Knowing the unknowns


  • Russell Seitz

    Corresponding author
    1. Department of Physics and the Center for International Affairs, Harvard University, Cambridge, Massachusetts, USA
    2. Now at Microbubbles LLC in Washington, District of Columbia, USA
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The Earth's atmosphere is not the only source of radiative forcing and anthropogenic climate change. As surely as people and civilizations have carbon footprints, they have albedo footprints as well. By altering the reflectivity of roughly half the land surface of the Earth in the past, mankind has made inadvertent geoengineering a part of the landscape of history. This worldwide alteration of reflectivity raises questions about the future of climate change, for albedo is a first-order determinant of the Earth's radiative equilibrium. As surfaces absorb roughly 100 times more solar energy than the CO2 in the atmosphere, future anthropogenic changes in both land and water albedo may figure significantly in climate policy outcomes.

At the turn of the 20th century, scientific controversy raged over the varying color of the planet Mars. Although a pleasure to look at, Victorian telescopes were of little use in taking pictures, delivering thumbnail images so dim that astronomers were reduced to sketching maps that differed so greatly that the red planet's fixed geography amounted to little more than two bright polar caps that came and went with the passing of the seasons.

Yet in retrospect, 19th century Mars gazers were better off than the 20th century geographers—at least they could see what they were arguing about (Figure 1). Although Percival Lowell's “canals” remained photographically elusive, they could easily gage how much sunlight Mars reflected, its “albedo,” by comparing its darker regions with its snow-white poles.

Figure 1.

NASA's Cassini Saturn probe looks back at the Earth and the Moon.

While we have no present say in the future of Mars' albedo, Earth is another story—the question is where will we get data enough to do it justice? Edwardian geophysicists did not enjoy the astronomer's bird's eye view of Mars. To quantify Earth's ever-changing albedo, they had to add up the sum of its parts by integrating cloud cover from distant weather reports, running soil samples through primitive photometers, and judging the reflection of watery earthshine from the Moon. Not until space-born instruments arrived could Earth scientists look back at the whole Earth directly, and wonder at how variably it reflected the light of the Sun. With weather in constant motion, and clouds, contrails, and currents crisscrossing seas and skies, you cannot see the same planet twice.

This has always been so—Homo sapiens is not the first species to boast of an albedo footprint. Just as the emergence of life changed the color of the oceans, the evolution of land plants transformed the albedo of the continents. Today, dark foliage is spreading not just in response to the physics of climate change, but the chemical spillover of anthropogenic nitrogen fertilizers into the wild and embossing nitrogen's green footprint on the borderlands of agriculture has shifted the balance of radiative forcing toward warming as surely as adding CO2 to the sky.

Oblivious to the risk of extending the dead zones too often seen where nitrogen-enriched rivers meet the sea, some blithely propose fertilizing the oceans with trace elements such as iron. But so great are the ensuing uncertainties that all that can be said with confidence is that they do not know what they are doing—increased marine carbon uptake can unpredictably alter albedo. As with clouds in the sky, clouds in the sea can trap solar energy as well as provide cooling shade; therefore, the vertical mobility of phytoplankton complicates the understanding of whether their solar energy absorption and backscattering warms or cools the surface. This complex problem comes in all colors, from red tides to chalky coccolithophore blooms white as the cliffs of Dover.

Microorganisms are not the only creatures with substantial albedo footprints. Farther up the evolutionary tree, Castor faber's mindless industry has turned the North Woods into a patchwork of black water beaver ponds whose combined radiative forcing rivals urban Canada's entire heat island effect.

Today, science basks in the space program's reflected glory. But while orbiting telescopes provide a lot of hyperspectral eye candy, our view of Earth's reflectivity remains dim—there is still some bickering over its second decimal place, for contemporary measurements of albedo span a range of ∼7 W/m2, a radiative uncertainty several times larger than total greenhouse gas forcing. Average values are easily estimated by integrating the data from orbiting radiometers, but having such numbers for yesterday tells us little about tomorrow. Albedo change is as dynamic as the weather on a planet where trees shed leaves and farmlands are plowed and harvested every day. There is more to climate modeling than backcasting, and until models incorporate the fine structure of how albedo varies, not just annually, but from sunrise to sunset, its impact on future climate variability will remain problematic. This lack of temporal detail cannot cheer those trying to wind down the Climate Wars, for albedo, not atmospheric chemistry, determines the first decimal place of the Earth's temperature.

Although this informational shortfall has been known for decades, it is hard to visualize because albedo maps remain surprisingly crude. Earth's ever-changing land and waterscape still robs albedo maps of levels of accuracy and resolution already achieved in land-use databases, and what maps there are provide little seasonal and less diurnal information about how the face of the Earth changes over time. This lack of dawn to dusk detail is an invitation to controversy. Knowing Mars' albedo to two decimal places did not calm the debate between those who saw only an airless desert and those who like Percival Lowell believed the same dim images justified his depiction of a canal-crossed planet transformed by a hydraulic civilization.

A hundred years later, the irony is clear. Airless, waterless Mars remains easier to model, and hence to understand, than the cloud-enshrouded, hydrosphere-covered Earth. Humans have been changing its scenery on continental scales ever since the discovery of fire, and the advent of hydraulic civilization has just made the problem harder—a Hubble telescope sent to Mars could look back and clearly image the rise and fall of water in our world's great reservoirs. Although nobody knows what the limits of our future influence on atmospheric dynamics and the play of color on the oceans may be, we are already seeing the creation of new journals to contain that debate, one in part begun by one of America's founders, Ben Franklin, who was the first statesman to note that nations have albedo footprints to begin with.

So far, roughly half the land surface of the Earth has been transformed by human action. Enough of that change was embarked upon by the early days of the Industrial Revolution for Franklin to assert in 1751 that his industrious countrymen, a hatchet-wielding Virginia youth named George Washington included, were:

Scouring our Planet, by clearing America of Woods, and so making this Side of our Globe reflect a brighter Light to the Eyes of Inhabitants in Mars or Venus.

Not content to alter albedo by cutting down trees, other Colonials took to transplanting them with a vengeance. By the middle of the 19th century, every continent save Antarctica was studded with plants from every other. Little did the founders of botanical gardens like Kew realize that catering to the craze for rhododendrons, eucalypts, and monkey-puzzle trees would turn the Earth into a botanical melting pot, but by 1900 America was anything but the Forest Primeval of Longfellow's imagination, and by the outbreak of the Great War the homogenization of the biosphere was a fait accompli, as Australian eucalypts covered southern California, and elms from Chinese Turkestan sprouted from the once treeless Great Plains.

Change happens but recording and integrating anthropogenic albedo changes is not an easy task. They operate over all climes and time scales, brightening and darkening willy-nilly. While some cultures zealously whitewash their cities and pave their highlands in conifer plantations, others consider clearing rainforest to plant soybeans as much a mission civilisatrice as laying black asphalt highways across the land. Within living memory humanity has turned blue inland seas into glaring salt deserts and salt flats into blue inland seas. We can scarcely guess the sea albedo changes wrought when coal-fired fleets fertilized the oceans by misadventure, fueling plankton blooms by spewing megatons of iron-rich fly ash along the shipping lanes, but those changes pale in comparison to those that began before the advent of industry.

Palaeoclimatologists are still trying to cope with the consequences of the discovery of fire, and its Ice Age deployment as a land management tool by long-gone hunter-gatherers.

Thanks to hyperspectral satellite imaging, and airborne LIDAR we are beginning to see high-resolution maps of standing biomass—here is the first high-resolution national map of aboveground carbon—the rain forest chopped in two by Panama's eponymous Canal (Figure 2).

Figure 2.

Changing biomass levels in Panama: red = highest and blue = lowest.

Historians now realize that the dark American woods Franklin complained of were shaped by millennia of native American understory burning, aimed both at giving hunters a clear shot at their quarry and fertilizing soils for cultivation. Such age-old patterns of agricultural fire setting persist alongside the cooking fires of the billion people who, though still lacking electricity, manage to compete with the car engines of their more prosperous neighbors in pouring out albedo-altering soot. The capacity of climate policy to manage albedo changes may ultimately be limited by the mere fact that not all of our contemporaries are living in the same time. Solar power and social engineering enthusiasts may be contemplating the next Industrial Revolution, but five millennia after the Bronze Age began, the Stone Age still lingers. Just as Neolithic tool manufacture goes on in remote tribal areas, some nations may still be burning coal when the rest of the world has gone more nuclear than France.

To understand anthropogenic albedo change to the point of sensible policy engagement, we must first master its natural variability, something easier said than done when so much depends reflexively on climate change. It is easy to see that wind velocity alters sea surface albedo by creating reflective whitecaps on the deep blue sea, but difficult to quantify the resulting feedback—whitecaps modulate ocean surface temperature, altering the radiative balance to create the thermoclines that drive the winds that whip the waves in an interwoven matrix of feedback loops that will go on running until the Sun burns out. There is no rest for climate modelers.

Meanwhile, in the seawater itself, planktonic life comes and goes, altering sea albedo as populations thrive or perish, and jockey up and down seeking their share of sunlight and nutrients, all the while altering the depth at which solar energy is absorbed, and water warmed or stratified. Yet while oceanographers are acutely aware of these processes and how they must inevitably affect the thermodynamics of the oceans, we have little knowledge of how they have varied over time. Scientists have yet to see a historical atlas of the oceans, charting on all time scales on how human events and natural processes have, locally, regionally, and globally, modulated the uptake of solar energy. While we have anecdotal accounts of outliers from white squalls and amaranthine reefs to Homer's wine-dark sea, nobody really knows the color of the ocean or the depth of the clouds scudding over it. All we have are averages of averages concealing another layer of unknowns.

The same is true of the other faces of the hydrosphere, from black ice to water droplet clouds in the sky and the brilliant but ephemeral reflectivity of fields of dew at dawn. Wherever air and liquid or solid water mix, the radiative equilibrium of the mixture can be profoundly altered, not just by physics but by biology. Plants, like people, are mostly water and their impact on albedo varies just as greatly. Higher light levels than photosynthesis demands invite the evolution of reflective vacuoles or bubbles into organisms to allow refractive index contrast to reflect or backscatter the rays of the Sun. Such structures, seen in seaweeds and saltworts at sea level and silverswords atop the volcanoes of Africa and Hawaii, are the antithesis of the light-absorbing conifer needles that allow trees to make a living in the boreal depths of Siberia's black taiga.

Because the albedo footprints of plants vary so greatly—some are darker, others lighter than the soil they grow upon, and man-made changes in vegetation necessarily alter local albedo and temperature as well. Modeling of the consequences of these and other human land-use changes has begun; but as with the color of the oceans, to explore the subject is to embark on a journey without maps—the wealth of hyperspectral satellite scans of Earth's land lakes and oceans that have been flowing down from orbit for the last several decades has yet to materialize in two dimensions. Go online, and you can get a day-by-day description of how Arctic sea ice is responding to climate change, but you can search the web in vain for forecasts of how light-absorbing or -reflecting plankton and forams are modulating the surface temperature of the sea. No agency exists to provide albedo forecasts like H. sapiens, every other species on Earth has an albedo footprint, but we scarcely know our own, and it is humbling to report that the most abundant marine organism came to light only decades ago.

The discovery of Prochlorococcus has sparked a revolution in marine biology, just as the evidence for past “snowball Earth” episode has necessitated new thinking in paleoclimatology. Today, the MAREDAT consortium has begun to chart photosynthetic plankton on a species-by-species and ocean-by-ocean basis, creating an atlas that lays the foundation for future studies of how life in the seas modulates marine albedo.

Although less studied than clouds, radiative forcing effects from marine organisms may rival anthropogenic atmospheric forcing when biological emissions of cloud-nucleating gases like dimethyl sulfide are taken into account. The reason is area—despite their often modest albedo contrast, plankton blooms are so vast that they outrank urban heat islands in the scheme of climate forcings (Table 1).

Table 1. The 20 Largest Cities in the World Ranked by Metropolitan Land Area (Excerpted From
RankCity/Urban AreaCountryPopulationLand Area (km2)Density (People per km2)
1New York MetroUSA17,800,0008,6832,050
7Los AngelesUSA11,789,0004,3202,750
8Dallas/Fort WorthUSA4,146,0003,6441,150
18Johannesburg/East RandSouth Africa6,000,0002,3962,500
19Minneapolis/St. PaulUSA2,389,0002,3161,050
20San JuanPuerto Rico2,217,0002,309950

While the greatest of cities cover less than 10,000 km2, blooms of well-named Nitschia frigida fill millions of square kilometers of the Arctic Ocean every summer, covering regions so large that the subtle—and often very beautiful differences in ocean color they create can outstrip the impact of cities on global albedo. But that is not the end of the story—N. frigida can modulate albedo on much the same scale as aerosols from human coal burning do, because it ranks as one of the world's great sources of cloud-nucleating dimethyl sulfide.

It is ironic that while acutely aware of black asphalts' capacity to soak up solar heat, few urban planners realize how absorbing water's features can be. Dark as a parking lot, the pond in Central Park undoes the cooling effect of the white roofs that overlook it by adding up to a gigawatt to the city's peak summer heat load. Like politics, all climate is local, and because local changes require local responses, new tools for solar radiation management are needed, which should operate on smaller scales of time and space than top-down geoengineering.

Knowing that the Earth will change is not the same as knowing how. Despite international scientific efforts to achieve the interdisciplinary ideal, the known unknowns of the atmosphere still get the lion's share of climate research funding, leaving the hydrosphere to languish as the element of geophysics we know the least about. Thousands of careers and billions of dollars are devoted annually to strengthening the scientific underpinnings of atmospheric policy. The economic wisdom of this prioritization seems materially questionable, for though the sky may be 30% larger in area than the sea, the hydrosphere remains 300 times more massive than the atmosphere.

So far, the huge physical mass and thermal inertia of the oceans has figured mostly as an excuse for inaction in the climate policy debate, which Presidential chief of staff John Sununu once put on hold by noting that, since the first 10 m of water in the oceans weigh as much as the atmosphere, climate change is foredoomed to be a pretty leisurely affair. His gambit might not have succeeded had shrewder conservative (and Tory) analysts observed that, though the turnover of ocean heat is slow, the meter has been ticking for three centuries—if the Republic merely survives for as long as it has already, it could literally end up in hot water.

Might it not be prudent therefore, for those inside the beltway and without having to reflect on the paramount importance of understanding water at least as well as air? Despite the antiquity of hydraulics, it is only in the last few decades that water has begun to be studied in earnest as a material, and a very surprising one.

Widely viewed as a dangerous conductor of electricity, it has been proven as an indispensible high-voltage dielectric in physics research. Long taught to be incompressible, it has been tamed by materials more incompressible still into becoming a superb high-pressure cutting medium. Yet for all this progress and its ubiquitous role in determining the color and climate of the Earth, we have scarcely begun to think of it as a medium whose optics can be manipulated much as we manipulate electronic materials to produce gross changes by minute additions. Even though natural microbubbles amount to just tens of parts per billion of the mixed layer of the oceans, they account for several parts in ten thousands of Earth's overall reflectivity. Just as a scant part per million of phosphorus can turn a dull grey expanse of silicon into a screen vivid enough to read, or induce a sea-spanning algal bloom, an intentionally manipulated volume part per million of air can do as much or more than an algal bloom to brighten the deep blue sea (Figure 3).

Figure 3.

Earth Observation Satellite Company (EOSAT)'s view of reflective coccolith bloom in the English Channel.

Just as traces of phosphorus and carbon can turn silicon and iron into semiconductors and steel, this new engineering salient uses air to brighten water. Driving this goal is the intersection of demography and economics. Not all of our contemporaries are living in the same time, and wars have been fought over lesser provocations than denying humanity's poorer half access to their just share of the carbon-intensive fruits of the Industrial Revolution.

Their reluctance to put climate model results ahead of their aspiration for their children is already politically manifest in Asia and Africa, and may render bien pensant schemes of carbon rationing and prohibition moot: As politics is the art of the possible, it falls on science and engineering to prepare for the real possibility of a global climate policy fiasco by beginning to think about the albedo of Earth and water as well as radiative forcing by fire and air.

Albedo's future is critical because water is so dark that even subtle changes in its reflectivity can alter the hydrogeological cycle as much as rising CO2. The flip side of Asia's life-giving monsoon is a long dry season that leaves whole nations starved for water. Last summer Pakistan's reservoirs fell to a 30-day supply.

The precautionary principle cuts both ways. While many have convinced themselves that it is morally hazardous to entertain policy alternatives to changing the composition of the atmosphere, their ardor will not quench the thirst of South Asia. Instead of conflating solar radiation management with changing the color of the sky, those whose policy focus is emissions control should acknowledge the separate existence of problems arising from uncontrolled evaporation. It seems bizarre that so much blood and money goes into creating national irrigation schemes and so little into advancing the fundamental study of the fluid that fills them (Figure 4).

Figure 4.

A drier future?

Local water conservation counts because water crises may be driven as much by demographic factors as climate change.

From the salt-blasted fields of Mesopotamia to the deserts of Rajasthan, South Asia is littered with the ruins of failed hydraulic states, and mighty as the Indus' flow may be, Pakistan's agricultural water supply is slated to slip below the existential minimum of 3 t a day per citizen just two decades hence.

Although anthropogenic albedo change is a fact of life, it is hard to predict what new unknowns in climate and water policy may arise from it. Some may be brought to light by the unreasonable power of mathematics before they outcrop as problems, but a prudent policy toolkit should accommodate some solutions that are still in search of problems.

Just as the natural history of clouds demonstrates the power of Mie scattering to transmute a little water into a great deal of reflective cloud cover, the prospect of creating clouds turned inside out raises the possibility of modulating our albedo footprint by design. How well and wisely we can learn to do so remains to be discovered, but this much is already clear—humanity has already transformed so much of the Earth that assuring the survival of biodiversity as we think we know today may demand more than the geoengineering by misadventure that has transformed the landscape of the past.

As new journals serve to expand the frame of scientific speculation, let me add two to the menagerie.

The first is the possibility of pre-empting the climatic eclipse of ecosystems by lending a pioneering hand to slow-moving species as they struggle to get out of harms way. Two centuries ago, von Humboldt kick-started the science of ecology by adducing a central principle of biogeography—altitude recapitulate latitude: temperatures fall when climbing a mountain as surely as in traveling north. His insight suggests a form of ecological insurance against climate shock—installing familiar flora in advance of species driven north or uphill by climate change could save many from extinction. At 1°C per 150 km, 20th century warming in effect shifted global biogeography toward the equator by a full degree, and the IPCC's future projections amount to a further shift of some miles a year (Figure 5).

Figure 5.

Climate change: Sometimes a degree is really a degree.

If so, conservationists might outrun the otherwise fatally slow northward motion of deeply rooted ecosystems by planting a scatter of southerly trees and plants 1° or 3° north of their present natural range. Putting slow-growing trees in place faster than climate can carry them could assure future refugia for many species as slow-growing forests mature.

Could the ample precedent of long-range forest planning in Europe be extended to transplanting key tree species northward faster than climate change can carry them? One unnecessary, unknown system ecologists could quantify is the number of species that might be saved by translocating temperate zone biological reserves northward at a rate of a few kilometers a year.

The hallmark of new knowledge is that it reminds us how much more remains to be discovered. Just as the rapid onset of Antarctic glaciation at the end of the Eocene has been linked to a drawdown of atmospheric CO2 by diatoms, silica-walled organisms today account for 40% of marine photosynthesis. That is too large a share of the biogeochemical cycle of carbon to ignore, but while 30 Si isotope data can inform us about changes in climate and marine biology 30 MA ago (cf.:, it leaves us guessing about the changes in ocean albedo that attended the last epochal shift in diatom ecology.

While the term “Anthropocene” debuted but a decade ago, the human transformation of land albedo began not with the Industrial Revolution but with the discovery of fire in Paleolithic times.

In the half-million years since, fire setting by hunter-gatherers, farmers, and pastoralists has altered the reflectivity and hydrology of roughly half the land surface of the Earth. We therefore face a policy paradox—if the CO2 forcing of recent centuries is reduced or reversed, albedo may once again become the dominant force in anthropogenic climate change, for our albedo footprint is the cumulative legacy of hundreds of generations, and the signature of land use can endure on the ground for just as long as CO2 lingers in the air.

As we advance into the unknown country of the Anthropocene, we must realize that new as the term may be, the anthropic transformation of the landscape of history has been progressing for a million years.

With so much already transformed by misadventure, we have a duty to consider whether the consequences of our prolonged interaction with land, sea and sky can be mitigated by design as well as eased by moderation. If population continues to grow, we will soon enough discover whether a civilization so globally dependent on agriculture, innovation, urbanization and trade can lighten, its geophysical footprint without unraveling the fabric of history.