"Global change". "Greenhouse effect". "Global warming". The media are full of statements, concerns, guesses, and speculation about these phenomena, as scientists and policy-makers around the world struggle to address recent scientific observations that indicate human activities impact our environment. And yet, each of these is a "natural" phenomenon, as are many others. Hurricanes, droughts, and monsoons all occur without any control by humans to initiate, forestall or moderate them.
Most readers of this Tutorial already know that global warming is literally a "hot" topic. For those still with relative unfamiliarity, we recommend this Wikpedia review: effects of global warming.
This page considers the nature of and evidence for global warming. There seems almost no doubt that the Earth's climate has been steadily warming overall in the last hundred years: global warming is a fact - a reality. The nagging question - still unanswered to everyone's satisfaction - is: How much of the quantified increase of about 1° C in that time interval is just due to a natural trend probably related to glacial cycles (which themselves are temperature-dependent) and how much is additional warming accelerated by human activity as the industrial revolution reaches rapidly increasing levels? The present page provides an overview of some of the answers to this question. Since it was first written, the writer (NMS) has encountered new lines of evidence - pro and con. Rather than try to splice these into the text, I have elected to add a new page (16-2a), accessed at the bottom of this page.
Most people who follow the news accounts of global warming have come to realize that the primary culprit is human contributions of heat-affecting gases into the atmosphere as these are released by everyday processes such as burning coal to produce electricity and consuming gasoline to run automobiles. The most blamed gas is carbon dioxide (CO2), but carbon monoxide, methane, sulphur dioxide, and other gases also contribute. The United States is often cited as the principal polluting nation, although now China and India - each with more than a billion people, many of whom are now driving cars - are becoming major players. In the U.S., cities and surrounding metropolitan areas are the chief sources of CO2, as evident in this map made from ground and aerial sounding monitors but also containing some input from satellite measurements:
These gases are the key factor in the Greenhouse Effect that is warming our atmosphere to the levels we now record. The Greenhouse Effect results from the trapping of solar radiation that reflects from the Earth’s surface by these (and other) gases. This is illustrated below in two complimentary versions.
The atmosphere is essentially transparent to incoming solar radiation. After striking the Earth's surface, the wavelength of this radiation increases as it loses energy. The gases that are involved are opaque to this lower energy radiation, and thus trap it as heat, thereby increasing the atmospheric temperature. As these gases increase, due to natural causes and human activity, they enhance the Greenhouse Effect, and may raise temperatures even more. If the climate warms, the vegetation belts will tend to move northward, changing global ecological and biome patterns. Other effects may be discerned in precipitation patterns, sea level changes, and more.
To fully understand global warming, one must first understand the operations of the Earth System (described on later pages), since natural sources of temperature-affecting gases play an important role. We can learn about our planet's interacting physical systems by observing the results of such natural phenomena, and use our knowledge to explore human-induced changes. Thus, one factor that may in itself induce and account for some of the regional and global temperature changes is volcanism. Remote sensing is effective at monitoring such events. Consider, for example, the eruption of a volcano, such as Mount Pinatubo in the Philippine Islands in 1991, that happened without human intervention. This volcano had been dormant for more than 600 years.
When a volcano erupts it spews millions of tons of ash, debris, and gases into the atmosphere, not to mention the lava flows from some volcanoes. Because of the presence of instruments - on the ground, at the ocean's surface, and in space - meteorologists/environmentalists observed a cloud of sulfur dioxide (SO2), emitted by Pinatubo, make its way westward, extending well past India within twelve days of the original eruption. Monitoring has been done by the UARS (Upper Atmosphere Research Satellite). By three months, that cloud had completely encircled the Earth, as shown from space (below), and inside of a year SO2 particles in the atmosphere were providing gloriously-colored sunsets all over the globe and lowering global temperatures, as well. Clearly, an erupting volcano impacts more people and places than just within its immediate vicinity.
16-4: Why does the Pinatubo ash tend to stay confined to a wide equatorial zone? ANSWER
Mt. Pinatubo offers a stellar example of how monitoring from space can continually update the status of transient ground events (that, in unpopulated areas [not the case in Luzon] may go unnoticed). Below are two SIR-C radar color composites (L-band HH = red; L-band HV = green; C-band = blue) taken in May (left) and September (right) of 1995, both showing the effects of the 1991 eruption. Ash from an earlier phase of the 1991 event appears in red just above the summit of the volcano.
16-5: There is one noticeable change in the right radar image. Find it. Make a guess as to its cause (clue: the volcano didn't erupt between May and September of 1995). ANSWER
Volcanoes are a main source of SO2 rises in atmospheric chemistry. The TOMS instrument (see page 14-9) has been making measurments of SO2 in the air for more than 30 years now. The plot below shows the amounts detected for major volcanic eruptions from those on island arcs and others not on a direct plate boundary for the period from 1979 to 2003:
Another fascinating example of a natural phenomenon we know as El Niño, because it occurs with some regularity (although not complete predictability) around Christmas time. El Niño refers to the baby Jesus, whose birthday we celebrate at the end of December. When an El Niño occurs, a pool of warm water from the western Pacific Ocean moves eastward to the western coast of South America. In the process, weather patterns around the world changes often to the detriment of human populations–as do South American fish populations. In non-El Niño years, fish are abundant in this region, because of the cold, nutrient-filled waters. When an El Niño occurs, that cold water flows deep into the Pacific, and fish populations decline dramatically, with concomitant effects on humans whose livelihood depends on those fish.
Other such phenomena abound. Some are readily observable by space sensors, particularly meteorological and oceanographic measurements (see Section 14). As examples, consider these near-global plots. The top from comes from the Seasat Radar Scatterometer and shows prevailing wind patterns over the oceans at supercontinental scales. The bottom one depicts the mean day, night, and day-nite temperatures of the Earth's land and sea surfaces, averaged for January of 1979, from Nimbus 6's High-resolution Infra-red Sensor (HIRS) 3.4 and 4.0 µm channels (page 14-4), integrated with MSU microwave and infrared data. Suffice to say that even without human contributions Earth is a dynamic system, one that changes routinely and often drastically.
Clearly, global mean annual temperatures are rising, and we continue to monitor this condition with space observations to help settle the question: how much is just a natural trend (e.g., inevitable interglacial warming) and how much is due to man's activities? Calculations show that the burning of fossil fuels (mainly coal, petroleum derivatives, and natural gas) add about 6 billion metric tons of carbon (as the element) to the air annually; each year also, deforestation permits an extra 1-2 billion metric tons of carbon to reach the atmosphere.
As an indication of how much worldwide temperatures have risen in the last few years, this next map of the globe shows the geographic distribution of temperature anomalies, with measurements from various sources as compiled by NASA GISS (Goddard Institute for Space Science), for the year 2006 - the fifth warmest on record. The plot of temperature increase since 1880 is shown but is hard to read. So, beneath it is a recent (Oct., 2009) version that clearly presents details:
GISS is continually refining its models. Daily inputs improve the credibility of the predictions. Now that Aqua (page 16-11) is operational, its AMSR instrument can provide global data covering very short time spans. This map is of worldwide surface temperatures on August 27, 2003:
Using temperature data, GISS has now published a map of sea surface heat content anomalies (measured to a water depth of 70 m) integrated over a ten year period from 1993 to 2003. From typical data (top illustration) they have derived a model distribution of temperature variations. Since the upper ocean waters serve as efficient reservoirs for heat storage, the patterns of distribution of excess, normal, and deficient heat (in Watts-year/m2) thus determined are proving helpful in weather and climate forecasting since the temperatures have strong effects on atmospheric heating.
The trend of temperature changes on annual and 5 year-running averages proves that in the second half of the 20th Century the global atmosphere has been slowly warming - cumulatively totaling about 0.5 ° C, as shown here:
A more recent graph released by GISS covers departures of global average annual temperatures from a 30 year average as baseline over the period from 1870 to 2005 (the latter being the warmest year on record):
GISS has designed its model to predict both sea surface and surface air temperatures up to 50 years in the future. Here is a recently published forecast, which suggests that under current trends the temperature rise will be somewhat greater (more than 1 °C) than the last 50 years:
Other models predict an even more extreme temperature rise - if little or nothing is done to alleviate the causes. One forecast holds the average temperature will increase by 3 to 5 degrees Fahrenheit in the next 50 years.
Perhaps most alarming of all is the result of an extended study by NASA's GISS. Using a sophisticated model, their researchers projected temperature rises by the year 2085 for the eastern U.S. For comparison, using this model they produced an estimate for the year 1993 (right map) which was closely matched by reality:
The colors denote peak July temperatures as follows: Violet = 79-85° F; Green = 86-92° F; Yellow = 93-99° F; Red = 100 - 107° F; Dark Purple = 108-114°F. Under the 2085 conditions, rainfall in this region will diminish and vegetation will become much like that around Phoenix, AZ today. Available water will become scarce, so that desalination of the Atlantic ocean would have to provide much that is needed.
That this warming trend is already happening elsewhere in North America is convincingly demonstrated by temperature data for the state of California. In the illustration, temperatures (in Fahrenheit) across the state and in Los Angeles have risen more than 2.5° F in places in the last four decades and more generally almost 4° F statewide in the last 120 years:
So, is there any sign this is happening everywhere? Much debate has started in the middle of 2006 because of very intense heat waves during July-August in both the U.S. and Europe (the latter seldom experiences the extremes that occurred then) as shown in these plots obtain from metsats:
These excess temperatures point up the problem that both advocates and disbelievers of global warming face in interpreting the abnormalities. Thus: Does this period of high temperatures (St. Louis, MO had almost a week of temperatures at or above 100° F) reflect the onset of the predicted hotter summers or is this just another of the drastic warm spells that have happened infrequently in the past? The "jury is still out", although the unusual European situation seems to favor the warming hypothesis.