-unappreciated ways. “There will be no immediate consensus on the details of the pattern of diversity,” he wrote. But “the pieces are falling into place.”

Enough pieces have come together for Alroy to speculate on his findings’ implication for the future, given that Earth is now experiencing another mass extinction. Starting with extinctions of large land animals more than 50,000 years ago that continued as modern humans proliferated around the globe, and picking up pace in the Agricultural and Industrial ages, current extinction rates are far beyond levels capable of unraveling entire food webs in coming centuries. Ecologists estimate that between 50 and 90 percent of all species are doomed without profound changes in human resource use.

In the past, many evolutionary biologists thought life would eventually recover its present composition, said Alroy. In 100 million years or so, the same general creatures would again roam the Earth. “But that isn’t in the data,” he said.

Instead Alroy’s analysis suggests that the future is inherently unpredictable, that what comes next can’t be extrapolated from what is measured now, no more than a mid-Cretaceous observer could have guessed that a few tiny rodents would someday occupy every ecological niche then ruled by reptiles.

“The current mass extinction is not going to simply put things out of whack for a while, and then things will go back to where we started, or would have gone anyway,” said Alroy. Mass extinction “changes the rules of evolution.”

Images: 1) A fossil skull of Dunkleosteus, an apex predator fish that lived between 380 million and 360 million years ago, and had what is believed to be history’s most powerful bite./Michael LaBarbera, courtesy of The Field Museum. 2) Graph of species diversity among marine animals of Cambrian, Paleozoic and Modern origin./Science.

Read More http://www.wired.com/wiredscience/2010/09/mass-extinction-dynamics/#ixzz0yPE4fDQc

Horned turtles butchered to extinction

* 20:00 16 August 2010 by Wendy Zukerman

The giant horned turtles of the Pacific became extinct later than we thought – and we were to blame.

The half-tonne meiolaniid turtles were thought to have died out 30 to 40,000 years ago. With no signs of human interference, climate change was blamed.

Now butchered turtle remains have been found in the South Pacific island nation of Vanuatu. Carbon dating shows that the most recent bones are between 2890 and 2760 years old. Humans arrived 3000 years ago: “Within 200 years, the turtles were gone,” says Trevor Worthy of the University of New South Wales in Sydney, Australia, who identified the bones.

via Horned turtles butchered to extinction – environment – 16 August 2010 – New Scientist.

Heat of the Moment: How Much Global Warming Are We Willing to Take?: Scientific American

Heat of the Moment: How Much Global Warming Are We Willing to Take?: Scientific American.

Some useful stats and harrowing predictions; also good fodder for the EvoElves Cycle….

The average temperature of the planet for the next several thousand years will be determined this century—by those of us living today, according to a new National Research Council report which lays out the impact of every degree of warming on outcomes ranging from sea-level rise to reduced crop yields.

“Because carbon dioxide is so long-lived in the atmosphere, it could effectively lock Earth and future generations into warming not just for decades and centuries, but literally for thousands of years,” atmospheric scientist Susan Solomon of the National Oceanic and Atmospheric Administration, who chaired the report, said at a July 16 press briefing held to release it. She compared CO2 to cheesecake: “If I knew that every pound of cheesecake that I ate would give me a pound that could never be lost, I think I would eat a lot less cheesecake.”

According to the report, for every degree Celsius of warming, impacts include:

* A 5 to 15 percent lower yield for some crops, including corn in Africa and the U.S., and wheat in India
* A 3 to 10 percent increase in heavy rainfall globally
* A 5 to 10 percent drop in rainfall in southwestern North America, southern Africa and the Mediterranean, among other precipitation changes
* A 5 to 10 percent change (increases in some regions, decreases in others) in stream flow in many river basins globally
* A 15 to 25 percent decrease in the extent of Arctic Ocean sea ice

The report’s authors were charged with evaluating a range of “greenhouse gas–stabilization targets and describe the types and scale of impacts likely associated” without any judgment on whether such targets are “technically feasible” or which is “most appropriate.” In essence, the scientists evaluated the impacts associated with a given final level of carbon dioxide in the atmosphere, but did so through the lens of temperature change.

This represents a shift in the usual analysis of climate change, particularly in international negotiations, which tend to focus on how much concentrations of greenhouse gases in the atmosphere will rise by a particular date. “Many impacts respond directly to changes in global temperature, regardless of the sensitivity of the planet to human emissions of CO2 and other greenhouse gases,” says geoscientist Katharine Hayhoe of Texas Tech University in Lubbock, a co-author of the report, excluding effects such as ocean acidification and CO2 as a fertilizer for plants. “Those impacts don’t ‘care’ about what the CO2 concentration is.”

It also eliminates much of the uncertainty surrounding potentially ill effects; whereas various mathematical models may disagree about when and at what concentrations Arctic Ocean sea ice disappears, they all agree that at roughly 3 degrees C of warming, the far north will be ice-free. “It’s amazing how consistent they become,” Solomon says. “At what point do you get to three to four degrees of warming, which is roughly the time when Arctic sea ice is mostly gone.”

Adds economist Gary Yohe of Wesleyan University, another co-author: “We will commit to an ice-free Arctic sometime this century. We won’t know definitively until 2090, but essentially there’s nothing we can do about it at that point in time and it changes the climate system dramatically.”

Already, the planet’s average temperature has warmed by 0.7 degree C, which is “very likely” (greater than 90 percent certain) to be a result of the rising concentrations of greenhouse gases in the atmosphere, according to the U.N. Intergovernmental Panel on Climate Change. That’s about half what can ultimately be expected from the roughly 390 parts per million of CO2 already in the atmosphere—the highest level the planet has experienced in at least 800,000 years.

CO2 in charge
One result of the survey of existing research undertaken by the scientists is the clear and dominant role played by CO2. Although a wide variety of greenhouse gases contribute to human-caused global warming, it is CO2, largely alone, that will determine the long-term climate, Solomon says. “If you reduce emissions of methane or black carbon, it would help you trim the peak warming that will be achieved in the next century or so,” Solomon says. But it’s the “cumulative carbon that will determine the long-term human footprint on this planet.”

That’s because it can take thousands of years to remove CO2 from the atmosphere without human intervention. So what matters most as far as total warming is the ultimate stabilized level of CO2. “It doesn’t matter what road you pick to get there,” Solomon notes. And achieving any stabilization target—whether 2 degrees C of warming or 450 ppm or 1,000 gigatons of carbon added to the atmosphere by human activity—will require at least an 80 percent cut in emissions from peak levels by the end of this century and, ultimately, zero emissions over the long term. “You can get there by cutting now at rates of 1 percent per year for the rest of the century or let carbon emissions rates grow for awhile and cut harder later to the tune of 4 percent per year,” Solomon explains.

Yohe estimates the cost of achieving a more modest goal of holding warming to roughly 2 degrees C at a cost of 0.5 to 1.5 percent of gross domestic product for the U.S. by 2050, thanks to the expense incurred by, for example, replacing existing coal-fired power plants with renewables or retrofitting them with carbon-capture technology. That hardly impacts the U.S. economy at all. “With usual growth, we’d get to the same level of GDP in 2051 that we would have gotten in 2050,” he says. “It’s not an awful disaster. The hyperbole of ‘all these green jobs’ or ‘we’re going to trash the economy’—neither one [is] true.”

Already, cities such as New York have adopted a risk-management approach to potential climate impacts—preparing for the prospects posed by already guaranteed global warming. By analyzing current building codes and the like, the New York City Panel on Climate Change determined the acceptable level of risk for its residents and is now prioritizing projects that hold to those same levels the perils from climate change impacts directly on the city, such as sea-level rise or more frequent heat waves. “You can’t actually climate-proof a city,” says Adam Freed, acting director of the city’s Office of Long-Term Planning and Sustainability. But “the benefits of the things that make sense to do today greatly increase as our climate changes.”

It remains to be seen whether there is economic value to the idea of overshooting a given target and then coming back down, given that the time frame of emission cuts matters less than the cumulative emissions of CO2, Yohe notes. It is clear that there is no environmental benefit to delay. “Climate change is already altering the character of the places we know and love,” Hayhoe says. “Unchecked, it has the potential to impact nearly every aspect of human infrastructure and our natural environment—from our cities and roads to our forests and fields.”

Regardless, the report notes that the planet has entered a new era, dubbed the Anthropocene, “during which the evolution of the planet’s environment will be largely controlled by the effects of human activities, notably emissions of carbon dioxide.” Hayhoe, for one, compares this report with a doctor’s visit for Earth—the chronic disease being human-emitted carbon dioxide. “Many of us have had the experience of going to the doctor and receiving advice on how to improve our health by making wise lifestyle choices,” she notes. “It’s up to us to decide how much we are willing to change.”

“There are all kinds of options: carbon sequestration, geoengineering, alternative energy,” Solomon notes. “How much can we adapt? Look at corn. Maybe we can choose to grow something else or genetically engineer that corn to make it more robust.”

The catch is: the decision is not just for the planet today and its present generations, it is also for the planet and generations to come. “The impacts we may be experiencing now and in the next few decades before choosing to stabilize CO2 levels,” Solomon notes, “would only be about half the eventual impacts.”