Over the past two decades, scientists have developed ways to predict how ecosystems will react to changing environmental conditions. Called ecological forecasts, these emerging tools, if used effectively, can help reduce pollution to our waterways.

Dead zone and toxic algae forecasts are similar to weather and climate forecasts. They can provide near-term predictions of ecosystem responses to short-term drivers such as this year’s nitrogen and phosphorus inputs. They can also be used in scenarios to analyze the impacts of controlling those drivers in the future.

These particular forecasts are important because when they match actual events well, they build confidence in using the models to guide policy and management decisions. Doing these forecasts annually also provides a regular check on whether these problems are being resolved.

While knowing the extent and location of these ecosystem conditions could allow decision-makers to adapt their management decisions, current ecological forecasts – at least those related to dead zones and toxic algae – are not sufficiently tuned in space and time to support that scale of adaptive management. Hopefully, someday they will be. In the meantime, their use provides powerful reminders of unsolved problems.

This year’s eco-forecast

Dead zones (hypoxia) are regions within lakes and oceans where oxygen concentrations drop to levels dangerous to marine life. They’re typically caused by decomposing algae, the growth of which is stimulated by nitrogen and phosphorus inputs from land. Toxic algae, also stimulated by these same excess nutrients, can poison aquatic life and humans when they contaminate the water supply.

In recent weeks, I contributed predictions to NOAA’s ensemble forecasts of this year’s dead zones in the Gulf of Mexico and the Chesapeake Bay, and the extent of toxic algae in Lake Erie.

The 2015 forecasts remind us that these persistent problems are not yet being addressed effectively. While the dead zone forecasts are for roughly “average” conditions, it is important to note that “average” does not mean natural, and in these cases, “average” is not acceptable. The toxic algae forecast is a clear reminder that long-term nutrient input reduction is critical.

Gulf of Mexico – In its 2001 action plan – confirmed in 2008 and again in 2013 – the federal, state and tribal Mississippi River/Gulf of Mexico Watershed Nutrient Task Force set a goal of reducing the five-year running average extent of gulf hypoxia, or oxygen deficiency, to 5,000 square kilometers (1930 square miles) by 2015.

But little progress has been made toward that goal. Since 1995, the gulf dead zone has averaged 15,323 square kilometers, not unlike this year’s prediction of the size of Connecticut. Nutrient-rich runoff from Midwest agriculture ends up in the Mississippi River and eventually makes its way to the gulf. The amount of nitrogen entering the Gulf of Mexico increased, mainly due to agricultural runoff, by about 300% between the 1960s and 1980s, and has changed little since then.

While the size of the gulf dead zone varies from year to year, mostly in response to changing weather patterns in the Corn Belt, the bottom line is that we will never reach the action plan goal of 5,000 square kilometers until more serious actions are taken to reduce the loss of Midwest nitrogen and phosphorus from agricultural lands, regardless of the weather.

Chesapeake Bay – Similar to the Gulf of Mexico, the Chesapeake Bay dead zone forecast of 5.7 cubic kilometers (1.37 cubic miles or 2.3 million Olympic-size swimming pools) is slightly lower than its long-term average. Also similar to the gulf, there is very significant year-to-year variability in inputs and thus, hypoxia. But, unlike the gulf, there appears to be some progress being made toward nutrient input reductions.

Why? Under an Environmental Protection Agency- (EPA) enforced regional compact, six states and the District of Columbia have agreed to reduce the nitrogen load 25% by 2025. Notice the word “enforced.” Having in place a two-year milestone check in 2017 under the agreement’s Total Maximum Daily Load (TMDL) Watershed Implementation Plan should make a difference. Those metrics will be graded by the EPA, and if they are missed, warnings will be issued to members of the regional compact, and the consequences could include additional regulatory measures. The EPA recently determined that the region is likely to miss that goal by half. So, real accomplishments will depend on the resolve of the EPA and the administration in power to be tough in 2017.

Lake Erie – This year’s Lake Erie toxic algae forecast is for a bloom larger than the one in 2014 that shut down the water supply to a half-million people in Toledo, and approaching the record-setting massive 2011 bloom. It’s worth noting that only a week or two before the formal forecast, NOAA was anticipating a relatively mild bloom, and the changed forecast was the result of one spring storm. Because these blooms are driven by diffuse phosphorus sources from the agriculturally dominated Maumee River watershed, this update is not surprising, and is a reminder of how much this issue is driven by these climate-induced increased storms.

In addition, unlike the dead zones, these blooms are highly dynamic in both time and space. In fact, while the 2014 bloom was much smaller than the massive 2011 bloom, it formed near Toledo’s water supply, and local winds mixed the bloom into the city’s deep-water intakes. So bloom predictions, regardless of size, do not necessarily correlate with risk. Until the phosphorus inputs are reduced significantly and consistently so that only the mildest blooms occur, the people, ecosystem and economy of this region are being threatened. We cannot cross our fingers and hope that seasonal fluctuations in weather will keep us safe.

Using ecological models for scenario analysis

We also participate in these annual forecasts because these same models are used to help guide decisions on long-term nutrient input targets needed to reduce dead zones and toxic blooms to acceptable levels.

We have been tracking the accuracy of some of these annual forecasts and find the models do a pretty good job in years without hurricanes or tropical storms that disrupt dead zones prior to taking measurements. This increases confidence in using these models for providing advice on needed nutrient load reductions.

In fact, some of these models have been used to guide policymakers who set nutrient input reductions, and most reach the surprisingly consistent recommendations of reducing inputs by 35%-45%. However, while some of these recommendations have been in place for over a decade, little progress has been made. Forecasts, scenarios, recommendations and agreements are obviously not enough.

So what to do?

In a recent posting, I suggested that while more extensive application of existing and new agricultural best management practices (BMPs), such as streamside buffers and wetlands restoration, are important, they alone may not be sufficient in reducing nitrogen and phosphorus inputs to the Gulf of Mexico, the Chesapeake Bay, and Lake Erie. Even if BMPs were effective, the current voluntary, incentive-based regime is not working, as outlined in a report by Marc Ribaudo, senior economist for the USDA Economic Research Service.

The fact is, our watersheds are overwhelmed by industrial-scale row crop agriculture, much of it corn, and real progress will be made only by reducing the demand for corn. That requires modifying the American diet, urging changes in the agricultural supply chain and cutting the production of corn-based ethanol.

While changing diets and supply chains requires long-term cultural change, cutting use of corn in our cars could be done more quickly.

A simple although apparently politically dangerous move would be for Congress to prohibit the use of corn for ethanol production. This has been proposed many times in many places. So why does the federal government continue to insist on burning corn in our gas tanks – especially since it has been demonstrated that ethanol produces more greenhouse gases than gasoline and it is not good for either consumers or the environment? Perhaps the answer lies with presidential hopefuls running to Iowa every four years proclaiming love for corn and ethanol, and an ethanol industry building a stronger roster of lobbyists.

These are, of course, political considerations worked out in Washington, DC and state capitals. In the meantime, the dead zones and algae blooms we forecast every year show the ongoing damaging effects of excessive nutrient runoff.

Donald Scavia is Graham Family Professor of Sustainability at University of Michigan.

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Scientists and policymakers are simultaneously looking for new ways to feed the world and save the oceans. Global seafood demands are increasing, and fisheries and aquaculture already have large impacts on marine ecosystems.

The concept of “balanced harvesting” aims to address both of these issues, and was the subject of several recent high-profile studies – including in Science, Proceedings of the National Academy of Sciences and Fish and Fisheries.

Balanced harvesting is a philosophy that advocates spreading fishing pressure evenly across the ecosystem instead of concentrating it on only a few sizes and species of fish. The idea is that we would harvest every size and species, each in proportion to its natural productivity. This would be a big change from current fisheries management, which focuses on a small number of species and often protects certain size-groups from fishing (usually the small young ones, but occasionally also the very largest old ones).

The commendable goal of balanced harvesting is to increase the fish supply while preserving the structure of ecosystems. While it’s an interesting idea, balanced harvesting has practical pitfalls that probably prevent it from being a viable fisheries management approach.

In a recent study published in Fish and Fisheries, my colleagues and I outlined some of these problems.

The promise of balanced harvesting

With balanced harvesting, we would get more fish mainly because we would be harvesting new species and sizes. We might also get more yield from the species and sizes we already harvest, but this possibility is still being debated. We would preserve the structure of ecosystems by maintaining the interactions and relative abundances of different sizes and species of fish. We might also reduce the evolutionary pressure on fish to be small that current fishing practices provide, with their emphasis on catching larger individuals.

On the strengths of these proposed benefits, balanced harvesting is now supported by a number of influential scientists within the International Union for Conservation of Nature, the Food and Agriculture Organization of the United Nations (FAO), and a few other fisheries management and conservation organizations. For now it remains a hypothetical framework, though it has been discussed by the European Parliament.

The problems with balanced harvesting

First, balanced harvesting is probably not technologically possible. Very few fisheries are able to target individual species selectively. When you dip your net into the ocean, you’re likely to get a wide variety of creatures, with little regard for just the one or two species you’re hoping for. That’s why micromanaging species’ relative harvest rates would be next to impossible.

Second, balanced harvesting would likely be very expensive. On the management side, it would require modern scientific monitoring of every species in the ecosystem; currently only the most commercially valuable species and a few by-catch species are monitored, via research surveys or on-board observers, for example. Modern fisheries management for a single species can already cost as much as 25% of the gross value of the catch. On the fishery side, balanced harvesting means expending significant effort catching sizes and species of fish that people may not want to buy or eat.

We don’t know exactly how much all of this would cost, but from simple calculations based on the evidence we do have, my colleagues and I concluded that global balanced harvesting would likely cause fisheries to lose money on the whole. This matters – even if our fishery management objectives are not focused on profits – because it means that balanced harvesting is probably a very inefficient way to protect ecosystems. For example, ceasing fishing altogether would therefore be better than balanced harvesting not just ecologically but also economically. It’s likely many other more desirable management strategies would be more cost-effective too.

What about food security? Some have argued that the prospects of extra protein from the sea, at a time when meat and fish demands are rapidly increasing, would make balanced harvesting worth its cost, even if the cost is high.

The problem, though, is that most of the new meat and fish demand projected to arise between now and 2050 will not come from new mouths to feed, but from people eating more protein – often much more than they need – as they become richer.

Because balanced harvesting would produce more food by introducing new types of fish to the menu, it might be a supply-side solution to a taste-driven demand problem that works only if we can change people’s tastes. This doesn’t make sense.

Even if we can use new seafood types from balanced harvests for aquaculture feeds – probably the most viable option at large scales – the economic and environmental costs would still need to be compared to the costs of other available solutions. Increasing the fish supply through balanced harvesting could have significant carbon costs, for example – perhaps much higher than the costs of producing extra chicken or pork, per pound of protein, depending on how new fish were caught.

More demand-side solutions needed

If we’re going to try to change people’s preferences anyway, why not work on giving people in rich countries incentive simply to eat less meat and fish? Recent research suggests this would have significant benefits for both the environment and human health. Changing diets would probably help the economy too, considering the enormous economic burden of obesity, heart disease and other overconsumption-related ailments in many countries.

How do we encourage diet change? There is no silver bullet solution, but one possibility would be to include food-related carbon emissions in carbon taxes. Because meat and fish have high carbon costs, carbon taxes on food would provide incentives for people to eat less meat- and fish-heavy diets. Making the taxes revenue-neutral could offset any adverse effects of new taxes on the economy, and avoid exacerbating food insecurity for the poor. A variant of this policy has been proposed in New Zealand, for example.

Regardless of whether carbon taxes specifically are the way to go, we need to start looking harder at demand-side solutions, which target the amounts of meat and fish required, rather than focusing on ways to produce more meat and fish. Demand-side solutions are likely our best shot at feeding the world and saving the oceans, not to mention ourselves.

Matt Burgess is Postdoctoral Scholar in Fisheries and Environment at University of California, Santa Barbara.

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We already knew about Venus. We had our suspicions about Mars. Now we’re sure.

Our two closest solar system neighbors once had oceans – planet-encircling, globe-girdling, Earth-like oceans. But waterbearing planets are fragile. Venus didn’t have the right stuff and lost her oceans to space. We have the smoking gun. And now we know that Mars, also, poor Mars, couldn’t hold on. Mars has lost to space at least 80% of all the water it once had.

Et tu, Earth? What about you? More to the point, what about us? Despite water’s apparent abundance, what does the future hold for the most precious material on our planet? Will we find a way to mistreat our reserve of irreplaceable water and turn our planet into a planetary desert, like our neighbors Venus and Mars? Kick the temperature up a few more notches, thanks to a runaway greenhouse effect, and the ultimate consequence of global warming could be ejecting the water from our planet.

Water on the atomic level

Let’s try our hand at interplanetary forensics. First, let me introduce you to the atomic constituents of that substance chemists call H2O, which most of us more commonly know as water. The H represents the atom hydrogen. The O represents the atom oxygen. The number two after the letter H tells us that a single molecule of water is composed of two hydrogen atoms and one oxygen atom.

In order to enter the world of CSI: Solar System, we need to understand the structure of atoms in a bit more detail. Hydrogen is hydrogen because its nucleus has one positively charged proton, which is orbited by one negatively charged electron. The nucleus, however, can also include one neutron, which lacks a charge. Even with one neutron, the atom still has a positive charge in the nucleus of +1. It’s therefore still hydrogen, but with one critical difference: it is much heavier, about twice as heavy, in fact, thanks to the additional neutron.

Chemists call this kind of heavy hydrogen deuterium. Deuterium behaves identically in chemical reactions to regular hydrogen; it’s just heavier. Remember that H2O molecule? When made with a deuterium atom, it’s an HDO molecule. It would taste the same, and it would provide the same sustenance to your flowers and gerbils, but it would weigh more.

That extra weight makes all the difference, because Isaac Newton’s and Albert Einstein’s unavoidable law of gravity says that deuterium is pulled downward toward the surface of a planet much more strongly than is regular hydrogen. When deuterium and regular hydrogen are both free to bounce around in a planet’s atmosphere, the regular hydrogen will bounce much higher. And if the planet’s gravity is weak enough – which is the case for Earth, Venus and Mars – regular hydrogen can bounce so high that it can escape into space, whereas the deuterium remains forever bound by gravity to the planet.

A base-level ratio for the solar system

In 1995, NASA’s Galileo probe measured the ratio of hydrogen to deuterium in the atmosphere of the giant planet Jupiter and found that ratio to be about 40,000-to-1.

Jupiter is such a massive planet that neither hydrogen nor deuterium can escape. Consequently, planetary scientists are quite certain that all the materials involved in the mixture of gases and dust that formed the sun and all the planets in our solar system formed with the same ratio of hydrogen to deuterium as the Galileo probe found for Jupiter’s atmosphere. We take it as a given that all the water originally deposited on Venus, on Earth, and on Mars also had that same ratio of hydrogen to deuterium.

Now let’s do some chemistry. If I wanted to make 20,000 water molecules, I would need a total of 40,000 hydrogen (H) and deuterium (D) atoms (of which 39,999 would be H and 1 would be D), plus, of course, 20,000 oxygen (O) atoms. In my mixture of 20,000 water molecules, I would be able to make 19,999 H2O molecules and one HDO molecule, given my initial ratio of hydrogen to deuterium atoms.

The real H-to-D ratios

In a cup of water scooped from any part of any of Earth’s oceans, in any local freshwater pond from any continent, in any cup of tea in any city, in an Alpine glacier or a hot spring in Yellowstone, the hydrogen-to-deuterium ratio is 6,250-to-1, not 40,000-to-1.

Why so low? The evidence suggests that early in Earth’s history, our planet lost a great deal of hydrogen (but not deuterium). As the hydrogen atoms escaped to space, the H-to-D ratio would have dropped from 40,000-to-1 to only 6,250-to-1. In fact, the Earth may have lost as much as 80% of its original population of hydrogen atoms, and since, on Earth, most hydrogen atoms are bound into water molecules, the loss of hydrogen atoms is likely equivalent to the loss of water.

NASA’s Pioneer Venus spacecraft, way back in 1978, dropped a probe that parachuted into and measured the properties of Venus’ atmosphere. One of its shocking discoveries was that the hydrogen-to-deuterium ratio on Venus is only 62-to-1, fully 100 times smaller than the ratio on Earth.

The clear implication of this discovery is that Venus was once wet but is now bone-dry. Venus, as we now know, has a surface temperature of 867 Fahrenheit (463 Celsius). Venus once had oceans, but Venus warmed up and the oceans boiled off the surface. Then ultraviolet light from the sun split the water molecules apart into their constituent atoms. As a result, the lighter hydrogen atoms bubbled up to the top of the atmosphere and escaped into space, while the heavier deuterium atoms were trapped by Venus’ gravitational pull. The hydrogen-to-deuterium ratio in Venus’ atmosphere is the crucial clue that provides the evidence for what happened a billion or more years ago on Venus.

Now, in research just published in Science this spring, a team of scientists led by G L Villanueva of NASA Goddard Space Flight Center has used powerful telescopes on Earth to map water (H2O) and its deuterated form (HDO) across the surface of Mars. They’ve confirmed the results obtained by NASA’s Curiosity/Mars Science Laboratory in 2013 that the hydrogen-to-deuterium ratio on Mars is smaller by a factor of about 7 compared to that on Earth. This measurement tells us that Mars, like Venus, has lost lots of hydrogen, which means Mars, like Venus, has lost lots of its water.

The total amount of water identified in all currently existing water reservoirs on Mars (the ice caps – which have some water but are mostly frozen carbon dioxide; atmospheric water; ice-rich regolith layer; near-surface deposits) would generate a global ocean about 21 meters (68 feet) deep. The deuterium measurements tell us that Mars once had about seven times more water, enough water to create an ocean that would have covered the entire planet to a depth of at least 137 meters (445 feet). The evidence is now clear: Mars has lost at least 85% of the water it once had. (And that estimate assumes the Earth has not lost any of its water; if the Earth also has lost 80% of its original water reservoir, then Mars has lost 97% of its original water reservoir.)

Whither goest Venus and Mars….

Venus and Mars. Mars and Venus. Planetary scientists know that both planets were wet and Earth-like in the beginning; they also know that neither Venus nor Mars could hold onto their water for long enough to nurture advanced life forms until they could flourish. The lessons from Venus and Mars are clear and simple: water worlds are delicate and fragile. Water worlds that can survive the ravages of aging, whether natural or inflicted by their inhabitants – and can nurture and sustain life over the long term – are rare and precious.

If we allow the temperature of our planet to rise a degree or two, we may survive it as a minor environmental catastrophe. But beyond a few degrees, do we know the point at which global warming sends our atmosphere into a runaway death spiral, turning Earth into Venus? We know what the endgame looks like.

David A Weintraub is Professor of Astronomy at Vanderbilt University.

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One of the most dramatic features of recent climate change is the decline of summer Arctic sea ice. The impacts of this summer ice loss on northern society, on Arctic ecosystems, and the climate both locally and further afield, are already being felt.

Less well known are the dramatic changes in winter sea ice in regions such as the Greenland and Iceland Seas, where the reduction over the past 30 years is unparalleled since 1900, when ice records in the region began.

In a study published in Nature Climate Change, we show that the loss of sea ice in this subpolar region is affecting the production of dense water that forms the deepest part of the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is an ocean circulation that carries warm water from the tropics northward in the upper layers of the Atlantic with a return flow of cold water southwards at depth. As such, the effect of these changes could mean a cooler climate in western Europe.

The loss of winter sea ice

Much of the dense water in the AMOC is produced in the Greenland and Iceland Seas through the transfer of heat and moisture from the ocean to the atmosphere. The heat transfer makes the surface waters in these regions colder, saltier and denser, resulting in a convective overturning of the water column. It also serves to warm the atmosphere in this part of the world, often resulting in distinctive cloud formations seen in satellite images of the region.

How much heat transfer, or atmospheric forcing, occurs depends on the magnitude of the air-sea temperature difference and the surface wind speed. As a result, it is typically largest near the sea ice edge where cold and dry polar air first comes into contact with the warm surface waters.

Sea ice retreat and ocean convection

In our study, we show that the retreat of winter sea ice has led to a large reduction in the intensity of oceanic convection in the Greenland and Iceland Seas. These changes raise the possibility of less heat being transferred from the ocean to the atmosphere in these regions, resulting in a weaker AMOC, which in turn means less subtropical water brought northwards and ultimately a possible cooling of Europe.

In addition to a large atmospheric forcing, oceanic convection typically occurs in regions where there is a weak vertical density contrast, usually within a closed ocean current known as a cyclonic gyre. This makes it easier for convective overturning to extend to greater depths in the ocean. Until recently, the gyres in the Greenland and Iceland Seas that are preconditioned for oceanic convection were situated close to the ice edge and, as a result, the atmospheric forcing was large, resulting in deep convective overturning.

However, the winter retreat of sea ice has now shifted the regions of largest atmospheric forcing away from these gyres. In other words, the regions where the forcing is largest and the regions most susceptible to deep ocean convection have moved apart. Since the 1970s, this has resulted in an approximate 20% reduction in the magnitude of this forcing, or heat transfer from the ocean to atmosphere, over the Iceland and Greenland Sea gyres.

Impact on the ocean and Europe

Using a mixed-layer ocean model, we have investigated the impact of this reduced atmospheric forcing. In the Greenland Sea we show that the decrease in forcing will likely result in a fundamental transition in the nature of oceanic convection there. Indeed our model results suggest a change from a state of intermediate depth convection to one in which only shallow convection occurs.

As the Greenland Sea provides much of the mid-depth water that fills the Nordic Seas, this transition has the potential to change the temperature and salinity characteristics of these seas. In the Iceland Sea, we demonstrate that a continued reduction in atmospheric forcing has the potential to weaken the local oceanic circulation that has recently been shown to supply a third of the dense water to the deep part of the AMOC.

Observations, proxies, and model simulations suggest that a weakening of the AMOC has recently occurred, and models predict that this slowdown will continue. Such a weakening of the AMOC would have dramatic impacts on the climate of the North Atlantic and western Europe. In particular, it would reduce the volume of warm water transported at the surface towards western Europe. This would reduce the heat source that keeps the region’s climate benign.

Although there is considerable debate regarding the dynamics of the AMOC, one proposed mechanism for its current and predicted decline is a freshening of the surface waters – for instance due to enhanced meltwater from the Greenland Ice Sheet. A lower salinity reduces the surface water’s density, making it more difficult for oceanic convection to occur.

However, much of this freshwater discharge is apt to be exported towards the equator via the boundary current system surrounding Greenland. This limits the direct spreading into the gyres in the Greenland and Iceland Seas where oceanic convection occurs. Further work is therefore required to determine how and where – and on what timescales – this freshwater pervades the North Atlantic.

However, our results suggest that other possible mechanisms for a slowdown in the AMOC may be at work, such as a reduction in the magnitude of the atmospheric forcing that triggers the convective overturning in the Greenland and Iceland Seas. This process would also result in a slowdown of the AMOC, again reducing the warming that Europe experiences. Our results reinforce the idea that a warm Europe requires a cold North Atlantic, which allows for large transfers of heat and moisture from the ocean to the atmosphere. A warming North Atlantic with the associated retreat of winter sea ice therefore has the potential to result in a cooling of Europe through a slowdown of the AMOC.

Whether these transfers continue to decline into the future is still an open question, as is their impact on the AMOC and European climate.

Kent Moore is Professor of Physics at University of Toronto.
Ian Renfrew is Professor of Meteorology at University of East Anglia.
Kjetil Våge is Research scientist in physical oceanography at University of Bergen.
Robert Pickart is Senior Scientist in Physical Oceanography at Woods Hole Oceanographic Institution.

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Vitus Bering, the famous explorer, led perhaps the most ambitious scientific expedition ever in the 1730s. Commanding 10,000 people, he was in charge of exploring the vast lands of Siberia and the unknown sea between Siberia and Alaska. In 1741, he was forced to land on what would be later known as Bering Island, where he would die. In his crew was a doctor and naturalist, Georg Steller, who discovered in the calm waters close to the island a massive three-ton marine mammal, similar to a manatee, that has the name of Steller sea cow.

The new species to science is famous because it became extinct only 27 years after it was discovered. Unfortunately, hundreds of other vertebrates have become extinct because of human activities in the last five centuries.

In our recent paper, we analyze whether the rate of modern extinctions caused by human activities is higher that the normal or natural extinction rate. This is important because it would let us understand if we are causing a mass extinction.

In the history of life on Earth, there have been five mass extinctions – episodes where large numbers of species became extinct in a short period of time. All mass extinctions have been caused by natural catastrophes, such as the impact of a meteorite.

Extinction rates

To do the study, we compared the normal – also known as background – extinction rates with the modern ones. In the normal rate, derived from a thorough analysis of thousands of mammal fossil and subfossil records from the last two million years, one would expect to lose two species for every 10,000 species present every 100 years. For example, if there are 40,000 species, we would expect to see eight extinctions in a century. A rate much higher than that would indicate a mass extinction.

We compiled the list of extinct and possibly extinct species from the International Union for Conservation of Nature (IUCN), an institution that compiles these data. We found that 477 species have become extinct in the last 100 years.

Under a normal extinction rate, we would have expected to have only nine extinctions; in other words, there were 468 more extinctions than would be expected in the last century! Putting it in a different way, the species lost in the last 100 years would have become extinct in more than 10,000 years under a normal extinction rate.

Ecosystem services

Our results are dramatic and tragic. We are losing species much more rapidly now than in the last two million years! At that pace, we may lose a large proportion of vertebrates, including mammals, birds, reptiles, amphibians and fishes, in the next two to three decades.

Those species are our companions in our travel across the universe. Losing them has many consequences. Those species are essential to maintain ecosystem services, which are all the benefits that we get for free from the proper function of nature. The combination of the gases of the atmosphere, the quality and quantity of water, soil fertilization, pollination and so on are ecosystem services. By losing species, we are eroding the conditions of Earth that are essential for human well-being.

There is still time to avert the most tragic consequences of a sixth mass extinction, because this one is caused by us. We need to curb the human population growth, social inequalities and more efficient of natural resources. We need to reduce habitat loss, overfishing and overhunting, pollution and other factors that are causing the current extinction episode.

We are the only species that has the capability to save all endangered animals. Paradoxically, saving them is the only way to save humanity.

Gerardo Ceballos is Researcher of Ecology and Zoology at Universidad Nacional Autónoma de México (UNAM).

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