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The Skeptic's Guide to the Medieval Warm Period

Sun ,01/04/2012

“How can science claim man is the cause of global warming when the temperature of the Earth was much warmer during the Medieval Warm Period (MWP)?”

The science Skeptics are quite adept at casting doubt on climate science by using clever arguments. One of their favorites is “How can science claim man is the cause of global warming when the temperature of the Earth was much warmer in the last millennium during the Medieval Warm Period (MWP)?”  You don’t really have to be a scientist to figure out that there were no accurate temperature records during the Medieval times and that much of the world was unknown.

The Skeptics usually point to historical records such as those by H.H. Lamb, which describe record heat waves in the known world during the MWP. Skeptics rather ignore the fact that H.H. Lamb was so concerned about the effect of global warming that he founded the Climate Research Unit (CRU) to study the Earth’s temperature records. The scientific questions comes down to  (1) whether the MWP was worldwide, (2) how warm the Earth actually was during the MWP, and (3) what caused the MWP?

Since there were no thermometers and no worldwide network of weather stations during the MWP, scientists have used a variety of proxy data from ice cores, isotope ratios, sediments, geological records, and even tree rings to try to reconstruct the temperatures for the last thousand years. Though there are large uncertainties in proxy records and they require careful calibration, they do show a similar pattern as you can see in the figure below, which is made up of 10 different reconstructions. The black line is the instrumental temperature record.* (See end of article for references.)

 

Though there are wide uncertainties in the proxy temperature records, taken together they form an overall pattern which answers the scientific questions. The proxy records show that that (1) there was a warm period from  AD 1000 to 1200, followed by a cooler period from  AD 1550 to1850  NASA identifies as the Little Ice Age, though it was not a true Ice Age. The record also show that (2) temperatures during the  MWP were quantitatively lower than the temperatures during the latter 20th century. To discover the cause of the MWP, it is necessary to look at another reconstruction.

While studying the cause of the past Ice Ages, scientists have identified the three  main factors which affect the Earth’s temperature, solar irradiance, greenhouse gases (primarily H2O and CO2), and particulates from volcanic activity. Below is a reconstruction of the three main factors controlling the Earth’s temperature. There are many interesting things in the records, but they show (3) the Earth was likely warmer worldwide during the MWP because of the higher solar radiation. It also shows that the solar radiation has been relatively constant during the last century while the other factors, primarily greenhouse gases, have increased.

 

 While it is possible to dispute or argue about the meaning of any of the individual records, it is rather disingenuous to claim that scientists “ have no data” or to dispute the obvious causes of the current global warming. To emphasize that, below is data from the last century, which is based on scientific records. While H20 and clouds accounts for about 75% of the greenhouse warming, their effect has increased only slightly( about 4%) while the amount of carbon dioxide has increased by about 39% in the last century.

CO2: Man is now putting about 30 billion tons of carbon dioxide into the air each year. About half of it dissolves in the ocean and they have become 20% more acidic in the last century. Much of the rest stays in the air, and  CO2 is building up in air:

Temperature: CO2 is one of the greenhouse gases that warm the Earth, and NASA’s graph shows how its increase is changing the Earth’s temperature. Note:  The effect of particulates from the explosion of Pinaturbo can be seen in the temperature decline from 1991 to 1995.

 

NASA GISS 2010A

 

The Sun: The current global warming is often wrongly attributed to an increase in intensity of the sun. The sunspot activity does not show up above the noise in the temperature record above. The solar irradiance increased slightly until 1960 and has declined slightly since then.

 

There is the scientific story. Disputes in science are settled by the data. Though Skeptics may dispute the evidence showing the current global warming is caused by man, the question is ” Where is the evidence?”

* References for temperature reconstructions:  The original version of this figure was prepared by Robert A. Rohde from publicly available data from NOAA and the references therein. The article stated: “For the purposes of this comparison, the author is agnostic as to which, if any, of the reconstructions of global mean temperature is an accurate reflection of temperature fluctuations during the last 2000 years. “ The reconstructions used, in order from oldest to most recent publication are:

  • (dark blue 1000-1991): P.D. Jones, K.R. Briffa, T.P. Barnett, and S.F.B. Tett (1998). “High-resolution Palaeoclimatic Records for the last Millennium: Interpretation, Integration and Comparison with General Circulation Model Control-run Temperatures”. The Holocene 8: 455-471. doi:10.1191/095968398667194956
  • (blue 1000-1980): M.E. Mann, R.S. Bradley, and M.K. Hughes (1999). “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations”. Geophysical Research Letters 26 (6): 759-762.
  • (light blue 1000-1965): Crowley and Lowery (2000). “Northern Hemisphere Temperature Reconstruction”. Ambio 29: 51-54. Modified as published in Crowley (2000). “Causes of Climate Change Over the Past 1000 Years”. Science 289: 270-277. doi:10.1126/science.289.5477.270
  • (lightest blue 1402-1960): K.R. Briffa, T.J. Osborn, F.H. Schweingruber, I.C. Harris, P.D. Jones, S.G. Shiyatov, S.G. and E.A. Vaganov (2001). “Low-frequency temperature variations from a northern tree-ring density network”. J. Geophys. Res. 106: 2929-2941.
  • (light turquoise 831-1992): J. Esper, E.R. Cook, and F.H. Schweingruber (2002). “Low-Frequency Signals in Long Tree-Ring Chronologies for Reconstructing Past Temperature Variability”. Science 295 (5563): 2250-2253. doi:10.1126/science.1066208.
  • (green 200-1980): M.E. Mann and P.D. Jones (2003). “Global Surface Temperatures over the Past Two Millennia”. Geophysical Research Letters 30 (15): 1820. doi:10.1029/2003GL017814.
  • (yellow 200-1995): P.D. Jones and M.E. Mann (2004). “Climate Over Past Millennia”. Reviews of Geophysics 42: RG2002. doi:10.1029/2003RG000143
  • (orange 1500-1980): S. Huang (2004). “Merging Information from Different Resources for New Insights into Climate Change in the Past and Future”. Geophys. Res Lett. 31: L13205. doi:10.1029/2004GL019781
  • (red 1-1979): A. Moberg, D.M. Sonechkin, K. Holmgren, N.M. Datsenko and W. Karlén (2005). “Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data”. nature 443: 613-617. doi:10.1038/nature03265
  • (dark red 1600-1990): J.H. Oerlemans (2005). “Extracting a Climate Signal from 169 Glacier Records”. Science 308: 675-677. doi:10.1126/science.1107046

(c) 2012 J.C. Moore

Note added on 07/09/2013 : In a new study by the PAGES 2k project, that was  published in Nature Geoscience , 78 researchers from 24 countries worked for seven years on the most extensive climate reconstruction to date of the past 2000 years.  It is based on 511 climate archives from around the world, from sediments, ice cores, tree rings, corals, stalagmites, historical documents and measurements.  Their  graph below confirms the basic “hockey stick” shape of the graph:Pages 2K

 

 

Science, Climate Change, and the Greenhouse Effect

Mon ,13/12/2010

In the 1800’s, scientist began to understand the role greenhouse gases  had in keeping the Earth warm. The greenhouse effect is now a well established scientific principle. Both the science and the data show that  current global warming is caused by the increasing CO2 in the atmosphere.

Greenhouse Effect: Most gardeners know how greenhouses work.  In the daytime, the sun’s radiation (visible and UV) comes in through the glass and warms the plants and soil.  The glass stops the heat radiation in the infrared (IR) region from passing back through and the greenhouse stays warm enough to keep the plants from freezing, even at night. The Earth works much the same way except greenhouse gases, primarily water and  carbon dioxide, play the role of the glass and trap some of the leaving IR radiation. Winter nights on Earth would be very cold without greenhouse gases.

Earth’s Energy Balance: Of the Sun’s energy coming to Earth, 30% is reflected immediately back into space by particles in the air, by clouds, and by the surface. Another 20% is absorbed by the atmosphere where it runs the weather cycle. The remaining 50% heats the land and oceans. All the absorbed heat is eventually radiated back into space as infrared radiation. It’s a balanced energy budget, 100% in and 100% back out. Anything that reflects more light back into space, such as an increase in particulate matter in the air, would cause the Earth to cool. Anything that delays the energy’s trip back to space, such as an increase in greenhouse gases, would cause the Earth to warm. There are many small things that affect the Earth’s energy balance, but the main three are the Sun, particulates, and greenhouse gases.The ash from the explosive eruption of Mt.Tambora in 1816 caused that year to be called the year without a summer, worldwide.

The Sun: Certainly a change in the Solar radiation the Earth receives would cause a change in  the Earth’s temperature. Small wobbles in the Earth’s orbit, the Milankovitch Cycles, are variations in the eccentricity, axial tilt, and precession of the Earth’s orbit. They affect the amount of solar radiation the Earth receives in predictable cycles. Both scientists and skeptics agree that these cycles are responsible for the Ice Ages that occur in roughly 100,000-year intervals. In the part of the cycle where the Earth receives more solar radiation, the oceans slowly warm and release CO2. The CO2 further amplifies the warming by the greenhouse effect. As the Earth moves into the part of the cycle where it receives less solar radiation, the oceans slowly cool, the CO2 dissolves back into the oceans and another ice age starts. The patterns of wobble in the Earth’s orbit are predictable and the model predicts that a minor cooling trend, which began some 6,000 years ago, will continue for the next 23,000 years. The current warming trend is too rapid and in the wrong direction to be a part of the Milankovitch Cycles.

The Sun also has cycles where its output varies slightly such as  Sunspots activity. They cause the amount of solar radiation to vary in approximately 11-year cycles. However, the effects of Sunspots are so small that they do not show up above the other small variations in NASA’s temperature record.(see below). Long term variations in the Sun’s intensity are not responsible for the current warming. The graph of solar irradiance from 1880 to the present in this article shows that the Sun’s intensity increased slightly from 1880 to 1960 and then has declined slightly since 1960.   Satellite measurements of solar radiation show also that the solar radiation reaching Earth has declined slightly over the last 30 years – yet the Earth still warmed.

Temperature Data: The best temperature data we have clearly shows the Earth is getting warmer. NASA has compiled the Earth’s average temperature for each year since 1880 by using ships logs, weather stations, and satellite measurements. In the graph below , each square dot shows how far that year’s average temperature was above or below the 1970 value.  Although the data varies widely from year to year because of random factors such as sunspots, weather events, ocean current, and particulates from volcanoes and man’s activities,  the trend is clearly upward. The solid red and blue lines are  moving averages, which make the trend easier to follow.

NASA's Temperature Data  Credit: NASA JPL GISS

Credit: NASA/JPL/MSSS

Temperature Trend: The greenhouse effect links some of the causes of the temperature trend to man’s activities. The trend took a turn upward in about 1920. That was when the automobile, industrialization, and energy production began further increasing the carbon dioxide concentration in the air. The trend was flat from about 1945 to 1975 and  that can be attributed mostly to particulates. There was an increase in particulates after 1945 from many sources such as WW II, atmospheric nuclear testing, and increased industrialization. Research during the early 1970’s showed a huge increase in aerosols from power production, increased industrialization, and vehicles and some alarmists even speculated that we might be causing another ice age.  Particulates are visible and cause immediate health problems so by 1980 most industrialized countries had restrictions on particulate release. During the period from1945 to 1975 the CO2 concentration had continued to rise but its effect had been masked by the particulates. Reducing the particulates in the air allowed the full effect of the CO2 to be felt, causing the Earth’s temperature to begin to rise again. The effect of particulates and the reliability of the temperature record can clearly be seen in the graph above. In 1991, Mt. Pinaturbo erupted spewing about 10 cubic kilometers of ash into the air which caused an immediate 0.3 °C temperature drop  for the entire Earth, lasting until about 1995.

Causality: Although the greenhouse effect is a well accepted principle, skeptics sometimes claim the correlation between global warming and CO2 does not constitute causality. However, G.N. Plass, in 1956, calculated the climate sensitivity of the Earth to CO2. He found that doubling the concentration of CO2 in the air would cause a 3 to 4 °C increase in the Earth’s temperature. A number of more recent studies have confirmed his work and have shown that, though the concentration of CO2 in the air is small, it accounts for about 25% of the greenhouse effect. No natural occurrences such as volcanoes, sunspots, fires, or dust storms can account for the major trend in the data. Certainly, the increasing amount of CO2 in the air is causing the Earth to warm.

Man’s Role: Man’s activities, mainly through deforestation and burning fossil fuels, have released large amounts of CO2 into the air. In the last century, man’s emission of CO2 from fossil fuels have increased to over 30 billion tons annually and the concentration of CO2 in the air has risen from 280 parts per million (ppm) to 385 ppm. The processes that remove carbon dioxide from the air takes decades or longer so as the carbon dioxide concentration slowly built up, the Earth became a better greenhouse. The concentration of carbon dioxide in the air is now 38% higher than in 1880 and the Earth’s temperature is about 0.8°C (or 1.3 °F) higher. Clearly, man’s activities are mainly responsible for increasing the CO2 concentration in the air – and the increasing CO2 concentration is causing global warming.

(C) 2010 J.C. Moore

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Is Global Warming Naturally Occurring?

Fri ,12/11/2010

The Earth goes through cycles of warming and cooling that occur about every 100,000 years. The present global warming we are experiencing is not part of those cycles. Is the cause natural or is it man’s activities?

The Earth is getting warmer. NASA has compiled the Earth’s annual mean temperatures since 1880 by using ships logs, weather station, and satellite data. Their data shows that the trend in the Earth’s temperature has been mostly upward and that the Earth is now about 1.3 °F warmer than it was a century ago. Scientists point to an increase in greenhouse gases from fossil fuel use as the major cause, but skeptics claim the warming observed is natural. The Sun, clouds, particulates, volcanoes, and greenhouse gases all affect the Earth’s temperature. The question is “What part does each play in global warming and what part does man play?”

Energy balance: The Earth has a solar energy balance. Of all the Sun’s energy coming to Earth, 30% is reflected immediately back into space by the surface, particles, and clouds. About 20% is absorbed into the atmosphere where it runs the weather cycle.The remaining 50% heats the land and oceans. All the absorbed heat is eventually radiated back into space as infrared radiation. It’s a balanced energy budget with 100% of the incoming energy eventually going back into space. Anything that reflects sunlight into space, such as an increase in particulate matter in the air, would cause the Earth to cool. Anything that increases the amount of sunlight received or delays the energy’s trip back to space such as greenhouse gases do, would cause the Earth to warm. What factors influence the energy balance?

The Sun: Solar radiation from the Sun seems to have varied little over the last million years. The Sun has Sunspot cycles where its output varies slightly and causes the amount of solar radiation to vary in approximately 11-year cycles. Increased sunspot activity causes the Earth to warm. However, the effect of Sunspots is so small that they do not show up above the other small variations in NASA’s temperature record. Long term variations in the Sun’s intensity does not seem responsible for the current warming. Satellite measurements of solar radiation show that the solar radiation reaching Earth has declined slightly over the last 30 years – yet the Earth still warmed. Also, this graph of solar irradiance from 1880 to the present as measured from the earth and later by satellite shows that the Sun’s intensity increased slightly from 1880 to 1960 and then has declined slightly since 1960.

Milankovitch Cycles: Small wobbles in the Earth’s orbit such as variations in the eccentricity, axial tilt, and precession of the Earth’s orbit affect the amount of solar radiation the Earth receives. Both scientists and skeptics seem to agree that these cycles are responsible for the Earth’s change from warmer to cooler periods and for ice ages that occur in roughly 100,000-year cycles. In the part of the cycle where the Earth receives more solar radiation, the oceans slowly warm and release CO2. The CO2 further amplifies the warming by the greenhouse effect. As the Earth moves into the part of the cycle where it receives less solar radiation, the oceans slowly cool, the CO2 dissolves back into the oceans and another ice age starts. The patterns of wobble in the Earth’s orbit are predictable and the model predicts that a minor cooling trend, which began some 6,000 years ago, will continue for the next 23,000 years. The current warming trend is too rapid and in the wrong direction to be a part of the Milankovitch Cycles.

Clouds: Clouds have a dual role. They cool the Earth in the daytime by reflecting solar radiation back into space but they also warm the Earth by their greenhouse effect. First frosts are more likely to occur on cold clear nights. The average cloud cover over the entire Earth remains relatively constant from year to year but cloud cover may increase as a feedback to the warming oceans. Clouds could be considered to be a cause of the current warming trend only if the average cloud cover could be shown to have increased over the years.

One unusual theory tries to do just that. It claims that the number of cosmic rays from the stars that strike the Earth is increasing. That would lead to more clouds since cosmic rays produce charged particles in the atmosphere that seed clouds. It’s an interesting theory, but there is no data showing that the number of cosmic rays are actually increasing cloud cover. Also, there are plenty of particulates in the air to seed clouds and any effect from cosmic rays would be small in comparison.

Greenhouse gases: If the Earth had no atmosphere, its average temperature would be much colder. Most of the Sun’s energy comes to Earth as light in the visible and ultraviolet region of the spectrum but it leaves as heat energy in the infrared region. The atmosphere’s clouds, water vapor, and CO2 traps some of the infrared radiation headed into space and directs it back to Earth, helping to warm the Earth. This is called the greenhouse effect, as it is similar to the effect that keeps greenhouses warm on cold nights. Recent research shows that about 75% of the greenhouse effect is due to water vapor and clouds and the rest is due mostly to CO2.

Water vapor is removed from the air whenever it rains so the average amount of water vapor in the air remains somewhat constant from year to year. However, the amount of CO2 in the air has been observed to be increasing. Measurements of the concentration of CO2 in the air have shown that it has increased from 280 ppm to 385 ppm in the last century. The physicist G. N. Plass determined the effect of the increase of CO2 on the Earth’s temperature. In 1956, he calculated that doubling the concentration of CO2 in the air would cause a 3 to 4 °C increase in the Earth’s temperature. A number of more recent studies have confirmed his results but the most important thing is that the rise in temperature he predicted 50 years ago in consistent with the increase in temperature we have observed. Certainly, the increasing amount of CO2 is a main factor global warming. But, is the increase in CO2 natural or man-made?

Volcanoes: Although impressive, the amount of heat released by volcanoes has a minuscule effect on the Earth’s temperature compared to that of the particulates and greenhouse gases they release. Particulates reflect sunlight back into space and cause the Earth to cool while the greenhouse gases released cause the Earth to warm. Particulates from large eruptions, such as Mt. Pinaturbo, cause a very quick decline in the Earth’s temperature. However, the particulates settle out or are removed by rain in a few years after the eruption stops. The CO2 released by the eruption causes greenhouse warming for a long time as it persists in the air for many decades. Volcanoes, however, play a small role in the current global warming compared to man. Man’s activities currently emit about seven times the particulates and 150 times the CO2 as all the world’s volcanoes together.

Fossil fuels: Since the 1800′s, scientists have been concerned with whether our use of fossil fuels might affect the temperature of the Earth. Burning carbon fuels releases large amounts of CO2, and there was speculation about whether an increase of CO2 in the air might actually change the energy balance of the Earth. Recent research has shown that CO2 plays a very important role in global warming and one scientist even labeled CO2 the “control knob” for the temperature of the Earth. The amount of CO2 man releases into the air is no longer minuscule as our emissions amount to 30 billion tons of CO2 each year. The concentration of CO2 in the air has increased from about 280 ppm to 385 ppm and the Earth’s temperature has increased by 1.3 °F. Clearly, CO2 is the main factor in global warming and our use of fossil fuels is the main cause of the increase in atmospheric CO2.

It’s not natural: Climate scientists agree. In 2004, a survey of 928 climate papers published in Science found that none of the authors argued for an entirely natural explanation of global warming. So there we have it. The main cause of the current global warming is not volcanoes, clouds, sunspots, changes in solar output, or cosmic rays from the stars – and it is not part of the natural cycles of nature. As much as we may dislike the idea, the major cause is man.


(C) 2010 J.C. Moore

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