The Scientific Evidence for Global Warming

Skepticism means doubt. Philosophical skepticism is sometimes misunderstood to be a stubborn refusal to believe anything that is generally accepted as true. 9/11 conspiracy theorists, UFO researchers, and holocaust deniers all consider themselves skeptics, but they are not. True skepticism is more than just contrarianism. There is a limit to how much human beings can know, and skepticism acknowledges that what is considered true today may be disproved tomorrow. However, it is impossible for us to go through life without accepting some things as true. What we believe should be based on the best evidence. For the skeptic, the wild speculation of conspiracy theorists does not compare to the carefully tested evidence provided by science. Skepticism’s use of empirical evidence to test beliefs is akin to the methods used by scientists to test their hypotheses.

Global warming denial is not skepticism. It does not use the best empirical evidence to support its contrarian claims. On the contrary, it disregards the mass of evidence supporting global warming, favoring any dubious evidence to the contrary. It borders on conspiracy theory when it complains that their alternative theories are not taken as seriously as the peer-reviewed writings of scientists. They use the same type of arguments that creationists use when they criticize the theory of evolution. As in all areas of science, the details are much more difficult to understand than the simplified overview that most of us learn in school. Global warming deniers use those details to their advantage, touting a few cherry-picked items to give their claims an air of legitimacy.

In this article we hope to cut through a lot of the noise and present a summary of the scientific evidence for global warming. In 2007, the International Panel on Climate Change (IPCC) issued a report on the physical science basis for climate change. It is extremely well-sourced, and provides a very readable overview of the science. On the other end of the spectrum, the minority Senators on the Environment and Public Works Committee, with James Inhofe as the ranking minority member, published a report that tried to poke holes in the consensus on global warming. The Inhofe report presented no actual science, and relied on cherry picking and quote mining to make its claims. It should be relatively easy for the skeptic to determine which is the best evidence.

Is the planet really getting warmer?

The temperature of the planet has been measured by humans for only a few hundred years. More recently, scientists have tried to piece together the recorded measurements from the historical record. In 1974, Professor Gordon Manley published a collection of temperatures in Central England from 1659 to 1973(1) . While the Central England Temperature (CET) was a great example of thorough research, more information was needed to measure worldwide temperatures. Measurements from more places around the planet, with standardized instruments, were needed to assemble a trustworthy measurement of the world’s temperature fluctuations.

By 1854, more useful measurements were available, taken with standardized instruments from many locations around the planet, including ships at sea. With this information, a data set could be assembled, and trends plotted. The International Meteorological Organization was formed in 1853, and was later replaced by the World Meteorological Organization, 1950. The information collected and catalogued by these organizations provided reliable data for climate models. “Over 400 million individual readings of thermometers at land stations and over 140 million individual in situ SST observations”(3) go into global climate calculations.

Ice core samples have given us a window to prehistoric global temperatures. The European Project for Ice Coring in Antarctica (EPICA) found that deep ice core from Antarctica “provides a climate record for the past 740,000 years.”(4) Other methods are used to determine the climate going father back, as well as to determine the levels of greenhouse gases present in the atmosphere.(5) For the purposes of determining the climate changes that are happening today, and comparing it to other changes in the planet’s past, we have a significant data set from which to draw.

When all this data is aggregated, what do we have? The EPA published a graph of the measured temperatures since 1880-2006:

(6)

Compared to the age of the Earth, or even the span of human civilization, this is a short period of time. Robert A. Rohde compiled a graph stretching back a thousand years:

(7)

What stands out in these graphs is the recent trend upward. The measurements of temperatures clearly show that the planet is getting warmer. We have already exceded the Medeival warming period. Global warming deniers who claim that it isn’t happening must contend with the hard data. As the late Senator Daniel Patrick Moynihan once said, “Everyone is entitled to their own opinion, but they are not entitled to their own facts.”(8) Temperatures have increased 0.76°C since 1850.(9) “Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature.”(10)

What is Causing Global Warming?

Scientists attribute global warming to the greenhouse effect. Certain constituent gases in a planet’s atmosphere absorb more solar radiation than other gases, so an increase in those constituents will cause the planet's surface to heat. The primary greenhouse gases on earth are: carbon dioxide, water vapor, methane, and nitrous oxide.(11) The greenhouse effect can be natural, and probably accounts for the various warming periods in the planet’s past. Carbon dioxide (CO₂) is frequently blamed for global warming, but it is only a contributing factor. It gets the ball rolling, so to speak.

Factors that change the climate are called “forcings”. When forcings create effects that, in turn, cause more changes, the snowballing of causes and effects is called a “feedback”. “A positive feedback intensifies the original process, and a negative feedback reduces it.”(12)

Just as scientists have measured global temperature changes through the ages, they have also measured changes in the makeup of the Earth’s atmosphere. As of 2007, the concentration of CO₂ in the atmosphere is 383 parts per million (ppm).(13) This compares to approximately 375 ppm in 2003. The measurements taken at the Mauna Loa Observatory in Hawaii show the steady upward trend.

(14)

Analysis of ice core samples provides the levels of atmospheric CO₂ going father back. A 400,000 year span shows peaks and valleys.

(15)

The levels of CO₂ has risen since the Industrual Revolution. Coal-burning power plants and other air-poluting industrial processes have been pumping carbon dioxide into the atmosphere nonstop for over a hundred and fifty years. It was initially thought that this excess carbon would be absorbed by the oceans. Calculations later showed that the process of carbon dioxide absorbtion by the oceans was far too slow to keep up with human activity. Roger Revelle concluded in the 1950s that “whatever CO₂ humanity added to the atmopshere would not be swallowed up promptly, but only over thousands of years.”(16)

Before we can blame global warming on carbon dioxide, a causal relationship must be established. An interest in the greenhouse effect was spurred by studies of other planets in our solar system. Carl Sagan’s predictions of climate on Venus and Mars inspired scientists to look at how the the same analysis could be applied to the Earth.(17) When temperature and CO₂ levels are looked at together, the correlation is apparent.

(18)

The peaks of the CO₂ chart closely match the global warming periods. Conversely, ice ages can be found in the low points of CO₂ levels.

There have been attempts to refute the causal relationship between carbon dioxide levels and temperature change. A British film called The Great Global Warming Swindle collected many of the contrarian claims into a documentary, which its critics are not hesitant to call propaganda. Among them is the assertion that there is an 800 year lag between the rise in temperature and the rise in carbon dioxide.(19) That would mean that CO₂ only began to rise after the planet had already started to warm, disproving it as a climate change forcing. While this lag can be seen in prehistoric warming periods, it is misleading when talking about today’s conditions. “On historical timescales, CO₂ has definitely led, not lagged, temperature.”(20) Today, carbon dioxide has most definitely not been lagging temperature rise. If anything, this assertion demonstrates the uniqueness of our current times. We are not in a typical warming period, a fact that should gravely concern us. Some of the scientists used in The Great Global Warming Swindle say that their contributions were misused and their true positions were misrepresented.(21)

As noted earlier, carbon dioxide only gets the ball rolling. The greater problem is water vapor. The temperature rise caused by increased carbon dioxide causes more water to be evaporated into the atmosphere.(22) You will recall that water vapor was included on the list of greenhouse gases. Water vapor is a positive feedback, causing a greater rise in temperature that, in turn, triggers other feedbacks that cause more heating.

Some scientists have downplayed the significance of greenhouse gases in global warming, instead focusing on other forcings. Besides carbon dioxide, another possible culprit is the sun itself. The Sun has cycles in which variations in solar radiation can be observed. The primary sunspot cycle rate is 11 years, in which solar radiation rises and falls by approximately 0.1%.(23) A related hypothesis attributes recent warming to cosmic rays that ionize in the atmosphere. These ideas have merit, in that these forcings do have an effect on climate. They do not, however, explain our current warming period. “Solar activity can account for nearly all of the early 20th century warming, but essentially none of the recent warming.”(24) The current climate changes are too big to be accounted for by solar cycles or cosmic rays.

Despite the attempts to explain global warming using other models, greenhouse gases are still considered the main factor in the measurable temperature increases we see today.

What are the Feedbacks?

The domino effect of global warming, by which one process effects another process, which effects another, and so on, intensifies the impact of small changes in the system. Some feedbacks are negative, meaning that they would tend to exert a cooling effect. Aerosols are a good example. The use of man-made aerosols, coupled with aerosols spewed into the atmosphere by volcanoes, led some scientists to predict a global cooling trend. Such predictions are correct about the physical properties of the aerosols: they cool, rather than warm. “However, because aerosols remain in the atmosphere only a short time compared to CO₂, the results were not applicable for long-term climate change projections.” (25)

Rain typically clears aerosols out of the atmosphere in a week or two, but when material from a violent volcanic eruption is projected far above the highest cloud, these aerosols typically influence the climate for about a year or two before falling into the troposphere and being carried away to the surface by precipitation. (26)

As the feedback dominos fall, the problems mount. A serious feedback that has been observed in the Polar Regions is the melting of the permafrost. The permafrost, as the name implies, is permanently frozen soil.(29) Global warning is beginning to thaw parts of the planet that are usually always frozen. The result of this thawing is the release of methane, a greenhouse gas. “A vast expanse of western Siberia is undergoing an unprecedented thaw that could dramatically increase the rate of global warming.”(30) These permanently frozen bogs have trapped large amounts of gas in the ground. “According to Larry Smith, a hydrologist at the University of California, Los Angeles, the west Siberian peat bog could hold some 70bn tonnes of methane, a quarter of all of the methane stored in the ground around the world.”(31)

Another feedback is cloud cover. Many global warming deniers have jumped on cloud cover, because it is admittedly the climate element about which there is the most uncertainty. “Clouds… do exert a blanketing effect similar to that of the greenhouse gases; however, this effect is offset by their reflectivity.”(32) Clouds have an albedo effect, like the sea ice, and a blanketing effect like greenhouse gases.

So which wins out? Are clouds a positive or negative feedback? Contrarians who wish to dismiss global warming say that clouds will have a cooling effect, negating any warming caused by other forcings. Such assertions are dubious, and may be intended to attack the otherwise strong global warming science at its weakest point. A paper in The Journal of Climate by Brian J. Soden and Isaac M. Held admitted that “intermodel differences in cloud feedback are found to provide the largest source of uncertainty in current projections of climate sensitivity.”(33) Though this is a serious admission, and it points out an area that needs more study, it does not support contrarian claims. The same report notes: “clouds appear to provide a positive feedback in all models.”(34) Everything we know so far tells us that the warming effect of clouds outweighs the cooling effect.

Are Humans to Blame?

Though there are many natural causes of prehistoric global warming, anthropogenic causes have become evident in our current warming period. Scientists have explored other causes, some of which were discussed above. Even accounting for non-human factors, we are beginning to see the hand of man imprinted on the environment. The Industrial Revolution led to massive consumption of fossil fuel and the burning of coal for electrical power. Approximately 150 years of constantly pumping carbon dioxide into the air has resulted in a change in the planet’s atmosphere. The earliest scientists to track global warming thought this would not be possible. They believed that the ocean would absorb the excess CO₂ from human sources, but the oceans absorb at a slow pace. The planet could not keep up with our rapid pollution.

The IPCC’s Working Group I summarized the rise of carbon dioxide:

The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores. (35)

Carbon dioxide is not the only poison humans have spewed into the air. Many of our industrial processes have produced aerosols, which perform the opposite work of greenhouse gases. In 1971, two scientists, S. Itchtiaque Rasool and Stephen Schneider, calculated that aerosols would cause a global cooling. “In fact, their equations and data were rudimentary, and scientists including Schneider himself soon noted crippling flaws.”(36) As observed earlier, aerosols do not stay in the atmosphere long enough to exert a significant cooling effect. They end up on the surface of the planet, even if propelled into the upper atmosphere by volcanoes. Carbon dioxide remains in the air much longer, more than offsetting any cooling caused by aerosols.

The facts are indisputable. Carbon dioxide levels in the atmosphere are measurable, and they are rising. Global temperature is measurable, and it is rising. Carbon dioxide has been found to be a greater factor in the rise of temperature than alternative forcings, such as solar variation. The rise in carbon dioxide since the Industrial Revolution is attributable to human activity. Global warming deniers, conspiracy theorists, and contrarians must contend with these basic, measurable, testable facts.

Is Global Warming a Big Deal?

Beyond the anthropogenic CO₂ forcing, the feedbacks are alarming. A small amount of warming creates factors that cause even more warming. This means that the problems that arise from global warming will accelerate. Environmental damage that once seemed far in the future will rush up to meet us.

Not all scientists think global warming is anything to worry about. Guy Stewart Callendar, one of the first scientists to discover global warming, considered it a “good thing for humanity, helping crops to grow more abundantly.”(37) Today’s scientists cannot take such a sanguine view, as they look at the long-term negative effects of climate change. One study predicts, “on the basis of mid-range climate-warming scenarios for 2050, that 15-37% of species in our sample of regions and taxa will be ‘committed to extinction’.”(38)

The temperature trend will change the timing and duration of the seasons. The IPCC Working Group II predicts “earlier timing of spring events, such as leaf-unfolding, bird migration and egg-laying.”(39) This may seem like a good thing, in that it might allow for a longer growing season, but it can be devastating to plants and animals that begin their spring rituals too early. Crops will be vulnerable if they bloom early, only to have temperatures drop back below freezing.(40) Farmers face a host of agricultural challenges. “Warmer climates and less soil due to increased evaporation may increase the need for irrigation. However, these same conditions could decrease water supplies.”(41)

Melting land ice causes sea level rise, threatening coastal cities. The sea level has already rising, and the rate is increasing. Between 1961 and 2003, the sea level rose at a rate of about 1.8 mm per year.(42) “The rate was faster over 1993 to 2003: about 3.1 [2.4 to 3.8] mm per year.”(43) Small islands are at risk of submersion, which will cause major economic catastrophes for island nations.

The redistribution of rain will result in droughts of increasing frequency and duration.(44) Parched land will be prone to wildfires. Increased rain in other areas will result in flash flooding. “The models predict that there will be greater periods of no rain, while single storms will produce more rainfall, much of which will not soak into the soil, but create flash floods.”(45)

Ironically, global warming may create temporary benefits for some interests. The melting of Arctic ice is creating new transit routes in previously impassible areas.(46) This is unfortunate, as it creates a financial incentive to perpetuate the thaw. The long-term wellbeing of the many could be sacrificed for the temporary profit of a few.

What Can be Done About It?

The simple answer is to reduce carbon dioxide emissions. This is not as easy as it sounds, and it will not cure the disease overnight. The feedbacks caused by the current levels of CO₂ will take longer to reduce, and the negative effects from them will be felt for some time. The ecosystem does not turn on a dime. It took over 150 years to get to this point. Persistent efforts to eliminate the causes and mitigate the effects of global warming will require cooperation on an unprecedented scale.

The challenge of global warming presents serious political problems. To reverse global warming, some activities simply must not continue, regardless of what market forces dictate. Plentiful, cheap sources of energy will have to be eschewed in favor of clean energy. Political ideologies that do not permit some regulation of industry will not have an adequate answer for global warming. This does not necessary advocate for socialist planned economies, but it does present difficulties for laissez faire economics.

Anarchic, libertarian, and conservative ideologues feel threatened by the political implications of global warming. To save themselves, they have led the charge of global warming denial. It is through their efforts that the spurious and contradictory claims of contrarians are publicized, creating the false impression that there is not a consensus about global warming in the scientific community. John Stossel, a journalist with libertarian leanings, claims that global warming is not caused by humans, and at any rate it is no big deal.(47) The U.S. Senator from Oklahoma, James M. Inhofe, believes that global warming is a hoax.(48) In dealing with a situation that requires great political will and intergovernmental cooperation, powerful and influential people are working in the opposite direction. When used as political tools, contrarianism and conspiracy theories can be dangerous.

The most important thing we can do to reverse global warming is to hold our elected officials and corporations accountable for their actions, and insist on positive movement in the right direction. Most importantly, we must accept that the problem is real. Like with all addictions, we cannot overcome the habitual use of carbon-dirty energy until we first accept that we have a problem. There are powerful interests that want us to remain addicted, and will encourage our denial. It is important that we approach their claims with a healthy skepticism.

© 2008 Bryan Lower

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