Biology Comics

Still no illustrators! Well here’s a hard one…


Climate Story

People say weather is what happens day to day while climate is what happens over thousands of days. But climate times also vary in length.

The sun warms the planet. Its radiance has increased by 25% over the last 4 billion years. Despite everything, the average temperature of the planet hovers above the freezing point of water.

Over a timescale of tens of millions to billions of years, volcanoes emit carbon dioxide and silicate rocks absorb it. If volcanism slows down and the earth cools, the chemical reaction governing the absorption of the carbon dioxide by the rocks also slows down (chemical reactions are dependent on temperature). Then carbon dioxide slowly accumulates in the atmosphere and the planet warms.

New carbon absorbing rocks are also created by volcanism, which is influenced by continental drift. Continental drift also uplifts new rocks from the mantle. It operates on a timescale of tens to hundreds of millions of years.

As plants evolved, they also took a hand in storing carbon dioxide. First they oxygenated the atmosphere, driving most organisms, for whom oxygen was a poison, underground. The plants were single cells floating on the surface of the sea. They took carbon into their bodies through photosynthesis, expelling oxygen as a waste. The carbon from the plants fell as dead plant bodies or animal poop to the bottom of the sea (the “poop pump”), where it remained.

Later, rooted land plants evolved. They pumped carbon dioxide into the soil as part of their strategy to obtain nutrients. This increased the rate of weathering in soil. Soils stored more and more carbon dioxide.

The ocean naturally takes up carbon dioxide from the atmosphere in a simple chemical reaction with water. The deep ocean circulation (the ocean conveyor) stores this carbon dioxide in the deep ocean. One round of the circulation takes 1000-10,000 years.

The placement of continents determines whether the oceans can circulate heat and so even out the temperature of the earth, cooling the tropics (which receive more direct solar radiation) and warming the poles. The narrow basin of the North Atlantic, bounded by continents on each side, lets warm surface water brought by currents from the Pacific and Indian oceans move north, and lets the return flow (at depth) move south toward Antarctica.


There’s more. Over the long term the earth’s temperature is determined by the sunlight falling on it, the position of its land masses, the heat capacity of its oceans and the characteristics of its atmosphere.

The atmosphere lets through the sun’s rays, which strike the earth, while various of its gases absorb the solar energy that is reradiated as heat to space. The gases act like a blanket.

Water vapor is the greatest greenhouse gas. Thus desert days and nights have wide temperature swings (less of a blanket there).

The other (known) gases that absorb heat are carbon dioxide (from combustion, logging, land clearance, animal respiration, plant decay), methane (from coal mining, landfills, rice paddies, cattle, termite mounds, bacterial action, swamps), nitrous oxide (from fertilizer, combustion, denitrifying bacteria), low level ozone (from air pollution and sunlight) and the halocarbons (from fires, bacterial action, chemical manufacture: these are the gases that ate the high altitude ozone shield).

These gases have different molecular weights and absorb infrared radiation at different frequencies. Thus their effect is greater than if they absorbed radiation at the same frequency: a symphony in infrared.

The warming gases are all produced naturally but human activity has become their largest source. The carbon dioxide content of the atmosphere has been measured continuously since the late 1950s from a mountaintop in Hawaii and is now 100 parts per million over the preindustrial level of 280 parts per million. It seems to me that if you know this and accept that carbon dioxide absorbs heat, you must believe in global warming. A hundred parts per million is a large part of 280 parts per million. (In fact, it is the difference between an ice age and an interglacial period.)

The warming gases are not common in the atmosphere and thus human activity can influence their abundance. Their effects are amplified by water vapor, the greatest greenhouse gas. A small warming from carbon dioxide leads to more evaporation from the oceans and more water vapor in the air, therefore a warmer atmosphere, which leads to more evaporation, and a still warmer atmosphere, in an ineluctable positive feedback.

From the steamy Cretaceous Period of 100 million years ago the earth has slowly cooled to our age of continental glaciations. The cooling has several causes: the storage of 8000 billion tons of carbon (10 times that in the modern atmosphere) during the damp, temperate Carboniferous of 300 million years ago; the current position of the continents, which lets massive ice sheets build up on land near both poles; the ongoing storage of methane by bacteria in sea bottom sediments; a moderate degree of volcanism (a volcano is erupting somewhere on earth every day); and an increased biological storage of carbon by higher animals, both in the poop pump of the oceans and on land.


A cooler earth is a dryer one and a drier one lets grasses outcompete trees. Over the last 70 million years grasslands have come to dominate much of the planet. Grassland and grazers accumulate much more carbon in their soils than forests. Grazers help by stimulating new growth and by recycling nutrients: the Great Plains were maintained by buffalo piss. Many of the most productive agricultural soils are former grasslands.

Man with his plow, his axe, his farm animals and his industries releases carbon to the atmosphere.

But back to the story: why the glacial cycles of our relatively low carbon world?

Imposed on the increasing radiance of the sun, the position of the continents, the state of the atmosphere and the biosphere, are the angle of the earth’s axis to incoming solar radiation and the earth’s distance from the sun: the Milankovitch cycles. While these cycles don’t affect the total amount of radiation the earth receives, they affect its seasonality. Glaciers grow when summers are short and cool (so snow doesn’t melt) and winters long and moderate (warmer winters mean more snow, which accumulates to become ice); that is, when there is less summer sun and more winter sun in the northern hemisphere (where there is more land at high latitudes for ice to form).

The timing of the Milankovitch cycles seems to correspond with the changes of climate expressed in cores from the Greenland ice sheet, in cores from the deep sea, from other glaciers and ice sheets, and from tree rings.

During a modern glaciation the surface of the planet goes from 10% to 30% ice. The ice forms mostly in the northern hemisphere, but also in Patagonia and New Zealand and wherever there are high mountains. Antarctica remains always covered with ice.

Glaciers take tens of thousands of years to grow and less than ten thousand to die. They seem to be self-limiting, perhaps because they depress the earth’s surface so much (up to half a mile) so any surface melting rapidly brings them down into warmer regions; and perhaps because they cover so much of the earth, whose plants no longer take up carbon dioxide, which then accumulates in the atmosphere, warming the planet.

As the planet warms it begins sucking up more carbon dioxide in its soils and oceans, both from increased biological activity on that newly uncovered 20% of the earth’s surface and because warmer temperatures speed up chemical reactions. This slowly lowers the carbon dioxide content of the atmosphere and sets the stage for the next cooling. These changes are reinforced (or not) by the planet’s position in the Milankovitch cycles.

The small changes in solar radiation during a Milankovitch cycle, and the not-so-small changes in carbon dioxide content of the atmosphere from glacial to interglacial period (about 50%) are amplified by the planet’s water cycle.

Our next glaciation is overdue. Glaciers should have begun forming about 8000 years ago, as the carbon dioxide content of the atmosphere began to fall. There are some signs of this on the rocky surface of northeast Labrador (the center of the last continental glaciation in North America).

But the ice melted. One theory is that deforestation to clear fields by humans in Eurasia 8000 years ago released enough carbon dioxide to stop the fall in atmospheric carbon dioxide and in fact raised it several parts per million.

The cultivation of paddy rice (which releases methane) and the keeping of cattle, as well as human population increases by 5000 years ago, raised it more.

(I should point out that the last long interglacial period, 30,000 years long rather than the more normal 10,000, was 400,000 years ago, when the orbit of the earth around the sun was nearly round, as now.)

Who knows? The numbers work. What is incontrovertible is that burning coal from the beginning of the industrial revolution in the 1700s began to raise the level of carbon dioxide in the atmosphere from its preindustrial level of 280 parts per million. The rise has accelerated lately, from 1.1% a year in the 1990s to 3% a year in 2000-2004. The current level of carbon dioxide in the atmosphere is 389 ppm.

Carbon dioxide shows a peak in the northern hemisphere fall and a trough in the spring. The seasonal variation is explained by the greater amount of land in the North Hemisphere. Plants use up carbon dioxide during the summer, while animals (and many plants) continue to produce it by respiration during the winter.

A warming climate causes many problems. Sea level rises, partly from melting of ice caps and glaciers, partly from thermal expansion of the water. Many continental glaciers, like the West Antarctic Ice Sheet, are grounded on the bottom of the sea. As the sea rises, ever so slightly, the glaciers lose their grip on the sea bottom and surge forward, speeding their collapse (and raising sea levels, in this case, 16-18 feet).

A warmer ocean also melts the glaciers faster where they meet the sea.

A likely estimate for sea level rise is 3-10 feet by 2100. Two feet will drown the Everglades. New Orleans is already a goner. (Because of subsidence, the relative sea level rise at the mouth of the Mississippi is now 4 feet per century.)

A rising sea also raises the heights of inland rivers, which overflow.

Salt water intrudes into coastal aquifers, such as the Magothy under Long Island, making them undrinkable.

Coastal communities build seawalls, move inland or are abandoned.

A warmer climate reduces grain crops in hot climates. Temperatures for growing maize are already marginal in much of the southeastern United States. Farmland moves north, where soils are rocky and poor.

Some regions become drier, some wetter. Reservoirs run out of water or dams must be reinforced. Heavy downpours are increasing in the eastern and middle western United States. A month’s rain comes in a few hours rather than being spread out. This is hard on crops and forests.

Mountain glaciers in the Himalayas and the Andes store winter snowfall and release it into summer springs and rivers. As they melt, summer flow falls and there is little water for people or crops. Similarly, as the winter snowpack in California’s Sierra Nevada melts earlier, summer water becomes more scarce.

Will this happen? Expressing all the known greenhouse gases as carbon dioxide, the current level of carbon dioxide is 430 ppm, a level not reached for a million years. (But carbon dioxide was about 800 ppm in the temperate Carboniferous 300 million years ago.) The last, recent time carbon dioxide was 350 ppm sea level was 80 feet higher.

Feedback processes are in play. Eastern forests are accumulating carbon much more quickly than 20 years ago; that is, their growth has speeded up. More carbon dioxide in the atmosphere makes it easier for the trees to photosynthesize (they lose less water obtaining carbon). A hopeful sign: but the trees will soon run out of nitrogen, an essential nutrient.

Forests throughout the western United States and Alaska are collapsing from a century of poor management, drought and an infestation of bark beetles. Drought and crowding stresses the trees. Winter temperatures no longer fall low enough to kill the beetles’ larvae and warm long summers let the beetles breed continuously for several months. Crowded stressed trees are not able to mobilize their defenses against the beetles. As these forest collapse and burn they release billions of tons of carbon dioxide to the atmosphere. Similar problems may affect the boreal forests (which may in any case be cleared for agriculture).

Moving societies north will require a lot of energy, which will put more carbon dioxide in the atmosphere.

Warming is most pronounced in polar regions. As the sea ice melts, the ocean absorbs heat from the sun, melting more ice, and the glaciers that touch the sea (as in Alaska and Greenland).

As the tundra warms, and the permafrost below it thaws, the soil releases carbon dioxide and methane. Methane now bubbles up through Siberian lakes all winter keeping them from freezing. As the permafrost around them melts, the lakes expand, then drain away as the land beneath them thaws, leaving the bare ground to warm in the sun, and release methane (perhaps 400 billion tons are stored in the tundra).

Off the northeastern coast of Siberia, methane is bubbling up from the sea from underwater clathrates, lattices of water and ice in which the methane is held by cold and pressure. The amount of methane held in underwater clathrates is enormous. Those under shallow seas will be released by a small increase in temperature of the water (of the order of 1° Centigrade).

A warming climate may shut down the Gulf Stream, part of the deep ocean circulation. Warm tropical water from the Indian and Pacific oceans is pulled north around Africa by the sinking of cold salty water around Iceland and Greenland. The warm water loses evaporated moisture to the Eurasian plains as it moves north, to the Pacific as easterly trades blow it across the Isthmus of Panama. It becomes salty and more dense. When it reaches a sufficient density of coldness and saltiness around the latitude of Iceland, it sinks into the oceanic abyss, helping drive the deep ocean circulation.

As the water moves north out of the tropics it also freshens from fresh water from rivers and melting ice. This lessens its density. If the density of the water is sufficiently reduced, it won’t sink. About every 1500 years over the last 100,000, the sinking has slowed and the earth’s climate has become colder, windier and more dry. A complete halt to the sinking and a disruption of the deep ocean circulation (which keeps the sea oxygenated) would be a disaster.

If we do nothing about the accumulation of greenhouse gases in the atmosphere we may be in for a wild ride.

Who knows? It may be interesting.