Overview
For the 2000 years prior to 1880, the earth was cooler than it now is, and was cooling. However, a growth curve best describes measured global temperatures since 1880. The ten warmest years on record have occurred in the 12-year period 1997-2008, and the pace of warming may be increasing. While mean temperature is now rising rapidly, seasonal variability appears to be decreasing.
Measured Global Annual Temperature Anomalies, 1880-2008
Measurements of the planet's temperature have been
recorded since 1880. In the graphs in this paper, we report
measurements as "anomalies." Anomalies are
computed relative to the base period 1951-1980. The reason to
work with anomalies, rather than absolute temperature, is that
absolute temperature varies markedly in short distances, while
monthly or annual temperature anomalies are representative of a
much larger region. Indeed, Hansen and Lebedeff (1987) have
shown that temperature anomalies are strongly correlated out to
distances of the order of 1000 km.
In the graph below, we fit linear and quadratic functions to the measurements. The linear function is -11.0263 + 0.00566083 x. A straight line fits the data well, with a correlation of temperature with time of .74.
The quadratic function is 161.594 - 0.171997 x + 0.000045694 x^2. This curve fits the data even better than the straight line, with a correlation of temperature with time of .79. Since 1995, though, 10 of the 13 measured years had global temperatures that were above the curve, suggesting that the pace of warming is increasing further.
:Source: GHCN 1880-11/2008 meteorological stations, using elimination of outliers and homogeneity adjustment. Downloaded from http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts.txt. 2008 data from http://data.giss.nasa.gov/gistemp/2008/.
Estimated Global Annual Temperature Anomalies for the past
Tree rings, ice cores, lake sediments, and other clues can be used as indices of temperature, and with tremendous effort and skill, can be used to estimate local temperature -- and with multiple points measured, global temperature. Scientific techniques used in these estimates continue to improve, but are already quite good. In this section, we look at estimates from multiple sources, including Briffa, Esper, Crowley and Lowery, Jones, Mann (1999), and three estimating procedures used by Jones and Mann (2004). Anomalies reported here are the smoothed series data.
In the graph below, the "Medieval Warm Period" (around 800-1300 AD) and the Little Ice Age which followed are discernable only with imagination, for they were European phenomena. As NOAA notes, "records that do exist show is that there was no multi-century periods when global or hemispheric temperatures were the same or warmer than in the 20th century."
Jones and Mann (2004) conclude "Our assessment affirms the conclusion that late 20th century warmth is unprecedented at hemispheric and, likely, global scales... Comparison of empirical evidence with proxy-based reconstructions demonstrates that natural factors appear to explain relatively well the major surface temperature changes of the past millennium through the 19th century (including hemispheric means and some spatial patterns). Only anthropogenic forcing of climate, however, can explain the recent anomalous warming in the late 20th century."
:Sources: GHCN 1880-11/2008 meteorological stations, using elimination of outliers and homogeneity adjustment. Downloaded from http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts.txt. Non-instrumental records retrieved from http://www.ncdc.noaa.gov/paleo/pubs/jones2004/jones2004.h tml and ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_auth or/jones2004/jonesmannrogfig5.txt.
Summarizing Estimated Global Annual Temperature Anomalies
The eight estimates of global temperature shown in the previous graph do not show any clear pattern visually, other than the rise in temperatures in recent years. To summarize the estimates, we fit a linear and quadratic trend line to each of the datasets from their earliest estimate to the year 1800. Every estimate shows cooler temperatures than in the base period (Anomalies are computed relative to the base period 1951-1980) and a gradual decline in mean global temperatures through the year 1800.
Eight additional datasets are available
from
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_auth
or/jones2004/jonesmannrogfig8.txt
Briffa et. al., 2001
Data based on tree ring density data that have been processed using a novel statistical technique (age band decomposition) designed to preserve greater long-timescale variability than in previous analyses. The 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period. In the analyses below, we examine the Briffa data only to 1880.
For the period 1402-1880, Brifa's estimates show cooler temperatures than in the base period (Anomalies are computed relative to the base period 1951-1980) and a gradual decline in mean global temperatures.
Source: http://www.ncdc.noaa.gov/paleo/pubs/briffa2001/briffa2001.ht ml
Esper et. al, 2002
Data based on carefully selected tree-ring chronologies from 14 sites in the Northern Hemisphere.
For the period 840-1880, Esper's estimates show cooler temperatures than in the base period (Anomalies are computed relative to the base period 1951-1980) and a gradual decline in mean global temperatures.
Source: http://www.sciencemag.org/cgi/content/abstract/295/5563/22 50
Crowley and Lowery (2000)
incorporating additional time series not used in earlier hemispheric compilations. Another difference is that the present reconstruction uses records that are only 900-1000 years long, thereby, avoiding the potential problem of uncertainties introduced by using different numbers of records at different times. Despite clear evidence for Medieval warmth greater than present in some individual records, the new hemispheric composite supports the principal conclusion of earlier hemispheric reconstructions and, furthermore, indicates that maximum Medieval warmth was restricted to two-three 20-30 year intervals, with composite values during these times being only comparable to the mid-20th century warm time interval. Failure to substantiate hemispheric warmth greater than the present con-sistently occurs in composites
For the period 1000-1880, Crowley and Lowery's estimates show cooler temperatures than in the base period (Anomalies are computed relative to the base period 1951-1980) and a gradual decline in mean global temperatures.
Source: Crowley and Lowery 2000 (Ambio 29, 51) Northern Hemisphere Temperature Reconstruction Modified as published in Crowley 2000 (Science v289 p.270, 14 July 2000. Available from ftp://ftp.ncdc.noaa.gov/pub/data/paleo/gcmoutput/crowley200 0/crowley_lowery2000_nht.txt.
Jones (1994)
Data based on land air temperature anomalies, expressed as anomalies from 1961-1990. The dataset has been extensively used in the various IPCC reports (see, e.g., Nicholls et al., 1996).
For the period 1000-1880, Jones' estimates show cooler temperatures than in the base period (Anomalies are computed relative to the base period 1961-1990) and a gradual decline in mean global temperatures.
Source: http://www.ncdc.noaa.gov/paleo/pubs/jones2004/jones2004.h tml
Mann et al (1999)
Building on recent studies, the authors attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. Results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 20th century warming counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing.
For the period 1000-1880, Mann et al's estimates show cooler temperatures than in the base period (Anomalies are computed relative to the base period 1951-1980) and a gradual decline in mean global temperatures.
Source: http://www.ncdc.noaa.gov/paleo/pubs/mann_99.html
Jones and Mann, 2004
Building on recent studies, the authors attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. Natural factors appear to explain relatively well the major surface temperature changes of the past millennium through the 19th century (including hemispheric means and some spatial patterns). The authors argue that only anthropogenic forcing of climate, however, can explain the recent anomalous warming in the late 20th century.
The GLB data from Jones and Mann were used for the graph and trend values.
For the period 200-1880, Mann et al's estimates show cooler temperatures than in the base period (Anomalies are computed relative to the base period 1951-1980) and a gradual decline in mean global temperatures.
Source: http://www.ncdc.noaa.gov/paleo/pubs/jones2004/jones2004.h tml
Global Temperature, 200-2008
In the graph below, measured temperature anomalies for the period 1880-2008 are combined with estimates of temperature anomalies for the past 2,000 years.
Trends in Seasonal Variability of Temperature Anomalies,
We might wonder whether the warming is occurring equally in each month, or whether the shift in mean temperature is accompanied by a shift in the variability of temperature. To determine this, we computed the standard deviation for the 12 month anomaly distribution for each year. The plot below suggests that the variability of temperature within a year has decreased over the period measured.
References
Briffa, K.R., T. J. Osborn, F.H. Schweingruber, I.C. Harris, P. D. Jones, S.G. Shiyatov, and E.A. Vaganov. Low-frequency Temperature Variations from a Northern Tree Ring Density Network Journal of Geophysical Research, 106 D3 (16-Feb-2001) pp. 2929-2941
Crowley, Thomas J. and Thomas S. Lowery (2000). "Northern Hemisphere Temperature Reconstruction". Ambio 29: 51-54.
Esper, Jan, Edward R.
Cook,and Fritz H. Schweingruber Low-Frequency Signals in
Long Tree-Ring Chronologies for Reconstructing Past
Temperature Variability Science 22 March 2002:
Vol. 295.
no. 5563, pp. 2250 - 2253 DOI: 10.1126/science.1066208.
http://www.sciencemag.org/cgi/content/abstract/295/5563/22
50
Hansen, J., R. Ruedy, J. Glascoe, and Mki. Sato, 1999: GISS analysis of surface temperature change. J. Geophys. Res., 104, 30997-31022, doi:10.1029/1999JD900835. http://pubs.giss.nasa.gov/abstracts/1999/Hansen_etal.html
Jones, P.D., 1994: Hemispheric surface air temperature variations: a reanalysis and an update to 1993. J. Climate 7, 1794-1802.
Jones P. D., M. E. Mann (2004), Climate over past millennia, Rev. Geophys., 42, RG2002, doi:10.1029/2003RG000143.
Mann, Michael E., Bradley, Raymond S., and Hughes, Malcolm K. Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations. © 1999, American Geophysical Union Geophysical Research Letters, Vol. 26, No. 6, p.759
Peterson, T.C., and R.S. Vose, 1997: An overview of the Global Historical Climatology Network temperature database. Bull. Amer. Meteorol. Soc. 78, 2837-2849.
Reynolds, R.W., and T.M. Smith, 1994: Improved global sea surface temperature analyses. J. Climate 7, 929-948.
Scientific Committee on Antarctic Research
(SCAR), www.scar.org.
Other Papers in this Series
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- Trends in Global Temperature: Appendix In this appendix to Trends in Global Temperature, we examine monthly anomalies, seasonal anomalies, and mean global temperature in degrees centigrade. The Northern Hemisphere has generally warmed more than the Southern. This gap emerged in about 1920, disappeared between 1965 and 1990, but has re-emerged. Warming in the Northern Hemisphere has been most severe in the Arctic. Between 1920 and 1960, the Arctic was warmer than normal, and the Antarctic was colder than normal. Since 1980, both Arctic and Antarctic have been warmer than normal, with the greatest warming occurring in the Arctic. Last revised Sunday, January 11, 2009.
- Trends in Global Temperature For the 2000 years prior to 1880, the earth was cooler than it now is, and was cooling. However, a growth curve best describes measured global temperatures since 1880. The ten warmest years on record have occurred in the 12-year period 1997-2008, and the pace of warming may be increasing. While mean temperature is now rising rapidly, seasonal variability appears to be decreasing. Last revised Wednesday, January 14, 2009.
- Trends in Snow and Ice Cover In the Northern Hemisphere, the average snow extent has decreased by 3.75 million square Km in the last 39 years. The greatest loss of snow cover has occurred in the months of June and July. In the Northern Hemisphere, the average extent of sea ice decreased by 1.5 million square Km during the period 1978-2008, from about 9.9 million square Km to about 8.4. In the Southern Hemisphere in this period, the extent of sea ice did not change significantly. Reduced snow and ice cover changes albedo, increasing the rate of warming. Last revised Friday, January 16, 2009.
- "Anthropogenic": A Look at Critical Correlates of Human Population Growth abstract Last revised Wednesday, January 14, 2009.
© 2009 David Stang. All rights reserved. Dr. Stang is available for presentations on this topic. Contact him by email. |
Last Revised: Wednesday, January 14, 2009