365 Days of Climate Awareness 20 - Greenhouse Earth

 

365 Days of Climate Awareness

20 - Greenhouse Earth

Greenhouse Earth is a climatic condition which has existed periodically throughout the planet's history, in which no continental (as opposed to alpine) glaciers exist. It is not to be confused with the intervening period between two glaciations. Greenhouse (and the opposite, icehouse) conditions persist for several million years and are due to longer-term causes than Milankovitch cycles.

Changes in carbon dioxide concentration are one component, but earth's atmosphere has changed tremendously since the planet's early existence (a future post). The sun's output--known as the solar constant--has not been constant over the last five billion years, but has increased by nearly 50% from its early level. And atmospheric carbon dioxide level itself is controlled by other factors, one in particular.

The major influence on carbon dioxide concentration over the earth's history is tectonics: the breakup, movement and collision of earth plates. The weathering of rock consumes carbon dioxide, and sequesters it in seafloor sediment. So when conditions arise exposing large amounts of bedrock to the environment, a drawdown of CO₂ occurs. When less weathering happens, carbon dioxide does not act to decompose rock as much, and atmospheric concentrations can rise.

There are a few ways this can happen. At times the earth is more tectonically active than at other times: plates are being generated and destroyed at faster rates, and mountain-building is more pervasive. The arrangements of continents matters too. Smaller continents in the tropical zones will be exposed to a great deal of weathering, being subjected to large amounts of rain. When tectonic activity is lower, or continents around the planet are in colder areas, or are larger and more arid, less CO₂ is consumed by rock weathering, and volcanic outgassing can continue to warm the planet.

Earth has been in greenhouse conditions for roughly 85% of its existence.

Tomorrow: icehouse earth.

Be well!

365 Days of Climate Awareness 19 - Milankovitch Cycles 4: Climatic Significance

 

365 Days of Climate Awareness

19 - Milankovitch Cycles 4: Climatic Significance

Studying Milankovitch cycles leads to the conclusion that changes in the earth's orbit can have significant effects on overall climate, but these are far from the only, or strongest, inflences. Over the past 800,000 years, northern hemisphere glaciations have occured almost with the regularity of tides, but that pattern does not hold further back in the geological record (though large glaciations certainly happened).

As we travel back in geological time, the records become sparser and our dating methods less precise. But the evidence for these regular climatic fluctuations doesn't exist from earlier times. Other conditions have recently set earth on a balancing point where the Milankovitch cycles have outsized effects on regional and global climate.

One idea centers on the formation around 3 million years ago of the isthmus of Panama, connecting North and South America with a complete land bridge where formerly were islands. The newly complete barrier of Central America cut off tropical circulation between the Atlantic and Pacific Oceans. (Ocean circulation will be the subject of many future posts.) This closure limited the transfer of heat around the equator and allowed greater temperature differences to build not only east to west, but also north to south.

A basic function of weather and ocean currents is to transfer heat from the equator to the poles. (Hurricanes play a measurable role in this process.) A barrier to that process like Central America makes the entire planet's poleward heat transfer system less efficient, less robust, and more subject to smaller influences like Milankovitch cycles.

At a moment like this it might seem like the floor drops out from climate science, that no conclusions or results can be trusted, if processes don't seem to operate effectively at all times. Is uniformitarianism valid? The answer, as far as we've yet seen, is yes. Uniformitarianism--the idea that the processes at work now have always been at work--has an important caveat: the conditions must exist for those processes to occur. Ocean tides could not occur before the ocean existed. Ozone and how it shields the earth's surface from UV rays were unlikely to have happened before oxygen existed in the atmosphere. And there are many more examples. Too narrow a focus of inquiry can cut off very relevant information.

These cycles which Milutin Milankovitch identified have operated for essentially all of earth's history (leaving aside the formation of the Solar System). But it is only occasionally that they have come to play a major role in earth's climate. Continental formations, and thereby sea and air circulation, have buried the cycles' effect before. It is very possible that rising the rising CO₂ concentration in the atmosphere will overwhelm them now, for at least a few thousand years.

Tomorrow's post: greenhouse earth.

Be well!

365 Days of Climate Awareness 18 - Milankovitch Cycles 3: Rotational Variations

 

365 Days of Climate Awareness

18 - Milankovitch  Cycles 3: Rotational Variations

The earth rotates on its axis, with slowly decreasing velocity. Earth's days were significantly shorter a few billion years ago, but the gradual effect of the moon's gravity has slowed the earth's spin rate. Eventually, so long as the solar system exists, earth's face will become locked to the moon, as the moon's face is locked to the earth (always showing the same face, with the same craters, to us).

There are two major types of regular variation in the earth's axis: the angle with respect to the ecliptic, and the orientation with respect to the (so to speak) fixed stars.

The orbit of the earth forms a plane through the sun known as the ecliptic. The earth currently rotates on an axis which is 23.5° away from perpendicular to that plane 63.5° away from the plane itself). However, that angle is not constant, but oscillates between 22.1° and 24.5°. (The angle is now decreasing with respect to perpendicular, and will reach its minimum inclination in about 11.8K years.) This decrease in inclination will lead to a smaller difference between summer and winter, and likely an overall cooling effect.

    

Rotational Milankovitch variations.

The other change is the gradual rotation of the axis itself on which the earth spins. It backs around in a circle with respect to the surrounding stars, clockwise when viewed from above the north pole, with a period of roughly 25.8K years. (Anyone who has spun a top and watched the orientation of the handle rotate slowly as the top spun rapidly can picture this motion perfectly.) After a few thousand years the earth's axis will no longer point at Polaris, and there will be no north star. The rotation of the axis itself changes where in earth's orbit the seasons occur, which changes not only which constellations are visible at night but also when during the year the earth is at perigee--closest to the sun, absorbing more solar energy--and apogee, furthest from the sun and absorbing less. (Currently perigee occurs on January 3, apogee on July 4.)

Tomorrow: Milankovitch cycles 4: climatic significance.

Be well!

365 Days of Climate Awareness 17 - Milankovitch Cycles 2: Orbital Variations

 

365 Days of Climate Awareness

17 - Milankovitch Cycles 2: Orbital Variations

The earth orbits the sun in an ellipse which is subject to three known types of variation, two discovered by Milankovitch, one more recently. These oscillations are in the eccentricity--the flatness--of the orbital ellipse, the precession or rotation of the ellipse, and the tilt of the orbital ecliptic against the invariable plane, i.e. the central plane of the Milky Way Galaxy.

Earth's orbit is very nearly circular. Eccentricity (e) is a measure of the semimajor (long) axis against the semiminor (short) axis, with 0 signifying a circle and 1 a straight line. Earth's orbit bounces between an almost perfectly circular value of 0.000055, and a slightly more stretched value of 0.0679. Right now, earth's orbital eccentricity is 0.017 and decreasing. This beat in eccentricity is caused primarily by the gravitational influence of Jupiter and Saturn. There are several different components in this variation, each with different periods (as long as 413,000 years), but they combine to an overall period of roughly 100,000 years.

When the orbit is less eccentric--more circular--there is less change in the amount of energy reaching earth (the solar constant) throughout the year, due to the earth's more constant distance from the sun. At this eccentricity perihelion (when the earth is closest to the sun) results in 6.8% more insolation energy than aphelion (when the earth is farthest away). Currently perihelion occurs on January 3 and aphelion on July 4. When the orbit is more eccentric perihelion results in 23% more solar energy reaching the planet than aphelion.

Rhythmic variations in Earth's orbit.

The final effect has only recently been discovered and is not well quantified. The Milky Way Galaxy has a central plane on which star systems tend to orbit--roughly the same as Jupiter's orbital plane around the sun--and the plane of earth's orbit wobbles slightly with respect to that. Its cause has not been identified but right now the angle of inclination is 1.57 degrees.The ellipse itself rotates around the solar focus once every 112,000 years, known as apsidal precession. Like eccentricity, this is caused by the gravitation of Jupiter and Saturn. The combination of this with another cycle, the rotation of the earth's axis (axial precession), causes the seasons to very slowly advance through the calendar, starting slightly earlier every year, completing a cycle every 23,000 years. This combined effect is known as the precession of the equinoxes, and is the reason why constellations visible in certain seasons now are not the same as they were several thousand years ago.

Tomorrow: Milankovitch cycles 3: rotational effects.

Be well!

365 Days of Climate Awareness 16 - Milankovitch Cycles 1: Introduction

365 Days of Climate Awareness

16 - Milankovitch Cycles 1: Introduction

The orbit and rotation of the earth are not static, but themselves pulse with various rhythms, and these rhythms can have large effects on our climate. The 100,000 year rhythm for glaciations which has held over the last several million years is correlated very closely to these slow oscillations. It is difficult to say whether these orbital factors have had effects of similar or greater magnitude in eons past, and it is not known why they do now.

The cycles are named after Milutin Milankovitch, a Serbian astronomer and geophysicist who worked out his theory for these variations in the 1920's. One portion of his work focused on regular changes in the earth's rotation, the other on changes in its orbit around the sun.

Basic aspects of Earth's rotation and orbit.

The earth rotates around a theoretical axis (which does not physically exist but explains the rotation mathematically). In defining any angle the axis takes, we must define what the axis is being measured against: what is the other side of the angle. In this case, the other side is the plane, or ecliptic, in which the earth orbits around the sun. Furthermore, we measure the attitude--the axis' orientation in space--as compared to the stars farther away in the galaxy, which we can consider fixed (as their extreme distance makes their motion too small for us to observe).

The earth's orbit around the sun is not a circle, but rather an ellipse. An ellipse is a curved shape with two foci (when the two foci are the same, the result is a circle). Two dimensions can characterize any ellipse: the distance between foci and the semiminor axis (shorter radius: see the illustration). A ratio between the semimajor and semiminor axes is called the ellipse's eccentricity, a very useful measure.

It's worth pointing out, before we move into more exotic effects, the very basics: our seasons are a function of the tilt of the earth's axis as the planet moves through its orbit. When one hemisphere--the half of the globe closest to one pole--is tilted toward the sun, it receives light and heat more directly, and therefore warms, causing summer. At the same time, the other hemisphere, closest to the opposite pole, is angled away and cools, resulting in winter. Six months later the situations are reversed.

However, as in all physics, reality is more complicated. Tomorrow we dive into some of the complications.

Tomorrow: Milankovitch cycles 2: orbital variations.

Be well!

365 Days of Climate Awareness 15 - Paleo Sea Level

365 Days of Climate Awareness 

15 - Paleo Sea Level

Reconstructing ancient sea levels is an extremely complicated task. The fundamental record is d¹⁸O. d¹⁸O is a very complex record, and it includes many components including, most importantly, global air temperature. But it also includes a signal from global sea level, which can be subtracted out by means of other proxies.

One of the most complicated aspects of paleoclimatology is untangling different components within a time series of data. Researchers examine the record for correspondence to other proxy records, such as geological deposits for ice extent. If a correspondence is found, the magnitude is identified and it can be subtracted. In this way different component signals are removed from the overall d¹⁸O signal. When sea level is the target value, d¹⁸O from marine sediment is used. The ocean temperature component is removed by another proxy: the variation by temperature of the ratio of magnesium (Mg) to calcium (Ca) in deposited calcite. The remaining signal corresponds to sea level.

It is calibrated against more sporadic data, such as benchmarks on ancient coral reefs, and modeling of ancient ice sheet behavior. The sea level curve is one of the trickiest proxy records we have, and only a few comprehensive examples exist. Much of the data used is not publicly available, being the geophysical and wellbore records obtained by oil companies during their global exploration. For that reason the best known paleo sea-level record is the Exxon Sea Level Curve.

Exxon-Hallam sea level curve.

Curves tracing sea level change within the last million years show a striking rhythm of roughly 100,000 years, corresponding to successive glaciations. Sea level falls as glaciers grow by taking up ocean water. As the glaciers melt back, they release that water back into the ocean. We are currently nearly 20,000 years into an interglacial period. This climatic rhythm is real, and though the causes for it are not fully understood, a main source is the variation in Earth's rotation and orbit around the sun: Milankovitch cycles.

Tomorrow: Milankovitch cycles.

Be well!

365 Days of Climate Awareness 14 - Relative Sea Level

 

365 Days of Climate Awareness

14 - Relative Sea Level

Relative sea level is the elevation of water compared to a datum (established baseline). Historically the datum would be local, a function of the natural tide range at any given location. Averaged over a long period of time--at the bare minimum, one lunar cycle of new moon to new moon, but ideally more than the 19-year metonic cycle, after which the moon's phases repeat on the same days of the year (important because the earth’s orbit around the sun is elliptical, and therefore the sun’s gravitational effect on tides is not uniform throughout the year).

From this data mean high and low tides can be computed and correlated to a benchmark, a nearby fixed object whose elevation is known geodetically. In this way local water level measurements can be correlated to others around the world. (The National Geodetic survey keeps a database of thousands of tidal benchmarks and geological monuments around the country as control points, so local work can be tied into the international geodetic system.)

It is local tidal gauge measurements, typically every six minutes, which provide NOAA, the National Oceanographic and Atmospheric Administration, with the high-resolution data needed for safety-of-navigation and research purposes. Satellite altimetry provides global extent of observations not possible with coastal instruments, but a typical spatial resolution is 0.25 degrees, or 15 nautical miles horizontally, and slightly over 1 cm vertically.

Vertically the accuracy of satellite altimetry is comparable in to tidal gauges. But local tidal stations are an important quality check on the satellite data and also ensure data at important coastal locations for shipping and research. Furthermore, tidal gauge data extends back as far as 1700 (Amsterdam), and where the data quality can be reliably estimated, forms an important data set for tracking sea level rise.

Tomorrow: estimating paleo sea levels.

Be well!

365 Days of Climate Awareness 13 - Eustatic Sea Level

 

365 Days of Climate Awareness

13 - Eustatic Sea Level

Eustatic sea level is the average height of the ocean as calculated from the center of the earth. This isn't something we can measure directly so it requires some geodetic modeling. (Geodesy is defined as the study of Earth's shape, gravitational field and orientation in space, and it relies heavily on satellite data.)

To a first order of approximation, the earth is a sphere, with an equatorial radius of 6378 km. The spherical model is generally used for modeling in fluid dynamics, since that shape makes the mathematics far simpler. To a second order of approximation, the rotating planet is an oblate spheroid, bulging slightly at the equator and slightly flattened at the poles, with an equatorial radius of 6378 km and a polar radius of  6357 km, a difference of 21 km. This shape is used for highly precise global mapping. To a third order of approximation, the planet is very slightly egg-shaped, with a small bulge in the southern hemisphere and a very slightly flattened northern hemisphere, on the order of a few tens of meters. This difference is so tiny that it is not used for practical applications, because it can be accounted for by other means.

DIfference between a sphere and an oblate spheroid.

Using the oblate spheroid model, satellite altimetry measurements taken of the world ocean are processed into a time-invariant model. And this must be modeled, because conditions in reality are constantly dynamic. Tides, from the sun, moon and even planets, are active not only on the ocean but on the rocky planet itself. Weather has a number of effects, between the wind building waves, and causing larger-scale pileups of water like storm surges, and via pressure differences. High air pressure pushes the surface of the ocean down, and low air pressure allows the sea surface to rise. Furthermore subsea features like mid-ocean ridges have a gravitational pull which lead to standing mounds of water overhead.

Eustatic sea level calculations average all of those effects out. In addition to satellites, a set of tidal gauges around the world are used to calibrate the satellite measurements. One such station is on Tahiti, an island in the Pacific far from the Kermadec Trench near New Zealand and the hotspot beneath Hawaii. It is assumed this area is tectonically stable enough to provide reliable sea level records. This and similar locations are used also to reconstruct paleo sea levels, for correlation with other climatic markers.

Tomorrow: relative sea level.

Be well!

365 Days of Climate Awareness 12 - Ocean Acidification

365 Days of Climate Awareness

12 - Ocean Acidification

As carbon dioxide continues to enter the atmosphere from combustion of fossil fuels, roughly 30% is absorbed by the ocean. CO₂ in the ocean does not have the insulating effect it does in the atmosphere--water is quite efficient enough itself in retaining heat--but it does affect the chemistry.

pH means "power of Hydrogen", and measures alkalinity versus acidity. Neutral pH is 7. Alkalines are higher on the scale (typically between 7 and 14, but very strong alkalines are higher). Acids typically measure between 0 and 7, but extremely strong acids have negative pH. The global mean for pH of the ocean was 8.2 prior to the Industrial Revolution, and is now 8.1. The ocean is becoming more acidic, because of the added CO₂. When carbon dioxide goes into solution in water, the reaction forming carbonic acid takes place:

    H₂O + CO₂ <--> H + HCO₃-

Only a low percentage of dissolved CO₂ becomes this acid, but it's been enough to make the global ocean more acidic, with widespread effects. Many animals and microscopic plankton produce their own shells from minerals absorbed from seawater. But in the increasing acidity--the dropping pH--of the ocean, those shells are dissolving again, leaving a growing number of sea creatures without their ordinary form of protection. The lowering pH also impacts coral reefs: along with rising temperatures it causes the corals to lose their color ("bleach") and then die.

Mean oceanic pH over the past 25 million years

Tomorrow: eustatic sea level.

Be well!

365 Days of Climate Awareness 11 - The Paleocarbon Record

365 Days of Climate Awareness

11 - The Paleocarbon Record

Tracking the atmospheric concentration of CO₂ through the earth's existence is nearly as important as tracking global mean temperature, in order to reconstruct overall conditions and processes. The discipline of geology is founded on the idea of uniformitarianism: that the processes active today are the same ones active throughout the earth's past. So we look for evidence of carbon dioxide's role in earth's changing climate of past millennia in order to better estimate what will happen in the future.

The paleocarbon record.

Paleo-carbon dioxide data can be found directly for the last 800,000 years in the air bubbles trapped in Antarctic ice cores, and via proxy data. The main source for paleo-CO₂ proxy data is through the ocean floor drilling program, where cores of unconsolidated (i.e. still loose, not lithified) ocean sediment are drilled, recovered and analyzed. Data from the last 65 million years--corresponding to the end of the Cretaceous, when the Chicxulub meteor struck the Gulf of Mexico and ended the era of the dinosaurs--is analyzed for proxy chemical signatures to reconstruct the ancient history of atmospheric CO₂.

The main type of proxy data is the ratio between two isotopes of boron (B), ¹⁰B (typical) and ¹¹B (rare), known as d¹¹B. Boron is found in the carbonate shells (tests) of microorganisms called foraminifera (forams). The ratio between boron isotopes (d¹¹B) depends on the acidity of seawater, and the acidity of seawater is directly affected by the CO₂ concentration in the atmosphere. A higher atmospheric CO₂ concentration means a lower oceanic pH (tomorrow's post), which means a lower value for d11B in the microscopic fossils.

Another type of proxy data is the d¹³C record, the ratio of ¹³C to ¹²C isotope, in  the remains of ancient algae. However, the d¹³C measure is sensitive to a wide range of influences beyond temperature and pH, so that record remains a work in progress.

Tomorrow: carbon dioxide and ocean acidity.

Be well!

365 Days of Climate Awareness 10 - The paleotemperature record

 

365 Days of Climate Awareness

10 - The paleotemperature record

Several different types of proxy data provide climatic information to us, particularly on a regional scale, such as pollen preserved in ice or sediment. However, biotic markers are not sufficient to estimate temperature, and are highly responsive to other factors like humidity and inter-species competition. The principal means of estimating temperature, in ice cores, sediment cores and sedimentary rock samples, is d¹⁸O (“delta-18-O”).

Larger version

O stands for oxygen, and the superscript 18 refers to the isotope. An atom consists of a nucleus of both protons and neutrons, and a cloud of orbiting electrons. An element is defined by the number of protons in the nucleus: 1 for hydrogen (H), 6 for carbon (C), 8 for oxygen (O), and so on. Usually the count of neutrons is equal to the number of protons, but not always. “Isotope” is a term describing the number of neutrons in the nucleus. Different isotopes act the same way chemically—they form the same bonds with other atoms—but the different atomic masses lead to some differences in behavior.

We refer to different isotopes with a superscript number: ¹⁶O (8 protons, 8 neutrons, typical), ¹⁸O (8 P, 10 N, rare); ¹²C (6 P 6 N, typical), ¹³C (6 P, 7 N, rare), ¹⁴C (6 P, 8 N, also rare). ¹⁸O occurs in the present atmosphere at a known concentration (roughly 0.2%), but this concentration depends on global temperature. The concentration of ¹⁸O (also known as “heavy oxygen”) in the atmosphere increases as the climate warms, and decreases as the climate cools.

In science and math the term d, or delta refers to a numerical change. So d¹⁸O refers to the change in atmospheric ¹⁸O concentration over time. By correlating estimates using this method with other proxies, we have been able to construct a temperature record spanning the last half-billion years (out of 4.7). Change is the norm, and the stability our species has been accustomed to during the last 2500 years of recorded history is not.

Tomorrow: the paleocarbon record.

Be well!

365 Days of Climate Awareness 9 - Intro to Paleoclimatology

365 Days of Climate Awareness

9 - Intro to Paleoclimatology

Paleoclimatology is defined as the study of Earth climate for which there are no direct measurements. Potentially this includes even the the twenty-first century in parts of the world where scientific work was seldom done, but many historical methods do not work for too-recent time. Temperature records were kept by scientists from the 1700's on, and institutions like the British Navy logged data such as temperature and magnetism (via compass combined with lat/long observations) which have added significantly to our understanding of the earth system. Preceding those, however, and in parts of the world where records like those were not kept, we are forced to use proxy data.

Proxy data are observations which can stand in for direct observations. The observed proxy has a physical relationship (sometimes horrifyingly complicated) to the target data which can be quantified theoretically. With this mathematical relationship, generally established by extensive testing and experiment, the proxy data can be analyzed and converted into the target quantity. Critical information including global temperature and atmospheric composition can be determined this way.

It goes without saying that having as clear and detailed an idea of the Earth's climatic history is very important for understanding the scope and causes of current climate change. With accurate historical data for comparison, we can judge the magnitude and quickness of temperature and greenhouse gas concentration increases in recent centuries. The most recent and high-resolution set of records uses tree rings from around the world. With knowledge of the species and changing growth rates in response to temperature, we can infer aspects of the climate for the past several thousand years. This data set is especially valuable because it overlaps modern observations and other proxy methods.

A way to look into the further past is via ice cores from Greenland and Antarctica. Antarctic ice is much thicker and older than Greenland's, and provides us with a much longer record: up to 800,000 years, versus about 130,000. Most of the information we want is locked inside of tiny air bubbles trapped in the ice as it froze, like so many miniature samples.

Looking back further than this, into the span of millions and then billions of years, we look at chemical signatures in sediment and sedimentary rock. (The other rock types, igneous and metamorphic, have been subjected to heat and pressure to the point that any prior chemical properties have been altered or lost.) The rock record is the lowest-resolution, with precisions of 2-5% of the sample age (going back into billions of years old). But it is our only window into the planet's distant past. [Note: the precision statement was edited as my original was for a best-case of younger samples.]

Tomorrow: the temperature record in paleoclimatology.

Be well!

365 Days of Climate Awareness

8 - Measuring Atmospheric Carbon Dioxide

The Mauna Loa observatory is at 3400m elevation in Hawaii, a location chosen for its isolation relative to cities and other large-scale CO₂ sources and sinks, and elevated enough that the atmosphere can be assumed to be globally well-mixed. The Pacific is used also for measuring another important benchmark, eustatic (global) sea level, the topic of a post to come. This record is not the same as the global average, which is calculated from a worldwide set of island-based CO₂ monitoring stations (including Mauna Loa). Island monitoring stations are considered to be the best means of avoiding biases from continental sources and sinks.

The record dates from 1958 and has been called "the backbone of climate change science". It gives not only a clear representation of the growing speed of CO₂ concentration in the atmosphere, but also provides a clear correlation with other data streams such as air and ocean temperature, ice mass (i.e. loss), sea level rise, and more.

Mauna Loa record of atmospheric carbon dioxide concentration.

Modern-day, highly precise measurements can be used to correlate with and statistically constrain methods of estimating prior atmospheric CO₂ concentrations, which is vital to climate change science. A feature of the trend is the almost metronomic annual rise and fall, averaging out to the smoothly increasing trend line. These annual fluctuations are due to seasonal effects on forests and other photosynthesizing regions. This effect is observed worldwide. Over the course of the sixty-plus years in which data has been tabulated, the atmospheric carbon dioxide concentration has increased from 315 ppm to nearly 420, an increase of almost exactly 33%.

Tomorrow: historical (paleo-) CO₂ measurements.

Be well!

Mauna Loa carbon dioxide record

365 Days of Climate Awareness 7 - Carbon Sources and Sinks

365 Days of Climate Awareness

7 - Carbon Sources and Sinks

One of the basic analytical tools in climate change is to identify carbon dioxide sources and sinks. These can be used to create a complete carbon dioxide budget for the atmosphere, but even without carrying things that far, it's useful, especially in terms of policy, to have an intelligent idea of where carbon dioxide comes from and where it goes to. Increasingly, it is necessary to include the ocean in these analyses, as it is by no means at all a limitless reservoir of either heat or CO₂ gas. It's currently estimated that world society emits roughly 55 gigatons of carbon dioxide equivalent (GtC) per year, and that total is climbing.

A carbon source is anything which supplies carbon dioxide to the atmosphere. It can be a point source, like a single factory or power plant, or a collective, such as the entire fleet of automobiles in the US. A carbon sink is anything which removes CO₂ from the atmosphere, and likewise can be point sinks (which are rare) such as carbon capture installations (which do exist), or distributed, such as forests.

Energy production, including for transportation, accounts for nearly 3/4 of global CO₂ emissions. Energy use for industry is the dominant component, 24% of overall emissions, with energy use to light, heat and cool buildings next at 17% and transportation a very close third at 16%. Agriculture collectively emits 18% of global CO₂, almost 6% due to livestock. Industrialized farming has turned that entire sector into a net carbon source. Curing of cement is another measurable source, 3% of global CO₂ emissions.

Carbon sources and sinks. (Pg = Petagrams = quintillion (10¹⁵) grams)

Photosynthesis, on land and in the ocean, is the dominant carbon dioxide sink, drawing down 120 GtC per year, but much of that carbon is not removed from the atmosphere for long, due to respiration and decomposition. Weathering of rock also consumes CO₂. There is a correlation in geological records between increased global orogenic (mountain-building) activity and decreased carbon dioxide content in the atmosphere (Earth's current level of mountain-building appears to be fairly moderate). The ocean itself removes about 90 GtC per year but most of that is returned, and only a small portion--2-3 GtC--is retained in the water. The added CO₂ in ocean water has other effects which will be discussed later.

Sinks and sources can be variable. In 2020, due to global COVID pandemic, economic activity was impaired and global emissions shrank. On the other hand, previously known carbon sinks such as the forests of western Canada and the Amazon basin in Brazil have lost much of their potency as sinks due to the persistent wildfires of British Columbia and the deliberate deforestation taking place in Brazil.

Tomorrow: measuring CO₂ concentration.

Be well!

365 Days of Climate Awareness 6 - Albedo

365 Days of Climate Awareness

6 - Albedo

Albedo is the reflectivity of any given surface on the planet. The surface might be exposed ground, desert sand, open water, ice and snow. clouds, boreal or temperate or jungle forests, farmland, grassland, or otherwise. It's measured on a scale from 0 to 1, being a percentage of total incoming radiation, with 0 for a perfect blackbody (total absorption) to 1 (perfectly reflective).

Reflectivity of the planet plays a huge role in climate, with a number of different feedbacks. "Feedback" is when the output of a system then becomes input again to the same system. (Hendrix pioneered this musically, playing in front of the speakers on stage.) A positive feedback is one which causes the process to accelerate, making the system less stable. A negative feedback causes the process to slow down, making the system more stable again.

Concept of albedo.

Both positive and negative feedbacks occur within the climate system with respect to
warming, though the positive feedbacks greatly outweigh the negative. In one instance, increased temperature causes more evaporation around the world, leading to higher water vapor content within the atmosphere, increasing the atmosphere's insulative effect: a positive feedback. However, more water vapor produces more clouds, which are highly reflective--having albedos between 0.4 and 0.8--and are a negative feedback on warming. (Therefore increased water content in the atmosphere has a variable effect.)

Some decidedly positive feedbacks resulting from changes in albedo: loss of ice and snow (from 0.3 to 0.85 albedo), being replaced by open water (albedo <0.1), soil (0.5-0.35) or small vegetation (0.1-0.25). (This is one big reason why loss of glacier and sea ice are very important, ominous developments.) All the radiation not reflected is absorbed and warms what absorbs it.

Replacement of forest with grass or farmland--being aggressively pursued in Brazil--increases the amount of CO₂ in the atmosphere, since trees consume far more carbon dioxide than smaller plants, but increase the albedo (forest: 0.5-0.15, vs. farmland, 0.15-0.25), a negative feedback. In this case the increase in albedo--the negative feedback--is overwhelmed by the change in vegetation and decreased CO₂ consumption.

Tomorrow: carbon dioxide sources and sinks.

Be well!

365 Days of Climate Awareness 5 - Greenhouse gases

365 Days of Climate Awareness  

5 - Greenhouse gases

The three most important greenhouse gases are carbon dioxide, methane, and water vapor. There is a handful of far more powerful, but fortunately more rare, chemicals which likewise trap heat and warm the planet. This is a brief summary.

Carbon dioxide (CO₂): The dominant greenhouse gas, with a much greater effect than the next most important, methane. CO₂ is produced by many forms of oxidation of carbon-based matter: burning wood, oil, gas or coal; and by respiration within our own cells. CO₂ is in turn consumed by plants during photosynthesis (though those same plants do then release some CO₂ again with their own cellular respiration). The concentration of CO₂ is currently 410 ppm (parts per million), and was 240 ppm before the industrial revolution. Carbon dioxide has been far from the only influence on earth's climate throughout the planet's 4.7 billion-year existence, but its role is significant.

Methane (CH₄): Methane is up to 100 times as effective as CO₂ in trapping and re-radiating IR energy, but its role is still smaller because its concentration is far lower (1500 parts per billion--ppb--up from 750 ppb before the industrial era), and because it tends to remain in the atmosphere for 100 years before descending back to the ground, as opposed to CO₂, which resides there for over 1000 years. Methane comes from a number of sources, including agriculture, oil and gas production, outgassing from decomposing materials (including human dumps), and other natural sources like permafrost melting (accelerating now with a warming planet).

Water vapor (H₂O): a powerful greenhouse gas in its own right, and a natural part of our weather as part of the worldwide water cycle. An increasingly warm planet means more water content in the atmosphere, increasing water vapor's warming potency. However, more vapor also leads to more clouds, which help block incoming UV radiation, a negative feedback.

Nitrous oxide (N₂O): Even more potent than methane, being about 114 times as effective as CO₂ in trapping heat, and remains in the atmosphere for over 100 years before disintegrating. Its main sources are agriculture, industry and energy production. Historically N₂O concentration has hovered around 265 ppb, but during the industrial era has increased quickly to 335 ppb.

Ozone (O₃): In the stratosphere (30-50 km altitude), ozone blocks UV radiation and so helps cool the planet and make it livable. It forms by chemical breakdown of breathable oxygen (O₂) via the sun's radiation. But closer to the ground, ozone is produced by combustion and is a pollutant, among other things, insulating the planet much like CO₂ does. Ozone is not a well-mixed atmospheric gas, constantly being made and breaking down, varying with time and distance.

Chlorofluorocarbons & perfluorocarbons: a number of gases (including the CFCs which were destroying the ozone layer in much of the 20th century) produced industrially. They can be hundreds of times more potent than CO₂ as insulators, but in concentrations generally measured in the parts per trillion (ppt) and with shorter atmospheric residence times.

Sulfur hexafluoride (SF₆): A frighteningly effective greenhouse gas--nearly 23,000 times as effective as CO₂, and highly stable chemically--produced industrially and with a number of applications in electronics. Its current atmospheric concentration is roughly 2.5 ppt.

Tomorrow: albedo.

Be well!