Synopsis
Global warming at about 0.8 degrees per century is happening but as yet there is no need to panic about the next 10 years, the immediate future. Whether or not to be concerned about the more distant future depends upon climate models. The rapid disappearance of the mass of the arctic sea ice (not just its area) is disturbing and suggests that sudden, large scale, unexpected events can occur. The failure of climate models to predict arctic melting means that these models are inaccurate. The ice-core data suggests that at present we are failing to experience an expected dip in global temperatures. The ice cores also show that 1-4 degrees of warming or cooling in a few centuries has happened very frequently in the past 400,000 years but has been rare in the past 5000 years. It appears to be the nature of the global climate system to have wild fluctuations in temperature. What is new is that there are now 7 billion people on the earth who might be affected by these changes so events on the scale of those in pre-history, such as North Africa changing from grassland to desert, will now affect millions or billions of people (See Is climate change a threat because of overpopulation?.
Sea level rise
Summary: sea level rise indicates a steady warming of the oceans that has almost halted in the past six years but is now continuing apace. The ice sheets and glaciers continue to melt.
Measurements of sea-level rise show that sea levels have risen between 1993 and 2007 at a steady rate of about 3.3 mm a year (Cazenave and Llovel 2010). Over the whole period about 30% of the observed rate of rise is due to ocean thermal expansion and 55% results from land ice melt of which 50% is melting glaciers and 50% melting ice-sheets (Casenave et al 2008).
The oceans act to some extent like a giant thermometer. If the seas are warmed they expand and sea levels rise. Sea level rise is also partly due to the melting of ice where that ice is not floating on the sea (for instance the ice in Greenland or Antartica).
The measurement of sea levels has been greatly expedited in recent years by satellite based measurements, especially the the TOPEX/Poseidon joint NASA/CNES (French Space Agency) Satellite, and the twin satellites of the Gravity Recovery And Climate Experiment (GRACE), a joint mission of NASA and the German Space Agency launched in 2002. The gravity measurements allow the contributions of thermal expansion and added water volume from melting ice to be separated.
"..thermal expansion contributed by ∼ 50% to the 3.1 mm/yr observed global mean sea level rise during the 1993–2003 decade, the remaining rate of rise being essentially explained by shrinking of land ice. Recently published results suggest that since about 2003, ocean thermal expansion change, based on the newly deployed Argo system, is showing a plateau while sea level is still rising, although at a reduced rate (∼ 2.5 mm/yr)."(Casenave et al 2008)
Antarctica is contributing about 0.54 to 0.56 mm a year to sea level rise and Greenland about 0.38 mm a year. Most interestingly the ice sheets of Greenland and Antarctica have both changed from net accumulation of water (as snow) to net liberation of water (as melt) in the past 3-5 years.
A summary of the satellite data. "Altimetry data are provided by the NOAA Laboratory for Satellite Altimetry."
Skeptics have claimed that the TOPEX/Poseidon instruments lacked the accuracy to perform sea level measurements, with wild claims that the accuracy was less than +/- a metre. The TOPEX/Poseidon measurements did indeed require a lot of statistical treatment but the JASON-2 missions can measure radial position to better than 1.5 cm on each sample and can achieve submillimetre accuracy on multiple measurements. Morner (2003) is often quoted as showing that the TOPEX/Poseidon data do not show any sea level rise but he chose a limited period and solely used TOPEX/Poseidon raw data without the statistical corrections required on the early instruments. If the subsequent, highly accurate missions had shown no sea level rise then Morner would have been vindicated but, as can be seen from the graph, the sea level rise is real. The current trend is for about 29 cm (1 foot) sea level rise in a century and there is, as yet, no sign of acceleration of the rise.
The historical data from sea level gauges is consistent with the modern satellite data:
Courtesy Wikimedia Commons.
The recent slow down in sea level rise due to expansion as a result of warming is clear from data from sea surface temperature measurements provided by NASA (GISS):
See Goddard Institute for Space Studies
It is also clear from this graph that 5 or 10 years of "bucking the trend" is normal but should not make us imagine that the trend will not resume.
Sea ice loss
Summary: the loss of arctic sea ice in recent years has been extraordinary. It is clear evidence of marked warming of the Arctic Ocean
Loss of sea ice is indicative of warmer poles and warmer ocean currents.
ICEsat satellite data shows that in the Arctic Ocean:
"Along with a more than 42% decrease in multiyear (MY) ice coverage since 2005, there was a remarkable thinning of ∼0.6 m in MY ice thickness over 4 years. In contrast, the average thickness of the seasonal ice in midwinter (∼2 m), which covered more than two-thirds of the Arctic Ocean in 2007, exhibited a negligible trend. Average winter sea ice volume over the period, weighted by a loss of ∼3000 km3 between 2007 and 2008, was ∼14,000 km3. The total MY ice volume in the winter has experienced a net loss of 6300 km3 (>40%) in the 4 years since 2005.."(Kwok et al 2009)
The image above is based on NASA data ( http://earthobservatory.nasa.gov/IOTD/view.php?id=8126 ). Image hosted by Wikimedia Commons.
Sea ice is less dense than sea water and the net effect of melting all the sea ice would be about a 40 mm rise in sea level (Noerdlinger and Brower 2007). Melting sea ice may also transiently reduce sea level because of the cooling effect (Jenkins and Holland 2007). The most important effect of melting the sea ice is likely to be a decrease in the reflectivity of the earth, leading to further heating (the arctic ice reflects almost all incident light, when it has gone the arctic ocean will absorb most incident light). It is disturbing that almost all the climate models used by the IPCC missed the recent, sudden change in the arctic (See Stroeve et al 2007. Arctic sea ice decline: Faster than forecast). (The loss of sea ice has remained in 2007-2010).
Tree ring studies
Summary: studies of bristlecone pine tree rings (dendroclimatology) appear to show that the tree line is getting warmer.
Dendroclimatology, the reconstruction of historical climate from the width of tree rings, was one of the first techniques to show that global warming might be occurring. The early work was done on Bristlecone Pines which are some of the oldest trees in the world (see for instance LaMarche et al 1984). There is an excellent, accessible, primary source of recent dendroclimatology data at Saltzer et al (2009). This seems to show that zone near the tree line, the maximum altitude at which bristlecones grow, is becoming warmer:
"Great Basin bristlecone pine (Pinus longaeva) at 3 sites in western North America near the upper elevation limit of tree growth showed ring growth in the second half of the 20th century that was greater than during any other 50-year period in the last 3,700 years. The accelerated growth is suggestive of an environmental change unprecedented in millennia."
However, they also say that:
"The growth surge has occurred only in a limited elevational band within ≈150 m of upper treeline, regardless of treeline elevation"
It is interesting that the original work of LaMarche et al was at 3400-3500 metres, near the tree line. As Saltzer et al point out, this near tree-line positioning may amplify the effects of warming. Bunn et al 2005 have also performed original work on foxtail and bristlecone pines at similarly high altitudes and found similar results.
Direct temperature measurements
Summary: the earth has warmed by about 0.8 degrees in the past century
Three separate, direct measurements of global surface temperature. NOAA (NCDC) and NASA (GISS) in the US, and the combined Hadley Centre and Climate Research Unit of the University of East Anglia in the UK (HadCRUT3).
The NOAA measurements use microwave measurements of atmospheric temperature and the GISS and HadCRUT3 are mainly due to land based observations. It is interesting to compare the satellite based data with ground observations:
It should be noted that the absolute value of the satellite temperatures (not the variation) is calibrated to be in line with balloon based observations (and vice versa). See Satellite temperature data and Christy and Norris 2006. The agreement between the remote (balloon and satellite) and ground based observations mean that the argument that corrected, earth-based temperature observations are confounded by the proximity of new urban developments is not true.
NASA GISS data for northern and southern hemispheres.
Ice Cores
Ice cores are cylinders of ice, often over a mile long, that have been extracted from the Greenland or Antarctic ice sheets. The cores have annual lines of discoloration caused by variations in dust between summer and winter. The ratio of oxygen isotopes and the amount of dust can be used to assess the rainfall and temperature for each year over thousands of years. The raw ice core data is available from the NOAA Paleoclimatology website, the problems confronting the analysis of the cores are considered in the Trieste 2008 Workshop Report on Ice Cores. A graph showing the calculated temperatures over the past 400,000 years is shown below:
Graph of temperature (blue) CO2 concentration (green) and dust (red) over the past 400,000 years. Vostok ice core (Antarctica). Courtesy Wikimedia Commons.
The average temperature over the past 400,000 years has been at least four degrees colder than the present. Notice how the warming phases are extremely abrupt (note: the time axis has the present on the left). Even the smaller fluctuation seem to occur over very short time scales. The data itself shows that there have been five periods of sudden warming represented in the cores. All except the last period of warming have been followed by prolonged periods of cooling that resulted in ice ages. The last period of warming has not been followed by the dip in temperatures that was typical of the previous warming periods. The ice core data seem to show that the current global temperatures indicate an incipient ice age avoided rather than simple global warming.
The most interesting aspect of the ice-core data for predicting the next 100 years is that sudden temperature swings of 2-4 degrees over a few decades, although rare over the past 5000 years, are quite usual in the longer term.
The Greenland ice cores show a lot of detail for the recent past:
Source Alley, R.B. (2000)
One can only wonder whether civilisation would have occurred if the past 5000 years had been as variable as the previous periods. Notice how the "little ice age" and the "medieval warming" are tiny fluctuations compared with what can naturally occur. Between 10 and 20000 years ago there were frequent swings in temperature that were as large as or greater than the modern "doomsday scenario" of 4 degrees change.
There is a remarkable correlation between the Antarctic and Greenland Oxygen isotope figures from which the temperature estimates are derived.
See also:
The Evidence for Global Warming. (1) the Himalayas
The Evidence for Global Warming. (2) Analysis using source data for global changes
Global warming (3) man or nature?
The strange case of the missing CO2
Global Warming: What Will Change?
Is climate change a threat because of overpopulation?
Blue Haze, Brown Clouds and the need to stop Geoengineering before it begins.
Global warming: what do we do now?
Alley, R.B. (2000) The Younger Dryas cold interval as viewed from central Greenland
Quaternary Science Reviews Volume 19, Issues 1-5, 1 January 2000, Pages 213-226.
Bunn AG, Graumlich LJ, Urban DL (2005) Trends in twentieth-century tree growth at high elevations in the Sierra Nevada and White Mountains, USA. Holocene 15:481–488. http://hol.sagepub.com/cgi/content/abstract/15/4/481
Cazenave et al (2008) Sea level budget over 2003–2008: A reevaluation from GRACE space gravimetry,satellite altimetry and Argo. Global and Planetary Change 65 (2009) 83–88
http://etienne.berthier.free.fr/download/Cazenave_et_al_GPC_2009.pdf
Cazenave, A. and Llovell, W. (2010) Contemporary Sea Level Rise. Annual Review of Marine Science
Vol. 2: 145-173 (Volume publication date January 2010)(doi:10.1146/annurev-marine-120308-081105)
First published online as a Review in Advance on September 28, 2009 http://arjournals.annualreviews.org/doi/abs/10.1146/annurev-marine-120308-081105
Jenkins, A., and D. Holland (2007), Melting of floating ice and sea level rise, Geophys. Res. Lett., 34, L16609, doi:10.1029/2007GL030784.
Kwok, R., G. F. Cunningham, M. Wensnahan, I. Rigor, H. J. Zwally, and D. Yi (2009), Thinning and volume loss of the Arctic Ocean sea ice cover: 2003–2008, J. Geophys. Res., 114, C07005, doi:10.1029/2009JC005312.
LaMarche VC, Jr, Graybill DA, Fritts HC, Rose MR. (1984) Increasing atmospheric carbon dioxide: Tree ring evidence for growth enhancement in natural vegetation. Science 225:1019–1021
Morner, N. (2003) Estimating future sea level changes from past records. Global and Planetary Change 40 (2004) 49 – 54. http://ruby.fgcu.edu/courses/twimberley/EnviroPhilo/PastRecords.pdf
Noerdlinger, P. D., and K. R. Brower (2007), The melting of floating ice
raises the ocean level, Geophys. J. Int., 170(1), 145 – 150, doi:10.1111/
j.1365-246X.2007.03472.x.
Matthew W. Salzer, Malcolm K. Hughes, Andrew G. Bunn, and Kurt F. Kipfmueller
Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes PNAS 2009 106: 20348-20353.
Global warming at about 0.8 degrees per century is happening but as yet there is no need to panic about the next 10 years, the immediate future. Whether or not to be concerned about the more distant future depends upon climate models. The rapid disappearance of the mass of the arctic sea ice (not just its area) is disturbing and suggests that sudden, large scale, unexpected events can occur. The failure of climate models to predict arctic melting means that these models are inaccurate. The ice-core data suggests that at present we are failing to experience an expected dip in global temperatures. The ice cores also show that 1-4 degrees of warming or cooling in a few centuries has happened very frequently in the past 400,000 years but has been rare in the past 5000 years. It appears to be the nature of the global climate system to have wild fluctuations in temperature. What is new is that there are now 7 billion people on the earth who might be affected by these changes so events on the scale of those in pre-history, such as North Africa changing from grassland to desert, will now affect millions or billions of people (See Is climate change a threat because of overpopulation?.
Sea level rise
Summary: sea level rise indicates a steady warming of the oceans that has almost halted in the past six years but is now continuing apace. The ice sheets and glaciers continue to melt.
Measurements of sea-level rise show that sea levels have risen between 1993 and 2007 at a steady rate of about 3.3 mm a year (Cazenave and Llovel 2010). Over the whole period about 30% of the observed rate of rise is due to ocean thermal expansion and 55% results from land ice melt of which 50% is melting glaciers and 50% melting ice-sheets (Casenave et al 2008).
The oceans act to some extent like a giant thermometer. If the seas are warmed they expand and sea levels rise. Sea level rise is also partly due to the melting of ice where that ice is not floating on the sea (for instance the ice in Greenland or Antartica).
The measurement of sea levels has been greatly expedited in recent years by satellite based measurements, especially the the TOPEX/Poseidon joint NASA/CNES (French Space Agency) Satellite, and the twin satellites of the Gravity Recovery And Climate Experiment (GRACE), a joint mission of NASA and the German Space Agency launched in 2002. The gravity measurements allow the contributions of thermal expansion and added water volume from melting ice to be separated.
"..thermal expansion contributed by ∼ 50% to the 3.1 mm/yr observed global mean sea level rise during the 1993–2003 decade, the remaining rate of rise being essentially explained by shrinking of land ice. Recently published results suggest that since about 2003, ocean thermal expansion change, based on the newly deployed Argo system, is showing a plateau while sea level is still rising, although at a reduced rate (∼ 2.5 mm/yr)."(Casenave et al 2008)
Antarctica is contributing about 0.54 to 0.56 mm a year to sea level rise and Greenland about 0.38 mm a year. Most interestingly the ice sheets of Greenland and Antarctica have both changed from net accumulation of water (as snow) to net liberation of water (as melt) in the past 3-5 years.
Skeptics have claimed that the TOPEX/Poseidon instruments lacked the accuracy to perform sea level measurements, with wild claims that the accuracy was less than +/- a metre. The TOPEX/Poseidon measurements did indeed require a lot of statistical treatment but the JASON-2 missions can measure radial position to better than 1.5 cm on each sample and can achieve submillimetre accuracy on multiple measurements. Morner (2003) is often quoted as showing that the TOPEX/Poseidon data do not show any sea level rise but he chose a limited period and solely used TOPEX/Poseidon raw data without the statistical corrections required on the early instruments. If the subsequent, highly accurate missions had shown no sea level rise then Morner would have been vindicated but, as can be seen from the graph, the sea level rise is real. The current trend is for about 29 cm (1 foot) sea level rise in a century and there is, as yet, no sign of acceleration of the rise.
The historical data from sea level gauges is consistent with the modern satellite data:
Courtesy Wikimedia Commons.
The recent slow down in sea level rise due to expansion as a result of warming is clear from data from sea surface temperature measurements provided by NASA (GISS):
See Goddard Institute for Space Studies
It is also clear from this graph that 5 or 10 years of "bucking the trend" is normal but should not make us imagine that the trend will not resume.
Sea ice loss
Summary: the loss of arctic sea ice in recent years has been extraordinary. It is clear evidence of marked warming of the Arctic Ocean
Loss of sea ice is indicative of warmer poles and warmer ocean currents.
ICEsat satellite data shows that in the Arctic Ocean:
"Along with a more than 42% decrease in multiyear (MY) ice coverage since 2005, there was a remarkable thinning of ∼0.6 m in MY ice thickness over 4 years. In contrast, the average thickness of the seasonal ice in midwinter (∼2 m), which covered more than two-thirds of the Arctic Ocean in 2007, exhibited a negligible trend. Average winter sea ice volume over the period, weighted by a loss of ∼3000 km3 between 2007 and 2008, was ∼14,000 km3. The total MY ice volume in the winter has experienced a net loss of 6300 km3 (>40%) in the 4 years since 2005.."(Kwok et al 2009)
The image above is based on NASA data ( http://earthobservatory.nasa.gov/IOTD/view.php?id=8126 ). Image hosted by Wikimedia Commons.
Sea ice is less dense than sea water and the net effect of melting all the sea ice would be about a 40 mm rise in sea level (Noerdlinger and Brower 2007). Melting sea ice may also transiently reduce sea level because of the cooling effect (Jenkins and Holland 2007). The most important effect of melting the sea ice is likely to be a decrease in the reflectivity of the earth, leading to further heating (the arctic ice reflects almost all incident light, when it has gone the arctic ocean will absorb most incident light). It is disturbing that almost all the climate models used by the IPCC missed the recent, sudden change in the arctic (See Stroeve et al 2007. Arctic sea ice decline: Faster than forecast). (The loss of sea ice has remained in 2007-2010).
Tree ring studies
Summary: studies of bristlecone pine tree rings (dendroclimatology) appear to show that the tree line is getting warmer.
Dendroclimatology, the reconstruction of historical climate from the width of tree rings, was one of the first techniques to show that global warming might be occurring. The early work was done on Bristlecone Pines which are some of the oldest trees in the world (see for instance LaMarche et al 1984). There is an excellent, accessible, primary source of recent dendroclimatology data at Saltzer et al (2009). This seems to show that zone near the tree line, the maximum altitude at which bristlecones grow, is becoming warmer:
"Great Basin bristlecone pine (Pinus longaeva) at 3 sites in western North America near the upper elevation limit of tree growth showed ring growth in the second half of the 20th century that was greater than during any other 50-year period in the last 3,700 years. The accelerated growth is suggestive of an environmental change unprecedented in millennia."
However, they also say that:
"The growth surge has occurred only in a limited elevational band within ≈150 m of upper treeline, regardless of treeline elevation"
It is interesting that the original work of LaMarche et al was at 3400-3500 metres, near the tree line. As Saltzer et al point out, this near tree-line positioning may amplify the effects of warming. Bunn et al 2005 have also performed original work on foxtail and bristlecone pines at similarly high altitudes and found similar results.
Direct temperature measurements
Summary: the earth has warmed by about 0.8 degrees in the past century
 |
| Three separate, direct measurements of global surface temperature. NOAA (NCDC) and NASA (GISS) in the US, and the combined Hadley Centre and Climate Research Unit of the University of East Anglia in the UK (HadCRUT3). |
Three separate, direct measurements of global surface temperature. NOAA (NCDC) and NASA (GISS) in the US, and the combined Hadley Centre and Climate Research Unit of the University of East Anglia in the UK (HadCRUT3).
The NOAA measurements use microwave measurements of atmospheric temperature and the GISS and HadCRUT3 are mainly due to land based observations. It is interesting to compare the satellite based data with ground observations:
It should be noted that the absolute value of the satellite temperatures (not the variation) is calibrated to be in line with balloon based observations (and vice versa). See Satellite temperature data and Christy and Norris 2006. The agreement between the remote (balloon and satellite) and ground based observations mean that the argument that corrected, earth-based temperature observations are confounded by the proximity of new urban developments is not true.
NASA GISS data for northern and southern hemispheres.
Ice Cores
Ice cores are cylinders of ice, often over a mile long, that have been extracted from the Greenland or Antarctic ice sheets. The cores have annual lines of discoloration caused by variations in dust between summer and winter. The ratio of oxygen isotopes and the amount of dust can be used to assess the rainfall and temperature for each year over thousands of years. The raw ice core data is available from the NOAA Paleoclimatology website, the problems confronting the analysis of the cores are considered in the Trieste 2008 Workshop Report on Ice Cores. A graph showing the calculated temperatures over the past 400,000 years is shown below:
Graph of temperature (blue) CO2 concentration (green) and dust (red) over the past 400,000 years. Vostok ice core (Antarctica). Courtesy Wikimedia Commons.
The average temperature over the past 400,000 years has been at least four degrees colder than the present. Notice how the warming phases are extremely abrupt (note: the time axis has the present on the left). Even the smaller fluctuation seem to occur over very short time scales. The data itself shows that there have been five periods of sudden warming represented in the cores. All except the last period of warming have been followed by prolonged periods of cooling that resulted in ice ages. The last period of warming has not been followed by the dip in temperatures that was typical of the previous warming periods. The ice core data seem to show that the current global temperatures indicate an incipient ice age avoided rather than simple global warming.
The most interesting aspect of the ice-core data for predicting the next 100 years is that sudden temperature swings of 2-4 degrees over a few decades, although rare over the past 5000 years, are quite usual in the longer term.
The Greenland ice cores show a lot of detail for the recent past:
Source Alley, R.B. (2000)
One can only wonder whether civilisation would have occurred if the past 5000 years had been as variable as the previous periods. Notice how the "little ice age" and the "medieval warming" are tiny fluctuations compared with what can naturally occur. Between 10 and 20000 years ago there were frequent swings in temperature that were as large as or greater than the modern "doomsday scenario" of 4 degrees change.
There is a remarkable correlation between the Antarctic and Greenland Oxygen isotope figures from which the temperature estimates are derived.
See also:
The Evidence for Global Warming. (1) the Himalayas
The Evidence for Global Warming. (2) Analysis using source data for global changes
Global warming (3) man or nature?
The strange case of the missing CO2
Global Warming: What Will Change?
Is climate change a threat because of overpopulation?
Blue Haze, Brown Clouds and the need to stop Geoengineering before it begins.
Global warming: what do we do now?
Alley, R.B. (2000) The Younger Dryas cold interval as viewed from central Greenland
Quaternary Science Reviews Volume 19, Issues 1-5, 1 January 2000, Pages 213-226.
Bunn AG, Graumlich LJ, Urban DL (2005) Trends in twentieth-century tree growth at high elevations in the Sierra Nevada and White Mountains, USA. Holocene 15:481–488. http://hol.sagepub.com/cgi/content/abstract/15/4/481
Cazenave et al (2008) Sea level budget over 2003–2008: A reevaluation from GRACE space gravimetry,satellite altimetry and Argo. Global and Planetary Change 65 (2009) 83–88
http://etienne.berthier.free.fr/download/Cazenave_et_al_GPC_2009.pdf
Cazenave, A. and Llovell, W. (2010) Contemporary Sea Level Rise. Annual Review of Marine Science
Vol. 2: 145-173 (Volume publication date January 2010)(doi:10.1146/annurev-marine-120308-081105)
First published online as a Review in Advance on September 28, 2009 http://arjournals.annualreviews.org/doi/abs/10.1146/annurev-marine-120308-081105
Jenkins, A., and D. Holland (2007), Melting of floating ice and sea level rise, Geophys. Res. Lett., 34, L16609, doi:10.1029/2007GL030784.
Kwok, R., G. F. Cunningham, M. Wensnahan, I. Rigor, H. J. Zwally, and D. Yi (2009), Thinning and volume loss of the Arctic Ocean sea ice cover: 2003–2008, J. Geophys. Res., 114, C07005, doi:10.1029/2009JC005312.
LaMarche VC, Jr, Graybill DA, Fritts HC, Rose MR. (1984) Increasing atmospheric carbon dioxide: Tree ring evidence for growth enhancement in natural vegetation. Science 225:1019–1021
Morner, N. (2003) Estimating future sea level changes from past records. Global and Planetary Change 40 (2004) 49 – 54. http://ruby.fgcu.edu/courses/twimberley/EnviroPhilo/PastRecords.pdf
Noerdlinger, P. D., and K. R. Brower (2007), The melting of floating ice
raises the ocean level, Geophys. J. Int., 170(1), 145 – 150, doi:10.1111/
j.1365-246X.2007.03472.x.
Matthew W. Salzer, Malcolm K. Hughes, Andrew G. Bunn, and Kurt F. Kipfmueller
Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes PNAS 2009 106: 20348-20353.
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- Norman