Introduction to Climate Change
Beginings of a Theory
Around 1900, a respected Swedish scientist named Svante Arrhenius published a series of papers and a book that included a crazy-sounding prediction. He and a colleague were studying the carbon cycle by estimating the changes in carbon dioxide ($CO_2$) produced by natural processes such as rock weathering, volcanic eruptions, and ocean absorption.
He also looked at a source no one had thought of before—humans. Was it possible that humans could change the climate? Arrhenius took his colleague's calculations of carbon emissions from human activities and crunched the numbers.
When atmospheric carbon doubled, he figured, it would be enough to raise Earth's temperature 9-11°F (5-6°C), but it would take thousands of years to do it at 1896 rates. By the time his book on the subject was published in 1908, so much more coal was being burned, Arrhenius revised his estimate to centuries. But he reasoned that a warmer climate would be a GOOD thing—understandable, perhaps, given his home in Stockholm, a few hundred miles shy of the Arctic Circle.
Although Arrhenius went on to win the Nobel Prize for Chemistry in 1903, his carbon calculations faded into obscurity. In the hundred years since Arrhenius made his estimate—remarkably close to today's best figure—scientists hotly debated the causes of past and current climate change. However today, most climate scientists agree that multiple lines of evidence clearly show that human-induced climate change is taking place.
Climate Influences: Natural Factors
Earth's climate shifts over time because so many different land, ocean, and space phenomena have a hand in it. The sun is the main driver of Earth's climate, as it provides most of the energy. The sun's energy output increased about a tenth of a percent from 1750 to 1950, which contributed about 0.2°F (0.1°C) warming in the first part of the 20th century. But since 1979, when we began taking measurements from space, the data show no long-term change in total solar energy, even though Earth has been warming.
$Figure$ $1.$ $Milankovitch$ $Cycles$
Repetitive cycles in Earth's orbit can influence the angle and timing of sunlight. The tilt and wobble of Earth's axis and the degree to which its orbit is stretched produce the Milankovitch cycles, which scientists believe triggered and shut off ice ages for the last few million years. But these changes take thousands of years, and so cannot explain the warming in this century.
Drifting continents make a big difference in climate over millions of years by changing ice caps at the poles and by steering ocean currents, which transport heat and cold throughout the ocean depths. These currents in turn influence atmospheric processes. Snow and ice on Earth also affect climate because they reflect more solar energy than darker land cover or open water.
This map shows currents as they may have existed with Pangea. We could expect to see both subtropical and polar gyres in the Panthalassa Ocean. We also could expect to see north and south subtropical gyres in the Tethys Sea.
Global average surface temperature change with volcanic eruptions
Huge volcanic eruptions can cool the Earth by injecting ash and tiny particles into the stratosphere. Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.
Major eruptions alter the Earth's radiative balance because volcanic aerosol clouds absorb terrestrial radiation, and scatter a significant amount of the incoming solar radiation, an effect known as "radiative forcing" that can last from two to three years following a volcanic eruption.
Incoming solar radiation and the effect of increasing greenhouse gases
also influence Earth's climate.
Climate Influences: Greenhouse Effect
Incoming and outgoing solar radiation
Earth's surface absorbs heat from the sun and then re-radiates it back into the atmosphere and to space.
We know the amount of incoming solar energy at the top of the atmosphere at diurnal, synoptic, and short-range climate timescales. But uncertainties arise in emulating the effects of the atmosphere and earth's surface on incoming solar and outgoing terrestrial radiation, which involves the following:
In the atmosphere:
- Reemission (for atmospheric longwave radiation)
- At the earth's surface
Transformation from shortwave into other forms of energy at the earth's surface, based on the state of that surface over the area covered by the model grid box
Net emission of longwave radiation from the earth's surface toward space
These uncertainties exist because:
- We can only crudely emulate the effects of the atmosphere and its constituents (for example, clouds, aerosols, and absorbing gases) on the incoming solar beam and outgoing terrestrial/longwave radiation
- We can only estimate the state of the land and sea surface in models and its effects on the absorption and subsequent conversion of incoming shortwave radiation into other forms of energy
- The real world data needed to fine-tune the emulation of land and sea surface physics (for example, soil moisture and surface fluxes) are incomplete
Even if a radiation model were perfect, model forecasts would be subject to errors in:
- Initial analyses of moisture and cloudiness
- Predicting the location and thickness of clouds
- Predicting the amount of moisture, aerosols, and trace gases in the atmosphere
- Analyzing, predicting, and/or prescribing the land and/or ocean surface state
Much of the outgoing longwave IR is absorbed by greenhouse gases, which then send the heat back to the surface, to other greenhouse gas molecules, or out to space. This is commonly called "the Greenhouse Effect", which is a misnomer and we are stuck with it, "the blanket effect" may be more appropriate, but is still inadequate. "Greenhouse gases", is even worse, as greenhouses are often deficient in $CO_2$ during the winter when tightly sealed.
Far less than 1% of atmospheric gases are greenhouse gases, but they are extremely powerful heat adsorbers.
Though carbon dioxide gets the most attention, it's certainly not the only greenhouse gas, nor even the most powerful. However, humans produce more of it than any other greenhouse gas, and it's very long-lasting (50-100+ years). In the United States, $CO_2$ comprises more than 80% of total greenhouse gas emissions.
Example of Human-Produced Greenhouse Gases
|Global Warming Potential (Relative to CO2)|
|Species||Lifetime (years)||20 years||100 years||500 years|
|Methane ($CH_4$)||12+/- 3||56||21||6.5|
|Nitrous Oxides ($NO_x$)||120||280||310||170|
|Sulfur hexafluoride ($SF_6$)||3,200||16,300||23,900||34,900|
|Carbon tetrafluoride ($CF_4$)||50,000||4,400||6,500||10,000|
The other greenhouse gases are both natural and human-made. The most common are methane, nitrous oxide, ozone, fluorinated gases, and water vapor. Methane, for example, is only about 8% of U.S. greenhouse gas output, but is 21 times more powerful than carbon dioxide per molecule, although it does not stay in the atmosphere as long. It is produced naturally in wetlands, melting permafrost, termites, belching cows, and by human activities, such as fossil fuel production, landfills, and rice cultivation.
Water vapor is by far the most important gas in the natural greenhouse effect, contributing 60% of the effect to carbon dioxide's 26%. Human activities don't directly increase water vapor. Instead, warming produced by other gases, such as $CO_2$, increases evaporation and allows the atmosphere to hold more water vapor. And in fact, satellites have detected an increase in atmospheric moisture over the oceans at a rate of 4% per degree F of warming (7% per degree C) since 1988. This additional water vapor then adds to the warming because water vapor is a greenhouse gas. More water vapor can also produce more clouds, which have a complicated effect of both cooling the atmosphere by reflecting light and warming it by trapping heat below the clouds.
The main factors that determine the effect of greenhouse gases on climate are:
- The amount and rate of greenhouse gas emissions
- The effectiveness of each gas in trapping heat, and
- The length of time each gas stays in the atmosphere (for example, $CO_2$, lingers in the atmosphere for hundreds of years)
Other greenhouse gases like water vapor are more powerful on a molecule for molecule basis, but the volume of manmade carbon dioxide emissions into the atmosphere this century and its atmospheric staying power are why carbon dioxide is the focus of concerns.
Climate Influences: The Carbon Cycle
Carbon dioxide isn't stuck in the atmosphere once it gets there. It moves into and out of living organisms, soil, rock, and water. For example, plants take up carbon dioxide in the air to make wood, stems, and leaves, and then release it back into the air when the leaves fall or the plants die. Forest fires release large amounts of $CO_2$, providing an important reason to preserve forests. Animals, including humans, take up carbon when they eat plants, and then release $CO_2$ back into the atmosphere via respiration.
Over very long time frames, the weathering of rocks can add carbon to surface waters that run into the ocean. Eventually, this carbon is removed from the water and forms limestone. It can later be released back into the atmosphere from volcanoes when the rock is melted.
In ancient times when Earth had a much warmer climate, huge swamps buried plant material faster than it could decay, and when the buried remains were subjected to heat and pressure, they became coal. In similar ways, microorganisms buried on lake and sea bottoms throughout Earth's history produced oil. These processes locked up lots of carbon as oil, gas, and coal. By burning these fuels in the last 150 years, we have released carbon dioxide into the atmosphere that took hundreds of millions of years to store.
Climate Influences: Past Climates
Graph of temperatures and $CO_2$ concentrations for the past 800,000 years (not including today's values)
Certainly, temperatures in Earth's past have been higher (and lower) than today, and $CO_2$ concentrations have varied considerably. At certain times, changes in the Earth's orbit caused warmer temperatures, which increased $CO_2$ and produced additional warming in a feedback process. But today, the $CO_2$ released by human activities is triggering the increase in temperatures. This is thought to be primarily due to the Milankovitch Cycles discussed previously.
Although increases of 1.0-1.6°F (0.6-0.9°C) over the last century or so may not sound very threatening, but it's a global average. The warming is stronger over land than over oceans and in the higher latitudes than in the tropics.
Let's look at the global map of surface temperature anomalies for the year 2008:
Notice that much of the world (and especially the Arctic) was warmer than normal (red and orange colors), although the U.S. was fairly close to normal. This illustrates the idea that you can't just look at the weather in one area—you need to see the picture over the entire planet.
Note also the large pool of cooler (blue and green) temperatures in the Pacific Ocean. These lower temperatures reflect a strong La Niña pattern, a naturally occurring oscillation of tropical ocean temperatures that also shape weather patterns. So how did the 2008 La Niña affect the picture?
Global seasonal temperature anomalies since 1950
Looking at the seasonal temperatures, we can see that the La Niña cooling cycle was strongest in the low latitudes and that it bottomed out in the winter (December 2007 through February 2008). Once the La Niña pattern relaxed, temperatures bounced upward again. This is a good example of natural climate variability superimposed on the global warming signal.
The top 1,000 feet (300 meters) or so of the ocean have warmed by 0.5°F (0.3°C) over the past 50 years. The deep sea, too, has warmed. A NOAA study that looked at the period from 1948 to 1998 found that every ocean warmed to at least 3,300 feet (1,006 m).
Graph of sea level rise (1993-2008)
Global average sea level since 1993 has risen at a rate of about 0.13 inches (3.3 millimeters) per year according to IPCC. Estimates of the levels of the oceans rose 4 to 10 inches (10-25 centimeters) in the 20th century, due to melting ice and snow and also the physical expansion of warmer water.
Snow and Ice
Snow and ice reflect the sun's energy back to space. Without this white cover, more water can evaporate into the atmosphere where it acts as a greenhouse gas, and the ground absorbs more heat. Snow and ice are melting at rates unseen for thousands of years. As with air temperature, most of the melting in our 100 years or so of official record keeping has occurred after 1980.
Spring snow cover has decreased since 1922 at an average rate of about 2% per decade in the Northern Hemisphere, including a steep 5% drop during the 1980s. River and lake ice don't last as long as they used to either. As permafrost melts in the vast northern tundra, trees locals colorfully call "drunken trees" are falling over and buildings are crumbling as the ground disintegrates beneath them.
Photo of Grinnell Glacier in 1900, Glacier National Park
Photo of Grinnell Glacier in 2008, Glacier National Park
With a few exceptions, glaciers have been shrinking across the globe. In Glacier National Park, for example, there were 150 glaciers in 1850. Today, there are 26. In Switzerland, the Tortin Glacier, which supported a local ski area, shrank so much that the Swiss put a city-block sized insulating sheet over the glacier's edge to slow its retreat.
Graph of Arctic sea ice extent (1979-2000 average, 2007. 2008)
Satellites have seen average Arctic sea ice shrink by 2.7% per decade from 1978 to 2006, with faster melting in summer.
Satellite image of the Northwest Passage 2007
In summer 2007, the Northwest Passage north of Canada became navigable for the first time as the polar cap melted to its lowest level on record—30 years faster than IPCC scientists had predicted; 2008's melt was second only to 2007.
Time lapse photography of major ice loss in the Arctic
Rain and Drought
Trend in contribution to total annual precipitation from very wet days
Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 3.39, Cambridge University Press.
More water vapor held by a warmer atmosphere also leads to heavier rains and more snowfall. In particular, heavy rains are increasing in temperate zones. For example, in the continental United States, intense precipitation increased by 20% over the past century, while total precipitation increased 7%.
Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. FAQ 3.2, Figure 1, Cambridge University Press.
More evaporative power doesn't just mean more rain—paradoxically, it also means that some areas get drier as storm tracks shift. A 2004 study by the National Center for Atmospheric Research found that the percentage of Earth's land experiencing serious drought had more than doubled since the 1970s.
Along with drought, wildfires have become more widespread recently across the U.S. and Canadian West.
Increased warmth has also affected living things. Plants and animals are experiencing longer growing seasons across the Northern Hemisphere. For example, the Japanese keep very detailed records on the blossoming of their Tokyo cherry trees, so they know they are blooming 5 days earlier on average than they were 50 years ago.
Mosquitoes, birds, and insects are also moving north in the Northern Hemisphere. The pine beetle, a native pest of lodge pole and jack-pine trees, has chewed up tens of thousands of square miles of forest across North America, staining the green forest with the rust of dead needles. A 2008 Nature article attributed their move northward and to higher elevation forests to warmer winters, hotter summers, and less rain. To make matters worse, the scientists projected that the beetles themselves could become a force for climate change. For example, the carbon released by the decay of the trees they have killed in British Columbia may cause the forest to emit more carbon than it absorbs.
The Human Element: $CO_2$
In pre-industrial times, carbon dioxide made up about 280 parts per million atmospheric molecules. In 2008, it makes up about 385 parts per million—30% more. So what's the big deal about a few hundred parts per million? Not only is the present atmospheric carbon dioxide concentration higher than it has been for at least 800,000 years, the rate of change is accelerating. Previously it never exceeded 30 parts per million per thousand years, but now, carbon dioxide has risen by 30 parts per million in the last 17 years.
Worldwide, burning fossil fuels is the major source of $CO_2$, but $CO_2$ can also come from cutting trees, burning forests or grasslands, agriculture, and interestingly, making cement.
According to data from the Netherlands Environmental Assessment Agency, China passed the U.S. in 2006 and is now the largest emitter of $CO_2$. Notice that the combined emissions of the EU-15 group and the large developing countries group are lower than the U.S. emissions.
Per capita $CO_2$ emissions in 2007
On a per person basis, the United States, with a population of 0.31 billion people, emits more than twice as much as the citizens of the EU-15 and about 10 times as much as 1.14 billion people in India.
The Human Element: Other Greenhouse Gases
Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 2.3, Geneva, Switzerland.
Carbon dioxide isn't the only greenhouse gas climbing alarmingly fast. Methane is now about 1,780 parts per billion. During the last 650,000 years, it never exceeded 790 parts per billion. And worryingly, methane has begun building up in the atmosphere again after stabilizing between 1999 and 2006. Some evidence points to melting permafrost across the Northern Hemisphere, where decaying plant material produced methane that was frozen in the tundra for millennia.
Nitrous oxides ($NO_x$), are other greenhouse gases produced by both natural and human-related sources. Less of this gas is emitted than $CO_2$, but it is a more powerful heat trapper on a per molecule basis and has an atmospheric lifetime of 120 years.
Some industrial chemicals, such as fluorinated gases, are considered to be "high global warming potential gases" because of their potency and long atmospheric lifetimes. For example, nitrogen trifluoride, a cleaning agent used in making flat panel TVs, computer monitors, and thin-film solar panels, has increased 30-fold since 1978 and is thousands of times more powerful as a greenhouse gas than carbon dioxide.
The Human Element: Are Humans the Cause?
Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 2.5, Geneva, Switzerland.
But couldn't the warming still be a natural variation? Scientists have looked for alternative explanations based on what caused climate changes in Earth's past. None of them fit the bill. Computer models help cinch the case for a human cause. The solar and volcanic conditions of the last 50 years would likely have produced cooling, based on our models' best estimates. Only the addition of the extra greenhouse gases and particles to our computer models can reproduce the pattern of warming we've seen over the past century.
Figuring out what influences climate, how it has changed in the past, whether it's changing now, and how it might affect our future has required the expertise of thousands of scientists and other experts from dozens of fields who were brought together in one body: the IPCC.
Since it was formed in 1988, the IPCC's purpose has been to bring together the experts who can provide decision makers and others with an objective source of information. It does not, however, conduct any research itself. The IPCC's Assessment Reports (issued in 1990, 1995, 2001, and 2007) also don't tell us what to do. They discuss options, but leave prescribing policies for dealing with climate change up to individual nations.
The specialists at IPCC, more than 2,000 of them, represent about 140 countries, a variety of fields, and a range of views. Their function is to assess the latest peer-reviewed literature and to achieve consensus about where the weight of the evidence points and where uncertainties lie.
Remember that this means they are looking at papers that have already been reviewed in depth by other experts before the articles were published in a scientific journal. For the 2007 Report, they examined 29,000 data sets. The assessment also included computer modeling to determine the cause of climate change and to predict future impacts.
Levels of Confidence
|Terminology||Degree of confidence in being correct|
|Very high confidence||At least 9 of 10 chances of being correct|
|High confidence||About 8 of 10 chances|
|Medium confidence||About 5 of 10 chances|
|Low confidence||About 2 out of 10 chances|
|Very low confidence||Less than 1 out of 10 chance|
|Terminology||Likelihood of occurrence/outcome|
|Virtually certain||>99% probability of occurrence|
|Very likely||>90% probability|
|About as likely as not||33-66% probability|
|Very unlikely||<10% probability|
|Exceptionally unlikely||<1% probability|
IPCC Confidence in Signs of Climate Change
|Warming of the climate system||Unequivocal||100%|
|Less frequent cold days, cold nights, and frosts, more frequent hot days and hot nights over most land areas||Very likely||>90%|
|During the second half of the 20th century, highest average Northern Hemisphere temperatures in at least the past 1,300 years||Likely||>66%|
|Global area affected by drought has increased since the 1970s||Likely||>66%|
IPCC Confidence in Observed Effects of Climate Change
|Effects||Confidence Level||Quantitative Confidence|
|Recent warming strongly affects terrestrial biological systems, including earlier spring events and poleward and upward shifts in plant and animal ranges||Very high||At least 9 out of 10 chance of being correct|
|Increases in number and sizes of glacial lakes||High||About 8 out of 10 chance of being correct|
|Increasing ground instability in mountain and other permafrost regions||High||About 8 out of 10 chance of being correct|
|Increased runoff and earlier spring peak discharge in many glacier and snow-fed rivers||High||About 8 out of 10 chance of being correct|
|Warming of lakes and rivers in many regions, with effects on thermal structure and water quality||High||About 8 out of 10 chance of being correct|
IPCC Confidence in Human Attribution
|Evidence humans are responsible||Certainty||% Probability or Quantitative Confidence|
|Global average net effect of human activities since 1750 is warming||Very high confidence||At least 9 out of 10 chance of being correct|
|Solar + volcanic forcings have produced cooling in last 50 years||Likely||>66%|
|Most of observed increase in global average temperatures since mid-20th century due to increases in greenhouse gases from human activities||Very likely||>90%|
|Human-caused warming over last 30 years has a had a visible influence on many physical and biological systems||Likely||>66%|
Two Positions: It's Not Happening
Although many statements that climate change is bogus are based on false or incomplete information, there have also been some legitimate scientific criticisms of climate change science. It's important to separate one from the other, particularly since misinformation can be stubbornly persistent, especially on the Internet. For example, you can find several websites and blogs where someone states that global warming is due to increased energy from the sun. However, observations of the sun for the past 30 years show no increase in its energy production.
Some sites still proclaim that volcanoes can issue more $CO_2$ than we as human are producing. While this is possible for the earth to have these kinds of eruptions, volcanoes to date have produced an equivalent of about 10% of the man-made emmissions.
The important questions posed by reputable climate scientists who disagree with most of their peers are these:
- Do we really know enough about the drivers of climate to be really sure that we know what causes change?
- Are scientists grossly underestimating the complexity of the Earth system?
- Are climate models good enough to accurately simulate the complex climate system?
- Are there processes that will limit warming naturally by producing a negative feedback?
These questions are important, and science thrives on debate that fuels further investigations. It is always possible that questions raised will lead to new information that will change our perspective. However, to date the majority of climate scientists are convinced that enough of the puzzle is visible to conclude that the world is warming and human activities are largely responsible.
Two Positions: It's Worse Than We Think
Since 2000, they have grown four times faster than in the 1990s. Some argue that the IPCC near-future assumptions about global energy use are too optimistic. A recent study also concludes that the IPCC estimates were too rosy with regard to how quickly developing countries will be able to afford technologies to reduce greenhouse gas emissions.
Scientists in this camp also say sea levels may rise far more than we anticipate because our calculations aren't taking into account the unexpectedly large melting and disintegration of the Greenland and Antarctic Ice Sheets. They also point out the last IPCC report was limited to already-published and peer-reviewed research, and the conclusions had to be reached by consensus. This is bound to produce a cautious, conservative estimate for the future, they say.