This page was written in June of 2007, well after the previous page was prepared (over several years). What has prompted the present page is a temporary DOUBT - induced by reading a science fiction novel by Michael Crichton (author of "Jurassic Park" and "The Andromeda Strain") entitled "State of Fear". Crichton uses the device of "ecology terrorists" trying to thwart a law suit against the U.S. government that would legitimize the global warming consequences. In the narrative, arguments both supporting and questioning the main premises of man-intensifying global warming are reviewed as conversations among the principals. In an Appendix, Crichton summarizes his personal views which, in their essence, moderate the extremist positions such as those put forth by Al Gore. There is a rebuttal against the Crichton position available on the Web, at this Union of Concerned Scientists site.
The essence of the Crichton message is this: We do not yet know enough about the periodic or cyclic natural trends in temperature changes over the globe to identify (quantitatively) the relative contributions of natural processes and manmade effects owing to atmospheric mismanagement to state unequivocally whether there is serious, (way of) life-threatening conditions resulting from human activities that require major modifications in policies and practices that release temperature-affecting gases into the atmosphere. Reading the book has prompted the writer to create this page, which represents my (current) thinking on the subject. Here are my thoughts:
The strongest evidence for global warming (often referred to as "climate change" to temper the implication of a one-directional trend) as a 20th Century temperature anomaly is this plot shown in two versions (the top has a "hockey stick" superimposed to provide a visual for remembering the shape of the curve)
Assuming the validity of this plot, what is most notable is the relative evenness of the temperature variations over the last thousand years - UNTIL THE 20TH CENTURY, when a pronounced upward spike occurs. Why this anomaly in the last hundred years? Is it the onset of interglacial warming or is it a response to the accentuated introduction of heat-moderating gases and other factors in a polluted atmosphere?
The next two plots show somewhat different plots covering only the last 140 years, first for the entire globe and then for one continent (Australia).
This diagram shows that the change patterns are nearly identical in the northern and southern hemispheres of Earth.
With the evidence so far, one is tempted to conclude that global warming is ubiquitous and entirely one of increasing temperatures. But look at this graph showing temperature changes in the Great Valley of California and the Sierra Nevadas to its east, during the winter season.
The two graphs indicate that the Valley has slowly warmed whereas the mountains have slightly cooled in the last 110 years. That raises the question: While there may be a worldwide trend towards temperature increase, are there different patterns in individual regions. This led the writer to survey temperature histories at various observing stations scattered over the globe. These are typical temperature-time plots retrieved off the Internet:
Similar plots can be inspected at this Web site.
Most regional stations having a history of decades of temperature records show a fairly steady trend of increases up to the present. Some have held steady, a few show declines, and many reveal non-uniform changes. Here are some typical results:
These four are characteristic of upward trends at stations in North America, South America, Africa, and eastern Asia. Below are four plots for stations as labeled that show interesting variations in Temperature-Time trends:
The first two are in China; the two stations are not far apart, yet the trends are different.
The next two are in Europe. Here the trends are similar until after the 1960s in which Geneva shows a marked upswing and Foggia a downturn:
Some regions have had little temperature change in well over 100 years. This (blurred) image shows the temperature history for three seasons at Death Valley in eastern California:
So, what can be concluded from these plots. First, individual temperature-recording stations can show change patterns that vary notably from one to the next. Second, overall, upward trends are the norm but a few have experienced slight to moderate cooling that may be anomalous and may represent local moderating variables (for example, certain kinds of air pollution can inhibit solar warming; see below). Third, in order to discern the general patterns of temperature change, station readings from many locations need to be convolved and averaged.
A fourth point: There is a plentitude of Temperature-Time graphs "out there". One can find all sorts of variants. Look at this set of graphs covering the last thousand years. Different methods of arriving at the data are involved. The variability is noteworthy. But they all show a pronounced upward swing during the last 100+ years.
Probably any advocate or critic of global warming can find a graph, or set of graphs, that support or attack their stance on the reality of this phenomenon. Consider these two plots:
This upper plot showing the U.S. as a whole would suggest that there was a warmer interval countrywide from about 1930 to 1960. This same interval is less evident in other plots, using different data sets. The trend is mirrored in the lower plot which covers only the winter months. Neither plot reveals any strong tendency towards steady warming. The key point: one must have appropriate knowledge of the source of the data used in producing these summary plots.
The astute reader may wonder whether the oceans are recording anomalous warming trends along with the atmosphere. After all, marine waters provide a huge heat sink. These three diagrams provide a partial answer:
The above plot is a fairly accurate measure of Sea Surface Temperatures (SST) in the North Atlantic ocean. The variations and trends roughly mirror those of atmospheric temperatures.
Using data of varying quality this curve estimates temperatures from the present back before 1800 to the (less reliable) temperatures measured back to the Middle Ages.
Topex-Poseidon and other temperature sensors on other spacecraft are now providing highly accurate readings of SST. Here is a worldwide average for a ten-year period ending in 2002. Note that Mean Sea Level (MSL) follows SST.
This sealevel rise is small but measurable. Most harbors show a rising but local circumstances can produce a falling trend:
As a generalization, one can discern a similar pattern of warming in the oceans compared with the atmosphere. This is expected since the air and surface waters approach thermal equilibrium. However, the "hockey stick" pattern of the last half century or so is not evident - the rise has been steadier.
So, with this suuperabundance of data, what can we believe as fact. And does this speak to the key question - how much of the warming is part of a natural cycle and how much is a "first time" contribution by the peoples of the Earth? This diagram is helpful:
The natural variation in gray is (from a model) a fairly flat plot suggesting that over just 120 years there should be no significant upward trend in temperatures. The red curve shows that there has been a notable departure from the predicted near constancy. This upward trend is "anthropomorphic", namely caused by human contributions to the meteorological environment. (Of course, other plots made from different data combinations may contradict this.)
Still, almost everyone seems to concur that in recent years much of the Earth seems to be having warmer summers and winters. We've seen a diagram like this next, on the previous page, but its idea - that our climate seems hotter - is repeated here for a different year.
The data from space measurements have been showing a worldwide, steady increase in temperature over the last 28 years. This is recorded by sounding readings from several satellites. This map set summarizes results on a global basis:
Models for the effects of CO2 on temperatures indicate a warming should occur in the upper troposphere - the 0.6+ °C shown here - but this heat buildup leads to a cooling of the next higher layer, the lower stratosphere. The actual results shown above bear this out. Note the strong temperature rises in the Arctic; the Antarctic shows a much less increase, consistent with the notably smaller amounts of carbon dioxide from the less industrialized southern hemisphere countries.
The United States has been experiencing a widespread and growing drought during the first years of the 21st Century. It started in the U.S. Southwest, then into the Great Plains, and in 2006-07 has developed in the Southeast. This map shows the situation at the start of summer over the 48 states:
This photo dramatises the extent of the current drought in Florida. Lake Okeechobee has had an overall drop in water level of more than 5 feet.
Is this consistent with global warming. Yes - Maybe. By itself the drought proves nothing. The Great Plains drought of the 1930s was even more severe, but it ended and decades of wet weather followed. An alarmist at the time would have cried "Global warming!" Not yet.
We also noted reference to the idea that certain factors associated with atmospheric pollution might be countermanding (negative feedback) the trend towards warming. One previous diagram showed the effects of particulates on temperatures. Here is a variant.
But, if anthropogenic global warming exists, then both science and technology have fingered the most likely culprits - the so-called Greenhouse gases. The identity of and relative proportions of the main gases is shown in this next pie chart diagram.
A survey of similar diagrams on the Internet discloses a wide variety of different versions. The choice of inputs has some effect. Location and time scale also contribute to the seemingly bewildering variants. This is a fundamental truism for much of the global warming data: the myriads of curves often seem different, but again, much depends on the specific inputs. Here is a pie chart that the writer found especially informative.
While Greenhouse gases are the principal culprit, other factors may be involved, as summarized in this diagram.
Not shown on the diagram are two suggested factors. The first relates to periodicity of certain orbital parameters of Earth with respect to the Sun. This may play a role but has not been scientifically demonstrated to most evaluators of causes.
These are closely related to the well-known Milankovitch solar cycles in which there seems to be a systematic variation in the Sun's energy output that may affect glaciation. One follows a roughly 100,000 year cycle. Their influence on terrestrial temperatures has not been fully demonstrated but remains a possibility.
Some investigators claim to find some correspondence between 11-year sunspot cycles and warming. The plot below implies this possibility but is not conclusive.
That other popular culprit is volcanism, which has been proposed to correlate glaciation with temperature changes. This plot seems to substantiate the idea:
Volcanism provides mechanisms that both increase and decrease atmospheric temperatures. Particulates would lower temperatures; gases should raise temperatures. Note that this plot runs counter to the volcanism plot four figures up.
So, there are checks and balances in the factors that affect local, regional, and global temperatures. The very premise of global warming is complex. But, if the past hundred years seems complicated and ambiguous, the story implicit in the last half million years may at first seem more so. Yet there is a pattern of regularity. And, a study of temperature histories over very long time spans gives some balance and insight into the causes of global warming.
On the previous page, you saw a very revealing diagram that shows a very strong correlation between CO2 gas bubbles in ice cores from deep holes in the Antarctic to calculated temperatures (based on the now proven relation between CO2 in the atmosphere and near surface atmospheric temperatures. We reproduce here the classic plot that has been used to explain the major Pleistocene continental glacial advances and retreats. It is evident that the world is currently in an upswing. But this diagram fails to single out what is shown on the next, very similar plot.
In the plot below, the curve as it moves near the right edge moves up, but then there is a very sharp upward spike (in green) over a small time period. This corresponds to the significant increase in CO2 over the last 100 years. Maybe this is within the range of allowable variation during an interglacial warming spell, but maybe this is a unique excess. Note from these plots that the range of temperatures between glacial minima and maxima is about 11° C.
In this plot, the ice data show that the CO2 levels have not been higher than about 285 ppm in the last 400000 years (in fact, not in the last 700000 years). The 2006 level has now reached 384 ppm. Temperatures are on the rise too, but currently lag behind what they will be in the next decade or so (the present temperatures are near the maxima for the several interglacials shown).
Another way to gauge temperature in the ice cores is to measure δ O18 (this is defined as the ratio of O18 to O16. The principle underlying variations in δO18 is embodied in this statement: In the hydrologic cycle, evaporation preferentially removes water with (i.e. light oxygen). This oxygen isotope is therefore rich in the gas phase of water in clouds. Precipitation and runoff returns water with high O16 to the Earth's surface. During glacial epochs, this precipitated O16 is preferentially stored in polar icecaps and continental ice sheets. This leaves the oceans enriched in O18. In the opposite case, when the cliamte is very hot, icecaps do not exist, and oceans are no longer enriched in O18. The abundance of O18compared to O16 is displayed in a ratio of the the two isotopes. The relative value of this ratio is compared to a standard so that the climate change with respect to time can be measured. Thus, when oceans are warm, and glaciation is minimal, δ O18 is lower because more O16 can be retained in the water (colder air reduces amount of O16 that evaporates). That δ O18 is a good estimator of temperature is evident from this graph:
Using ice cores collected at Vostok, Antarctica and elsewhere, this pattern emerges for the Cenozoic time frame:
These measurements were performed on the calcium carbonate (CaCO3; Calcite) making up foraminera shells collected from the benthic zone (deeper ocean waters). The warmest interval in the Cenozoic was the Eocene Epoch. Note in the above plot that there is a large downswing in δ O18 beginning in the late Tertiary and climaxing in the Pleistocene (lower left). A more detailed plot covering the last 5 million years shows a gradual increase in δ O18 that steepened in the last million years.
Studies of δ O18 variations during the Pleistocene indicate a clear correlation with periods of warming and cooling. The curve on the left below depicts changes in one deep sea sediment core; on the right, a combination of results from various cores brings out the fact that, in addition to major times of glaciation, there were periodic shifts in δ O18 that indicate less extensive glaciation (such as is recorded as the little Ice Age around 1400 A.D.
This is an important observation, since it suggests that there have been frequent periods of temperature change associated with notable climate shifts. The obvious question: Is the current and recent temperature rise just one of these?
A logical followup question would be addressed to patterns of longer climate changes, well back into geologic time. It is harder to be specific and accurate but δ O18 measurements in age-dated deep sea sediments going back 100 million years indicates a general cooling trend since mid-Cretaceous.
Similar δ O18 measurements have now been carried out back through the Phanerozoic (to the Cambrian Period at the beginning of the Paleozoic. The left part of the plot below coincides with the 100 million year trend.
Paleoclimatologists have attempted to broadly define temperature and carbon dioxide variations in the terrestrial atmosphere during all of the Paleozoic. Here is one such reconstruction:
The four major glaciations are shown on this plot at the same time as on the previous plot. What is especially interesting is the high CO2 content in the past. Assuming this is a generally reliable estimate, the obvious point to be made: Carbon dioxide seems to have been much higher in the distant geologic past than at present - BUT LIFE SURVIVED AND EVEN THRIVED. (Also note: natural interglacials and alternating glacial advances both influenced life forms in each time span; some species disappeared and new species and even higher taxonomic levels emerged; thus, the current trends of loss of some life forms - cited as calamities induced by global warming - may in fact be natural consequences.
The above plot is too "coarse" to show one major temperature event that may be a precursor model to the projected rises due to global warming. At the end of the Paleocene - start of the Eocene, about 50 million years ago, there was an increase of about 10°C (18°F) over a span of about 10000 years. During this the Paleocene-Eocene Temperature Maximum (PETM), in North America tropical vegetation (e.g., Palmetto trees) spread as far north as today's Wyoming and Minnesota. A spurt in mammalian evolution resulted. The cause(s) are not precisely known but there was a notable increase in CO2, as recorded in the rocks. In the scenario so far reconstructed, life on Earth was affected but on the whole in a positive way. After about 80000 years, average temperatures began to drop and eventually in the last few million years polar ice formed and survived, leading to glaciation. Whether PETM implies that mankind can survive, and perhaps prosper, such changes, has not been settled to scientists' satisfaction. At first glance this would seem to score one for the anti-global warmers who say "don't worry". But, keep in mind the time frame: over 10000 years, the averaged rate of increase is only 0.18° F per 100 years - far slower than the present and forecast rates that are now several degrees in the 21st century.
All of the above is quite interesting but, what does it tell us about the future (does the apparent trend seem so dire that humans must start acting decisively to curb global warming?). Various investigators have put forth predictions extending to the end of the 21st Century. Observe:
The worst case scenario predicts about a 5° C increase by 2100 A.D. That would place global temperatures in a range experienced by the dinosaurs at the beginning of the Mesozoic. Life flourished then, including vegetation. We find no evidence of drastic negative environmental conditions at that time. But, then, it didn't much matter whether coastlines were drowned and tropical plants encroached upon the poles. Changes must have occurred. Many would be detrimental to large segments of the world population if similar ones happened in this century. However, Man has grown ingenious at adjusting to situations that appeared adverse at the time. As long as mankind doesn't face starvation, or some other form(s) of extinction, the writer believes that, even if global warming has threatening aspects, we on this planet will accommodate much of the global warming threat.
The upfront conclusion reached by the Panel: The evidence for abnormal increases in carbon dioxide, methane, nitrous oxide, and halocarbons - the threats to temperature increase and ozone destruction - in the last century is overwhelming. That is dramatically portrayed by this illustration:
The best evidence for the responsibility of humankind for this increase is the measurements of carbon isotopes in gases collected (directly and from ice cores) prior to and subsequent to the last 200 years. Also, there is a strong correlation between rising gas concentrations and regions where industrial activity has been strongest.
The article introduces the term "radiative forcing". This is defined as the change in atmospheric energy balance (expressed in watts/sq. meter) brought about by human activity since the preindustrial times. A positive forcing induces warming; negative, cooling. Natural change drivers are principally contributions from volcanic activity and by variations in solar activity that affect its thermal output. This illustration plots the best estimates of relative forcings:
The chief contributors to negative radiative forcing (promoting cooling) are the aerosols released by volcanic eruptions and by human activity that reside in the atmosphere. But these are notably offset by the positive forcings from anthropogenic gases that have resulted in a net increase of about 1.5 W/m2.
The IPCC has put forth the conclusion that the anthropogenic gas-release forcing is about 10 times the natural forcing caused by detected solar output increase in the last 200 years; they conclude that volcanic aerosols are episodic and average out to a small contribution in this period.
Other notable statements: 11 of the last 12 years have been the warmest globally since 1850; the mean average global temperature has increased by 0.74 +/- 0.18 degree Celsius since 1906 (most of that has occurred since 1956); sealevel rise since 1993 averages 3.1 +/- 0.7 mm per year; Arctic sea ice is being areally reduced at a rate of 2.7 +/- 0.6 percent per decade (but is now accelerating). The IPCC also has raised its confidence in stating that the bulk of global warming is manmade to the 90% level.
Some of the sea level rise results from direct contributions of water from melting ice. But part is due to expansion of surficial ocean water that is heated further by the temperature rise. About 80% of the added heat to the atmosphere is eventually absorbed by marine waters. But that in turn evaporates these waters so that the water vapor content of the atmosphere is rising.
The IPCC conclusions are based in large part on a series of global warming simulations using 18 different models produced by investigators worldwide. The model outputs are broadly similar but use some different inputs and assumptions. Many models include contributions by natural processes. Taken together, the expected global temperature changes can be represented as follows:
The most salient feature of these plots is the blue line - changes expected solely from natural causes. Since 1900 this amount to almost nil - the changes are nearly constant rather than the result of interglacial warming.
One general prediction from the models: If solar heating were the prime factor, both the upper troposphere and lower stratosphere should warm; if greenhouse gases are the main factor, the stratosphere would cool. This latter case is the actual observation.
Some of the models extrapolate into the future - primarily, through the 21st century.
The results of combining the models have been expressed in a series of global maps showing changes in various parameters expected by the end of the 21st century. Here are two examples:
The principal conclusions reported in the IPCC's Fourth Assesment (April 2007) are: 1) unless drastic action is taken, atmospheric CO2 will rise to 445 to 1130 ppm, 2) even with some abatement of gas release, the rates of changes will be greater in the 21st century than in the 20th; 3) the temperature rise is likely to be about 0.2° C per decade; 4) by the end of the century, the increase will be between 1.8 and 4.0° C, depending on what actions the world takes; 5) over time the oceans will become less efficient in taking up carbon dioxide in solution, leaving more in the atmosphere; 6) the buffering roles of vegetation and soil uptake may provide negative feedback (incorporating more of the CO2) but in as yet poorly known quantities; 7) sealevel rise should be between 30 and 40 centimeters, flooding most deltas and many islands; and 8) at higher latitudes, temperature will rise, permafrost will dissipate, and sea ice largely disappear whereas at lower latitudes summer heat waves may become severe, rainfall increases, and hurricanes/typhoons will strengthen. The article does not expand upon one serious effect: the shift in geographic location of agriculture and overall productivity, but does postulate that mid- and high-latitude regions could become important producers (requiring, however, extensive removal of boreal forests).
This diagram effectively summarizes the prime argument for global warming:
In the top panel, temperature variations have remained nearly the same for 900 years starting in 1000 BCE; they have strongly spiked upwards in the last 100 years. The middle panel shows that there has been a strong upward trend in CO2 since the 1800s; physical chemistry indicates that this gas causes heating and, by inference, mirrors the rise in temperatures. The bottom panel indicates a growth curve for human population that closely resembles the CO2 plot. As they say in Latin: QED!!
So, from all this, what is the bottom line: The writer believes these two premises have now been substantiated: First, we are most likely within the increasing phase of a global warming trend, probably a repeat of warming patterns following previous Pleistocene glaciations. Second, humans are almost certainly responsible for some of the (seemingly larger and faster) increases in warming. But the relative proportion of the first to the second still is not quantitatively ascertained to everyone's satisfaction. And, while one can safely specify some of the consequences of warming, and ice melting, we do not yet know the degree of "direness" nor whether we can moderate or even reverse the apparent inevitable. Thus, it may be wise just to accept warming as a part of the future even as we try to control the negatives. Is reducing the rate of CO2 gases into the atmosphere a bad thing. Probably the opposite. Good! But at what price. Stick around - the younger readers should see some answers to these concerns.
Primary Contact: Nicholas M. Short, Sr.