Ocean acidification is one of the most significant threats to ocean health.  The implications are far-reaching and dire, and are likely more pervasive and far more threatening to marine life than initially envisioned.  This article, generously shared with us by Oceanography Magazine, focuses on the threats that ocean acidification poses to coral reefs, a precious and vulnerable marine resource.

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Coral Reefs and Ocean Acidification

By Joan A. Kleypas and Kimberly K. Yates
Special Issue Feature from: Oceanography, Vol.22, No.4 (December 2009)

Abstract

Coral reefs were one of the first ecosystems to be recognized as vulnerable to ocean acidification. To date, most scientific investigations into the effects of ocean acidification on coral reefs have been related to the reefs’ unique ability to produce voluminous amounts of calcium carbonate. It has been estimated that the main reef-building organisms, corals and calcifying macroalgae, will calcify 10–50% less relative to pre-industrial rates by the middle of this century. This decreased calcification is likely to affect their ability to function within the ecosystem and will almost certainly affect the workings of the ecosystem itself. However, ocean acidification affects not only the organisms, but also the reefs they build. The decline in calcium carbonate production, coupled with an increase in calcium carbonate dissolution, will diminish reef building and the benefits that reefs provide, such as high structural complexity that supports biodiversity on reefs, and breakwater effects that protect shorelines and create quiet habitats for other ecosystems, such as mangroves and seagrass beds. The focus on calcification in reefs is warranted, but the responses of many other organisms, such as fish, noncalcifying algae, and seagrasses, to name a few, deserve a close look as well.

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Today we are anchored in a little bay on the west coast of the Baja peninsula. We are hiding from 30-40 knot north winds and a nasty swell that have been building for the past two days. The little fishing village at the head of the bay, San Juanico, is enveloped in a sand storm. It is a mile away and a dinghy ride in these winds would be far too wet for a visit. So, we will hide here at least for the afternoon. These north winds are called the “Baja Bash,” because any boater foolish enough to try to get to San Diego by this coastal route is bashed unmercifully. Adding to the wind bashing is the surprising cold; sea and air temperatures have dropped by 10°C (18°F) in the past two days and we have said goodbye to tropical nights.

In this report I will describe how the this sudden change in our conditions — polar fleece, sleeping bags, wool caps — is the end result of a chain of air and ocean currents that begins on a global scale with solar heating of the tropics and ends on a local scale with coastal winds and the upwelling of deep, cold ocean water.

The famous Coriolis force lies at the heart of it all. We will talk about this mysterious force then we will use a simple weather chart to demonstrate the interrelationship between the tropics, the Mid-Pacific Gyre, the Garbage Patch, coastal north winds, and, finally, coastal upwelling.

Figure 1. Current weather. A weather chart for the North Pacific ocean shows the surface pressure lines, wind barbs, and colored areas for precipitation. Our hiding spot is marked by the black circle at 26°N on the Baja coast. Labels have been added to the chart for reference. The marked features are the Mid-Pacific High (H), the North Pacific Gyre, the trade wind regimes (T), the Intertropical Convergence Zone (ITCZ), and upwelling areas along the coast (U) which are driven by the northerly winds. Click on the graph for a detailed view. Image credit: buoyweather.com.

Reading the weather chart.

The weather chart, Figure 1, was downloaded as we left Cabo San Lucas and prepared for a week-long transit to San Diego. This chart has, in one graphic, just about everything I have mentioned in many previous reports on air and ocean currents.

Symbols on the chart define the barometric pressure, winds, and areas of rain. Constant pressure lines are called isobars and, in the chart, the isobar circles define the mid-Pacific high pressure which is centered at 34°N, 150°W and has a central pressure of about 1028 millibars. The green wind barbs show wind speed and direction. Barbs are like arrows with only half their feathers. They point in the direction of the surface wind, and the number of barbs give the wind speed. One half a barb is 10 knots, a full barb is 20 knots, and one and a half barbs show a 30 knot wind. Precipitation areas are colored according to the scale on the right hand side. Rates from zero to forty-eight millimeters (2 inches) per hour are shown.

What follows here is a brief and highly simplistic description of the labeled features on the weather chart. However, before anything else you must understand the very important Coriolis force.

Coriolis force is central to understanding how winds and currents interact and none of the explanations here make sense without the most rudimentary understanding of this mysterious force.

Coriolis force is not actually a force, like gravity. In actuality it is a complicated gyroscopic reaction to the rotating Earth that pushes air or water in motion to the right of its path. It seems like a force because of its effect on motion. As an example, if water tries to move in a westerly direction, it veers off course, toward the north, as though a force was pushing it. (In the Southern Hemisphere the veer is to the left.)

The next time you are in the playground — in raising four sons I have spent quite a bit of time in the playground — stand on the merry-go-round and have someone turn it gently in a clockwise direction. Now close your eyes and take a small step in any direction. Surprise; you will not step forward, rather your foot will land to the right of where you planned. That is Coriolis force.

Figure 2. Global wind fields. A schematic of the major global wind fields. The red dashed boxes are the area of interest. The equatorial Hadley Cell (HC) is created by solar heating at the equator. Starting at the equator, warm, moist air rises to a high altitude. Winds at the surface converge to replace the rising air (the trade winds). The region of convergence is called the doldrums or the ITCZ. The rising air loses all its water as precipitation, then moves in a northwesterly direction to mid latitudes where it descends back to the surface as dry, cold (relatively) air. The area of descending air concentrates into the mid-ocean high regions. Image credit: David Jessey, Cal Poly, Pomona.

Trade winds (T). Figure 2 shows the equatorial Hadley Cell (marked by HC) which is made up of the Intertropical Convergence Zone (ITCZ), the northwest moving “westerlies” at high altitude, the descending dry air at mid latitudes, and the trade winds. The trade winds are steady winds, about 15-20 knots, that are directed to the equator. In the northern hemisphere they are directed to the south and in the southern hemisphere they are directed to the north. Due to Coriolis force (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere), the trades bend to the west in both hemispheres. The resulting wind fields (by convention winds are described by the direction they come from) are the NE trades above the equator and the SE trades below the equator. Or simply the Easterlies.

Intertropical Convergence Zone (ITCZ). The popular name for the ITCZ is the doldrums. The doldrums is a region near the equator where the north and south trade winds converge. As the Northeast and the Southeast trade winds flow toward each other they take up water by evaporation and become saturated with water. They warm in the intense tropical sun. The name doldrums means low spirits, a feeling of boredom or depression which is well suited to these conditions.. Clouds in this region reflect the high humidity and the sudden release of energy that accompanies the rainfall.

When the winds meet (converge) they rise up into the atmosphere and lose their water by precipitation. The doldrums is the narrow band of towering clouds, squalls, and high humidity that is formed from the convergence. Because of differences in ocean coverage between the two hemispheres, the trade winds do not converge exactly at the equator, but usually they between two and seven degrees north approximately. In Figure 1, the doldrum band is clearly identified by the high precipitation region around +5°N. The trade winds and the doldrums were discussed in a previous report.

Mid-Pacific High (H). Coriolis forces concentrate the dry, descending air of the Hadley Cell into mid-ocean high pressure areas. In the Pacific Ocean this is the Mid-Pacific High. Figure 1 shows the Mid-Pacific High centered around 30-40°N latitude. The center of the high and its central pressure vary in time according to season and in response to other processes such as the Jet Stream to the north.

As a rule, air flows from high pressure toward low pressure and we expect to see winds flowing away from the high. However, Coriolis force pushes the flow to the right, so as the winds blow away from the high they are turned to the right. As a result, the winds around the high blow in a clockwise direction (see the green wind barbs) around the high cell.

Note; the same thing happens in reverse for a hurricane. Air that is heated by a warm ocean forms a low pressure center. As air flows toward the low it is turned to the right and begins a counter-clockwise vortex. As the vortex becomes stronger, winds increase and the storm intensifies to become a hurricane.

North Pacific Gyre, the Garbage patch. Around the Mid-Pacific High, the clockwise winds blow across thousands of miles of ocean. There is a small amount of friction between the air and the water so, by friction, the winds try to push the ocean surface in the same circle. However, water flow has the same Coriolis force to the right that air does, so the resulting ocean currents, on the surface, are toward the high center (there is no shame in doodling these directions on a piece of scrap paper).

Floating objects, pollution, garbage, and plastics are carried with the resulting ocean currents, and over time they spiral into the high. The garbage is captured in the gyre and remains there for years. The famous “garbage patch” has attracted considerable attention lately. At last accounting the garbage patch covered an area roughly the size of the United States. In Figure 1 the North Pacific Gyre roughly occupies the area shown by the dashed line.

Coastal winds and the Baja Bash. Finally, we come to the important coastal upwelling region on the west coast of the U.S.A. and Mexico. As I write this report Ocean Watch is hiding in a little bay called Bahia San Juanico on the west coast of the Baja peninsula.

You can see from the isobars and the wind barbs in Figure 1 that for the entire trip up the coast Ocean Watch has sailed against winds of 20 to 30 kts. These winds have been blowing across at least 500 miles of open ocean and over such a long distance a large ocean swell can develop. They have and we can vouch for it.

This route up the Baja coast is well known to sailors, and is avoided as a matter of course. The ride is notoriously known as “The Baja Bash.” It’s fun to sail down to Mexico; not so much fun coming home.

Last night the isobars tightened more and the head winds increased to above 30 knots. Ocean Watch was riding over the waves with ease, but for the crew the ride was not comfortable. Our mainsail was pulled down into its third and final reef, and as conditions worsened, we decided to hide in this little bay, at least until tomorrow.

Along with the winds, waves and the bashing, it has become cold. For the first time in months we are wearing our polar fleece jackets on deck, at least at night. The air temperature has dropped from 30°C (86°F) to 14°C (57°F), and in the wind over the ocean, that is bone chilling.

Coastal Upwelling (U) and Sea-Surface Temperature

The reason the air is cold is that the ocean is cold. We are in the regime of coastal upwelling. The upwelling area, see ‘U’ on the chart, extends up the coast of California and is one of several upwelling regions around the world. My previous report on upwelling and El Niño talked about upwelling around the world. Figure 3 tells the story for the California coast. Since leaving Puerto Vallarta the sea temperature has dropped from about 25°C (77°F) to 15°C (59°F) and most of that drop occurred in just the last few hundred miles, from Cabo San Lucas to our current anchorage in Bahia San Juanico.

Figure 3. SST on the coast. A map of the sea surface temperature (SST) along the coast of Baja. The anchorage at San Juanico is shown marked with “OW” and the Cabo San Lucas at the end of the Baja peninsula. The temperature decrease along the Baja western shore is a result of coastal upwelling, a response to the strong northerly coastal winds. Click on the image for a larger view. Image credit: buoyweather.com.

Coastal upwelling is brought about by north winds and Coriolis force. As discussed above, wherever the wind blows, there is a tendency for the resulting current to flow to the right of the wind. (Even icebergs flow about 20° to the right of the wind.) The wind blows along the coast and the surface water moves offshore. Something has to replace the water leaving the coast and that is water from below; cold fertile water from below the thermocline. The thermocline was introduced in science report 24 April 2010).

Finally…

The world and its workings cannot be taken in pieces. Everything in Nature, the winds, currents, animals, chemicals, and even the motion of the stars, play a role in how this grand machine plays its song. In this simple description I wanted to show how the cold water on our coast, the coastal winds, the oceanic pressure fields, the trade winds, and the Sun’s heat in the tropics (heat from the Sun drives the whole engine) all relate to each other. That is the lesson we must remember. Tinker with any tiny piece of the whole at a risk to us all.

Remember: all views, ideas, and comments here are ad hoc, off the cuff, minimally researched, and subject to revision at any moment.

Michael Reynolds, Ph.D., RMR Company

In Costa Rica Ocean Watch took on two new instruments: a second thermosalinograph for underway measurements of surface water temperature and salinity, and a profile instrument called a CTD for conductivity-temperature-depth. The CTD has a high-precision pH sensor. The CTD is lowered by a rope so a profile of the upper ocean structure can be obtained. The profile is necessarily shallow, but still reveals interesting near-surface structure including solar heating layers, mixed layers, and a strong thermocline. We will use the profiles to demonstrate many important concepts in physical oceanography.

Layers in the ocean

The waters of the oceans are layered and the structure of the layering tells oceanographers a great deal about the ocean currents, the origins of the water and its fate. To discuss ocean layers one needs to know about density. Density of a substance is the weight of a set volume of that substance. For example, a cup of mercury weighs much more than a cup of water because the density of mercury is about 14 times the density of water. Oil floats on water because oil density is slightly less than water.

In the ocean, the water density is determined, primarily, by two factors: the temperature and the salinity. As the temperature is increased the water becomes lighter. Salinity is the amount of salt in a volume of water. Oceanographers measure salinity in parts per thousand and use the symbol “°/_°°“. The average salinity in the ocean is about 34°/_°°, meaning 3.4% by volume. The saltier water, e.g. higher salinity water, is heavier. That is, it has a greater density. Density is a measure of weight for a specific volume. For comparison, the salinity of water in the Persian Gulf can reach more than 40°/_°°, while the salinity in a river-fed bay might be a brackish 25°/_°°.

Physical Oceanography pays particular attention to density and to the layers that form in the sea. The interface between layers is important because they form barriers to exchange between layers. There are thousands of examples and we certainly will not be covering them in detail here. I want to discuss instead the structure of the water we are currently transiting, where coastal upwelling breaks down the layers and brings nutrients to the surface thereby fueling the high productivity that supports a fishing industry.

The ocean surface is a region where layers are complex and variable. This is where the ocean and atmosphere exchange heat, moisture (rain and evaporation), and gases (approximately one-third of all CO₂ emitted into the atmosphere by humans is absorbed into the ocean through its surface). Figure 2 shows that the top of the ocean in our current location off the Mexican coast is a warm layer over a deeper layer of cold water. Our cast does not go deep enough to register deeper layers, but the very strong thermocline is apparent.

Life in the ocean depends on the plants. In a pyramidal fashion, called the food chain, the plants (algae) come first and are followed by plant eaters such as shrimp, krill, shellfish, and tiny fishes. Then, eating these small creatures are larger fish, squid, and the other carnivores. Finally come the large fish, the tuna, swordfish, dolphin, and porpoises. The food chain depends on the plants, and the plants grow where there is sunlight and nutrients. Productivity is a measure of the rate of creation of carbon in the food chain. Satellites can measure ocean color and determine the amount of chlorophyll, and from that estimate the productivity in different regions. Murkiness in the water is related to the concentration of algae.

Plants grow in the ocean layers where there is ample sunlight, that is, near the surface. The euphotic zone is the top layer of water where there is enough sunlight for photosynthesis. The rest of the food chain follows and life in the upper ocean is good until the nutrients run out. Nutrients for ocean plants are not much different than nutrients needed in your garden. Plants need nitrogen, phosphorous, iron, and other ingredients. If any one of these runs out, productivity comes to a halt. As the food chain prospers there is a slow sinking of nutrients to the bottom. Fish and plants die and animals excrete waste, creating a steady snow of particulates, falling downward, and importantly, through the thermocline barrier. Indeed, the falling debris is called “marine snow” and it is reported by deep divers in all the world’s oceans.

Figure 3. Wikipedia describes marine snow as follows: “In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone. Consequently, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic activity and ocean currents.” Image credit: Planktos Science

When the snow falls through the density barrier, in this case the thermocline, it is removed from the surface water and cannot provide nutrients for more algae production. The thermocline is an interface that bars mixing of water. When the nutrients run out the whole food chain is diminished, the water clears and turns blue to the observer, and the euphotic zone becomes much deeper. Productivity will increase only when more nutrients are injected into the euphotic zone, and a major way this happens is by upwelling.

We have described upwelling in previous reports. From the point of view of oceanography, upwelling is caused when the mixed layer is pushed offshore by the winds (along with gyroscopic forces called Coriolis forces) and deeper water moves shoreward lifting the thermocline up to the surface where it can be broken by wind and wave turbulence. With the thermocline broken deeper water can mix to the surface.

Measuring the ocean from Ocean Watch

This spring, Ocean Watch was sailing out of an El Niño region and up the west coast of the U.S., one of the world’s productive upwelling regions. For an ersatz oceanographer like me, it is just too frustrating to be sailing over such fascinating water without trying to look under the surface. Therefore, while I was back in Seattle in April I contacted my friends at Sea-Bird Electronics, Inc. and without hesitation they agreed to loan us two instruments. Sea-Bird is one of the most respected names in oceanographic instrumentation and we are deeply thankful for their support.

First they provided us with an instrument called the thermosalinograph. A thermosalinograph is a device that sucks seawater through a fitting below the water line in the boat’s hull, through a special sensor, and back out to sea. It continuously measures the temperature and salinity of the water as the ship travels. Other sensors, such as oxygen, pH, and opacity can be included. Our SeaKeeper thermosalinograph, described in many of our reports, has operated since we left Seattle. After such a long time we had some concerns about calibration drift, i.e. the accuracy of our measurements. The Sea-Bird thermosalinograph was connected into the same pumping system as the SeaKeeper sensors and it provides a check on any error from the SeaKeeper system.

And I am happy to say that the two key measurements, salinity and temperature, agree nearly perfectly on the two instruments. We are pleased and reassured by this result.

CTD profiles from Ocean Watch

Ocean sub-surface density profiles are measured by an instruments called a CTD. The name stands for conductivity-temperature-depth, the primary sensors. The CTD is the heart of any oceanographic research cruise. Other optional sensors for pH, oxygen, and other chemicals are often included with the package The Ocean Watch probe has a pH sensor (see Figure 2) so it is possible see differences to pH through the surface layers. Figure 1 shows the SeaCat being deployed and Figure 2 shows the profile of salinity, temperature, and pH for a typical cast. A discussion of the CTD program on Ocean Watch is a topic for a future report.

Michael Reynolds, Ph.D., RMR Company

Volcanic ash from Mt. Eyjafjallajökull in Iceland is disrupting air travel. Could it also disrupt the climate? “Ahh,” say the skeptics, “that should solve the global warming issue for awhile.” Any excuse for business as usual. But the fact is that this eruption, and the aerosols they disperse into the atmosphere, are small by comparison to previous events. Any global dimming, and associated temperature decrease, they produce is a short-term adjustment to the continuously increasing global temperature. Aerosols do, in fact, counteract the warming from increased greenhouse gasses, but they are short lived and they can never completely stop the warming process. We review aerosols, global dimming, and nuclear winter. The measurement program on Ocean Watch is discussed.

What is an Aerosol?

Very simply, an aerosol in the atmosphere is any particle small enough to be kept aloft by air currents. Aerosols include water droplets (clouds are aerosols), ash, dust, salt crystals from ocean spray, and smoke of any kind. The smoke from the burning of tropical rain forests is a major source of aerosols as is automobile exhaust. Aerosols largely come from man-made sources from many countries and also some natural sources. African dust and its impact on climate and the health of the West Indies (including corals) is of growing concern.

In general, aerosols are continuously falling to back to Earth. How fast they fall depends on their size, shape, and density. Nevertheless, when compared to other processes, such as increasing CO₂ or orbit changes, their impact on climate, weather, or human activities is short-term (days to months) and is generally localized near major sources rather than being spread globally.

Some major volcanic aerosol emissions, such as Mt. Pinatubo, are injected high into the stratosphere and are carried around the world. But this is not the norm. The effect of aerosols on climate change is highly uncertain compared to what we know about the effect of “greenhouse gasses”. The ability of global climate computer models to describe or predict climate change is limited by our ability to define aerosol effects.

Mt. Eyjafjallajökull, Iceland: today’s volcano

On about April 14, Mt. Eyjafjallajökull, Iceland, began a major eruption. The volcano had been venting several weeks prior to the eruption. Extreme Icelandic cold caused the hot emission to cool into a gritty ash plume which was carried by high winds right over Europe (see figure below). The eruption is far from finished, and at this writing has kept much of Europe land-bound. New mini-eruptions raise concerns about longer-term damage to world air travel and trade.

As volcanic eruptions go, Mt. Eyjafjallajökull is not a big event. It pales in comparison to past climate-cooling eruptions. Eyjafjallajökull’s ash has reached a height of 55,000 feet, according to press reports. By contrast, ash from the 1991 Mt. Pinatubo eruption, one of the biggest in the 20th century, reached 78,740 feet.

Overall, Pinatubo, which is in the Philippines, ejected 14 to 26 million metric tons of sulfur dioxide that produced a significant global cooling effect for a few years. Following the eruption, this temporary cooling also slowed sea level rise rates temporarily. Nevertheless, in an April 18 news article (Headland SatNews) it was reported “Modern Europe has never seen such a travel disruption. Air space across a swath from Britain to Ukraine was closed and set to stay that way until Sunday or Monday in some countries, affecting airports from New Zealand to San Francisco. Millions of passengers have had plans foiled or delayed. Activity in the volcano at the heart of this increased early Saturday, and showed no sign of abating. ‘There doesn’t seem to be an end in sight,’ Icelandic geologist Magnus Tumi Gudmundsson said on Saturday.”

Climate Change, Global Dimming and Nuclear Winter

The setting sun in Los Angeles turns different shades of orange because the aerosols, from traffic mainly, filter incoming sunlight. Refer to Figure 2 and imagine the atmosphere as a dirty window. The solar radiation that strikes the Earth’s atmosphere (Referred to as the “top of the atmosphere radiation,” or TOA.) is measured by satellites and solar observatories and is well known.

As the radiation penetrates the atmosphere it interacts with air molecules, ozone, and aerosols. Some of the radiation passes through without interaction; this is the sometimes orange sun you see in a clear sky. Some of the sunlight interacts with air molecules and is scattered in different directions. (Think of light as a stream of individual particles, called photons, and think of collisions between the photons and aerosol and air molecules as a game of billiards.) Some of the scattered photons are sent back to space. Therefore, aerosols reduce the heating effect of solar radiation and, in a small way, help to cool our climate and offset the warming effects of greenhouse gasses (CO₂, methane, ozone).

On the other hand, aerosols can add heat to the atmosphere which partially offsets the cooling effect. As the Earth heats up from the sun, it radiates heat back to space. Aerosols absorb some of the heat radiation and reduce the amount of heat radiation escaping out to space. This is the same heat-blocking effect attributed to greenhouse gasses, and in this way aerosols can have a heating effect on global climate.

Nevertheless, the net effect of aerosols is to reduce the rate of global warming from greenhouse gasses. Does this mean we should all go build fires and drive our cars? No, because the offset that aerosols make on all of all these activities is smaller than the impact those activities make on global warming. Models and data now show that aerosols reduce the increase in global temperature by a factor of approximately 50% (there is uncertainty in the actual amount). So, they slow down the process but do not stop it. And they create pollution and effect health at the same time.

Global dimming is a term used for the reduction of sunlight as it passes through the atmosphere. When aerosols are concentrated the sunlight is reduced, heat reaching the surface is reduced, and global warming is reduced.

The TOA (top of the atmosphere) radiation (Figure 2) is scattered and absorbed during its passage through the atmosphere. The fraction of sunlight reaching the Earth’s surface depends on two things: the aerosol loading and the angle of the sun. As the sun nears the horizon the beam cuts through more and more atmosphere thickness and the impact of the aerosol increases. This explains why the setting sun turns different colors in a smoggy or hazy sky.

A numerical measure of the aerosol loading factor is the “optical depth.” Optical depth is a logarithmic measure like the Richter scale for earthquake intensity and pH for acidity. The total optical depth is related to the total reduction in sunlight reaching the Earth’s surface. When the sun altitude is 60° above the horizon and the optical depth is 0.2, roughly 82% of incoming radiation reaches the ground. Alternately, when the optical depth is 1.0, as under thin cloud, only 13% reaches the ground.

The total optical depth is the sum of effects from ozone, air molecules, and aerosols. When all other contributions are removed from the measure the resulting number is the “aerosol optical depth,” which is the dominant term for solar radiation. As aerosol optical depth is increased due to particles from anthropogenic sources, the atmosphere becomes less clear. This is easily observed by eye as a haziness in the atmosphere. The sky looks whiter rather than sky-blue, and visibility is reduced, i.e. distant objects can no longer be seen.

Excessive aerosol optical depths are found around and downwind from major urban areas. For example the Ohio River Valley, the northeast U.S. coast, Los Angeles, Phoenix, Mexico City, and Beijing are regions with high optical depths over five-hundred to one-thousand mile distances. These plumes of aerosol are released near ground level where they are often effected by local weather. For instance, aerosol concentrations at low levels and are drastically reduced from rainfall (hence the clear skies following a rain storm).

Figure 5 is a graph of optical depth in recent history showing volcano events in 1982 and in 1991. The increase in optical depth is apparent and the graph shows that the effects of major events last only a couple of years. Of interest is the fact that the optical depth since 1981 and especially after 1991 has been dropping by a considerable amount. The decrease is attributed to increased clean air legislation in North America and Europe. Reducing aerosol optical depths means increased global warming.

Will Mt. Eyjafjallajökull significantly reduce global warming?

No.

The Union of Concerned Scientists recently published an opinion on how the Iceland volcano might affect climate change. The headline was “Iceland Volcano Eruption Too Small to Have Significant Climate Effect, Science Group Says.”

“Even if a volcanic eruption were big enough to temporarily cool the planet, heat-trapping carbon dioxide from burning fossil fuels and destroying rain forests (burning and decomposition) would still pose a significant threat, says UCS climate scientist Brenda Ekwurzel. ‘Unlike volcanic ash that will leave the atmosphere within a few months or years, carbon dioxide remains there for decades and even centuries,’ Ekwurzel said. ‘Overloading the atmosphere with carbon dioxide has put us on the path toward a long-term warming trend, so we really can’t pin our hopes on occasional volcanic eruptions to solve the problem.’

“The short-term cooling effects of the Mt. Pinatubo eruption are long gone, and global warming is continuing unabated, she said. ‘In fact, we just experienced the hottest decade on record.’”

Aerosol Measurements from Ocean Watch

The Around the Americas voyage is collaborating with the Joint Institute for the Study of the Atmosphere and Ocean of the University of Washington to support a NASA project to measure the sun’s brightness at sea level and to record the aerosol optical depth along the coast of the Americas. These data complement ongoing observations from a network of continental and island stations. The observation network is called Aeronet. All of these measurements together combine to provide validation points for long-term NASA satellite measurements of optical depth on the worldwide scale.

NASA is using satellites to measure optical depth by a different method: by measuring the radiation reflected back to space. NASA operates a number of satellites to take images of the Earth, and then use the images to estimate the brightness of the Earth-atmosphere. This brightness, as measured from space, is related to the optical depth that a sun photometer measures directly at the surface.

The particular value of the sun photometry measurements from the Ocean Watch AtA voyage is that they are made near the coast where satellite measurements area difficult and where validation of the satellite data is particularly important.

Michael Reynolds, Ph.D., RMR Company