What we know

New discoveries – what we know, how we know it, and what it means.

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June 28, 2011 | 9:57 PM | By

Is climate change causing sea level rise?

Salt marshes, beaches, and coastal development face the threat of accelerating sea level rise. Marshes can also help scientists uncover past sea level changes.

A new study – published in a high-profile scientific journal with an author list that reads like a who’s who of ocean and climate science – finds that sea level is rising faster than at any point in the past two thousand years and human-caused global warming is to blame.

If you’ve been following the sea level rise story closely, that kind of declaration may prompt a weary “Another one?” Virtually every new scientific study or academic review released in the past year has concluded that the rate of sea level rise is accelerating toward the unprecedented thanks to rising water temperatures and ever more rapidly melting glaciers. As a result, middle-of-the-road estimates of sea level rise for the coming century have increased from a foot or more (just a few years ago) to at least three feet.

Nonetheless, a new sea level rise study published last week caused quite a ruckus, both in the media and among academics. At the heart of both the media frenzy and the scientific debate is the claim that the new data clearly links current – unprecedented – sea level rise to climate change. It’s hard to argue that rising air and water temperatures won’t raise sea level. That’s basic physics – warmer water expands, and rising air and water temperatures drive melting of ice that adds water to the oceans. What’s at issue is essentially the question of how much sea level rise can be attributed to human greenhouse gas emissions.

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FILED UNDER: Ocean Change, sea level rise
June 1, 2011 | 11:18 AM | By

Ocean acidification confuses clownfish

flickr/sarniebill1

Okay, search your memory here. Remember ocean acidification? It’s been a while, but as you may recall, ocean acidification is the phenomenon in which carbon dioxide from the atmosphere dissolves in the surface waters of the ocean, producing carbonic acid that (in sufficient quantities) shifts the pH balance of the ocean toward acidity. In the past 200 years, the ocean has absorbed nearly a third of carbon dioxide emissions, resulting in a 30% increase in ocean acidity.

The extra acidity impairs the ability of animals like oysters and corals to extract the calcium carbonate they need to build their skeletons or shells. And there’s evidence that ocean acidification is already impacting the health and survival of bay scallops and other shellfish.

But get this: a new study published in the journal Biology Letters suggests that ocean acidification may affect basic life-saving behaviors in fish.

WHAT WE KNOW and HOW WE KNOW IT

The researchers at University of Bristol raised clownfish in modern seawater (390 ppm CO2) and in seawater enriched with carbon dioxide (600-900 ppm CO2). The elevated carbon dioxide situations mimicked levels expected by mid- to late-century if greenhouse gas emissions continue to grow at the current rate.

At the point when juvenile clownfish would normally be choosing a coral reef to settle down on, the researchers played recordings of a busy, predator-filled reef during the day – normally a big turn-off for small fish in the market for a home – and then watched whether the young fish swam away from the speaker. As expected, fish reared in modern-day water showed a strong inclination to get away from the threatening sound. But fish reared in any of the elevated carbon dioxide conditions did not; they spent more than twice as much of their time at the end of the fish tank closest to the speaker.

WHAT WE DON’T KNOW

The results of the experiment were pretty clear-cut, but there’s nothing in this study to explain why the fish failed to respond to the threatening sounds as expected. The researchers didn’t see any major changes in the ear bones of the fish, so speculate that it’s more likely changes in how the brain processes or responds to sounds than an actual change in hearing capabilities. But that’s only speculation at this point.

Also, this is just one study of one species of fish. We have no idea how widespread this type of response might be. Maybe it’s all fish. Maybe it’s just clownfish.

WHAT IT MEANS

As with many laboratory studies of the impacts of ocean acidification, the real-world implications are hard to know. But failing to avoid predators could have drastic consequences (we’ve all seen those nature shows, right?). Lead author Dr Steve Simpson of the School of Biological Sciences at the University of Bristol says the concern is whether fish will be able to adapt to increasingly acidic waters quickly enough to avoid worst-case scenarios.

“What we have done here is to put today’s fish in tomorrow’s environment, and the effects are potentially devastating. What we don’t know is whether, in the next few generations, fish can adapt and tolerate ocean acidification. This is a one-way experiment on a global scale, and predicting the outcomes and interactions is a major challenge for the scientific community.”

UPDATED: A lot of media reports about this study have been saying that ocean acidification leaves clownfish deaf. I just want to be absolutely clear that that is not what the study says. The authors stress that they do not know why the clownfish don’t respond to the threatening sounds, but they suspect it might be due to a change in neural function – how messages about the sound are transmitted to or processed by the brain – brought about by disrupted acid-base balance in the fish. In other words, nerves and brain might be the culprits rather than the ear.

FILED UNDER: Ocean Change, ocean acidification
April 4, 2011 | 11:40 AM | By

Rising wind and waves: is it climate change?

flickr/tylerdurden1

A new study suggests that oceanic wind speeds and extreme wave heights have increased over the past twenty years.

A new study of oceanic winds and waves has been getting a lot of attention. The authors conclude that wind speeds and wave heights have risen steadily over the past twenty years. But there’s still a lot of room for uncertainty. So let’s break it down.

WHAT WE KNOW and HOW WE KNOW IT

Three scientists from Swinburne University of Technology in Melbourne, Australia, used 23 years worth of global satellite data to study trends in wind speeds and wave heights. The idea here was that satellite measurements would be more reliable than previous observations from a host of different ships and buoys, particularly since one member of the team recently led efforts to cross-calibrate measurements from different satellite systems to ensure consistency. When looking at both wind and waves, the researchers considered average speeds/heights as well as the most extreme conditions.

Globally, average wave heights didn’t appear to have changed much. In fact, some regions showed very slight decreases in average wave heights, but nothing that can be considered significant. But looking at the top 10% or 1% of waves presented a different story. While there was no change in tropical regions, extreme wave heights at higher latitudes (both north and south) showed steady increases. All told, the top 1% of waves appear to be 10% bigger than they were 20 years ago.

The story for wind speed is simpler: there were global increases in both average and extreme wind speeds, with the top 1% showing the biggest increases. Extreme wind speeds increased significantly over half to two-thirds of the ocean, particularly the southern hemisphere.

The take-home? The current analysis suggests that extreme wind and wave conditions – particularly at high latitudes – appear to be getting more extreme.

WHAT WE DON’T KNOW

This study isn’t the first to look at trends in wind speeds and wave heights, and it’s unlikely to be the last. Data from both buoys and satellites suggest that wind speeds are increasing. But the picture is a lot fuzzier when it comes to wave heights. While there are plausible explanations for increases in extreme conditions (more severe storms), measurements are least reliable under extreme conditions. So pinning an argument entirely on the top 1% of waves is a bit risky.

Then there’s the elephant in the corner: If wind and waves are increasing, is it because of climate change? The authors of the study are careful to point out that they only have two decades’ worth of data. Since there are long-term up-and-down cycles – oscillations – in many climate factors, including temperatures, winds, and waves, it’s impossible to know whether a twenty-year increase is part of a longer trend related to climate change or whether it’s just the upswing in a multi-decade cycle. The only way to figure that out is to keep watching. If it’s climate change, the increases should continue and become more pronounced; if it’s a natural oscillation, the trend should start to reverse itself.

WHAT IT MEANS

The authors point out that increases in wind speeds and wave heights could actually impact climate change by affecting the transfer of heat between the ocean and the atmosphere – one of the great unknowns in climate calculations. There are also more concrete implications: coastal erosion and property damage, shipping, even offshore wind energy development. Climate Central’s Michael Lemonick writes that, while increases in average wind speeds might be a boon to wind energy developers, increases in extreme winds and waves are unlikely to help (turbines are usually shut down at very high wind speeds to avoid damage) and may pose damage risks that would make development less feasible.

FILED UNDER: Ocean Change, extreme weather
February 23, 2011 | 2:53 PM | By

Ocean fertilization: Could a greener ocean stop climate change?

NASA image by Norman Kuring, MODIS Ocean Color Team / NASA

Phytoplankton blooms large enough to be seen from space occur naturally, like this 2009 bloom near Hokkaido, Japan. Fertilizing even larger, longer-lasting blooms may be one way to counter rising carbon dioxide emissions.

We usually think of the oceans as blue. But, in fact, what’s out there is a sea of green. Microscopic plant-like organisms – phytoplankton – produce half the world’s oxygen. In the process, phytoplankton take in carbon dioxide, locking the carbon in their cells and carrying it to the depths of the ocean when they, or the animals that eat them, die and sink. This so-called biological pump is a life-support system for the entire planet. With a little encouragement – usually in the form of fertilization with iron – it may also be one way to combat rising carbon dioxide emissions.

THE BIG IDEA

On land, we’re used to fertilizing our lawns and gardens with nitrogen and phosphorus because those are the nutrients in shortest supply. The same is true in much of the ocean. But in the Southern Ocean that surrounds Antarctica and in portions of the Pacific Ocean, nitrogen and phosphorus are in ready supply; the nutrient that limits phytoplankton growth in these areas is often iron. Areas that are naturally iron-rich support more algal growth and draw down more carbon dioxide. So, the idea behind ocean iron fertilization, as the process is known, is simple: more iron = more phytoplankton = less carbon dioxide = less climate change.

WHAT WE KNOW and HOW WE KNOW IT

That simple equation has been put to the test in thirteen major open-ocean experiments over the past two decades, each lasting a few weeks and covering areas of a few hundred square kilometers (for perspective, Buzzards Bay is almost 600 square kilometers). These relatively small, short experiments have confirmed one thing – iron addition can stimulate algal blooms and temporarily reduce carbon dioxide levels near the ocean’s surface. The best-case scenario, according to a recent review by UNESCO and the Intergovernmental Oceanographic Commission, is that ocean iron fertilization could pull out of the atmosphere a little less than an eighth of the carbon dioxide that human activity adds each year.

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FILED UNDER: Mitigation, geoengineering
January 7, 2011 | 4:27 PM | By

Discovery of the year: phytoplankton decline linked to warming seas

NASA image created by Norman Kuring, Ocean Color Web

NASA’s Aqua satellite captured this image of a massive phytoplankton bloom off of the Atlantic coast of Patagonia on December 21, 2010.

Today’s “Discovery of the Year” goes to a study that several Climatide commenters, ClimateCentral, and Wonk Room’s Brad Johnson voted the hands-down discovery of the year – evidence of a 40% decline in phytoplankton (the microscopic marine plants that generate half of the oxygen on Earth) coinciding with rising ocean temperatures. It’s big news, but not without controversy.

WHAT WE KNOW (and HOW WE KNOW IT)

Since 1979, scientists have used satellite images (like the one above) to track the color of the ocean’s surface and infer the amount of chlorophyll – a green pigment produced by phytoplankton and other plants for use in photosynthesis (the production of biological energy from sunlight). Such studies have revealed variation that might be related to climate change, but a few decades of data isn’t enough to really see long-term trends. So a group of scientists at Dalhousie University used data collected with a much older and lower-tech device called a Secchi disk – essentially, a plate-sized white disk on a rope that is used to gauge the transparency of the ocean by measuring the depth at which it becomes invisible. Standardized Secchi disks have been used to measure ocean transparency for over a century. The authors of the new study used “established models” to convert hundreds of thousands of publicly available transparency measurements into chlorophyll counts.

The results were striking: eight out of ten major ocean regions have seen dramatic declines in phytoplankton abundance since 1950. Globally, the rate of decline was estimated at 0.6% per year (compared to the global median, or most commonly measured level) – a total drop of almost 40% in 60 years. The decline was most notable in open-ocean regions where phytoplankton productivity is highest. Sea surface temperature was the single factor most tightly linked to the phytoplankton decline; water temperatures could affect phytoplankton growth in a number of direct and indirect ways, including altering the mixing and flow of nutrients.

The results were striking: a total global drop of almost 40% in 60 years.
As I mentioned before, this study sparked some debate among experts in the field. A lot of data from a lot of different sources went through a lot of blending, filtering, and modeling, leaving some with doubts about the reliability of the results. The study may also have suffered from guilt by association; one of the authors – Boris Worm – has drawn criticism for past dire predictions about marine ecosystems. An accompanying commentary in the same issue of Nature compared the strengths and weaknesses of Secchi disk and satellite measurements; the two authors (both scientists in this field) conclude that satellite data present their own challenges and that the study is “a sorely needed contribution to our knowledge of historical changes in the ocean biosphere.”

WHAT IT MEANS

Phytoplankton are the foundation of the oceanic food web, draw down carbon dioxide from the atmosphere, and produce half the oxygen we need to survive. In other words, they’re important – really important. In the words of lead author Daniel Boyce, “Phytoplankton is the fuel on which marine ecosystems run. A decline of phytoplankton affects everything up the food chain, including humans.”

WHAT WE DON’T KNOW

Aside from lingering questions about methodology (which are considerable), the single biggest question left hanging is whether phytoplankton abundance will continue to decline as temperatures continue to rise. The ocean’s response to climate change is anything but simple. Rising carbon dioxide could spur phytoplankton growth. Ocean circulation could change dramatically, throwing current trends (no pun intended) out the window. This is certainly not the last word on the subject, but it was a major development.

FILED UNDER: Ocean Change, 2010 in review, phytoplankton, sea surface temperature
January 5, 2011 | 2:33 PM | By

Discovery of the year: ocean acidification is happening NOW

The greatest threat facing the ocean, namely rising levels of carbon dioxide in the atmosphere, actually poses two distinct threats – warming and acidification. Each will get its day in the “Discovery of the Year” spotlight. Today, it’s ocean acidification’s turn.

As you may recall, ocean acidification is the phenomenon in which carbon dioxide from the atmosphere dissolves in the surface waters of the ocean, producing carbonic acid that (in sufficient quantities) shifts the pH balance of the ocean toward acidity and impairs the ability of animals like oysters and corals to extract the calcium carbonate they need to build their skeletons or shells. In the past 200 years, the ocean has absorbed nearly a third of carbon dioxide emissions, resulting in a 30% increase in ocean acidity.

Stony Brook University

The impacts of ocean acidification - slowed growth and thinner shells - are strikingly visible in a side-by-side comparison of quahogs raised in water simulating past (250ppm), present (390ppm) and future (750ppm and 1500ppm) levels of atmospheric carbon dioxide.

Scientists have generally considered the impacts of ocean acidification to be a problem of the (near) future. But in September 2010, two marine scientists from Stony Brook University – Stephanie Talmage and Christopher Gobler – published a paper in the Proceedings of the National Academy of Sciences that forced a revision of that thinking, suggesting that ocean acidification may already be (and have been for some time) taking a toll on shellfish.

WHAT WE KNOW (and HOW WE KNOW IT)

Talmage and Gobler reared quahogs (Mercenaria mercenaria) and bay scallops (Argopecten irradia) under conditions simulating past, present, and likely future carbon dioxide levels. Not surprisingly (because numerous previous studies have documented similar findings), the shellfish of the future had severe shell defects, higher death rates, and slower growth than their modern-CO2 counterparts. What was less expected was the observation that modern conditions produced shellfish with thinner shells, slower growth, and death rates almost double those of shellfish grown in pre-industrial water conditions.

WHAT IT MEANS

Talmage and Gobler conclude that ocean acidification “may [already] be inhibiting the development and survival of larval shellfish and contributing to global declines of some bivalve populations.” In fact, since shellfish grown in the laboratory are granted a relatively luxurious life with abundant food and no predators or competitors, the authors say their data represent conservative estimates of the impacts of acidification. In the wild, slow-growing, thin-shelled animals would likely be vulnerable to any number of untimely ends – predation, over-crowding, incidental crushing. Stressed animals may also be more susceptible to diseases, like those that have ravaged east coast oyster populations in recent decades.

WHAT WE DON’T KNOW

With ocean acidity reaching levels not seen in over twenty million years, perhaps the biggest question on many scientists’ minds is whether evolutionary adaptation will be able to keep pace with rising acidity.

FILED UNDER: Ocean Change, 2010 in review, ocean acidification
January 4, 2011 | 9:36 AM | By

Discovery of the year: new indicator of fisheries health needed

flickr/Pieter Pieterse

Selecting the overfishing-related discovery of the year was a no-brainer. When this story hit, I overheard people talking about it in the local coffee shop. Granted, that’s in Woods Hole, where the vast majority of people in the coffee shop are likely to be ocean scientists. Still, this was big news. Here’s a lightly edited version of my original post:

“Trophic level” is a fancy term for whether you’re more likely to eat or be eaten. In the ocean, microscopic algae sit at the bottom of the food chain – a trophic level of one – while large predators such as sharks and tuna are on the top of the world, around trophic level four or so. Currently, many fisheries managers use the average trophic level of fish harvests as a leading indicator of ecological health. The system is based on a 1998 study of over four decades of data that indicated the average trophic level of catches was declining as we “fished down the food web” by overharvesting the highest trophic levels, then moving on to fish lower and lower on the chain. It’s a familiar storyline here on Cape Cod – when the cod dried up, fishermen moved on to dogfish and herring. But a high-profile study published in the journal Nature upended this theory and left scientists and managers fishing for a new measure of ecosystem health (sorry, subscriptions needed for both articles).

Continue reading →

FILED UNDER: Ocean Change, 2010 in review, overfishing
January 3, 2011 | 11:55 AM | By

Discovery of the year: plastic in the Atlantic

SEA/Skye Moret

Plastic debris collected by Sea Education Association scientists and students as part of their study of plastic pollution in the Atlantic Ocean.

This week, I’m highlighting my (and your) picks for the most important ocean science discoveries of 2010 with a special series of “What We Know” posts – one for each of the greatest threats facing the ocean – starting with plastic pollution.

WHAT WE KNOW

Scientists have long known that plastic debris accumulates in parts of the northern Pacific Ocean bounded by circular ocean currents, or gyres; these regions have been called The Great Pacific Garbage Patches.

This year, scientists from Sea Education Association and Woods Hole Oceanographic Institution confirmed what many have suspected for years: that there is a similar accumulation of plastic debris in the Atlantic Ocean. In the Subtropical Gyre, plastic debris – most of it less than half an inch in diameter – reached concentrations of 20,000 pieces per square kilometer. To put that in perspective, imagine walking down a sidewalk (about 4ft wide). You’d encounter a piece of plastic debris every 100-150ft.

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FILED UNDER: Ocean Change, 2010 in review, plastic pollution
November 22, 2010 | 3:11 PM | By

Fishing down the food chain? Maybe not.

flickr/Pieter Pieterse

It's a fish-eat-fish ocean out there. Whether humans are catching too many fish at the top of the food chain is the subject of a new study that calls into question over a decade of fisheries assessments.

“Trophic level” is a fancy term for whether you’re more likely to eat or be eaten. In the ocean, microscopic algae sit at the bottom of the food chain – a trophic level of one – while large predators such as sharks and tuna are on the top of the world, around trophic level four or so. Currently, many fisheries managers use the average trophic level of fish harvests as a leading indicator of ecological health. The system is based on a 1998 study of over four decades of data that indicated the average trophic level of catches was declining as we “fished down the food web” by overharvesting the highest trophic levels, then moving on to fish lower and lower on the chain. It’s familiar storyline here on Cape Cod – when the cod dried up, fishermen moved on to dogfish and herring. But a new study published last week in the journal Nature has upended this theory and left scientists and managers fishing for a new measure of ecosystem heath (subscriptions needed for these articles).

Continue reading →

FILED UNDER: Ocean Change, overfishing