The Natural and Enhanced Greenhouse Effect

Since Tyndall’s time, scientists have understood the basics of the greenhouse effect, which describes how greenhouse gas molecules trap solar radiation (heat) near Earth’s surface. Some solar radiation never reaches Earth’s surface because it is reflected out into space by clouds and dust high in the atmosphere. Some solar radiation is reflected back into space by Earth’s ice- and snow-covered surfaces, which have high reflectivity, or albedo. Some solar radiation is absorbed by the land and the oceans. The solar radiation that is neither reflected away from the planet nor absorbed by the planet’s surface is sent back toward space as infrared radiation. Some of this infrared radiation escapes into space. However, some of it is absorbed by GHGs in the atmosphere and then reradiated back to Earth’s surface, where it warms the planet. Thus, the more GHGs there are in the atmosphere, the warmer the planet’s surface will be.

The greenhouse effect is not necessarily negative. In fact, every living thing on Earth owes its life to the natural greenhouse effect. Without heat-trapping gases in its atmosphere, Earth would be a frozen, lifeless wasteland. The GHGs that are emitted naturally into the atmosphere (water vapor from evaporation; volcanic CO2, for example) maintain the world’s warm, life-sustaining climate.

The main naturally occurring GHGs are: water vapor, CO2, and methane. (The primary components of the atmosphere—nitrogen and oxygen—are thermally neutral and have no impact on the greenhouse effect.)

• The enhanced greenhouse effect refers to GHGs that have been added to the atmosphere by human activity. The enhanced greenhouse effect leads to global warming because the additional GHGs reradiate more infrared radiation and heat back to Earth’s surface. Carbon dioxide is not the only GHG in Earth’s atmosphere.

• Water vapor and methane have been mentioned as vital GHGs. Methane levels in the atmosphere increase with the number of livestock raised and the amount of rice grown. In the 1980s, it was found that deforestation also adds methane to the atmosphere. These activities have resulted in an increase in atmospheric methane concentrations from 791 ppb (parts per billion) in 1850 to 1,847 ppb in 2004.12 CFCs (chlorofluorocarbons) are a thoroughly anthropogenic (humanmade) source of greenhouse warming. CFCs are a family of chemicals that were used as propellants (in aerosol cans such as hairspray) and as refrigerants in air conditioners and refrigerators from the 1950s to the 1980s.

• After it was discovered that CFCs destroy stratospheric ozone, creating an annual “ozone hole” over Antarctica, in 1987 nearly all the nations of the world signed on to the Montreal Protocol, an international agreement to phase out production and use of CFCs.

• CFCs are thousands of times more potent than CO2 at trapping heat and they remain in the atmosphere for centuries. So CFCs (and to some extent the hydrofluorocarbons, or HFCs, that replaced them) continue to act as GHGs. Nitrous oxide (N2O), coming mainly from fertilizers and disturbed soil, was identified in the 1970s as another powerful GHG.

All in all, by 1985 more than 30 trace gases were found that amplify the greenhouse effect. Most occur in minute amounts, but together they can cause significant warming. Though it is not listed among the GHGs that are affected by human activity, water vapor is one of the most potent GHGs on Earth. The heattrapping capacity of water vapor is largely responsible for the natural greenhouse effect that created the life-giving warmth of Earth’s climate. The intimate relationship between air temperature and the amount of water vapor in the air (via evaporation) is one vital mechanism that drives global warming. Further, water vapor amplifies the effects of atmospheric CO2; thus it has a major impact on climate change. However, its short residence in the atmosphere (about 10 days), among other factors, means that water vapor has not been assigned a numerical global warming potential (GWP), comparing its heat-trapping capacity to that of carbon dioxide. This lack of designation should not lead one to underestimate the potency of this important GHG.

As scientists gained more understanding of climate cycles and the greenhouse effect, pressing questions arose: How do minor changes in the amount of sunlight reaching Earth cause climate changes as drastic as ice ages? What relationship, if any, does CO2 have to climate changes caused by orbital variations? Is there some trigger or strong feedback mechanism that provides the necessary push to propel a small change due to orbital or axial variations into a major climate shift? Earth’s climate is a nonlinear system in which seemingly insignificant, step-by-step changes can suddenly cross a threshold and snowball to cause dramatic climate shifts. Even relatively small alterations in some aspect of the climate can initiate feedbacks that amplify the effects of these changes.

Once a feedback mechanism begins, it may send the climate hurtling over a threshold that causes irreversible climate change. Scientists speculate that CO2 might be one of the triggers that flips the sensitive and delicately balanced climate into a new regime. Ice and sediment core studies would reveal how closely coupled CO2 and shifts in Earth’s climate system really are.

Core Confirmations

Even back in the 1950s and 1960s, it seemed logical to some observers to correlate higher CO2 concentrations with fossil fuel burning—where else could all that extra carbon be coming from? Yet there was no conclusive evidence either that human activity was solely responsible for the excess CO2 or that was a bad thing. Maybe a warming climate would keep the next ice age at bay and save civilization. Until incontrovertible evidence showed that a warming climate was dangerous and undesirable and that it was being caused by human burning of fossil fuels, societies would resist the economic and lifestyle disruptions that abandoning fossil fuels would entail. After all, everything in modern industrial society is powered by fossil fuels, from electricity generation (mostly coal powered) to home heating (mainly oil) to transportation (gasoline).

Obviously, more research was needed. That research delved deep into Earth’s ice and sediment. Ice Cores One way to determine if today’s climate changes are the result of human activity is to dredge up data from ancient climates and then compare what happened then with what is happening now. If paleoclimate conditions resemble what is happening today, then the argument that a natural cycle is causing today’s observed warming is supported. If climate conditions observed today, particularly in terms of the rate and degree of atmospheric CO2 increase, are absent from the paleoclimate record, then the climate changes currently observed can likely be attributed to human activity.

Ice sheets are a perfect place to look for clues about ancient climates. When snow falls on an ice sheet and is compacted into ice, it contains minute bubbles of the air through which it fell. So every snowflake that has fallen on an ice sheet over time deposits in the ice a minute sample of Earth’s air at the time the snow fell. Scientists can analyze those ice-bound air bubbles to find out the chemical composition of the atmosphere in the distant past.

To travel really far back in time, scientists must analyze ice from an enormously thick ice sheet. That is why most ice core research is conducted in Greenland or Antarctica. Greenland’s ice sheet is several kilometers thick, and its lower layers formed hundreds of thousands of years ago.

The milesthick ice sheets in Antarctica contain ice more than a million years old. To get at ancient ice, intrepid teams drill into the ice to remove a core that is usually a 10–12 centimeter- (4–5 in.) diameter cylinder of ice.

• The first ice core, drilled in 1961 at Camp Century in Greenland, was only a few feet long and revealed little about ancient climates. By 1966, advances in drilling technology allowed these researchers to extract an ice core 1.4 kilometers (0.87 mi.) long, representing 100,000 years of Earth’s climate.

• Two years later, a 1.6 kilometer- (1-mi.) long ice core was removed from the Ross Ice Shelf in Antarctica. By the late 1980s, scientists in Greenland were able to extract cores of increasing length (and therefore age), as were drilling teams in Antarctica, especially at the research station at Lake Vostok. Removing a cylinder of ice from a glacier is not simply a matter of drilling a hole and yanking out a core.

• As ice is removed from the depths, it must be lifted with extreme care or the lessening of pressure on the ice as it nears the surface will cause it to explode. After refrigerating and examining the core, scientists carve it up into thin slices that are easy to handle and whose microscopic characteristics can be minutely analyzed. Scientists first assess a core’s visible characteristics. For example, ice is laid down in layers that are comparable to tree rings. Scientists can measure the size of each layer to determine which periods got more or less snow and the opacity of the layers to see which layers contain the most dust (indicating dry, windy conditions or volcanic eruptions). Unfortunately, for a number of years, two of the most important clues held in the ice—the chemical composition of its air bubbles and the temperature at which it formed—were technologically impossible to unravel.

• Then in the 1960s, Danish paleoclimatologist Willi Dansgaard (1922– ) discovered a way to use isotopes of oxygen to determine the temperature at which ancient ice formed. Scientists knew that a rare isotope of oxygen, oxygen-18, is heavier than “normal” oxygen-16. When the climate is cold, O-18 will condense before O-16, and O-18 will also precipitate out of clouds before O-16. Dansgaard showed that it is possible to determine the precise temperature at which various ratios of O-16 to O-18 will occur. An analysis of the ratio of O-16 to O-18 in ice tells scientists the atmospheric temperature at the time the ice was laid down. Determining temperature at the time of ice formation was further refined by Jeffrey Severinghaus (1959– ), who, in 1999, showed that analyzing the amounts of argon and nitrogen isotopes in the air bubbles enabled scientists to date changes in surface temperature at the time of ice formation to within a decade—a remarkable achievement and a key to understanding abrupt climate change.

• In the 1970s, scientists developed a dependable way to retrieve and analyze the air bubbles trapped in ancient ice. The method involved crushing a squeaky-clean ice sample in a vacuum chamber that contained gas-analyzing equipment. The equipment was able to accurately analyze the chemical composition of the tiny, rapidly exploding air bubbles. Using these two vital analytical tools, climatologists finally were able to conduct the crucial analyses of past climates that would put our own changing climate into perspective. What they found was momentous, astonishing, and troubling.

• In 1985, researchers in central Antarctica published their study of a 2-kilometer- (1.24-mi.) long ice core taken from the huge ice sheet at Lake Vostok. This core contained a record of the temperature and composition of the atmosphere over the past 150,000 years (a grand climate cycle of ice age, warm period, ice age). Significantly, the study results showed that the globally averaged temperature rose and fell in step with concentrations of CO2 in the atmosphere. These results prompted one expert to conclude that there is an  “emerging consensus that CO2 is an important component in the system of climatic feedbacks” and that future research would “require treating climate and the carbon cycle as parts of the same global system rather than as separate entities.”

Scientists were impressed by these findings, but hesitated to use them to declare that “global warming is real.” Though the data were compelling, they revealed only one grand climate cycle. Perhaps, scientists speculated, this grand climate cycle was in some way abnormal. So instead of claims of certainty, climatologists called for more and longer cores to reveal conditions through several grand climate cycles. It was not long before deeper ice cores were drilled and subjected to the same analyses. By 1987, a Vostok core dating back more than 160,000 years showed the same CO2-temperature coupling.

A few years later, the Vostok team removed an ice core dating back 420,000 years that revealed the climate through four grand climate cycles. Analysis of this core showed that during the coldest part of the four previous ice ages, atmospheric concentrations of CO2 leveled out at about 180 ppm. During the warmest part of the four interglacial periods, CO2 concentrations never exceeded 280 ppm. Antarctic drilling teams continued to pull longer and older ice out of the ice sheet. All the Antarctic cores—from 600,000 years ago, from 850,000 years ago—confirmed the CO2 concentration data. At no time during the last eight interglacial warm periods had CO2 concentrations topped 280–300 ppm.

At the time these scientists were conducting their analyses, the air they were breathing contained CO2 concentrations of 345–382 ppm—truly unprecedented elevations of CO2. These studies revealed that CO2 was a significant factor in amplifying the changes in the global paleoclimate caused by orbital variations. The research underscored the crucial difference between natural climate variations in the ancient past and climate change today. During past grand climate cycles, as the ice age waned, the ocean warmed along with the climate.

The warmer ocean emitted to the atmosphere large quantities of CO2, which amplified the natural climate change, but did not induce it. In our current situation, CO2 is a causative factor that is enhancing the greenhouse effect and warming the global climate at a rate and to a degree not seen before. Based on their ice core study, the Vostok scientists stated that continued emissions of CO2 would produce “a warming unprecedented in the past million years, and [would occur] much faster than previously experienced by natural ecosystems.