Recent Satellite-observed Trends and the “Pause” in Global Warming

A recent slow rise in the Earth’s mean temperatures observed by surface reporting stations since 1979 has been supported by microwave satellite data processed by NASA (Figure 1) that obtains an estimate of lower atmospheric temperature from the emission of thermal energy by oxygen molecules.[1] Originally, it was thought that the satellite data did not show any warming, but after some recent corrections in the conversion of this data to temperature, the trend fell nearly in line with surface temperature readings for the same period. In Figure 1 it can be seen that since the late 1990’s the global mean atmospheric temperature was nearly steady, but has begun to increase again from 2015-2017, likely due to a strong El Niño event such as that which occurred in 1998. In fact, the years 2014 and 2015 were considered global records for the modern instrumented period (1850 to the present). The total warming during the period of satellite measurements has been about 0.8°C.

Figure 1. Mean temperature difference of the global lower atmosphere (as of March 2018) from the 1981-2010 average (degrees C) as measured by polar-orbiting environmental satellites. The red line is a smooth version of the data obtained by calculating a running 13-month average. (Source: University of Alabama-Huntsville)

There has been considerable debate about this recent “pause in global warming” from about 1998 to 2014. Some contend that it doesn’t really exist at all, or that the global temperatures really increased during the period, but at the rate of only 0.01°C per year, which is not at all significant. A recent paper went so far as to re-analyze historical temperature data from ship reports to remove perceived biases, resulting in a steady temperature rise during that period. [2]  Ships measure the temperature of intake water used for ballast and engine coolant. However, since the depth at which this water originates varies from ship to ship depending on size and weight, it must be adjusted but wasn’t in the study.

Finally, sound judgement prevailed in a recent consensus article stating that the slowdown in global warming was real,[3] and was believed to be caused mainly by a cool phase in the slow moving North Pacific ocean currents, referred to as the Pacific Decadal Oscillation (PDO) (see a later blog on this topic). This has resulted in excess atmospheric heat being absorbed into the oceans, leading to a rise in sea levels. They insist that “human-caused warming is continuing, but it is masked by natural factors.” It should be noted however, that absorption of heat and CO2 into the oceans is a normal process that goes on continually.

A recent, plausible theory for this pause in global warming was the discovery that aerosols in the lower stratosphere (above 8 km) have increased during the period from 2005 to 2012 due to volcanic eruptions. [4] The measurements were made on board instrumented commercial jets and by the CALIPSO satellite which is equipped with a earth-pointing lidar system. The volcanic aerosol which effectively scatters solar radiation into space is sulfur dioxide (SO2) gas. A similar, more dramatic event occurred 200 years ago when the eruption of Tambora volcano in Indonesia resulted in a “year without a summer” in 1816.

Was this pause in global warming predictable? A recent statistical study finds that it was not, due to the inability to accurately predict changes in the North Pacific sea surface temperatures (SST), a primary cause, and North Atlantic SST, a secondary driver. [5]

In short, satellite measurements support a gradual rise in global mean surface temperature (0.8 °C) since the late 1970’s, and little or no rise in global mean temperature from 1998 to 2014. A recent spike in temperatures may be partly due to a strong El  Niñthat has faded by mid-2016.

References:

  1. Spencer, R. W., J. Christy, and W. Braswell, 2015: Version 6.0 of the UAH Temperature Dataset Released. (Available at: http://www.drroyspencer.com/2015/04/version-6-0-of-the-uah-temperature-dataset-released-new-lt-trend-0-11-cdecade/)
  2. Karl, T. R., A. Arguex, B. Huang, J. Lawrimore, J. McMahon, M. Menne, T. Peterson. R. Vose, and H-M Xhang, 2015. Possible Artifacts of Data Biases in the Recent Global Surface Warming Hiatus. Science, Vol. 348, issue 6242, pages 1469-1472.
  3. Fyfe, J. C., G. Meehl, M. England, M. Mann, B. Santer, G. Flato, E. Hawkins, N. Gillett, S-P Xie, Y. Kosaka, and N. Swart, 2016. Making Sense of the Early-2000s Warming Slowdown, Nature Climate Change, 6, pages 224-228.
  4. Andersson, S. M., Martinsson, B. G., Vernier, J-P, Friberg, Brenninkmeijer, C., Hermann, M., van Velthoven, P. F. J., and Zahn, A. Significant radiative impact of volcanic aerosol in the lowermost stratosphere. Nature Communications, 2015; 6: 7692 DOI: 10.1038/ncomms8692
  5. Mann, M. E.,B. A. Steinman,S. K. Miller, L. M. Frankcombe, M. H. England, and A. H. Cheung (2016), Predictability of the recent slowdown and subsequent recovery of large-scale surface warming using statistical methods, Geophys. Res. Lett., 43, 3459–3467, doi:10.1002/2016GL068159

Next: Review of the “Greenhouse Effect”

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Review of the “Greenhouse Effect”

Just to review, the “greenhouse effect” occurs when incoming solar ultraviolet (UV) radiation penetrates the atmosphere, is reflected by the earth’s surface, clouds, etc., but is prevented from completely escaping the atmosphere as infrared (IR) thermal energy by intervening gases that absorb the energy and re-radiate it back toward the Earth’s surface, similar to what occurs in a glass greenhouse (Figure 1). The greenhouse gases can include carbon dioxide (CO2), methane (CH4), water vapor (H2O), and man-made chlorofluorocarbons. The greenhouse effect is essential to our existence. Without it, the Earth would be nearly uninhabitable, resulting in an average temperature below freezing. The question is can an imbalance caused by excessive greenhouse gases cause a rise in the surface temperatures to dangerous levels? As to the specific role of CO2, there is even some conjecture as to whether increases in CO2 alone can even produce a significant increase in the Earth’s average temperature.  This was discussed in more detail in the “Hockey Stick” blog.

Greenhouse_effect_Smithsonian

Figure 1. A schematic depiction of the “Greenhouse Effect.” (Source: Smithsonian Institute)

While CO2 is the most publicized greenhouse gas, it is not the most prevalent (that is water vapor), nor is it the most efficient absorber (that is methane).  Fortunately, methane (CH4) is not nearly as abundant as water vapor and CO2. Methane is less than 2 parts per million (ppm), whereas CO2 is more than 400 ppm. Water vapor is highly variable but can range from 10 ppm in deserts to 50,000 ppm in the tropics. Other significant absorbing gases in the atmosphere are nitrous oxide (N2O) and man-made chemicals such as chlorofluorocarbons (CFCs) that are used in refrigerant and fire suppression systems (their use in aerosol cans was banned by the Montreal Protocol in the late 1970’s).

Another important factor to consider is the longevity of the greenhouse gases.  Airborne CO2 and N2O can persist for many years (30 to more than 100) whereas water vapor has an average residence time of nine days. Thus, the effects of CO2 are longer-lasting. Man-made chemicals such as CFCs are extremely long-lasting (100 to thousands of years).

The efficiency of greenhouse gases has been combined with their lifetimes to form a Global Warming Potential (GWP) which has been calculated for various time periods. [1] While CO2 is much less important than other gases at short time scales (~20 years), it comes closer to CH4 over periods greater than 500 years. Due to its persistence and greater radiative efficiency, N2O has GWP significantly greater than CH4 and CO2 at all time periods. [1]

GWP_table_DOE

TABLE I. Global Warming Potential (GWP) of well know natural and man-made greenhouse gases relative to CO2 from the IPCC 2007 report. The GWP of CO2 is set to one for comparison. (Source: U. S. Dept. of Energy)

It is important to note that CO2 is produced by both natural and man-made causes. The latter include emissions from motor vehicles and other forms of fossil fuel burning (especially coal burning power plants that account for more than 40% of emissions). Natural emissions can come from volcanoes, decaying vegetation and the oceans.  The rise in CO2 is partly alleviated by absorption from plant photosynthesis and the oceans, a sort of checks and balances system. Plants thrive in a CO2-rich environment.

It appears that CH4 has been increasing globally over the past decade, with a 30% increase in the United States, according to a recent study based on satellite data and surface observations. [2] A third of the U.S. increase is from oil and gas emissions, another third from livestock (yes, cow flatulence), while the remainder comes from landfill emissions and coal burning.

Although man-made emissions of CO2 are relatively small compared to other sources in the carbon cycle, not all of it can be absorbed, leaving a net increase in atmospheric CO2. Current levels (around 400 ppm) are believed to be the highest in more than 600,000 years. [3] This is not that surprising, since the Earth’s population is continuing to increase in an exponential fashion. Figure 2 below clearly shows that the trends of CO2 emissions and population match almost exactly. But as the many variations in global temperature over the last century show (Figure 3), CO2 increases do not always result directly in a rise in mean temperature, as a portion of the excessive heat can be absorbed by “sinks” such as the oceans, or incoming solar radiation can be blocked by increased aerosol concentrations.

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Population-vs-emissions

Figure 2. Trend of CO2 emissions (millions of tons) versus global population (millions) from 1850-2005 (Source: Steve Easterling, U. of Toronto)

NOAA_global_land_sea_temperatures+CO2

Figure 3. Trend of CO2 concentration (ppm) since 1880 (solid black line) is compared with annual mean global temperature (°F) for the same period. Horizontal reference line is mean temperature for the period (57.6°F). (Source: NOAA/National Climate Data Center)

The role of CO2 in significantly warming the Earth’s climate has been questioned by some. One researcher, the late Dr. William Gray, argued that the expected increase in CO2, even a doubling of CO2, will not bring the anticipated increase in global temperatures. Instead, he believed that warming of only half a degree C or less will occur, not the 2-4°C or more expected by the IPCC.[4]  This claim has historical basis. Around 1900, Swedish physicist Knut Angstrom determined that CO2 concentration beyond about 50 ppm has little effect on the Earth’s temperature, although the results of that experiment were disputed. Dr. Gray believed that changes in the strength of the inter-ocean circulation (such as the Atlantic Thermohaline Current) which controls the salinity of the oceans has led to the 0.8°C increase we have seen in the past century, not man-made greenhouse gases.

References:

  1. Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report Climate Change, 2007, Working Group 1: The Physical Science Basis, Table 2.14, Section 2.10.2 Direct Global Warming Potentials
  2. Turner, A. J. and co-authors, 2016: A large increase in U. S. methane emissions over the last decade inferred from satellite data and surface observations. Geophysical Research Letters, Vol. 43, pages 2218-2224.
  3. Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report Climate Change, 2007, Synthesis
  4. Gray, William M., 2012: Physical Flaws of the Global Warming Theory and Deep Ocean Circulation Changes as the Primary Climate Driver. Unpublished manuscript prepared for the Heartland Institute’s 7th Int’l Conference on Climate Change, Chicago, May 21-23, 2012.

Next: Accuracy of Long Range Global Temperature Forecasts

“97% of Climate Scientists Agree That…”

You’ve no doubt heard the statement made by politicians and others (maybe even some of your friends) that “97% of climate scientists agree that man-made global warming is real, and that it is caused mainly by human activity.” Is that statement true, and if so, where did it originate? The apparent source is an article by John Cook and co-authors (2013)[1] that actually states that 97% of published articles (that they surveyed) on climate change over a 20 year period endorsed the view that the Earth is warming up, and that human emissions of greenhouse gases are mostly to blame. Since some of the papers were written by the same authors, it is difficult to leap to that 97% figure based on this sample. Not only that, many of the authors felt that the conclusions of their papers were misrepresented. After the results were challenged, someone calculated that only about 1.6% of the articles stated explicitly that man-made greenhouse gases caused at least 50% of global warming. [2]

The American Meteorological Society (AMS) recently completed a survey of its membership to learn more about their views on climate change.[3] About 39% of the AMS members who responded consider themselves “climate scientists,” and the rest are mostly degreed scientists specializing in various atmospheric or technology disciplines. The results indicated that while the large majority (89%) of the more than 4,000 respondents believe that climate change is occurring, a little more than two-thirds (69%) believe that the changes are mostly or entirely due to human activity, while 14% think natural and man-made causes are about equal. A small but significant number believe that climate change is either completely due to natural causes (5%), isn’t happening at all (1%), or don’t know (6%).

Opinions in the AMS study on the extent to which future climate change can be averted if all nations agree to mitigation measures are also varied. Only 18% think a large portion of future changes can be prevented while many more think that a moderate (42%) or small (25%) additional amount of climate change can be averted.

In summary, while a majority of atmospheric scientists (including climate scientists) believe that human- caused warming is occurring, the percentage that believe that the majority of recent climate change is due to manmade causes is much less than 97%, and is probably more like two-thirds. Also, less than half believe that a significant portion of the expected warming can be prevented through global mitigation efforts.

References:

  1. Cook, J. , D. Nuccitelli, S. Green, M. Richardson, B. Winkler, R. Painting, R. Way, P. Jacobs, and A. Skuce, 2013: Quantifying the consensus on global warming in the scientific literature. Environ. Research Letters, Vol. 8, 7 pages.
  2. Epstein, A., 2015: ‘97% of Climate Scientists Agree’ is 100% Wrong, Forbes /Energy and the Environment, Jan. 15, 2015
  3. Maibach, E., Perkins, D., Francis, Z., Myers, T., Englbom, A., 2016. A 2016 National Survey of American Meteorological Society Member Views on Climate Change: Initial Findings. George Mason University, Fairfax, VA: Center for Climate Change Communication

Next: The “Hockey Stick” Controversy

Climate Change Series Intro

A balanced view of the scientific evidence related to climate change

A Closer Look at Global Warming

Introduction

You’ve all seen or heard the alarming news stories about the impending dangers to our planet due to the expected warming caused by the continued release of man-made greenhouse gases.  Global warming, also known as “climate change,” is expected to accelerate during this century, leading to a variety of environmental hazards such as rising sea levels, extreme drought, more severe storms, etc.  While the majority of scientists believe this is going to happen, a smaller, less vocal group believes that the threats are exaggerated, or worse, a complete hoax.  These “deniers” as they are called, are sometimes treated with contempt, resulting in little or no open dialog. There have even been recent attempts by the U. S. Department of Justice and state attorney generals to subpoena various climate skeptics for their records of past positions and research. It did not help that thousands of emails leaked to the public in 2009 (the so-called “Climate-gate” scandal) revealed a tendency among some climate scientists to try to hide or alter unfavorable data and silence dissenting views. Even among supporters, there is perceived to be an attitude of complacency, called “neo-skepticism,” that feels there is little we can or should do to alter the trend. So who’s right, or do both sides have valid points?  My goal as a non-climate scientist is to present a more balanced view of the scientific evidence related to this topic through a series of blogs. To view the blog in the correct order, follow the links at the bottoms of the posts. (References are included in each section, denoted in the text by a superscript numeral)

  1.  “97% of Climate Scientists Agree That ….”
  2. The Hockey Stick Controversy
  3. Recent Satellite-observed Temperature Trends and the “Pause” in Global Warming
  4. A Review of the Greenhouse Effect
  5. Accuracy of Long Range Global Temperature Predictions
  6. Looking at Climate in the Distant Past
  7. Short Period Natural Climate Variations
  8. Variations in Solar Output and Their Effects on Climate
  9. Drought Cycles and Crop Yields
  10. Heavy Precipitation and Flooding
  11. Trends in Hurricane Frequency and Intensity

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The “Hockey Stick” Controversy

One of the most significant climate change controversies started when researchers noted that a sharp increase in the carbon dioxide (CO2) concentration in the atmosphere beginning in the 1970’s was accompanied by a significant rise in global mean temperature (Figure 1). [1] The shape of the resulting trend of surface temperature over the past 1000 years became known as the “hockey stick” diagram (see right panel in Figure 2 below).  An inference was made that the rise in temperature was directly caused by a simultaneous increase in CO2 concentration. There are several problems with this conclusion: (1) an equally sharp rise in temperature in the early 1900’s was only accompanied by a modest increase in CO2 (Figure 1), (2) global temperatures from land-based observing stations ceased rising in the late 1990’s (more on this in a later blog), (3) a Medieval warm period observed in historical temperature records from about 1100 to 1400 AD disappeared from the Intergovernmental Panel on Climate Change (IPCC) reports between 1990 and 2001 (Figure 2), and (4) some question whether CO2 increases have much if any effect on the Earth’s temperature.

A recent unpublished report [2] questioned the reliability of the Global Average Surface Temperature (GAST) data set used for decision-making worldwide. The GAST data set extends from 1880 to the present and is based on temperature observations over land (mostly from airport sites) and from ocean-going vessels and buoys. The report shows that cyclical patterns in the raw temperature data have been removed by adjustments that reduced the magnitude of warm periods in the early to mid 20th century (such as the 1930’s and early 1950’s) while increasing mean temperatures in more recent decades. The purpose of the adjustments was to correct for changes in reporting locations and instruments over land, and methods of collecting temperatures from ships over oceans. The result has been a sharpening of the upward trend in the records to one that resembles the “hockey stick.” The claims are supported by graphs originally from official sources such as NOAA, EPA, the U. K. Met Office, the Hadley Centre at East Anglia University, and NASA.

While the 2001 IPCC report included additional types of data in the longer period of record (~1000 years), such as tree ring analyses, it was thought that the conclusion of a modern spike in global temperatures was exaggerated relative to historical temperature records.  Further doubt was cast when researchers found that the statistical method used to create the updated trend line (Principle Component Analysis) was flawed, emphasizing those data series (referred to as “proxies”) that showed a temperature trend favoring the “hockey stick” shape with time. [3,4] The result is that the medieval warm period and a cool period known as the Little Ice Age in the early 1800’s were nearly eliminated (Figure 2). Other scientists have been unable to replicate the “hockey stick” trend in their independent analyses.

NOAA_global_land_sea_temperatures+CO2

Figure 1. Trend of CO2 concentration (ppm) since 1880 (solid black line) compared with annual mean global temperature (°F) for the same period. Horizontal reference line is mean temperature for the period (57.6°F). (Source: NOAA/National Climate Data Center)

IPCC_Comparison-charts_1990vs2001

Figure 2. Trends of global temperature (°C) for the past 1000 years as shown by the Intergovernmental Panel on Climate Change (IPCC) reports for 1990 (left) and 2001 (right).(From McKitrick, 2005)

In regards to the longer, 1000 year trend of global temperatures, an independent study based on data from over 6,000 sub-surface borehole records (formed by layers of petrified mud in lake bottoms) that go back 20,000 years clearly showed that the Medieval Warm Period and Little Ice Age existed. [5] In fact, the medieval warm period was significantly warmer than what we have seen in the late 20th century. Nevertheless, the results of this study were ignored by the IPCC. It is interesting to note that the “Hockey Stick” figure was removed entirely in both the 2007 and 2013 IPCC reports. The only temperature trends shown in those reports were in the 1850-2005 year range. So while we should still be concerned about future climate change, recent global trends in surface temperature do not appear to be extreme based on trends over the past millennium.

Borehole_temperature_trend_Huang_etal_1998

Figure 3. Trend in global mean temperatures (°C) over the past 1,000 years inferred from over 6,000 soil cores (adapted from figure in Huang et al. 1997). The large peak centered between 1300 and 1400 is the Medieval Warm Period, and the dip centered near 1800 is the Little Ice Age. Top and lower lines show the range of uncertainty.

The role of CO2 in warming the Earth’s climate has been questioned by some. One researcher, the late Dr. William Gray, argued that the expected increase in CO2, even a doubling of CO2, will not bring the anticipated increase in global temperatures. Instead, he believed that warming of only half a degree C or less will occur, not the 2-4°C or more expected by the IPCC.[6]  This claim has historical basis. Around 1900, Swedish physicist Knut Angstrom determined that CO2 concentration beyond about 50 ppm has little effect on the Earth’s temperature, although the results of that experiment were disputed. Dr. Gray believed that changes in the strength of the inter-ocean circulation (such as the Atlantic Thermohaline Current) which controls the salinity of the oceans has led to the 0.8°C increase we have seen in the past century, not man-made greenhouse gases.

Another interesting theory that support’s a diminished role of CO2 in climate change is that the recent increases in the Earth’s temperature were not due to increased CO2 concentrations, but to the depletion of ozone in the upper atmosphere [7]. Our ozone layer, with peak concentrations at around 20 km altitude, absorbs most of the dangerous ultraviolet-B (UV-B) radiation from the sun before it reaches the Earth. According to the Planck-Einstein relationship, energy is proportional to the frequency of electromagnetic radiation (EMR). Thus, EMR from the sun (UV-B), provides almost 50 times more energy than infrared EMR emitted from Earth which has a much lower frequency (i.e., longer wavelength), and thus is more effective in producing temperature change at the surface.

Depletion of atmospheric ozone due to release of man-made chlorofluorocarbons (CFCs) into the upper atmosphere, which began in the 1960’s, continued into the early part of the 21st century.  A plot of Northern Hemisphere temperature anomalies versus total column ozone (Dobson Units – DU) from 1927 to 2013 is plotted in Figure 4. Although there is considerable scatter, the decrease in ozone of 30 DU explains a temperature increase of 0.9°C, which is close to that observed. The scatter in the diagram is believed to be due to (1) emission of volcanic aerosols and (2) sudden, large ozone emissions due to tests of nuclear devices and volcanic lava flows. The Montreal Protocol, signed in 1989, began to alleviate this problem by reducing emission of CFCs.

Figure 4. A decrease in yearly total column ozone at mid-latitudes of the northern hemisphere by 30 Dobson Units (DU) from 1927 to 2013 corresponds  to a 0.9°C increase in temperature anomaly. Numbers are the years of observations. (from Ward, 2016)

In short, a rise in global mean temperature of about 1°C has occurred since the beginning of the Industrial Revolution over the past 150 years but that is not an alarming amount based on historical temperature records for the past millennium, which includes a medieval warm period believed to be significantly warmer than today. Carbon dioxide levels, meanwhile, have risen significantly to about 400 ppm since the latter part of the 20th century, but the resulting temperature increase (0.8 °C) has not been consistently in step. For example, there has been little or no rise in global mean temperature from 1998 to 2013.

References:

1, Mann, M. E., Bradley R. S., and Hughes, M. K., 1998: Global-scale temperature patterns and climate forcing over the past six centuries. Nature, 392, pages 779-787

2. Wallace, J. P., J. S. D’Aleo, and C. D. Idso, 2017: On the validity of NOAA, NASA and Hadley CRU Global Average Surface Temperature data and the validity of EPD’s CO2 endangerment finding. Abridged Research Report. June 2017, 30 pages. (available online at:  https://thsresearch.files.wordpress.com/2017/05/ef-gast-data-research-report-062717.pdf 

3. McKitrick, R., 2005: What is the hockey stick debate about? Asia-Pacific Economic Cooperation (APEC) Study Group, Canberra, Australia, April 4, 2005, invited paper

4. Muller, R., 2004: Global Warming Bombshell. MIT Technology Review, https://www.technologyreview.com/s/403256/global-warming-bombshell/

5. Huang, S., Pollack, H. N., and P. Y. Shen, 1997: Late Quaternary Temperature Changes Seen in Worldwide Continental Heat Flow Measurements. Geophys. Res. Letters 24, pages 1947-1950.

6. Gray, William M., 2012: Physical Flaws of the Global Warming Theory and Deep Ocean Circulation Changes as the Primary Climate Driver. Unpublished manuscript prepared for the Heartland Institute’s 7th Int’l Conference on Climate Change, Chicago, May 21-23, 2012.

7. Ward, Peter L., 2016: Ozone Depletion Explains Global Warming. Current Physical Chemistry, Volume 6, 21 pages.

Next: Recent Satellite-observed Trends and the “Pause” in Global Warming Continue reading “The “Hockey Stick” Controversy”