Did climate change kill off the Neanderthals? Not likely…

The reasons why the Neanderthals died off remain a mystery. One of the major theories says that they were the victims of major climate changes, but new evidence suggests that this is an unlikely scenario.

In an age where climate change driven by our own hand is poised to cause catastrophic changes to human life, it seems fitting to work out if shifting climates also doomed our evolutionary cousins, the Neanderthals.

A Neanderthal hunter - did he fail to compete with humans?The question of why the Neanderthals died out has been the source of fierce debate since the first bones were discovered in the early 19th century. Some scientists turned the finger of blame inwards, suggesting that early humans killed them off, either directly, through violence and the spread of new diseases, or indirectly by gradually out-competing them.

Others have accused changing climates. According to them, Neanderthals were adapted to cold environments and, being less flexible than humans, they were unable to cope with a warming post-Ice Age world.

Now, Chronis Tzedakis from the University of Leeds has found compelling evidence that the Neanderthals extinction was unlikely to have coincided with the extreme shifts in climate at the end of the last Ice Age.

While their findings don’t rule out the climate change model completely, they strongly suggest that it wasn’t a major factor in the Neanderthals’ downfall.

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Bleached corals recover in the wake of hurricanes

Hurricanes can physically cool coral reefs but they can also save them, by cooling the surrounding ocean and reversing the effects of bleaching.

Hurricanes can reverse coral bleaching by cooling surrounding waterIn 2005, corals in the large reef off the coast of Florida were saved by four hurricanes. Tropical storms seem to be unlikely heroes for any living thing. Indeed, coral reefs directly in the way of a hurricane, or even up to 90km from its centre, suffer serious physical damage. But Derek Manzello from the National Oceanic and Atmospheric Administation has found that corals just outside the storm’s path reap an unexpected benefit.

Hurricanes can significantly cool large stretches of ocean as they pass overhead, by drawing up cooler water from the sea floor. And this cooling effect, sometimes as much as 5°C, provides corals with valuable respite from the effects of climate change.

Rising temperatures, dying corals

As the globe warms, the temperature of its oceans rises and that causes serious problems for corals. Their wellbeing depends on a group of algae called zooxanthellae that live among their limestone homes and provide them with energy from photosynthesis. At high temperatures, the corals eject the majority of these algae, leaving them colourless and starving.

These ‘bleached’ corals are living on borrowed time. If conditions don’t improve, they fail to recover their algae and eventually die. But if the water starts to cool again, they bounce back, and Manzello found that hurricanes can help them to do this.

Together with scientists from the Universities of Miami and the US Virgin Islands, he measured the extent of bleaching in reefs off Florida and the US Virgin Islands over the course of 2005.

Corals bleach when temperatures riseBy September, both reefs were suffering from equal amounts of bleaching. But while the situation continued to worsen in the storm-free Virgin Islands, the advent of four hurricanes in Florida turned the tide in the reefs’ favour.

Hurricanes vs bleaching

The storms – Dennis, Rita, Wilma and the infamous Katrina – each left behind an imprint of cooler water and the seas within 400km of their paths cooled by up to 3.2°C and stayed that way for up to 40 days. Two weeks after the fourth hurricane, Wilma, had passed, the corals had almost completely recovered.

Manzello’s study shows that the benefits of hurricanes on coral reefs can sometimes outweigh the localised physical wear and tear they cause. The question now is whether this is an isolated incident or a common occurrence.

Manzello isn’t sure. Based on the numbers of bleaching events and hurricane landfalls in Florida since the 19th century, the odds of both happening at the same time (as in 2005) is about one in seven. But the actual probability is likely to be higher especially since the same factors that cause bleaching, such as warmer water, also encourage the growth of hurricanes.

Even so, it would be extremely foolish to expect hurricanes to bail corals out completely – only conservation projects and addressing rising temperatures can do that.

Reference: Manzello, Brandt, Smith, Lirma, Hendee & Nemeth. 2007. Hurricanes benefit bleached corals. PNAS doi.10.1073/pnas.0701194104.

Related posts on corals:
Hope for corals – swapping algae improves tolerance to global warming
Corals survive acid oceans by switching to soft-bodied mode

Related posts on the hidden effects of climate change: Icebergs are hotspots for life
Human nitrogen emissions indirectly capture carbon by fertilising forests
When the heat is on, male dragons become females
Climate change responsible for decline of Costa Rican amphibians and reptiles

Icebergs are hotspots for life

Icebergs are hotspots for Antarctic life. The water around them teems with nutrients, plankton and animals – a mobile community dragged along by the drifting ice. Together, they enrich over a third of Antarctic waters.

Icebergs are hotspots for Antarctic life.Say the word iceberg, and most people are likely to free-associate it with ‘Titanic’. Thanks to James Cameron (and, well, history too), the iceberg now has a reputation as an cold murderous force of nature, sinking both ships and Leonardo DiCaprio. But a new study shows that icebergs are not harbingers of death but hotspots of life.

In the late 1980s, about 200,000 icebergs roamed across the Southern Ocean. They range in size from puny ‘growlers’, less than a metre long, to massive blocks of ice, larger than some small countries.

They may be inert frozen lumps, but icebergs are secretly in the business of nutrient-trafficking. As the ice around Antarctica melts in the face of global warming, some parts break free from the parent continent and strike out on their own. As they melt, they release stored minerals into the water around them, and these turn them into mobile homes for a variety of life.

A tale of two icebergs

Kenneth L. Smith Jr, from the Monterey Bay Aquarium Research Institute, and other scientists from San Diego discovered the true extent of these icy ecosystems by studying two icebergs floating in the Antarctic Weddell Sea.

Even the smaller of the two, W-86, has a surface area larger than 17 football pitches. The larger one, A-52 was over a thousand times bigger, with a surface area of 300 km2 and extending 230 metres into the freezing waters.

Smith and crew identified the duo through satellite imaging, and tracked them down by boat. Their ship spiralled around the blocks of ice collecting water samples as it went, from a dangerously close distance of a few hundred feet to a safer five miles away.

Hotspots for life

Diatoms cover the underside of icebergs and form a ring of life around them.The skies above the two icebergs were patrolled by seabirds, including Cape petrels and Antarctic fulmars. Below the water, Smith explored the icebergs’ undersides with a remote-operated vehicle and found them teeming with life.

Below W-86, he saw a lattice-like surface, and the ridges of these were home to diatoms (right). These single-celled algae are part of the phytoplankton, microscopic creatures that make their energy from the sun and form the basis of the ocean’s food web. In between these diatom-covered ridges, baby icefish and segmented worms swam among the lattice’s nooks and crannies.

A-52 was even more varied, with large caves extending deep into the iceberg’s core. The team found diatoms here too, along with Antarctic krill (below), small shimp-like animals with a taste for diatoms. Among these were various invertebrates – comb jellies, colonial jellyfish-like animals called siphonophores, and predatory torpedo-shaped worms called chaetognaths.

Further out, the area immediately around the iceberg was void. But just beyond that, the ice was encircled by another halo of phytoplankton. These creatures, along with the diatoms on the ice itself, were thriving on the nutrients released by the melting ice, such as iron.

When Smith exposed diatoms to tiny mineral-rich particles filtered from his collected water samples, they grew slowly and steadily, while other diatoms cultured in normal water did not.

Diatoms fed by meltwater nutrients from icebergs, act as food for krill.These drifting islands of ice were dragging entire communities along with them. As they drift and melt, they release small amounts of important nutrients. That triggers the growth of creatures at the bottom of the food chain and provides the foundations for larger animals like krill and seabirds. A-52 alone enriched a massive ring of water about the size of the Isle of Man.

To estimate the effect of other icebergs, Smith used satellites to count the number of bergs in a sample area. Within this space, the satellites spotted almost a thousand individual icebergs that, together, covered less than 0.5% of the ocean’s surface. But even this small amount was enough to enrich over 39% of the Southern Ocean!

Icebergs and climate change

By providing support for phytoplankton, the icebergs were also inadvertently helping to mitigate the effects of climate change. Just like land plants, phytoplankton make their own energy through photosynthesis. And just like land plants, they absorb carbon dioxide to do so. By eating the phytoplankton and excreting the remains, krill cause carbon to fall down into the ocean depths in a rain of droppings.

So even as their parent continent melts and releases carbon into the atmosphere, icebergs serve to draw planet-warming carbon away from the air and transfer it to the deepest sea. Smith believes that climate modellers need to take this into account to better predict the effects of melting Antarctic ice.

The disappearing ice can reveal underlying rock which absorbs more heat, hastens melting and releases even more trapped carbon – this is known as ‘positive feedback’. But as the ice melts, icebergs break off and these help to suck in carbon from the atmosphere – this is ‘negative feedback’. The next task is to understand how these two processed balance out.

Reference: Smith Jr, Robison, Helly, Kaufmann, Ruhl, Shaw, Twining and Vernet. 2007. Free-drifting icebergs: hot spots of chemical and biological enrichment in the Weddell Sea. Science doi.10.1126/science.1142834.

Other posts on ecology:
Shark-hunting harms animals at bottom of the food chain
The fox and the island – an Aleutian fable
Parasites can change the balance of entire communities
Human nitrogen emissions indirectly capture carbon by fertilising forests

Human nitrogen emissions indirectly capture carbon by fertilising forests

Human activity has greatly increased the levels of active nitrogen in the environment. By acting as a fertiliser and speeding the growth of forests, this extra nitrogen has indirectly locked up more carbon dioxide in the world’s trees.

There is no doubt that of all the elements in the entire periodic table, carbon is currently hogging the limelight. As it cycles through our environment, the policy decisions and economic futures of entire countries hang in the balance.

Industrial exhausts pump huge amounts of nitrogen into the atmosphere.For all its media-whoring, you might be forgiven for forgetting that carbon is not the only element we are belching into the environment. Over the last century, we have greatly overwhelmed the natural nitrogen cycle too.

Nitrogen – the neglected element

Through the manufacture of nitrogen-based fertilisers and the exhausts of our cars, power plants and factories, we have more than doubled the natural levels of active nitrogen in the atmosphere.

Nitrogen is a valuable commodity in many parts of the world, and restricts the growth of local plant life. As such, the recent man-made influx has led to large increases in plant growth. In some cases like algal blooms that choke rivers and lakes, it’s too much of a good thing. But there is a silver lining.

Federico Magnani from the University of Bologna, together with an international team of scientists, have found that the changes in the nitrogen cycle may have been inadvertently fertilising our forests.

Carbon and nitrogen

The world’s forests act as massive carbon sinks, delaying the global warming effects of carbon dioxide by trapping it in prisons of wood and leaves. And larger forests mean more trapped carbon. The temperate forests of the Northern Hemisphere alone could store a massive 600 megatonnes of carbon every year.

The carbon and nitrogen cycles dance around each other in complex ways. When nitrogen levels increase, forests respond by channelling growth from roots to leaves and trunks. These above-ground organs are more enduring than roots and retain sequestered carbon for a longer time. More leaves also means increased photosynthesis, which serves to draw more carbon dioxide in from the air.

The extra nitrogen also delays the decay of leaf litter, further halting the release of organic carbon into the atmosphere.

The world’s forests act as massive carbon stores.Magnani’s colleagues are not the first group to try and look at the interplay between nitrogen levels and carbon capture. But other studies have found it difficult or impossible to account for the effects of nitrogen alone.

Carbon balance

A forest’s carbon balance – the amount of carbon trapped versus the amount released – depends on a variety of factors, including its age, logging, fires, and more. Some of these are easy to account for at a small scale. For example, when logging or fires kill off patches of forest, they become net sources of carbon as they start to regrow.

But after a couple of decades or so, the mature forest turns into a carbon sink, and the amount it stores outweighs the amount it releases. Clearly, a forest’s carbon balance changes as it matures, but real forests consist of patches of vegetation are very different ages.

To look at the overall picture, Magnani’s group took direct measurements of the carbon balance over a long period of time, from a network of forest sites in Western Europe and the USA. This allowed them to account for short-term sources of variation. And by using direct measurements, they have surpassed the models and simulations of previous studies.

The group found that carbon balance corresponds well with nitrogen levels in the area. In fact, the prowess of some forests at carbon capture seem to be overwhelmingly driven by their extra nitrogen boost. Our effects on the nitrogen cycle may have been acting like an unexpected carbon offset scheme.


By fertilising forests, nitrogen emissions could offset carbon emissions.So should we start pumping nitrogen in our forests to trap more carbon dioxide? Certainly, Magnani’s results suggest that small extra amounts of nitrogen can cause unexpectedly large levels of carbon capture. But his view and those of other commentators is a resounding “Not yet”.

There are still many questions left to be answered, particularly about the exact relationship between nitrogen addition and carbon levels. There is some evidence that some temperate forests are suffering from nitrogen saturation. Could adding more nitrogen damage them, or prevent them from returning to a situation where nitrogen is limited and not free-flowing?

And what of the other risks and benefits? The extra wood from the faster-growing trees could find a use as a replacement for concrete, a notoriously eco-unfriendly building material. But additional nitrogen could affect other animals and plants in the local environment. Would biodiversity suffer if certain species monopolise the newfound nitrogen bonuses?

As future research addresses these questions, those involved in forest management would do well to heed the importance of the world’s forests in sequestering carbon dioxide. There are other ways of increasing forest coverage besides mass-fertilisation, and the most obvious one is safeguarding the forests that we already have!

Reference: Magnani, Mencuccini, Borghetti, Berbigier, Berninger, Delzon, Grelle, Hari, Jarvis, Kolari, Kowalski, Lankreijer, Law, Lindroth, Loustau, Manca, Moncrieff, Rayment, Tedeschi, Valentini & Grace. 2007. The human footprint in the carbon cycle of temperate and boreal forests. Nature 447: 848-850.

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When the heat is on, male dragons become females

It seems almost fashionable now to blame everything on climate change, but the most unusual claim yet is that it could lead to sex-changing lizards.

At higher temperatures, male bearded dragons turn into females.For humans and other mammals, sex is neatly determined by the X and Y chromosomes. If you have a Y you are male, and without it you are female. Reptiles however, use a variety of strategies, and the mammalian X/Y system is just one of them.

In some species, the female is the one with different chromosomes, in this case Z and W, and the male has two Zs. And some reptiles ignore sex chromosomes altogether. For them, an individual’s sex is determined by the temperature that their eggs were incubated at.

Scientists had long believed that these strategies were mutually exclusive with each species choosing one of the other.

But Alexander Quinn and colleagues form the University of Canberra have found that an Australian lizard, the central bearded dragon (Pogona vitticeps) flouts this rule. It has become the first animal known to use two separate methods to determine the sex of individuals.

Bearded dragons use two methods of sex determination.The dragon uses the Z/W system, where the males carry two Z chromosomes and the females have a Z and a W. But Quinn found that these genes are only the dominant influence on gender if eggs are incubated between 20 and 32 degrees Celsius.

At higher temperatures, males ignore their genetic heritage and become females instead. When Quinn incubated broods of eggs between 34 and 37 degrees Celsius, the hatchlings were almost invariably female. And as predicted, about half of these sisters were genetically male. For dragons at least, when the heat is on, the men turn into women

Quinn believes that the key to the manliness of boy dragons lies in a temperature-sensitive protein produced by the Z chromosome. The protein’s activity needs to surpass a certain threshold before a dragon can become male. For that, there need to be two copies of Z, and the temperature must be just right.

Reptiles that use temperature to assign gender must have fine-tuned their systems over time to cope with an ever-changing environment. But Quinn fears that the current pace of climate change may be too rapid for these animals to adapt to.

If temperatures rise far enough to bias an entire species over to a single gender, extinction would be all but inevitable. These warnings have been sounded before, and Quinn’s work suggests that they should be shouted a little bit louder.

More about animal sex and reproduction: 
Virgin birth by Komodo dragons
Butterflies evolve resistance to male-killing bacteria in record time 
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

More on the effects of climate change: 
Icebergs are hotspots for life
Climate change responsible for decline of Costa Rican amphibians and reptiles
Hope for corals – swapping algae improves tolerance to global warming
Corals survive acid oceans by switching to soft-bodied mode



Reference: Quinn, Georges, Sarre, Guarino, Ezaz & Graves. 2007. Temperature sex reversal implies sex gene dosage in a reptile. Science 316: 411.

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Climate change responsible for decline of Costa Rican amphibians and reptiles

Amphibians around the world are facing extinction from habitat loss and a killer fungus. Now, climate change joins their list of enemies. In Costa Rica, warmer and wetter days have led to a loss of rainforest leaf litter that has sent amphibian and reptile populations crashing.

Miners used to take canaries into unfamiliar shafts to act as early warning systems for the presence of poisons. Today, climate scientists have their own canaries – amphibians.

The golden toad was one of the first casualties in the great amphibian decline.Amphibians – the frogs, toads and salamanders – are particularly susceptible to environmental changes because of their fondness for water, and their porous absorbing skins. They are usually the first to feel the impact of environmental changes.

And feel it they have. They are one of the most threatened groups of animals and one in three species currently faces extinction. The beautiful golden toad (right) was one of the first casualties and disappeared for good in 1989. Even though they are less glamorous than tigers, pandas or polar bears, amphibians are a top priority for conservationists.

The usual factors – introduced predators and vanishing habitats – are partially to blame, but many populations have plummeted in parts of the world untouched by pesky humans.

More recently, a large number of these deaths have been pinned on a fatal fungal disease called chytridiomycosis. Hapless individuals become infected when they swim in water used by diseased peers, and fungal spores attach to their skins. The disease had decimated amphibians across the Americans.

A third of the world’s amphibians face extinction, if not more.The extent of the damage may be even worse than we think. We have very little long-term data on the population sizes of many amphibian species, particularly in the tropics, where the greatest diversity exists. One of the few sites to buck the trend of ignorance is La Selva Biological Station in Costa Rica, which has been monitoring amphibian populations since the 1950s.

Steven Whitfield and colleagues from Florida International University used the La Selva data to analyse the populations of a species living among the leaf litter that covers the local rainforest floor. The team ran their census of about 30 species of amphibians, as well as many reptiles (lizards and snakes).

To their astonishment, the populations of these species had plummeted by 75% in 35 years. This massive decline is worrying for many reasons, the least of which is that La Selva sits within a protected area. Habitat destruction is non-existent here, so something else must be happening.

Nor is chytridiomycosis to blame. The fungus doesn’t tolerate high temperatures and only grows in temperate regions or mountainous ones. La Selva is neither. The killer fungus marks its presence with rapid falls in amphibian numbers within months, but these declines took place over decades.

And most tellingly of all, the reptiles suffered population losses as great as those of the amphibians. With their dry, scaly skins, reptiles lack the amphibian vulnerability to chemicals and chytridiomycosis. Something else is afoot.

Warmer and wetter days are diminishing the leaf litter that amphibians and reptiles call home.Whitfield believes that climate change is the answer. Over the past 35 years, La Selva has experienced wetter and warmer days. Temperatures have gone up by one degree Celsius, which slows the growth of local trees, and reduces the volume of leaves that they shed. The number of dry days has halved, and with more rainfall, the leaves that do fall decay faster.

So these combined climate changes have conspired to reduce the levels of leaf litter in the forest, robbing amphibians and reptiles alike of their homes. Even in this protected area, habitat destruction is going on right under our feet.

The climate change idea explains another odd finding. Whitfield saw that amphibian and reptile numbers had not declined in nearby abandoned cacao plantations. That’s because cacao trees shed their leaves throughout the year and provide a continual supply of new leaf litter.

The picture for the world’s amphibians seemed bleak enough, but it seems that we have been ignoring a larger simmering danger in the face of the immediate threat of chytridiomycosis. It is telling that all but one of the disappearing species in this study are listed as ‘least concern’ by the World Conservation Union (IUCN). Whitfield’s study should be a call to action for conservationists.

Reference: Whitfield, Bell, Phillippi, Sasa, Bolanos, Chaves, Savage & Donnelly. 2007. Amphibian and reptile declines over 35 years at La Selva, Costa Rica. PNAS doi.0611256104.

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Carbon offset schemes worsen global warming if trees are planted in the wrong places

Many carbon offset schemes rely on planting new trees to counteract rising carbon dioxide levels and the climate change they cause. But new research shows that these schemes only work if trees are planted in the tropics. Plant elsewhere, and you’ll only be adding to global warming.

Plant a tree and save the planet. If it sounds too good to be true, it’s usually because it is.

Temperate forests actually keep the world warmer by absorbing solar radiation into their shaded undergrowthIt is now crystal clear that modern global warming is a man-made phenomenon. But with this acceptance comes guilt, and the quest to find ways of mitigating our energy-hungry lifestyles

On the surface, carbon offset schemes appear to offer a win-win solution. People can assuage their guilt over yet another business flight, or drive to the shops, by paying for trees to be planted or investing in renewable technology.

Trees act as carbon sinks, sucking the gas in from the air and shunting the carbon atoms across into the plants’ own molecules. So plant enough trees, and the emissions you are responsible for will effectively be negated. You can whistle a jaunty tune and slap a carbon-neutral sticker on your car.

That’s the theory anyway. But Govindasamy Bala and colleagues form the Lawrence Livermore National Laboratory have found that it’s not just what you plant that matters, it’s where.

They ran complex simulations of how the planet’s climate would change if trees in different parts of the world were removed or restored. Unexpectedly, they found that overall, deforestation cools the planet down, and adding new trees in some regions may actually fuel global warming.

Carbon offset schemes are a poor alternative to not flying at all.In a simulation where all the world’s trees were removed, the global temperature fell by about 0.3 degrees Celsius.

Why should this be? After all, trees soak up carbon dioxide and store carbon in their bodies – this keeps the planet cool. They release water vapour into the air, which forms clouds that reflect solar radiation away form the earth, again resulting in cooling.

But forests are also dark and by absorbing the energy from sunlight, they heat the planet too. According to Bala’s simulation, this heating effect outweighs the cooling ones.

When Bala looked at the effect of deforestation in specific areas, a clearer picture emerged. The tropical rainforests are doing their bit in fighting global warming by forming clouds and absorbing carbon dioxide. Their loss led to a rise in global temperature.

In contrast, the temperate and polar forests aren’t pulling their weight. These verdant slackers heat the planet themselves by absorbing solar radiation. Without them, the underlying snow would reflect more of the sun’s energy into space and we’d get a cooler planet.

Carbon offset schemes only fight global warming if trees are planted in the tropics.These experiments suggest that tree-planting will only help to restrain global warming as planned if it happens in the tropics. In other parts of the world, it could even do more harm than good. When it comes to carbon offset schemes, the devil’s in the details.

Bala and his co-workers are modest on their work and are quick to point out that it is based on a single simulation. And they are careful to quickly stem the inevitable backlash from anti-environmental groups, who may well perversely suggest that this data warrants declaring war on trees.

Forests clearly have value beyond their influence on temperature. They harbour a great richness of life, keep the soil together and stop the oceans from acidifying by storing carbon dioxide – the list goes on. Deforestation is clearly not a solution to global warming, but wanton re-forestation won’t do any good idea.

Bala’s study gives pause for thought to those of us who seek to placate our environmental consciences by paying into carbon offset schemes.

At the very least, the details of any schemes should be checked carefully. Even better, serious thought should be given to preventive measures, like reducing car or plane use, rather than cures.


Reference: Bala, Caldeira, Wickett, Phillips, Lobell, Delire & Mirin. 2007. Combined climate and carbon-cycle effects of large-scale deforestation. PNAS 104: 6550-6555.

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